Streptomyces: implications and interactions in plant growth promotion

Biocontrol potential of Streptomyces sp. N2 against green and blue mold disease in postharvest navel orange and the action mechanism

The objective of this study was to provide a promising alternative to chemical fungicides for management of postharvest citrus decay, thereby promoting sustainable citrus fruit production. The postharvest decay of citrus fruit caused by Penicillium digitatum and Penicillium italicum results in substantial economic losses in citrus industry worldwide. With growing fungal resistance issues in P. digitatum and P. italicum, there is an urgent need for searching new methods to address above problems in a safe and environmentally friendly way. Streptomyces sp. N2, a new species from Streptomyces genus, exhibits significant antagonistic activity against Rhizoctonia solani. However, its biocontrol efficacy against postharvest decay caused by P. digitatum and P. italicum and its action mechanism remain unknown. In this study, Streptomyces sp. N2 was found to have significant potential in controlling green and blue mold diseases in postharvest navel oranges. Moreover, the action mechanism of Streptomyces sp. N2 against both P. italicum and P. digitatum was elucidated. On the one hand, Streptomyces sp. N2 stimulated fruit resistance to fight against invading fungal pathogens. It significantly reduced ROS content in navel orange upon the infection of mold disease, increased the production of defense-related enzymes including peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) and pathogenesis-related proteins of chitinase and β-1,3-glucanase. On the other hand, Streptomyces sp. N2 secreted bioactive substances to inhibit the growth of P. italicum and P. digitatum so as to prevent the development of postharvest decay. The bioactive substances secreted by Streptomyces sp. N2 significantly inhibited the spore germination and mycelial growth and led to microstructural damages to the cell wall and membrane, ROS burst, and mitochondrial dysfunction in both P. italicum and P. digitatum. This study provides a theoretical reference and application potential for the biological control of Streptomyces sp. N2 on green and blue mold diseases.

Exploration of a multifunctional biocontrol agent Streptomyces sp. JCK-8055 for the management of apple fire blight

Apple fire blight, caused by the bacterium Erwinia amylovora, is a devastating disease of apple and pear trees. Biological control methods have attracted much attention from researchers to manage plant diseases as they are eco-friendly and viable alternatives to synthetic pesticides. Herein, we isolated Streptomyces sp. JCK-8055 from the root of pepper and investigated its mechanisms of action against E. amylovora. Streptomyces sp. JCK-8055 produced aureothricin and thiolutin, which antagonistically affect E. amylovora. JCK-8055 and its two active metabolites have a broad-spectrum in vitro activity against various phytopathogenic bacteria and fungi. They also effectively suppressed tomato bacterial wilt and apple fire blight in in vivo experiments. Interestingly, JCK-8055 colonizes roots as a tomato seed coating and induces apple leaf shedding at the abscission zone, ultimately halting the growth of pathogenic bacteria. Additionally, JCK-8055 can produce the plant growth regulation hormone indole-3-acetic acid (IAA) and hydrolytic enzymes, including protease, gelatinase, and cellulase. JCK-8055 treatment also triggered the expression of salicylate (SA) and jasmonate (JA) signaling pathway marker genes, such as PR1, PR2, and PR3. Overall, our findings demonstrate that Streptomyces sp. JCK-8055 can control a wide range of plant diseases, particularly apple fire blight, through a combination of mechanisms such as antibiosis and induced resistance, highlighting its excellent potential as a biocontrol agent. KEY POINTS: • JCK-8055 produces the systemic antimicrobial metabolites, aureothricin, and thiolutin. • JCK-8055 treatment upregulates PR gene expression in apple plants against E. amylovora. • JCK-8055 controls plant diseases with antibiotics and induced resistance.

