Endophytic colonization of Vitis vinifera L. by plant growth-promoting bacterium Burkholderia sp. strain PsJN

Exploring endophytic bacteria communities of Vanilla planifolia

BACKGROUND

Rhizosphere bacterial community and endophytes are now known to influence plant health and response to environmental stress. Very few studies have reported the diversity of endophytic bacterial communities of Vanilla planifolia and their potential roles in promoting plant growth or contributing to aromatic quality.

RESULTS

In this study, the composition and diversity of the Vanilla rhizosphere bacterial community were explored by analyzing rhizosphere soil and root tissue samples as well as green pods of three accessions of Vanilla planifolia grown on different types of substrates (compost and leaf litter). In addition, the endophytic bacterial diversity of roots and green pods as well as the evolution of endophytic bacteria after the curing process of vanilla green pods were analyzed based on a metabarcoding approach. The results showed that bacterial species richness and diversity were higher in the compost. The analysis of the soil bacterial composition displayed that Halomonas, Pseudoalteromonas, Enterobacter and Bradyrhizobium were the most abundant genera. Moreover, the results indicated that the soil bacterial community structure was linked to the host plant genotype. Regarding the roots endophytic bacteria composition, the genera Halomonas, Pseudoalteromonas, Bacillus and Carboxydocella genera were present in all samples, independently from the substrate nature. Several genera including Bacillus, Bradyrhizobium, Burkholderia and Halomonas were transmitted internally from the roots to the green pods. The curing process reduced the bacterial richness and bacterial diversity associated with the green pods. Halomonas, Pseudoalteromonas, Bacillus, and Carboxydocella are the dominant genera in the pods after the curing process.

CONCLUSIONS

This study provides an overview of changes of the bacterial communities dynamics especially endophytic in the roots and the green pods. It highlighted bacterial genera (Halomonas, Pseudoalteromonas, Bacillus, and Carboxydocella) potentially implicated in the formation of aroma compounds of vanilla beans.

Global insights into endophytic bacterial communities of terrestrial plants: Exploring the potential applications of endophytic microbiota in sustainable agriculture

Endophytic microorganisms are indispensable symbionts during plant growth and development and often serve functions such as growth promotion and stress resistance in plants. Therefore, an increasing number of researchers have applied endophytes for multifaceted phytoremediation (e.g., organic pollutants and heavy metals) in recent years. With the availability of next-generation sequencing technologies, an increasing number of studies have shifted the focus from culturable bacteria to total communities. However, information on the composition, structure, and function of bacterial endophytic communities is still not widely synthesized. To explore the general patterns of variation in bacterial communities between plant niches, we reanalyzed data from 1499 samples in 30 individual studies from different continents and provided comprehensive insights. A group of bacterial genera were commonly found in most plant roots and shoots. Our analysis revealed distinct variations in the diversity, composition, structure, and function of endophytic bacterial communities between plant roots and shoots. These variations underscore the sophisticated mechanisms by which plants engage with their endophytic microbiota, optimizing these interactions to bolster growth, health, and resilience against stress. Highlighting the strategic role of endophytic bacteria in promoting sustainable agricultural practices and environmental stewardship, our study not only offers global insights into the endophytic bacterial communities of terrestrial plants but also underscores the untapped potential of these communities as invaluable resources for future research.

Recent progress in the role of seed endophytic bacteria as plant growth-promoting microorganisms and biocontrol agents

The importance of microorganisms residing within the host plant for their growth and health is increasingly acknowledged, yet the significance of microbes associated with seeds, particularly seed endophytic bacteria, remains underestimated. Seeds harbor a wide range of bacteria that can boost the growth and resilience of their host plants against environmental challenges. These endophytic associations also offer advantages for germination and seedling establishment, as seed endophytic bacteria are present during the initial stages of plant growth and development. Furthermore, plants can selectively choose bacteria possessing beneficial traits, which are subsequently transmitted through seeds to confer benefits to future generations. Interestingly, even with the ongoing discovery of endophytes in seeds through high-throughput sequencing methods, certain endophytes remain challenging to isolate and culture from seeds, despite their high abundance. These challenges pose difficulties in studying seed endophytes, making many of their effects on plants unclear. In this article, a framework for understanding the assembly and function of seed endophytes, including their sources and colonization processes was outlined in detail and available research on bacterial endophytes discovered within the seeds of various plant species has also been explored. Thus, this current review aims to provide valuable insights into the mechanism of underlying seed endophytic bacteria-host plant interactions and offers significant recommendations for utilizing the seed endophytic bacteria in sustainable agriculture as plant growth promoters and enhancers of environmental stress tolerance.

Trophic interactions in microbiomes influence plant host population size and ecosystem function

Plant microbiomes that comprise diverse microorganisms, including prokaryotes, eukaryotes and viruses, are the key determinants of plant population dynamics and ecosystem function. Despite their importance, little is known about how species interactions (especially trophic interactions) between microbes from different domains modify the importance of microbiomes for plant hosts and ecosystems. Using the common duckweed Lemna minor, we experimentally examined the effects of predation (by bacterivorous protists) and parasitism (by bacteriophages) within microbiomes on plant population size and ecosystem phosphorus removal. Our results revealed that the addition of predators increased plant population size and phosphorus removal, whereas the addition of parasites showed the opposite pattern. The structural equation modelling further pointed out that predation and parasitism affected plant population size and ecosystem function via distinct mechanisms that were both mediated by microbiomes. Our results highlight the importance of understanding microbial trophic interactions for predicting the outcomes and ecosystem impacts of plant-microbiome symbiosis.

Evaluation of Antifungal Activity of Endophytic Bacillus spp. and Identification of Secondary Metabolites Produced Against the Phytopathogenic Fungi

Endophytic bacteria serve as a rich source of diverse antimicrobial compounds. Recently, there has been a growing interest in utilizing endophytic Bacillus spp. as biological agents against phytogenic fungi, owing to their potential to produce a wide range of antimicrobial substances. The objective of this research was to investigate the protective abilities of 15 endophytic Bacillus spp. isolated from previous study from wheat plant, against the phytopathogenic fungi, Fusarium graminearum and Macrophomina phaseolina. A dual culture plate assay was conducted as a preliminary analysis, revealing that 7 out of 15 endophytic Bacillus spp. demonstrated inhibition against one or both of the phytopathogenic fungi used in this study. All seven endophytes were further assessed for the presence of diffusible antifungal metabolites. The cultures were grown in potato dextrose broth for 120 h, and the cell-free supernatant was extracted and analyzed using the cup plate method. The methanolic extract yielded similar results to the dual culture plate analysis, except for WL2-15. Additionally, deformities in the mycelial structure were examined under the light microscope upon exposure to methanolic extract. Furthermore, the analysis and identification of metabolites were carried out via gas chromatography-mass spectrometry of methanolic extract from selected seven endophytic Bacillus spp. The chromatogram revealed the presence of some major peaks such as tridecanoic acid, methyl ester, hydroperoxide, 1-methylbutyl, 9-octadecenamide, (z)-, hexane-1,3,4-triol, 3,5-dimethyl- tetradecanoic acid. To the best of our knowledge, this is the first report of these biocontrol agents in endophytic Bacillus spp. Interestingly, volatile organic compound production was also seen in all the isolates against the phytopathogenic fungi.

Bioprospecting Study of Plant Growth Promoting Rhizospheric Bacteria from Oil Palm Plantation as Biological Control Agent of Ganoderma boninense

<b>Background and Objective:</b> The prioritisation of oil palm studies involves the exploration of novel bacterial isolates as possible agents for suppressing <i>Ganoderma boninense</i>. The objective of this study was to evaluate and characterise the potential of rhizospheric bacteria, obtained from the rhizosphere of oil palm plants, in terms of their ability to demonstrate anti-<i>Ganoderma </i>activity. <b>Materials and Methods:</b> The study began by employing a dual culture technique to select hostile bacteria. Qualitative detection was performed to assess the antifungal activity, as well as the synthesis of chitinase and glucanase, from certain isolates. The candidate strains were molecularly identified using 16S-rRNA ribosomal primers, specifically the 27F and 1492R primers. <b>Results:</b> The findings of the study indicated that the governmental plantation exhibited the highest ratio between diazotroph and indigenous bacterial populations in comparison to the other sites. Out of a pool of ninety bacterial isolates, a subset of twenty-one isolates demonstrated the ability to impede the development of <i>G. boninense</i>, as determined using a dual culture experiment. Twenty-one bacterial strains were found to exhibit antifungal activity. Nine possible bacteria were found based on the sequence analysis. These bacteria include <i>Burkholderia territorii</i> (RK2, RP2, RP3, RP5), <i>Burkholderia stagnalis</i> (RK3), <i>Burkholderia cenocepacia</i> (RP1), <i>Serratia marcescens</i> (RP13) and <i>Rhizobium multihospitium</i> (RU4). <b>Conclusion:</b> The findings of the study revealed that a significant proportion of the bacterial population exhibited the ability to perform nitrogen fixation, indole-3-acetic acid (IAA) production and phosphate solubilization. However, it is worth noting that <i>Rhizobium multihospitium</i> RU4 did not demonstrate the capacity for phosphate solubilization, while <i>B. territory</i> RK2 did not exhibit IAA production.

Interspecific hybridization and inoculation with Pantoea eucalypti improve forage performance of Lotus crop species under alkaline stress

This study was designed to elucidate the physiological responses of three Lotus forage accessions to alkaline stress, and the influence of inoculating with Pantoea eucalypti endophyte strain on alkaline stress mitigation. A diploid L. corniculatus (Lc) accession, L. tenuis (Lt), and the interspecific hybrid Lt × Lc obtained from these two parental lines were exposed to alkaline stress (pH 8.2). Both Lt and the Lt × Lc hybrid are alkaline-tolerant compared to Lc, based on observations that dry mass was not reduced under stress, and there were no chlorosis symptoms on leaf blades. In all three Lotus accessions, Fe2+ concentration under stress decreased in aerial parts and simultaneously increased in roots. Inoculation with P. eucalypti considerably increased Fe2+ content in shoots of all three Lotus forage species under alkaline treatment. Photochemical efficiency of PSII was affected in Lc accession only when exposed to alkaline treatment. However, when cultivated under alkalinity with inoculation, plants recovered and had photosynthetic parameters equivalent to those in the control treatment. Together, the results highlight the importance of inoculation with P. eucalypti, which contributes significantly to mitigating alkaline stress. All results provide useful information for improving alkaline tolerance traits of Lotus forage species and their interspecific hybrids.

Antioxidant and Antibacterial Properties of 1,3-Dihydroxy-,2', 2'-Dimethylpyrano-(5,6)-Xanthone from Streptomyces sp. SU84

<b>Background and Objective:</b> The SU84 was isolated from the rhizosphere of <i>Curcuma longa</i> and identified to be <i>Streptomyces</i> sp. via analysis of its 16S rDNA sequence, chemotaxonomy and morphology. This study aimed to isolate major compounds from the extract culture of strain SU84 and evaluate their antibacterial activity. <b>Materials and Methods:</b> The TLC and silica gel column chromatography were used to purify major compounds, elucidate 1,3-dihydroxy-,2',2'-dimethylpyrano-(5,6)-xanthone (compound <b>1</b>) and lupeol (compound <b>2</b>) using mass spectrometry and nuclear magnetic resonance. One new chemical, compound <b>1</b>, was first isolated from microbial sources. Antibacterial, antioxidant and cytotoxic properties of these compounds were carried out. <b>Results:</b> Various bioassays showed that compound <b>1</b> displayed antibacterial property against Gram-positive bacteria, with a minimum inhibitory concentration of 8-32 μg/mL and minimum bactericidal concentration of 32-128 μg/mL. In addition, the purified compounds were tested against normal cell lines using tetrazolium assay. The results did not show cytotoxic property against L929 and Vero cells, with IC₅₀ values of >512.00 μg/mL. Compounds <b>1</b> and <b>2</b> have also antioxidant properties, with IC₅₀ values of 16.67±7.48 and 38.86±8.45 μg/mL, respectively. <b>Conclusion:</b> The findings suggested that compounds of <i>Streptomyces</i> sp. SU84 displayed antibacterial and antioxidant properties without cytotoxic activity. Extensive studies of compound <b>1</b> may be useful for the advancement of improved methods for avoidance, control and management of bacterial infections and metabolic-related free radical contribution.

Biosynthesis of polyhydroxybutyrate by Methylorubrum extorquens DSM13060 is essential for intracellular colonization in plant endosymbiosis

Methylorubrum extorquens DSM13060 is an endosymbiont that lives in the cells of shoot tip meristems. The bacterium is methylotrophic and consumes plant-derived methanol for the production of polyhydroxybutyrate (PHB). The PHB provides protection against oxidative stress for both host and endosymbiont cells through its fragments, methyl-esterified 3-hydroxybutyrate (ME-3HB) oligomers. We evaluated the role of the genes involved in the production of ME-3HB oligomers in the host colonization by the endosymbiont M. extorquens DSM13060 through targeted genetic mutations. The strains with deletions in PHB synthase (phaC), PHB depolymerase (phaZ1), and a transcription factor (phaR) showed altered PHB granule characteristics, as ΔphaC had a significantly low number of granules, ΔphaR had a significantly increased number of granules, and ΔphaZ1 had significantly large PHB granules in the bacterial cells. When the deletion strains were exposed to oxidative stress, the ΔphaC strain was sensitive to 10 mM HO· and 20 mM H2O2. The colonization of the host, Scots pine (Pinus sylvestris L.), by the deletion strains varied greatly. The deletion strain ΔphaR colonized the host mainly intercellularly, whereas the ΔphaZ1 strain was a slightly poorer colonizer than the control. The deletion strain ΔphaC lacked the colonization potential, living mainly on the surfaces of the epidermis of pine roots and shoots in contrast to the control, which intracellularly colonized all pine tissues within the study period. In earlier studies, deletions within the PHB metabolic pathway have had a minor effect on plant colonization by rhizobia. We have previously shown the association between ME-3HB oligomers, produced by PhaC and PhaZ1, and the ability to alleviate host-generated oxidative stress during plant infection by the endosymbiont M. extorquens DSM13060. Our current results show that the low capacity for PHB synthesis leads to poor tolerance of oxidative stress and loss of colonization potential by the endosymbiont. Altogether, our findings demonstrate that the metabolism of PHB in M. extorquens DSM13060 is an important trait in the non-rhizobial endosymbiosis.

Root colonization by beneficial rhizobacteria

Rhizosphere microbes play critical roles for plant's growth and health. Among them, the beneficial rhizobacteria have the potential to be developed as the biofertilizer or bioinoculants for sustaining the agricultural development. The efficient rhizosphere colonization of these rhizobacteria is a prerequisite for exerting their plant beneficial functions, but the colonizing process and underlying mechanisms have not been thoroughly reviewed, especially for the nonsymbiotic beneficial rhizobacteria. This review systematically analyzed the root colonizing process of the nonsymbiotic rhizobacteria and compared it with that of the symbiotic and pathogenic bacteria. This review also highlighted the approaches to improve the root colonization efficiency and proposed to study the rhizobacterial colonization from a holistic perspective of the rhizosphere microbiome under more natural conditions.

Comparative Genome Analyses Provide Insight into the Antimicrobial Activity of Endophytic Burkholderia

Endophytic bacteria are endosymbionts that colonize a portion of plants without harming the plant for at least a part of its life cycle. Bacterial endophytes play an essential role in promoting plant growth using multiple mechanisms. The genus Burkholderia is an important member among endophytes and encompasses bacterial species with high genetic versatility and adaptability. In this study, the endophytic characteristics of Burkholderia species are investigated via comparative genomic analyses of several endophytic Burkholderia strains with pathogenic Burkholderia strains. A group of bacterial genes was identified and predicted as the putative endophytic behavior genes of Burkholderia. Multiple antimicrobial biosynthesis genes were observed in these endophytic bacteria; however, certain important pathogenic and virulence genes were absent. The majority of resistome genes were distributed relatively evenly among the endophytic and pathogenic bacteria. All known types of secretion systems were found in the studied bacteria. This includes T3SS and T4SS, which were previously thought to be disproportionately represented in endophytes. Additionally, questionable CRISPR-Cas systems with an orphan CRISPR array were prevalent, suggesting that intact CRISPR-Cas systems may not exist in symbiotes of Burkholderia. This research not only sheds light on the antimicrobial activities that contribute to biocontrol but also expands our understanding of genomic variations in Burkholderia's endophytic and pathogenic bacteria.

Probing the potential of salinity-tolerant endophytic bacteria to improve the growth of mungbean [Vigna radiata (L.) Wilczek]

Soil salinity is one of the major limiting factors in plant growth regulation. Salinity-tolerant endophytic bacteria (STEB) can be used to alleviate the negative effects of salinity and promote plant growth. In this study, thirteen endophytic bacteria were isolated from mungbean roots and tested for NaCl salt-tolerance up to 4%. Six bacterial isolates, TMB2, TMB3, TMB5, TMB6, TMB7 and TMB9, demonstrated the ability to tolerate salt. Plant growth-promoting properties such as phosphate solubilization, indole-3-acetic acid (IAA) production, nitrogen fixation, zinc solubilization, biofilm formation and hydrolytic enzyme production were tested in vitro under saline conditions. Eight bacterial isolates indicated phosphate solubilization potential ranging from 5.8-17.7 μg mL-1, wherein TMB6 was found most efficient. Ten bacterial isolates exhibited IAA production ranging from 0.3-2.1 μg mL-1, where TMB7 indicated the highest potential. All the bacterial isolates except TMB13 exhibited nitrogenase activity. Three isolates, TMB6, TMB7 and TMB9, were able to solubilize zinc on tris-minimal media. All isolates were capable of forming biofilm except TMB12 and TMB13. Only TMB2, TMB6 and TMB7 exhibited cellulase activity, while TMB2 and TMB7 exhibited pectinase production. Based on in vitro testing, six efficient STEB were selected and subjected to the further studies. 16S rRNA gene sequencing of efficient STEB revealed the maximum similarity between TMB2 and Rhizobium pusense, TMB3 and Agrobacterium leguminum, TMB5 and Achromobacter denitrificans, TMB6 and Pseudomonas extremorientalis, TMB7 and Bradyrhizobium japonicum and TMB9 and Serratia quinivorans. This is the first international report on the existence of A. leguminum, A. denitrificans, P. extremorientalis and S. quinivorans inside the roots of mungbean. Under controlled-conditions, inoculation of P. extremorientalis TMB6, B. japonicum TMB7 and S. quinivorans TMB9 exhibited maximum potential to increase plant growth parameters; specifically plant dry weight was increased by up to 52%, 61% and 45%, respectively. Inoculation of B. japonicum TMB7 displayed the highest potential to increase plant proline, glycine betaine and total soluble proteins contents by 77%, 78% and 64%, respectively, compared to control under saline conditions. It is suggested that the efficient STEB could be used as biofertilizers for mungbean crop productivity under saline conditions after field-testing.