Removal of formaldehyde from indoor air by potted Sansevieria trifasciata plants: dynamic influence of physiological traits on the process

Plant-based removal of indoor formaldehyde is a widely studied method, yet little is known about the dynamic changes in this process. In this study, potted Sansevieria trifasciata Prain plants were exposed to 5-ppm formaldehyde gas concentration for 7 days. The results showed that formaldehyde exposure led to plant stress, affected photosynthesis, and damaged membrane lipids, as evidenced by a decrease in chlorophyll content, an increase in Chl a/b ratio and malondialdehyde content. However, the formaldehyde removal ability of the plants increased over the first 5 days, peaking at 18.02 mg h-1 kg-1 dry weight on the 5th day. This trend was correlated with changes in various indicators in the plant roots, including phytohormone and antioxidant enzymes. Notably, catalase activity in the roots behaved differently from other indicators. The indicators in the leaves showed turning points around the 3rd day due to the direct exposure of the leaves to formaldehyde. The relative abundance of endophytes indicated an increase in plant growth-promoting bacteria, which helped the plant cope with formaldehyde stress. The study suggests that under formaldehyde stress, plants manage active oxygen content by increasing phytohormones and regulating redox reactions. This enhances their tolerances to formaldehyde, thereby improving their ability to remove formaldehyde and aiding recovery after formaldehyde exposure.

Unveiling the hidden world: How arbuscular mycorrhizal fungi and its regulated core fungi modify the composition and metabolism of soybean rhizosphere microbiome

BACKGROUND

The symbiosis between arbuscular mycorrhizal fungi (AMF) and plants often stimulates plant growth, increases agricultural yield, reduces costs, thereby providing significant economic benefits. AMF can also benefit plants through affecting the rhizosphere microbial community, but the underlying mechanisms remain unclear. Using Rhizophagus intraradices as a model AMF species, we assessed how AMF influences the bacterial composition and functional diversity through 16 S rRNA gene sequencing and non-targeted metabolomics analysis in the rhizosphere of aluminum-sensitive soybean that were inoculated with pathogenic fungus Nigrospora oryzae and phosphorus-solubilizing fungus Talaromyces verruculosus in an acidic soil.

RESULTS

The inoculation of R. intraradices, N. oryzae and T. verruculosus didn't have a significant influence on the levels of soil C, N, and P, or various plant characteristics such as seed weight, crude fat and protein content. However, their inoculation affected the structure, function and nutrient dynamics of the resident bacterial community. The co-inoculation of T. verruculosus and R. intraradices increased the relative abundance of Pseudomonas psychrotolerans, which was capable of N-fixing and was related to cry-for-help theory (plants signal for beneficial microbes when under stress), within the rhizosphere. R. intraradices increased the expression of metabolic pathways associated with the synthesis of unsaturated fatty acids, which was known to enhance plant resistance under adverse environmental conditions. The inoculation of N. oryzae stimulated the stress response inside the soil environment by enriching the polyene macrolide antifungal antibiotic-producing bacterial genus Streptomyces in the root endosphere and upregulating two antibacterial activity metabolic pathways associated with steroid biosynthesis pathways in the rhizosphere. Although inoculation of pathogenic fungus N. oryzae enriched Bradyrhizobium and increased soil urease activity, it had no significant effects on biomass and N content of soybean. Lastly, the host niches exhibited differences in the composition of the bacterial community, with most N-fixing bacteria accumulating in the endosphere and Rhizobium vallis only detected in the endosphere.

CONCLUSIONS

Our findings demonstrate that intricate interactions between AMF, associated core fungi, and the soybean root-associated ecological niches co-mediate the regulation of soybean growth, the dynamics of rhizosphere soil nutrients, and the composition, function, and metabolisms of the root-associated microbiome in an acidic soil.

Shotgun metagenomic dataset of root endophytic microbiome of citrus (Citrus nobilis L.)