Deciphering the mechanisms, hormonal signaling, and potential applications of endophytic microbes to mediate stress tolerance in medicinal plants

The global healthcare market in the post-pandemic era emphasizes a constant pursuit of therapeutic, adaptogenic, and immune booster drugs. Medicinal plants are the only natural resource to meet this by supplying an array of bioactive secondary metabolites in an economic, greener and sustainable manner. Driven by the thrust in demand for natural immunity imparting nutraceutical and life-saving plant-derived drugs, the acreage for commercial cultivation of medicinal plants has dramatically increased in recent years. Limited resources of land and water, low productivity, poor soil fertility coupled with climate change, and biotic (bacteria, fungi, insects, viruses, nematodes) and abiotic (temperature, drought, salinity, waterlogging, and metal toxicity) stress necessitate medicinal plant productivity enhancement through sustainable strategies. Plants evolved intricate physiological (membrane integrity, organelle structural changes, osmotic adjustments, cell and tissue survival, reclamation, increased root-shoot ratio, antibiosis, hypersensitivity, etc.), biochemical (phytohormones synthesis, proline, protein levels, antioxidant enzymes accumulation, ion exclusion, generation of heat-shock proteins, synthesis of allelochemicals. etc.), and cellular (sensing of stress signals, signaling pathways, modulating expression of stress-responsive genes and proteins, etc.) mechanisms to combat stresses. Endophytes, colonizing in different plant tissues, synthesize novel bioactive compounds that medicinal plants can harness to mitigate environmental cues, thus making the agroecosystems self-sufficient toward green and sustainable approaches. Medicinal plants with a host set of metabolites and endophytes with another set of secondary metabolites interact in a highly complex manner involving adaptive mechanisms, including appropriate cellular responses triggered by stimuli received from the sensors situated on the cytoplasm and transmitting signals to the transcriptional machinery in the nucleus to withstand a stressful environment effectively. Signaling pathways serve as a crucial nexus for sensing stress and establishing plants' proper molecular and cellular responses. However, the underlying mechanisms and critical signaling pathways triggered by endophytic microbes are meager. This review comprehends the diversity of endophytes in medicinal plants and endophyte-mediated plant-microbe interactions for biotic and abiotic stress tolerance in medicinal plants by understanding complex adaptive physiological mechanisms and signaling cascades involving defined molecular and cellular responses. Leveraging this knowledge, researchers can design specific microbial formulations that optimize plant health, increase nutrient uptake, boost crop yields, and support a resilient, sustainable agricultural system.

Core Endophytic Bacteria and Their Roles in the Coralloid Roots of Cultivated Cycas revoluta (Cycadaceae)

As a gymnosperm group, cycads are known for their ancient origin and specialized coralloid root, which can be used as an ideal system to explore the interaction between host and associated microorganisms. Previous studies have revealed that some nitrogen-fixing cyanobacteria contribute greatly to the composition of the endophytic microorganisms in cycad coralloid roots. However, the roles of host and environment in shaping the composition of endophytic bacteria during the recruitment process remain unclear. Here, we determined the diversity, composition, and function prediction of endophytic bacteria from the coralloid roots of a widely cultivated cycad, Cycas revoluta Thunb. Using next-generation sequencing techniques, we comprehensively investigated the diversity and community structure of the bacteria in coralloid roots and bulk soils sampled from 11 sites in China, aiming to explore the variations in core endophytic bacteria and to predict their potential functions. We found a higher microbe diversity in bulk soils than in coralloid roots. Meanwhile, there was no significant difference in the diversity and composition of endophytic bacteria across different localities, and the same result was found after removing cyanobacteria. Desmonostoc was the most dominant in coralloid roots, followed by Nostoc, yet these two cyanobacteria were not shared by all samples. Rhodococcus, Edaphobacter, Niastella, Nordella, SH-PL14, and Virgisporangium were defined as the core microorganisms in coralloid roots. A function prediction analysis revealed that endophytic bacteria majorly participated in the plant uptake of phosphorus and metal ions and in disease resistance. These results indicate that the community composition of the bacteria in coralloid roots is affected by both the host and environment, in which the host is more decisive. Despite the very small proportion of core microbes, their interactions are significant and likely contribute to functions related to host survival. Our study contributes to an understanding of microbial diversity and composition in cycads, and it expands the knowledge on the association between hosts and symbiotic microbes.

Rhizobiome Transplantation: A Novel Strategy beyond Single-Strain/Consortium Inoculation for Crop Improvement

The growing human population has a greater demand for food; however, the care and preservation of nature as well as its resources must be considered when fulfilling this demand. An alternative employed in recent decades is the use and application of microbial inoculants, either individually or in consortium. The transplantation of rhizospheric microbiomes (rhizobiome) recently emerged as an additional proposal to protect crops from pathogens. In this review, rhizobiome transplantation was analyzed as an ecological alternative for increasing plant protection and crop production. The differences between single-strain/species inoculation and dual or consortium application were compared. Furthermore, the feasibility of the transplantation of other associated micro-communities, including phyllosphere and endosphere microbiomes, were evaluated. The current and future challenges surrounding rhizobiome transplantation were additionally discussed. In conclusion, rhizobiome transplantation emerges as an attractive alternative that goes beyond single/group inoculation of microbial agents; however, there is still a long way ahead before it can be applied in large-scale agriculture.

Concurrent application of bacterial-mediated and mycosynthesized ZnO nanofungicides to maintain high ascorbic acid and delay postharvest decay of apricot

Quality of apricot fruit is affected by different biotic stresses during growth, harvesting and storage. Due to fungal attack, huge losses of its quality and quantity are observed. The present research was designed for the diagnoses and management of postharvest rot disease of apricot. Infected apricot fruit were collected, and the causative agent was identified as A. tubingensis. To control this disease, both bacterial-mediated nanoparticles (b-ZnO NPs) and mycosynthesized nanoparticles (f-ZnO NPs) were used. Herein, biomass filtrates of one selected fungus (Trichoderma harzianum) and one bacterium (Bacillus safensis) were used to reduce zinc acetate into ZnO NPs. The physiochemical and morphological characters of both types of NPs were determined. UV-vis spectroscopy displayed absorption peaks of f-ZnO NPs and b-ZnO NPs at 310-380 nm, respectively, indicating successful reduction of Zinc acetate by the metabolites of both fungus and bacteria. Fourier transform infrared (FTIR) determined the presence of organic compounds like amines, aromatics, alkenes and alkyl halides, on both types of NPs, while X-ray diffraction (XRD) confirmed nano-size of f-ZnO NPs (30 nm) and b-ZnO NPs (35 nm). Scanning electron microscopy showed flower-crystalline shape for b-ZnO NPs and spherical-crystalline shape for f-ZnO NPs. Both NPs showed variable antifungal activities at four different concentrations (0.25, 0.50, 0.75 and 1.00 mg/ml). Diseases control and postharvest changes in apricot fruit were analyzed for 15 days. Among all treatments, 0.50 mg/ml concentration of f-ZnO NPs and 0.75 mg/ml concentration of b-ZnO NPs exhibited the strongest antifungal activity. Comparatively, f-ZnO NPs performed slightly better than b-ZnO NPs. Application of both NPs reduced fruit decay and weight, maintained higher ascorbic acid contents, sustained titratable acidity, and preserved firmness of diseased fruit. Our results suggest that microbial synthesized ZnO NPs can efficiently control fruit rot, extend shelf life, and preserve the quality of apricot.

Community-based mechanisms underlying the root cadmium uptake regulated by Cd-tolerant strains in rice (Oryza sativa. L)

In recent years, the problem of Cd pollution in paddy fields has become more and more serious, which seriously threatens the safe production of food crops and human health. Using microorganisms to reduce cadmium pollution in rice fields is a green, safe and efficient method, the complicated interactions between the microbes in rice roots throughout the process of cadmium absorption by rice roots are poorly understood. In this investigation, a hydroponic pot experiment was used to examine the effects of bacteria R3 (Herbaspirillum sp) and T4 (Bacillus cereus) on cadmium uptake and the endophytic bacterial community in rice roots. The results showed that compared with CK (Uninoculated bacterial liquid), the two strains had significant inhibitory or promotive effects on cadmium uptake in rice plant, respectively. Among them, the decrease of cadmium content in rice plants by R3 strain reached 78.57-79.39%, and the increase of cadmium content in rice plants by T4 strain reached 140.49-158.19%. Further investigation showed that the cadmium content and root cadmium enrichment coefficient of rice plants were significantly negatively correlated with the relative abundances of Burkholderia and Acidovorax, and significantly positively correlated with the relative abundances of Achromobacter, Agromyces and Acidocella. Moreover, a more complex network of microbes in rice roots inhibited rice plants from absorbing cadmium. These results suggest that cadmium uptake by rice plants is closely related to the endophytic bacterial community of roots. This study provides a reference scheme for the safe production of crops in cadmium contaminated paddies and lays a solid theoretical foundation for subsequent field applications.

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

Recombination of endophytic bacteria in asexual plant Ligusticum chuanxiong Hort. caused by transplanting

Background

Long-term asexual reproduction can easily lead to the degradation of plant germplasm, serious diseases and insect pests, reduction of production and even catastrophic crop failure. "Mountain Breeding and Dam Cultivation" is the main cultivation mode of Ligusticum chuanxiong Hort., which successfully avoided the germplasm degradation caused by long-term asexual reproduction. The recombination of endophytic fungi of L. chuanxiong caused by off-site transplantation was considered to be an important reason for its germplasm rejuvenation. However, whether bacteria have the same regularity is not yet known.

Methods

In this study, we carried out the experiment of cultivating propagation materials of L. chuanxiong in different regions and transplanting them to the same region. High-throughput sequencing was performed to analyze the bacterial communities in L. chuanxiong and its soil.

Results

The results showed that after transplanting, the plant height, tiller number, fresh weight, etc. of L. chuanxiong in mountainous areas were significantly higher than those in dam areas. At the same time, significant changes had taken place in the endophytic bacteria in reproductive material stem nodes (Lingzi, abbreviated as LZ). The diversity and abundance of bacteria in dam area LZ (YL) are significantly higher than those in mountainous area LZ (ML). The relative abundance of bacteria such as Xanthobacteraceae, Micromonosporaceae, Beijerinkiaceae, Rhodanobacteria, in ML is significantly higher than YL, mainly classified in Proteobateria and Actinobacteriota. In addition, the abundance advantage of Actinobacteriota still exists in MY (underground mature rhizomes obtained by ML). Meanwhile, the bacterial community was different in different area of transplanting. The diversity of bacterial communities in dam soil (YLS) is significantly higher than that in mountain soil (MLS). MLS had more Acidobacteriota than YLS. Comparative analysis showed that 74.38% of bacteria in ML are found in MLS, and 87.91% of bacteria in YL are found in YLS.

Conclusions

We can conclude that the community structure of endophytic bacteria recombined after the transplantation of L. chuanxiong, which was related to the bacterial community in soils. Moreover, after transplanting in mountainous areas, LZ accumulated more potentially beneficial Actinobacteriota, which may be an important reason for promoting the rejuvenation of germplasm in L. chuanxiong. However, this hypothesis requires more specific experiments to verify. This study provided a new idea that off-site transplanting may be a new strategy to restore vegetative plant germplasm resources.

In Vitro Evaluation of Some Endophytic Bacillus to Potentially Inhibit Grape and Grapevine Fungal Pathogens

Romania has a long history of grapevine culturing and winemaking. However, like any agricultural sector, viticulture faces devastating biological threats. Fungi responsible for grapevine trunk diseases (GTDs) and grape spoilage lead to considerable yield losses and a decline in grapevine quality. In the actual context, many countries, including Romania, have reoriented their approaches to minimize chemical inputs, which have been proven to be toxic and to have negative impacts on the environment, and to replace them with sustainable biocontrol strategies for the wine-growing sector. Within biocontrol strategies, Bacillus spp. is a well-known plant-protective bacteria with antifungal properties. Within this paper, six endophytic bacteria from various plant sources were studied. The bacterial strains were identified as B. pumilus, B. subtilis, and B. velezensis by sequencing their 16S rDNA region. Regardless of the in vitro test methods (using living bacterial cells, bacterial-cell-free supernatant (CFS), and volatile active compounds (VOCs)), B. velezensis strains revealed strong and broad antifungal activity against grape and grapevine fungal pathogens such as Aspergillus spp., Botrytis cinerea, Penicillium expansum, Diplodia seriata, Eutypa lata, Fusarium spp., Clonostachys rosea, Neofusicoccum parvum, and Stereum hirsutum. The functional antifungal genes encoding for difficidin, fengycin, iturins, macrolactin, and mycosubtilin were molecularly detected, which could support the proven antifungal activity of the endophytic strains. Lytic enzymes involved in fungal growth inhibition, such as chitinase, cellulase, and proteases, were also revealed to be produced by some of these bacterial strains. Various other in vitro tests, such as phosphate and phytate solubilization, phytohormone synthesis, the production of enzymes involved in the polyamine biosynthetic pathway, and pH as well as temperature tolerance tests were carried out to reveal the plant-beneficial potential of these bacterial strains. These results revealed that the B. velezensis strains, especially BAHs1, are the most suitable endophytes for grapevine biologic control, which could lead to the future development of sustainable management strategies.

Paraburkholderia phytofirmans PsJN colonization of rice endosphere triggers an atypical transcriptomic response compared to rice native Burkholderia s.l. endophytes

The plant microbiome has recently emerged as a reservoir for the development of sustainable alternatives to chemical fertilizers and pesticides. However, the response of plants to beneficial microbes emerges as a critical issue to understand the molecular basis of plant-microbiota interactions. In this study, we combined root colonization, phenotypic and transcriptomic analyses to unravel the commonalities and specificities of the response of rice to closely related Burkholderia s.l. endophytes. In general, these results indicate that a rice-non-native Burkholderia s.l. strain, Paraburkholderia phytofirmans PsJN, is able to colonize the root endosphere while eliciting a markedly different response compared to rice-native Burkholderia s.l. strains. This demonstrates the variability of plant response to microbes from different hosts of origin. The most striking finding of the investigation was that a much more conserved response to the three endophytes used in this study is elicited in leaves compared to roots. In addition, transcriptional regulation of genes related to secondary metabolism, immunity, and phytohormones appear to be markers of strain-specific responses. Future studies need to investigate whether these findings can be extrapolated to other plant models and beneficial microbes to further advance the potential of microbiome-based solutions for crop production.

Role of microbial inoculants as bio fertilizers for improving crop productivity: A review

The world's population is increasing and is anticipated to spread 10 billion by 2050, and the issue of food security is becoming a global concern. To maintain global food security, it is essential to increase crop productivity under changing climatic conditions. Conventional agricultural practices frequently use artificial/chemical fertilizers to enhance crop productivity, but these have numerous negative effects on the environment and people's health. To address these issues, researchers have been concentrating on substitute crop fertilization methods for many years, and biofertilizers as a crucial part of agricultural practices are quickly gaining popularity all over the globe. Biofertilizers are living formulations made of indigenous plant growth-promoting rhizobacteria (PGPR) which are substantial, environment-friendly, and economical biofertilizers for amassing crop productivity by enhancing plant development either directly or indirectly, and are the renewable source of plant nutrients and sustainable agronomy. The review aims to provide a comprehensive overview of the current knowledge on microbial inoculants as biofertilizers, including their types, mechanisms of action, effects on crop productivity, challenges, and limitations associated with the use of microbial inoculants. In this review, we focused on the application of biofertilizers to agricultural fields in plant growth development by performing several activities like nitrogen fixation, siderophore production, phytohormone production, nutrient solubilization, and facilitating easy uptake by crop plants. Further, we discussed the indirect mechanism of PGPRs, in developing induced system resistance against pest and diseases, and as a biocontrol agent for phytopathogens. This review article presents a brief outline of the ideas and uses of microbial inoculants in improving crop productivity as well as a discussion of the challenges and limitations to use microbial inoculants.

Insights into the seed microbiome and its ecological significance in plant life

In recent years, the microbiome has attracted much attention because of the multiple roles and functions that microbes play in plants, animals, and human beings. Seed-associated microbes are of particular interest in being the initial microbial inoculum that affects the critical early life stages of a plant. The seed-microbe interactions are also known to improve nutrient acquisition, resilience against pathogens, and resistance against abiotic stresses. Despite these diverse roles, the seed microbiome has received little attention in plant ecology. Thus, we review the current knowledge on seed microbial diversity, community structure, and functions obtained through culture-dependent and culture-independent approaches. Furthermore, we present a comprehensive synthesis of the ecological literature on seed-microbe interactions to better understand the impact of these interactions on plant health and productivity. We suggest that future research should focus on the role of the seed microbiome in the establishment, colonization and spread of plant species in their native and non-native ranges as it may provide new insights into conservation biology and invasion ecology.

Discovery of Brassica Yellows Virus and Porcine Reproductive and Respiratory Syndrome Virus in Diaphorina citri and Changes in Virome Due to Infection with 'Ca. L. asiaticus'

Detection of new viruses or new virus hosts is essential for the protection of economically important agroecosystems and human health. Increasingly, metatranscriptomic data are being used to facilitate this process. Such data were obtained from adult Asian citrus psyllids (ACP) (Diaphorina citri Kuwayama) that fed solely on mandarin (Citrus ×aurantium L.) plants grafted with buds infected with 'Candidatus Liberibacter asiaticus' (CLas), a phloem-limited bacterium associated with the severe Asian variant of huanglongbing (HLB), the most destructive disease of citrus. Brassica yellows virus (BrYV), the causative agent of yellowing or leafroll symptoms in brassicaceous plants, and its associated RNA (named as BrYVaRNA) were detected in ACP. In addition, the porcine reproductive and respiratory syndrome virus (PRRSV), which affects pigs and is economically important to pig production, was also found in ACP. These viruses were not detected in insects feeding on plants grafted with CLas-free buds. Changes in the concentrations of insect-specific viruses within the psyllid were caused by coinfection with CLas. IMPORTANCE The cross transmission of pathogenic viruses between different farming systems or plant communities is a major threat to plants and animals and, potentially, human health. The use of metagenomics is an effective approach to discover viruses and vectors. Here, we collected buds from the CLas-infected and CLas-free mandarin (Citrus ×aurantium L. [Rutaceae: Aurantioideae: Aurantieae]) trees from a commercial orchard and grafted them onto CLas-free mandarin plants under laboratory conditions. Through metatranscriptome sequencing, we first identified the Asian citrus psyllids feeding on plants grafted with CLas-infected buds carried the plant pathogen, brassica yellows virus and its associated RNA, and the swine pathogen, porcine reproductive and respiratory syndrome virus. These discoveries indicate that both viruses can be transmitted by grafting and acquired by ACP from CLas+ mandarin seedlings.