Citrus (Citrus nobilis L.) is one of the main fruit crops in Dak Lak Province of Vietnam; however, a dataset on the endophytic microbiome of this plant has yet to be discovered. This article presented the endophytic microbial dataset from roots of healthy Citrus nobilis L. collected in Dak Lak for the first time. We found that 4 kingdoms, 30 phyla, 58 classes, 125 orders, 242 families, 722 genera, and 1637 species of endophytic microorganisms were identified from the sample. Actinomycetota was shown to be the main phylum (64.36 %) and biosynthesis to be the most abundant function (55.64 %) of the endophytic microbial community. Data provided insights into the composition and functional diversity of the Citrus nobilis L. endophytic microbiome, especially novel microbial resources. They could be used for the next works towards applying the endophytic microbiome for sustainable citrus production.

Plant G × Microbial E: Plant Genotype Interaction with Soil Bacterial Community Shapes Rhizosphere Composition During Invasion

It is increasingly recognized that different genetic variants of hosts can uniquely shape their microbiomes. Invasive species often evolve in their introduced ranges, but little is known about the potential for their microbial associations to change during invasion as a result. We asked whether host genotype (G), microbial environment (E), or their interaction (G × E) affected the composition and diversity of host-associated microbiomes in Centaurea solstitialis (yellow starthistle), a Eurasian plant that is known to have evolved novel genotypes and phenotypes and to have altered microbial interactions, in its severe invasion of CA, USA. We conducted an experiment in which native and invading plant genotypes were inoculated with native and invaded range soil microbial communities. We used amplicon sequencing to characterize rhizosphere bacteria in both the experiment and the field soils from which they were derived. We found that native and invading plant genotypes accumulated different microbial associations at the family level in each soil community, often counter to differences in family abundance between soil communities. Root associations with potentially beneficial Streptomycetaceae were particularly interesting, as these were more abundant in the invaded range field soil and accumulated on invading genotypes. We also found that bacterial diversity is higher in invaded soils, but that invading genotypes accumulated a lower diversity of bacteria and unique microbial composition in experimental inoculations, relative to native genotypes. Thus variation in microbial associations of invaders was driven by the interaction of plant G and microbial E, and rhizosphere microbial communities appear to change in composition in response to host evolution during invasion.

Bioactive Streptomycetes: A Powerful Tool to Synthesize Diverse Nanoparticles With Multifarious Properties

Nanobiotechnology has gained significant attention due to its capacity to generate substantial benefits through the integration of microbial biotechnology and nanotechnology. Among microbial organisms, Actinomycetes, particularly the prominent genus Streptomycetes, have garnered attention for their prolific production of antibiotics. Streptomycetes have emerged as pivotal contributors to the discovery of a substantial number of antibiotics and play a dominant role in combating infectious diseases on a global scale. Despite the noteworthy progress achieved through the development and utilization of antibiotics to combat infectious pathogens, the prevalence of infectious diseases remains a prominent cause of mortality worldwide, particularly among the elderly and children. The emergence of antibiotic resistance among pathogens has diminished the efficacy of antibiotics in recent decades. Nevertheless, Streptomycetes continue to demonstrate their potential by producing bioactive metabolites for the synthesis of nanoparticles. Streptomycetes are instrumental in producing nanoparticles with diverse bioactive characteristics, including antiviral, antibacterial, antifungal, antioxidant, and antitumor properties. Biologically synthesized nanoparticles have exhibited a meaningful reduction in the impact of antibiotic resistance, providing resources for the development of new and effective drugs. This review succinctly outlines the significant applications of Streptomycetes as a crucial element in nanoparticle synthesis, showcasing their potential for diverse and enhanced beneficial applications.

Physiological and biochemical characteristics and microbial responses of Medicago sativa (Fabales: Fabaceae) varieties with different resistance to atrazine stress