Inoculation of ACC deaminase-producing endophytic bacteria down-regulates ethylene-induced pathogenesis-related signaling in red pepper (Capsicum annuum L.) under salt stress

Pathogenesis-related (PR) signaling plays multiple roles in plant development under abiotic and biotic stress conditions and is regulated by a plethora of plant physiological as well as external factors. Here, our study was conducted to evaluate the role of an ACC deaminase-producing endophytic bacteria in regulating ethylene-induced PR signaling in red pepper plants under salt stress. We also evaluated the efficiency of the bacteria in down-regulating the PR signaling for efficient colonization and persistence in the plant endosphere. We used a characteristic endophyte, Methylobacterium oryzae CBMB20 and its ACC deaminase knockdown mutant (acdS^- ). The wild-type M. oryzae CBMB20 was able to decrease ethylene emission by 23% compared to the noninoculated and acdS^- M. oryzae CBMB20 inoculated plants under salt stress. The increase in ethylene emission resulted in enhanced hydrogen peroxide concentration, phenylalanine ammonia-lyase activity, β-1,3 glucanase activity, and expression profiles of WRKY, CaPR1, and CaPTI1 genes that are typical salt stress and PR signaling factors. Furthermore, the inoculation of both the bacterial strains had shown induction of PR signaling under normal conditions during the initial inoculation period. However, wild-type M. oryzae CBMB20 was able to down-regulate the ethylene-induced PR signaling under salt stress and enhance plant growth and stress tolerance. Collectively, ACC deaminase-producing endophytic bacteria down-regulate the salt stress-mediated PR signaling in plants by regulating the stress ethylene emission levels and this suggests a new paradigm in efficient colonization and persistence of ACC deaminase-producing endophytic bacteria for better plant growth and productivity.

Mechanism for Utilization of the Populus-Derived Metabolite Salicin by a Pseudomonas-Rahnella Co-Culture

Pseudomonas fluorescens GM16 associates with Populus, a model plant in biofuel production. Populus releases abundant phenolic glycosides such as salicin, but P. fluorescens GM16 cannot utilize salicin, whereas Pseudomonas strains are known to utilize compounds similar to the aglycone moiety of salicin-salicyl alcohol. We propose that the association of Pseudomonas to Populus is mediated by another organism (such as Rahnella aquatilis OV744) that degrades the glucosyl group of salicin. In this study, we demonstrate that in the Rahnella-Pseudomonas salicin co-culture model, Rahnella grows by degrading salicin to glucose 6-phosphate and salicyl alcohol which is secreted out and is subsequently utilized by P. fluorescens GM16 for its growth. Using various quantitative approaches, we elucidate the individual pathways for salicin and salicyl alcohol metabolism present in Rahnella and Pseudomonas, respectively. Furthermore, we were able to establish that the salicyl alcohol cross-feeding interaction between the two strains on salicin medium is carried out through the combination of their respective individual pathways. The research presents one of the potential advantages of salicyl alcohol release by strains such as Rahnella, and how phenolic glycosides could be involved in attracting multiple types of bacteria into the Populus microbiome.

Salinity alleviator bacteria in rice (Oryza sativa L.), their colonization efficacy, and synergism with melatonin

In this study, rhizospheric and endophytic bacteria were tested for the alleviation of salinity stress in rice. Endophytic isolates were taken from previous studies based on their salt stress-alleviating traits. The rhizospheric bacteria were isolated from rice and screened based on salt tolerance and plant growth-promoting traits. Molecular identification indicated the presence of class Gammaproteobacteria, Bacillota, and Actinomycetia. Two-two most potential isolates each from rhizospheric and endophytic bacteria were selected for in planta trials. Results showed that microbial inoculation significantly improved germination and seedling vigor under elevated salinity. The confocal scanning laser microscopy showed higher bacterial colonization in inoculated rice roots than in control. Based on this experiment, rhizospheric bacteria Brevibacterium frigoritolerans W19 and endophytic Bacillus safensis BTL5 were selected for pot trial along with a growth-inducing compound melatonin 20 ppm. Inoculation of these two bacteria improved the levels of chlorophyll, proline, phenylalanine ammonia-lyase, catalase, superoxide dismutase, polyphenol oxidase, root-shoot length, and dry weight under elevated salt concentration. The gene expression studies showed modulation of SOD1, CATa, NHX1, and PAL1 genes by the bacterial strains and melatonin application. The inoculation was found to have additive effects with 20 ppm melatonin. This enhancement in dry matter accumulation, compatible solute production, and oxidative stress regulation could help plants in mitigating the ill effects of high salinity. Exploring such a combination of microbes and inducer molecules could be potentially useful in developing stress-alleviating bioformulations.

Understanding the sugar beet holobiont for sustainable agriculture

The importance of crop-associated microbiomes for the health and field performance of plants has been demonstrated in the last decades. Sugar beet is the most important source of sucrose in temperate climates, and-as a root crop-yield heavily depends on genetics as well as on the soil and rhizosphere microbiomes. Bacteria, fungi, and archaea are found in all organs and life stages of the plant, and research on sugar beet microbiomes contributed to our understanding of the plant microbiome in general, especially of microbiome-based control strategies against phytopathogens. Attempts to make sugar beet cultivation more sustainable are increasing, raising the interest in biocontrol of plant pathogens and pests, biofertilization and -stimulation as well as microbiome-assisted breeding. This review first summarizes already achieved results on sugar beet-associated microbiomes and their unique traits, correlating to their physical, chemical, and biological peculiarities. Temporal and spatial microbiome dynamics during sugar beet ontogenesis are discussed, emphasizing the rhizosphere formation and highlighting knowledge gaps. Secondly, potential or already tested biocontrol agents and application strategies are discussed, providing an overview of how microbiome-based sugar beet farming could be performed in the future. Thus, this review is intended as a reference and baseline for further sugar beet-microbiome research, aiming to promote investigations in rhizosphere modulation-based biocontrol options.

Use of beneficial bacterial endophytes: A practical strategy to achieve sustainable agriculture

Beneficial endophytic bacteria influence their host plant to grow and resist pathogens. Despite the advantages of endophytic bacteria to their host, their application in agriculture has been low. Furthermore, many plant growers improperly use synthetic chemicals due to having no or little knowledge of the role of endophytic bacteria in plant growth, the prevention and control of pathogens and poor access to endobacterial bioproducts. These synthetic chemicals have caused soil infertility, environmental contamination, disruption to ecological cycles and the emergence of resistant pests and pathogens. There is more that needs to be done to explore alternative ways of achieving sustainable plant production while maintaining environmental health. In recent years, the use of beneficial endophytic bacteria has been noted to be a promising tool in promoting plant growth and the biocontrol of pathogens. Therefore, this review discusses the roles of endophytic bacteria in plant growth and the biocontrol of plant pathogens. Several mechanisms that endophytic bacteria use to alleviate plant biotic and abiotic stresses by helping their host plants acquire nutrients, enhance plant growth and development and suppress pathogens are explained. The review also indicates that there is a gap between research and general field applications of endophytic bacteria and suggests a need for collaborative efforts between growers at all levels. Furthermore, the presence of scientific and regulatory frameworks that promote advanced biotechnological tools and bioinoculants represents major opportunities in the applications of endophytic bacteria. The review provides a basis for future research in areas related to understanding the interactions between plants and beneficial endophytic microorganisms, especially bacteria.

Rhizosphere Priestia species altered cowpea root transcriptome and enhanced growth under drought and nutrient deficiency

Priestia species isolated from the cowpea rhizosphere altered the transcriptome of cowpea roots by colonization and enhanced nutrient uptake, antioxidant mechanisms, and photosynthesis, protecting cowpea from drought and nutrient deficiency. Cowpea is a significant grain legume crop primarily grown in sub-Saharan Africa, Asia, and South America. Drought and nutrient deficiency affect the growth and yield of cowpea. To address this challenge, we studied the phyto-beneficial effects of stress-tolerant rhizobacteria on the biomass yield of cowpea under water- and nutrient-deficit conditions. Among the bacteria isolated, two rhizobacillus genotypes, C8 (Priestia filamentosa; basonym: Bacillus filamentosus) and C29 (Priestia aryabhattai; basonym: Bacillus aryabhattai) were evaluated for the improvement of seed germination and growth of cowpea under stress. Our study revealed that C8 protected cowpea from stress by facilitating phosphorus and potassium uptake, protecting it from oxidative damage, reducing transpiration, and enhancing CO₂ assimilation. A 17% increase in root biomass upon C8 inoculation was concomitant with the induction of stress tolerance genes in cowpea roots predominantly involved in growth and metabolic processes, cell wall organization, ion homeostasis, and cellular responses to phosphate starvation. Our results indicate a metabolic alteration in cowpea root triggered by P. filamentosa, leading to efficient nutrient reallocation in the host plant. We propose inoculation with P. filamentosa as an effective strategy for improving the yield of cowpea in low-input agriculture, where chemical fertilization and irrigation are less accessible to resource-poor farmers.

Effects of manure fertilization on human pathogens in endosphere of three vegetable plants

Pathogens can colonize plant endosphere and, be transferred into human beings through the food chain. However, our understanding of the influences of agricultural activities, such as fertilization, on endophytic microbial communities and human pathogens is still limited. Here, we conducted a microcosm experiment using the combination of 16 S rRNA gene amplicon sequencing and high-throughput qPCR array to reveal the effects of manure fertilization on microbiomes of soils and plants and how such impact is translated into endophytic pathogens. Our results showed that manure fertilization significantly altered soil microbiomes, whereas with less influence on endophytic microbial communities. Soil is a vital source of both bacterial communities and human pathogens for the plant endosphere. The abundance of pathogens was increased both in soils and endosphere under manure fertilization. These findings provide an integrated understanding of the impact of manure fertilization on endophytic pathogens.

Plant-soil-microbes: A tripartite interaction for nutrient acquisition and better plant growth for sustainable agricultural practices

Plants can achieve their proper growth and development with the help of microorganisms associated with them. Plant-associated microbes convert the unavailable nutrients to available form and make them useful for plants. Besides nutrient acquisition, soil microbes also inhibit the pathogens that cause harm to plant growth and induces defense response. Due to the beneficial activities of soil nutrient-microbe-plant interactions, it is necessary to study more on this topic and develop microbial inoculant technology in the agricultural field for better crop improvement. The soil microbes can be engineered, and plant growth-promoting rhizobacteria (PGPR) and plant growth-promoting bacteria (PGPB) technology can be developed as well, as its application can be improved for utilization as biofertilizer, biopesticides, etc., instead of using harmful chemical biofertilizers. Moreover, plant growth-promoting microbe inoculants can enhance crop productivity. Although, scientists have discussed several tools and techniques by omics and gene editing approaches for crop improvement to avoid biotic and abiotic stress and make the plant healthier and more nutritive. However, beneficial soil microbes that help plants with the nutrient acquisition, development, and stress resistance were ignored, and farmers started utilizing chemical fertilizers. Thus, this review attempts to summarize the interaction system of plant microbes, the role of beneficiary soil microbes in the rhizosphere zone, and their role in plant health promotion, particularly in the nutrition acquisition of the plant. The review will also provide a better understanding of soil microbes that can be exploited as biofertilizers and plant growth promoters in the field to create environmentally friendly, sustainable agriculture systems.

Plant Growth Promotion and Heat Stress Amelioration in Arabidopsis Inoculated with Paraburkholderia phytofirmans PsJN Rhizobacteria Quantified with the GrowScreen-Agar II Phenotyping Platform

High temperatures inhibit plant growth. A proposed strategy for improving plant productivity under elevated temperatures is the use of plant growth-promoting rhizobacteria (PGPR). While the effects of PGPR on plant shoots have been extensively explored, roots-particularly their spatial and temporal dynamics-have been hard to study, due to their below-ground nature. Here, we characterized the time- and tissue-specific morphological changes in bacterized plants using a novel non-invasive high-resolution plant phenotyping and imaging platform-GrowScreen-Agar II. The platform uses custom-made agar plates, which allow air exchange to occur with the agar medium and enable the shoot to grow outside the compartment. The platform provides light protection to the roots, the exposure of it to the shoots, and the non-invasive phenotyping of both organs. Arabidopsis thaliana, co-cultivated with Paraburkholderia phytofirmans PsJN at elevated and ambient temperatures, showed increased lengths, growth rates, and numbers of roots. However, the magnitude and direction of the growth promotion varied depending on root type, timing, and temperature. The root length and distribution per depth and according to time was also influenced by bacterization and the temperature. The shoot biomass increased at the later stages under ambient temperature in the bacterized plants. The study offers insights into the timing of the tissue-specific, PsJN-induced morphological changes and should facilitate future molecular and biochemical studies on plant-microbe-environment interactions.

Endophyte-Mediated Stress Tolerance in Plants: A Sustainable Strategy to Enhance Resilience and Assist Crop Improvement

Biotic and abiotic stresses severely affect agriculture by affecting crop productivity, soil fertility, and health. These stresses may have significant financial repercussions, necessitating a practical, cost-effective, and ecologically friendly approach to lessen their negative impacts on plants. Several agrochemicals, such as fertilizers, pesticides, and insecticides, are used to improve plant health and protection; however, these chemical supplements have serious implications for human health. Plants being sessile cannot move or escape to avoid stress. Therefore, they have evolved to develop highly beneficial interactions with endophytes. The targeted use of beneficial plant endophytes and their role in combating biotic and abiotic stresses are gaining attention. Therefore, it is important to experimentally validate these interactions and determine how they affect plant fitness. This review highlights research that sheds light on how endophytes help plants tolerate biotic and abiotic stresses through plant–symbiont and plant–microbiota interactions. There is a great need to focus research efforts on this vital area to achieve a system-level understanding of plant–microbe interactions that occur naturally.

Endophytes and their potential in biotic stress management and crop production

Biotic stress is caused by harmful microbes that prevent plants from growing normally and also having numerous negative effects on agriculture crops globally. Many biotic factors such as bacteria, fungi, virus, weeds, insects, and nematodes are the major constrains of stress that tends to increase the reactive oxygen species that affect the physiological and molecular functioning of plants and also led to the decrease in crop productivity. Bacterial and fungal endophytes are the solution to overcome the tasks faced with conventional farming, and these are environment friendly microbial commodities that colonize in plant tissues without causing any damage. Endophytes play an important role in host fitness, uptake of nutrients, synthesis of phytohormone and diminish the injury triggered by pathogens via antibiosis, production of lytic enzymes, secondary metabolites, and hormone activation. They are also reported to help plants in coping with biotic stress, improving crops and soil health, respectively. Therefore, usage of endophytes as biofertilizers and biocontrol agent have developed an eco-friendly substitute to destructive chemicals for plant development and also in mitigation of biotic stress. Thus, this review highlighted the potential role of endophytes as biofertilizers, biocontrol agent, and in mitigation of biotic stress for maintenance of plant development and soil health for sustainable agriculture.

Spatial variations of root-associated bacterial communities of alpine plants in the Qinghai-Tibet Plateau

Exploring the geospatial variation of root-associated microbiomes is critical for understanding plant-microbe-environment interactions and plant environmental adaptability. Root-associated bacterial communities from the three compartments [rhizosphere surrounding soil (RSS), rhizosphere soil (rhizosphere), and root endosphere (endophytic)] are influenced by multiple factors, including plant species and geographical locations. Nonetheless, these communities remain poorly understood under harsh conditions. In this study, we selected four dominant alpine plants on the Qinghai-Tibet Plateau (i.e., Elymus nutans, Festuca sinensis, Kobresia pygmaea, and Kobresia humilis) to investigate their root-associated bacterial communities across 11 geographical locations and determine the factors driving spatial variation. The results showed that the microbiota of the three compartments had significantly different community compositions, with more Pseudomonadaceae and Enterobacteriaceae present in the endosphere. Spatial variations in root endophytic microbiota were mainly governed by stochastic processes, which were different from the deterministic processes in the other two compartments. Meanwhile, the geographical location had greater effects on bacterial communities than plant species, and the spatial variation of α-diversity in the endosphere was much higher than that in the RSS and rhizosphere. We further found that the differentiation of bacterial diversity in the endosphere among sympatric plant species was enhanced by higher annual precipitation, lower soil nutrients (carbon and nitrogen), and pH. For example, the coefficient of variation of endosphere Pseudomonadaceae abundance was positively correlated with annual mean precipitation, whereas that of Enterobacteriaceae abundance was negatively correlated with soil pH. The co-occurrence network analysis identified a higher proportion of bacterial coexistence in the endosphere (70.9%) than in the RSS (49.5%) and rhizosphere soil (50.9%). Finally, we revealed the relative convergence of endophytic communities among sympatric plant species in the alpine grasslands.

Endophytic Burkholderia: Multifunctional roles in plant growth promotion and stress tolerance

The genus Burkholderia has proven potential in improving plant performance. In recent decades, a huge diversity of Burkholderia spp. have been reported with diverse capabilities of plant symbiosis which could be harnessed to enhance plant growth and development. Colonization of endophytic Burkholderia spp. have been extensively studied through techniques like advanced microscopy, fluorescent labelling, PCR based assays, etc., and found to be systemically distributed in plants. Thus, use of these biostimulant microbes holds the promise of improving quality and quantity of crops. The endophytic Burkholderia spp. have been found to support plant functions along with boosting nutrient availability, especially under stress. Endophytic Burkholderia spp. improve plant survival against deadly pathogens via mechanisms like competition, induced systemic resistance, and antibiosis. At the same time, they are reported to extend plant tolerance towards multiple abiotic stresses especially drought, salinity, and cold. Several attempts have been made to decipher the potential of Burkholderia spp. by genome mining, and these bacteria have been found to harbour genes for plant symbiosis and for providing multiple benefits to host plants. Characteristics specific for host recognition and nutrient acquisition were confirmed in endophytic Burkholderia by genomics and proteomics-based studies. This could pave the way for harnessing Burkholderia spp. for biotechnological applications like biotransformation, phytoremediation, insecticidal activity, antimicrobials, etc. All these make Burkholderia spp. a promising microbial agent in improving plant performance under multiple adversities. Thus, the present review highlights critical roles of endophytic Burkholderia spp., their colonization, alleviation of biotic and abiotic stresses, biotechnological applications and genomic insights.