Atrazine, a commonly employed herbicide for corn production, can leave residues in soil, resulting in photosynthetic toxicity and impeding growth in subsequent alfalfa (Medicago sativa L.) crops within alfalfa-corn rotation systems. The molecular regulatory mechanisms by which atrazine affects alfalfa growth and development, particularly its impact on the microbial communities of the alfalfa rhizosphere, are not well understood. This study carried out field experiments to explore the influence of atrazine stress on the biomass, chlorophyll content, antioxidant system, and rhizosphere microbial communities of the atrazine-sensitive alfalfa variety WL-363 and the atrazine-resistant variety JN5010. The results revealed that atrazine significantly reduced WL-363 growth, decreasing plant height by 8.58 cm and root length by 5.42 cm (p < 0.05). Conversely, JN5010 showed minimal reductions, with decreases of 1.96 cm in height and 1.26 cm in root length. Chlorophyll content in WL-363 decreased by 35% under atrazine stress, while in JN5010, it was reduced by only 10%. Reactive oxygen species (ROS) accumulation increased by 60% in WL-363, compared to a 20% increase in JN5010 (p < 0.05 for both). Antioxidant enzyme activities, such as superoxide dismutase (SOD) and catalase (CAT), were significantly elevated in JN5010 (p < 0.05), suggesting a more robust defense mechanism. Although the predominant bacterial and fungal abundances in rhizosphere soils remained generally unchanged under atrazine stress, specific microbial groups exhibited variable responses. Notably, Promicromonospora abundance declined in WL-363 but increased in JN5010. FAPROTAX functional predictions indicated shifts in the abundance of microorganisms associated with pesticide degradation, resistance, and microbial structure reconstruction under atrazine stress, displaying different patterns between the two varieties. This study provides insights into how atrazine residues affect alfalfa rhizosphere microorganisms and identifies differential microbial responses to atrazine stress, offering valuable reference data for screening and identifying atrazine-degrading bacteria.

Streptomyces pratensis S10 Promotes Wheat Plant Growth and Induces Resistance in Wheat Seedlings against Fusarium graminearum

Fusarium graminearum, a devastating fungal pathogen, causes great economic losses to crop yields worldwide. The present study investigated the potential of Streptomyces pratensis S10 to alleviate F. graminearum stress in wheat seedlings based on plant growth-promoting and resistance-inducing assays. The bioassays revealed that S10 exhibited multiple plant growth-promoting properties, including the production of siderophores, 1-aminocyclopropane-1-carboxylic acid deaminase (ACC), and indole-3-acetic acid (IAA), phosphate solubilization, and nitrogen fixation. Meanwhile, the pot experiment demonstrated that S10 improved wheat plant development, substantially enhancing wheat height, weight, root activity, and chlorophyll content. Consistently, genome mining identified abundant genes associated with plant growth promotion. S10 induced resistance against F. graminearum in wheat seedlings. The disease incidence and disease index reduced by nearly 52% and 65% in S10 pretreated wheat seedlings, respectively, compared with those infected with F. graminearum only in the non-contact inoculation assay. Moreover, S10 enhanced callose deposition and reactive oxygen species (ROS) accumulation and induced the activities of CAT, SOD, POD, PAL, and PPO. Furthermore, the quantitative real-time PCR (qRT-PCR) results indicated that S10 pretreatment increased the expression of SA- (PR1.1, PR2, PR5, and PAL1) and JA/ET-related genes (PR3, PR4a, PR9, and PDF1.2) in wheat seedlings upon F. graminearum infection. In summary, S. pratensis S10 could be an integrated biological agent and biofertilizer in wheat seedling blight management and plant productivity enhancement.

Effects of actinomycetes on the growth, antioxidant and genes expression in Fritillaria taipaiensis P. Y. li