Halotolerant rhizobacteria isolated from a mangrove forest alleviate saline stress in Musa acuminata cv. Berangan

Saline soils resulting from anthropogenic activity and climate change present a challenge to future food security. Towards addressing this, we isolated and characterized halotolerant bacteria from a Malaysian mangrove forest, and explored their effect on morpho-physiological and biochemical parameters of banana plantlets under salt stress. A total of 88 rhizobacterial and 16 endophytic bacterial isolates collected from the roots and rhizosphere of Rhizophora apiculata, Avicennia alba and Sonneratia alba, were found to tolerate up to 400 mM of sea salt. Based on best performance in multiple plant growth traits, three rhizobacterial strains RB1, RB3 and RB4 and three endophytic bacterial strains EB1, EB2 and EB3 were used for further analysis. The rhizobacterial strains were identified as Bacillus sp. and endophytic bacteria as Pseudomonas sp. based on 16 S rRNA gene sequence. SEM observation confirmed colonization of each strain on banana plantlet roots. When colonized plantlets were subjected to 90 mM salt and compared to uninoculated (control) and mock inoculated plants, improved plant growth was observed with each of the strains, especially with bacterial strains EB3 and RB3. Biochemical analysis of plantlets revealed that root colonization with EB3 and RB3 enhanced levels of plant chlorophyll (> 5-fold), carotenoid (> 2.85-fold) and proline (2.6-fold and 2.3-fold), while plantlets also showed reduced MDA content (0.45-fold and 0.51-fold), significantly reduced generation of ROS (0.23-fold and 0.47-fold) and lower levels of electrolyte leakage (0.77 and 0.51-fold). Antioxidant enzymes also showed enhanced activity with EB3 and RB3. Our results indicate that these halotolerant Bacillus and Pseudomonas strains from the mangrove have multifunctional plant growth promoting activity and can reduce salt stress in bananas. This data provides a reference for exploring halotolerant microbes from hypersaline environments to overcome salt stress in plants.

The antifungal activity of a serine protease and the enzyme production of characteristics of Bacillus licheniformis TG116

The growth of Phytophthora capsica, Rhizoctonia solani, Fusarium graminearum, Fusarium oxysporum and Botrytis cinerea were all inhibited by the fermentation supernatant of Bacillus licheniformis TG116, a biocontrol strain isolated from Typhonium giganteum Engl. previously with broad-spectrum resistance to plant pathogens. The fermentation supernatant of the TG116 has a great stability on temperature and UV, and shows the biological activity of protease and cellulase. The antifungal protease produced by B. licheniformis TG116 was purified to homogeneity by ammonium sulfate precipitation, DEAE Sepharose Fast Flow column chromatography and Sephadex G-50 column chromatography. The inhibition of protease by the three surfactants increased with increasing concentration inhibition. Among these surfactants, EDTA showed the strongest inhibition, with only 25% protein activity at a concentration of 1.1 mmol·L^-1. Gene amplification verified the presence of a gene fragment of serine protease in the strain TG116. The antimicrobial substance isolated from the fermentation broth of TG116 is a serine protease component.

Effects of single- and mixed-bacterial inoculation on the colonization and assembly of endophytic communities in plant roots

The introduction and inoculation of beneficial bacteria in plants have consistently been considered as one of the most important ways to improve plant health and production. However, the effects of bacterial inoculation on the community assembly and composition of the root endophytic microbiome remain largely unknown. In this study, 55 strains were randomly isolated from tomato roots and then inoculated into wheat seeds singly or in combination. Most of the isolated bacterial strains showed an ability to produce lignocellulose-decomposing enzymes and promote plant growth. The results demonstrated that bacterial inoculation had a significant effect on the wheat root endophytic microbiome. The wheat root samples inoculated with single-bacterial species were significantly separated into two groups (A and B) that had different community structures and compositions. Among these, root endophytic communities for most wheat samples inoculated with a single-bacterial strain (Group A) were predominated by one or several bacterial species, mainly belonging to Enterobacterales. In contrast, only a few of the root samples inoculated with a single-bacterial strain (Group B) harbored a rich bacterial flora with relatively high bacterial diversity. However, wheat roots inoculated with a mixed bacterial complex were colonized by a more diverse and abundant bacterial flora, which was mainly composed of Enterobacterales, Actinomycetales, Bacillales, Pseudomonadales, and Rhizobiales. The results demonstrated that inoculation with bacterial complexes could help plants establish more balanced and beneficial endophytic communities. In most cases, bacterial inoculation does not result in successful colonization by the target bacterium in wheat roots. However, bacterial inoculation consistently had a significant effect on the root microbiome in plants. CAP analysis demonstrated that the variation in wheat root endophytic communities was significantly related to the taxonomic status and lignocellulose decomposition ability of the inoculated bacterial strain (p < 0.05). To reveal the role of lignocellulose degradation in shaping the root endophytic microbiome in wheat, four bacterial strains with different colonization abilities were selected for further transcriptome sequencing analysis. The results showed that, compared with that in the dominant bacterial species Ent_181 and Ent_189 of Group A, the expression of lignocellulose-decomposing enzymes was significantly downregulated in Bac_133 and Bac_71 (p < 0.05). In addition, we found that the dominant bacterial species of the tomato endophytic microbiome were more likely to become dominant populations in the wheat root microbiome. In general, our results demonstrated that lignocellulose-decomposing enzymes played a vital role in the formation of endophytes and their successful colonization of root tissues. This finding establishes a theoretical foundation for the development of broad-spectrum probiotics.

Rhizobiome engineering: Unveiling complex rhizosphere interactions to enhance plant growth and health

Crop plants are affected by a series of inhibitory environmental and biotic factors that decrease their growth and production. To counteract these adverse effects, plants work together with the microorganisms that inhabit their rhizosphere, which is part of the soil influenced by root exudates. The rhizosphere is a microecosystem where a series of complex interactions takes place between the resident microorganisms (rhizobiome) and plant roots. Therefore, this study analyzes the dynamics of plant-rhizobiome communication, the role of exudates (diffusible and volatile) as a factor in stimulating a diverse rhizobiome, and the differences between rhizobiomes of domesticated crops and wild plants. The study also analyzes different strategies to decipher the rhizobiome through both classical cultivation techniques and the so-called "omics" sciences. In addition, the rhizosphere engineering concept and the two general strategies to manipulate the rhizobiome, i.e., top down and bottom up engineering have been revisited. In addition, recent studies on the effects on the indigenous rhizobiome of inoculating plants with foreign strains, the impact on the endobiome, and the collateral effects on plant crops are discussed. Finally, understanding of the complex rhizosphere interactions and the biological repercussions of rhizobiome engineering as essential steps for improving plant growth and health is proposed, including under adverse conditions.

Bioprospects of Endophytic Bacteria in Plant Growth Promotion and Ag-Nanoparticle Biosynthesis

In this study, five endophytic bacterial strains, namely Rhizobium pusense (MS-1), Bacillus cereus MS-2, Bacillus flexus (MS-3), Methylophilus flavus (MS-4), and Pseudomonas aeruginosa (MS-5), were used to investigate their potential role in the enhancement of growth yields of two types of tomato varieties, viz. hybrid and local, and in the biosynthesis of silver nanoparticles (AgNPs). The inoculation of bacterial strains enhanced the root and shoot length, biomass, and leaf chlorophyll contents. The fruit weight of the tomato (kg/plant) was also higher in the bacteria inoculated plants of both hybrid and local varieties than in the control (untreated). A significant increase was recorded in the fruit yield (g/plant) in all the treatments, whereas Methylophilus flavus (MS-4) inoculated plants yielded nearly 2.5 times more fruit weight compared to the control in the hybrid variety and two times higher in the local variety. The response to M. flavus as a microbial inoculant was greater than to the other strains. Biosynthesis of Ag nanoparticles was also carried out using all five endophytic bacterial strains. The weakest producers of AgNPs were Rhizobium pusense (MS-1) and Methylophilus flavus (MS-4), while Bacillus cereus MS-2, Bacillus flexus (MS-3), and Pseudomonas aeruginosa (MS-5) were strong producers of AgNPs. Nanoparticles were further characterized using high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), UV-Vis spectrophotometry, and X-ray diffraction (XRD) analysis, and revealed cuboidal shaped AgNPs in the Bacillus cereus MS-2 strain. In addition, the biosynthesized AgNPs showed antibacterial activity against various pathogenic and endophytic bacterial strains.

A highly conserved core bacterial microbiota with nitrogen-fixation capacity inhabits the xylem sap in maize plants

Microbiomes are important for crop performance. However, a deeper knowledge of crop-associated microbial communities is needed to harness beneficial host-microbe interactions. Here, by assessing the assembly and functions of maize microbiomes across soil types, climate zones, and genotypes, we found that the stem xylem selectively recruits highly conserved microbes dominated by Gammaproteobacteria. We showed that the proportion of bacterial taxa carrying the nitrogenase gene (nifH) was larger in stem xylem than in other organs such as root and leaf endosphere. Of the 25 core bacterial taxa identified in xylem sap, several isolated strains were confirmed to be active nitrogen-fixers or to assist with biological nitrogen fixation. On this basis, we established synthetic communities (SynComs) consisting of two core diazotrophs and two helpers. GFP-tagged strains and ¹⁵N isotopic dilution method demonstrated that these SynComs do thrive and contribute, through biological nitrogen fixation, 11.8% of the total N accumulated in maize stems. These core taxa in xylem sap represent an untapped resource that can be exploited to increase crop productivity.

The plant xylem microbiota remains understudied. Here, the authors characterise the xylem microbiota in maize plants finding that some bacteria carried N fixing genes. By using synthetic communities the authors confirm that xylem inhabiting and N fixing bacteria provide the host plant with N.

Unraveling Nitrogen Fixing Potential of Endophytic Diazotrophs of Different Saccharum Species for Sustainable Sugarcane Growth

Sugarcane (Saccharum officinarum L.) is one of the world’s highly significant commercial crops. The amounts of synthetic nitrogen (N₂) fertilizer required to grow the sugarcane plant at its initial growth stages are higher, which increases the production costs and adverse environmental consequences globally. To combat this issue, sustainable environmental and economic concerns among researchers are necessary. The endophytic diazotrophs can offer significant amounts of nitrogen to crops through the biological nitrogen fixation mediated nif gene. The nifH gene is the most extensively utilized molecular marker in nature for studying N₂ fixing microbiomes. The present research intended to determine the existence of novel endophytic diazotrophs through culturable and unculturable bacterial communities (EDBCs). The EDBCs of different tissues (root, stem, and leaf) of five sugarcane cultivars (Saccharum officinarum L. cv. Badila, S. barberi Jesw.cv Pansahi, S. robustum, S. spontaneum, and S. sinense Roxb.cv Uba) were isolated and molecularly characterized to evaluate N₂ fixation ability. The diversity of EDBCs was observed based on nifH gene Illumina MiSeq sequencing and a culturable approach. In this study, 319766 operational taxonomic units (OTUs) were identified from 15 samples. The minimum number of OTUs was recorded in leaf tissues of S. robustum and maximum reads in root tissues of S. spontaneum. These data were assessed to ascertain the structure, diversity, abundance, and relationship between the microbial community. A total of 40 bacterial families with 58 genera were detected in different sugarcane species. Bacterial communities exhibited substantially different alpha and beta diversity. In total, 16 out of 20 genera showed potent N₂-fixation in sugarcane and other crops. According to principal component analysis (PCA) and hierarchical clustering (Bray–Curtis dis) evaluation of OTUs, bacterial microbiomes associated with root tissues differed significantly from stem and leaf tissues of sugarcane. Significant differences often were observed in EDBCs among the sugarcane tissues. We tracked and validated the plethora of individual phylum strains and assessed their nitrogenase activity with a culture-dependent technique. The current work illustrated the significant and novel results of many uncharted endophytic microbial communities in different tissues of sugarcane species, which provides an experimental system to evaluate the biological nitrogen fixation (BNF) mechanism in sugarcane. The novel endophytic microbial communities with N₂-fixation ability play a remarkable and promising role in sustainable agriculture production.

Identification and Characterization of a Multifunctional Biocontrol Agent, Streptomyces griseorubiginosus LJS06, Against Cucumber Anthracnose

Twenty-eight bacterial strains isolated from Chinese herb extracts, beer fermentation waste, and raw oyster shells were evaluated for their efficacy in controlling cucumber anthracnose. Four bacterial strains, namely TG01, TG02, LJS06, and LJS08, were found to effectively reduce the mycelial growth of Colletotrichum orbiculare COC3 on PDA media. Spraying or drenching LJS06 spore suspension before inoculation significantly p < 0.05 reduced disease severity; thus, LJS06 was subject to further characterization. On the basis of the morphological, physiological, and biochemical characteristics and a multilocus sequence analysis of partial 16S rRNA, atpD, rpoB, and trpB genes, LJS06 was identified to be Streptomyces griseorubiginosus (Ryabova and Preobrazhenskaya) Pridham et al. Physiological and biochemical tests revealed that S. griseorubiginosus LJS06 can produce amylase, cellulase, chitinase, protease, siderophore, polyamines, and indole-3-acetic acid. Thus, a culture filtrate of LJS06 (specifically SL06) was formulated and evaluated for its efficacy against conidial germination, appressorium formation, and anthracnose management. Diluted SL06 was found to significantly (p < 0.05) inhibit conidial germination and appressorium formation, which can be attributed to impaired membrane integrity, accumulated reactive oxygen species (ROS), and impaired energy metabolism in the conidia. In addition, the spraying and drenching of diluted SL06 before inoculation consistently and significantly (p < 0.05) reduced anthracnose severity. These results jointly suggest that S. griseorubiginosus LJS06 can aid in the management of cucumber anthracnose by directly inhibiting conidial function and priming the plant defense system.

Plasmopara viticola the Causal Agent of Downy Mildew of Grapevine: From Its Taxonomy to Disease Management

Plasmopara viticola (P. viticola, Berk. & M. A. Curtis; Berl. & De Toni) causing grapevine downy mildew is one of the most damaging pathogens to viticulture worldwide. Since its recognition in the middle of nineteenth century, this disease has spread from America to Europe and then to all grapevine-growing countries, leading to significant economic losses due to the lack of efficient disease control. In 1885 copper was found to suppress many pathogens, and is still the most effective way to control downy mildews. During the twentieth century, contact and penetrating single-site fungicides have been developed for use against plant pathogens including downy mildews, but wide application has led to the appearance of pathogenic strains resistant to these treatments. Additionally, due to the negative environmental impact of chemical pesticides, the European Union restricted their use, triggering a rush to develop alternative tools such as resistant cultivars breeding, creation of new active ingredients, search for natural products and biocontrol agents that can be applied alone or in combination to kill the pathogen or mitigate its effect. This review summarizes data about the history, distribution, epidemiology, taxonomy, morphology, reproduction and infection mechanisms, symptoms, host-pathogen interactions, host resistance and control of the P. viticola, with a focus on sustainable methods, especially the use of biocontrol agents.

Phytostimulating Potential of Endophytic Bacteria from Ethnomedicinal Plants of North-East Indian Himalayan Region

North-East Indian Himalayan Region has a humid subtropical climate having diverse ecosystems. The majority of the population of the region depends on agriculture for sustainable livelihood. However, it can produce only 1.5% of the country’s food grains, thereby importing from other parts of the country for consumption. To feed the increase in the population of the region, there is an urgent need to augment the agricultural and allied products to sustain the population and uplift the economic conditions. Plant beneficial endophytes isolated from ethnomedicinal plants of North-East India play an important role as a plant growth promoter by the production of phytohormones, solubilization and mobilization of mineral nutrients. It also indirectly promotes growth by protecting the plants from diseases through the production of antibiotics, enzymes and volatile compounds. The bacteria also have the potential to induce systemic resistance against various abiotic stresses. Since the region has various agro-climatic conditions, the plants are continuously affected by abiotic stress particularly, acidity, drought and waterlogging, there is a need to explore the indigenous endophytes that can mitigate the stress and enhance the sustainable development of agricultural products.

Cytobacts: Abundant and Diverse Vertically Seed-Transmitted Cultivation-Recalcitrant Intracellular Bacteria Ubiquitous to Vascular Plants

We have recently described ‘Cytobacts’ as abundant intracellular endophytic bacteria inhabiting live plant cells based on the observations with callus and cell suspension cultures of grapevine and other plant species with the origin ascribable to field explants. In this study, we investigated the prevalence of such cytoplasmic bacterial associations in field plants across different taxa, their cultivability, and the extent of taxonomic diversity and explored the possibility of their embryo-mediated vertical transmission. Over 100 genera of field plants were surveyed for ‘Cytobacts’ through bright-field live-cell imaging as per our previous experience using fresh tissue sections from surface-sterilized shoot-tissues with parallel cultivation-based assessments. This revealed widespread cellular bacterial associations visualized as copious motile micro-particles in the cytoplasm with no or sparse colony forming units (CFU) from the tissue-homogenates indicating their general non-cultivability. Based on the ease of detection and the abundance of ‘Cytobacts’ in fresh tissue sections, the surveyed plants were empirically classified into three groups: (i) motile bacteria detected instantly in most cells; (ii) motility not so widely observed, but seen in some cells; and (iii) only occasional motile units observed, but abundant non-motile bacterial cells present. Microscopy versus 16S-rRNA V3–V4 amplicon profiling on shoot-tip tissues of four representative plants—tomato, watermelon, periwinkle, and maize—showed high bacterial abundance and taxonomic diversity (11–15 phyla) with the dominance of Proteobacteria followed by Firmicutes/Actinobacteria, and several other phyla in minor shares. The low CFU/absence of bacterial CFU from the tissue homogenates on standard bacteriological media endorsed their cultivation-recalcitrance. Intracellular bacterial colonization implied that the associated organisms are able to transmit vertically to the next generation through the seed-embryos. Microscopy and 16S-rRNA V3–V4 amplicon/metagenome profiling of mature embryos excised from fresh watermelon seeds revealed heavy embryo colonization by diverse bacteria with sparse or no CFU. Observations with grapevine fresh fruit-derived seeds and seed-embryos endorsed the vertical transmission by diverse cultivation-recalcitrant endophytic bacteria (CREB). By and large, Proteobacteria formed the major phylum in fresh seed-embryos with varying shares of diverse phyla. Thus, we document ‘Cytobacts’ comprising diverse and vertically transmissible CREBs as a ubiquitous phenomenon in vascular plants.

An Overview of the Mechanisms Against "Candidatus Liberibacter asiaticus": Virulence Targets, Citrus Defenses, and Microbiome

Citrus Huanglongbing (HLB) or citrus greening, is the most destructive disease for citrus worldwide. It is caused by the psyllid-transmitted, phloem-limited bacteria "Candidatus Liberibacter asiaticus" (CLas). To date, there are still no effective practical strategies for curing citrus HLB. Understanding the mechanisms against CLas can contribute to the development of effective approaches for combatting HLB. However, the unculturable nature of CLas has hindered elucidating mechanisms against CLas. In this review, we summarize the main aspects that contribute to the understanding about the mechanisms against CLas, including (1) CLas virulence targets, focusing on inhibition of virulence genes; (2) activation of citrus host defense genes and metabolites of HLB-tolerant citrus triggered by CLas, and by agents; and (3) we also review the role of citrus microbiome in combatting CLas. Finally, we discuss novel strategies to continue studying mechanisms against CLas and the relationship of above aspects.