F. taipaiensis P. Y. Li represents a significant asset within traditional Chinese medicinal flora, though it confronts the challenge of germplasm deterioration during its cultivation phase. This study aimed to discern the implications of single strains or combinations of diverse growth-promoting actinomycetes on the growth metrics, antioxidant competence and pertinent gene expression in the leaves of F. taipaiensis. The result revealed that the malondialdehyde content within the plant's leaves notably diminished in the treatment groups compared to the CK group, with the S6 group showcasing the most pronounced malondialdehyde reduction, amounting to approximately one-third of the CK's value. Leaf area, length and width peaked in the S5 cohort, registering values 4.55, 2.46 and 1.85 times surpassing the CK group. Concurrently, plant height and stem thickness were maximal in the S6 group, being 2.29 and 1.75 times that of the CK group, whereas leaf thickness reached its zenith in the S7 group, marking a 2.17-fold elevation compared to the CK. Photosynthetic pigments, soluble sugars and soluble proteins in the leaves, exhibited augmentation across the inoculated groups to varying magnitudes. Specifically, the S5 group was superior in photosynthetic metrics and pigments, while the S6 group manifested the highest soluble sugar concentration, which was 1.35 times that of the CK. The S3 group demonstrated the pinnacle of soluble protein content, an impressive 5.86-fold increment relative to the CK group. The enzymatic activities of superoxide dismutase, peroxidase and catalase, along with their affiliated gene expressions, were observably augmented in the inoculated groups, with the S5 group standing out. To encapsulate, the actinomycete inoculation holds potential in fostering the growth and maturation of F. taipaiensis, amplifying its environmental resilience. The revelations from this study extend valuable insights for the judicious choice of microbial fertilizers in the cultivated propagation of Fritillaria taipaiensis P. Y. Li.

Spent Mushroom Substrate Improves Microbial Quantities and Enzymatic Activity in Soils of Different Farming Systems

Organic fertilizers, such as spent mushroom substrate (SMS), improve soil fertility, but studies comparing their effects on different agricultural soils are limited. In this study, the effects of standard, SMS and composed fertilizers on soils from conventional-integrated, organic and biodynamic farming were investigated. Soil samples were analyzed for microorganisms and the activity of β-glucosidase (β-GLU), β-1,4-N-acetylglucosaminidase (NAG), urease (URE), arylamidase (ARN), phosphatase (PHOS), acid phosphatase (PAC), alkaline phosphatase (PAH) and arylsulphatase (ARS). Biodynamic soil showed the highest microbial counts and enzyme activities, followed by organic and conventional soils. SMS significantly increased the number of microorganisms and enzyme activities, especially in biodynamic and organic soils. Seasonal variations affected all microorganisms and most enzymes in all soils, except NAG in conventional and organic soils. Biodynamic soil showed stable activity of enzymes and microorganisms throughout the year, indicating greater stability. This study concludes that soil microorganisms and enzyme activities respond differently to fertilization depending on the soil type, with SMS demonstrating beneficial effects in all tested soils.

ZnO nanoparticles encapsulated cellulose-lignin film for antibacterial and biodegradable food packaging

Foodborne illness caused by consuming foods contaminated by pathogens remains threating to the public health. Despite considerable efforts of using renewable source materials, it is highly demanding to fabricate food packaging with multiple properties including eco-friendliness, bactericidal effect and biocompatibility. Here, sodium lignosulfonate (SL) and ZnO nanoparticles (ZnO NPs) were used as functional filler and structure components, respectively, on the cellulose nanofibers (CNFs)-based films, which endows the produced membrane (CNF/SL-ZnO) the UV-light blocking, antioxidant, and antimicrobial characteristics. Due to the interconnected polymeric structure, the prepared CNF/SL-ZnO films possessed considerable mechanical properties, thermal stability, and good moisture barrier capability. Moreover, the tested samples exhibited an improved shelf life in food packaging. Furthermore, metagenome analysis revealed superior biodegradability of obtained films with negligible side effect on the soil microenvironment. Therefore, the biocompatible, degradable, and antibacterial CNF/SL-ZnO film holds enormous potential for sustainable uses including food packaging.

Combined effects of cadmium and simulated acid rain on soil microbial communities in the early cultivation of Populus beijingensis seedlings

The combined cadmium (Cd) and acid rain pollution poses a significant threat to the global ecological environment. Previous studies on the combined adverse effects have predominantly focused on the aboveground plant physiological responses, with limited reports on the microbial response in the rhizosphere soil. This study employed Populus beijingensis seedlings and potting experiments to simulate the impacts of combined mild acid rain (pH=4.5, MA) or highly strong acid rain (pH=3.0, HA), and soil Cd pollution on the composition and diversity of microbial communities, as well as the physiochemical properties in the rhizosphere soil. The results showed that Cd decreased the content of inorganic nitrogen, resulting in an overall decrease of 49.10 % and 46.67 % in ammonium nitrogen and nitrate nitrogen, respectively. Conversely, acid rain was found to elevate the content of total potassium and soil organic carbon by 4.68 %-6.18 % and 8.64-19.16 %, respectively. Additionally, simulated acid rain was observed to decrease the pH level by 0.29-0.35, while Cd increased the pH level by 0.11. Moreover, Cd alone reduced the rhizosphere bacterial diversity, however, when combined with acid rain, regardless of its intensity, Cd was observed to increase the diversity. Fungal diversity was not influenced by the acid rain, but Cd increased fungal diversity to some extend under HA as observed in bacterial diversity. In addition, composition of the rhizosphere bacterial community was primarily influenced by the inorganic nitrogen components, while the fungal community was driven mainly by soil pH. Furthermore, "Metabolism" was emerged as the most significant bacterial function, which was markedly affected by the combined pollution, while Cd pollution led to a shift from symbiotroph to other trophic types for fungi. These findings suggest that simulated acid rain has a mitigating effect on the diversity of rhizosphere bacteria affected by Cd pollution, and also alters the trophic type of these microorganisms. This can be attributed to the acid rain-induced direct acidic environment, as well as the indirect changes in the availability or sources of carbon, nitrogen, or potassium.

Metabarcoding expands knowledge on diversity and ecology of rare actinobacteria in the Brazilian Cerrado

Rare and unknown actinobacteria from unexplored environments have the potential to produce new bioactive molecules. This study aimed to use 16 s rRNA metabarcoding to determine the composition of the actinobacterial community, particularly focusing on rare and undescribed species, in a nature reserve within the Brazilian Cerrado called Sete Cidades National Park. Since this is an inaccessible area without due legal authorization, it is understudied, and, therefore, its diversity and biotechnological potential are not yet fully understood, and it may harbor species with groundbreaking genetic potential. In total, 543 operational taxonomic units (OTUs) across 14 phyla were detected, with Actinobacteria (41.2%), Proteobacteria (26.5%), and Acidobacteria (14.3%) being the most abundant. Within Actinobacteria, 107 OTUs were found, primarily from the families Mycobacteriaceae, Pseudonocardiaceae, and Streptomycetaceae. Mycobacterium and Streptomyces were the predominant genera across all samples. Seventeen rare OTUs with relative abundance < 0.1% were identified, with 82.3% found in only one sample yet 25.5% detected in all units. Notable rare and transient genera included Salinibacterium, Nocardia, Actinomycetospora_01, Saccharopolyspora, Sporichthya, and Nonomuraea. The high diversity and distribution of Actinobacteria OTUs indicate the area's potential for discovering new rare species. Intensified prospection on underexplored environments and characterization of their actinobacterial diversity could lead to the discovery of new species capable of generating innovative natural products.

The Co-Inoculation Effect on Triticum aestivum Growth with Synthetic Microbial Communities (SynComs) and Their Potential in Agrobiotechnology

The use of rhizospheric SynComs can be a new and sustainable strategy in the agrobiotechnology sector. The objective of this study was to create the most appropriate SynCom composition; examine the ability to dissolve natural rock phosphate (RP) from Morocco in liquid-modified NBRIP medium; determine organic acids, and phytohormones; and verify plant growth promoting and nutrition uptake effect in the pot experiments of winter wheat (Triticum aestivum). A total of nine different microorganisms were isolated, which belonged to three different genera: Bacillus, Pseudomonas, and Streptomyces. Out of the 21 treatments tested, four SynComs had the best phosphate-dissolving properties: IJAK-27+44+91 (129.17 mg L-1), IIBEI-32+40 (90.95 µg mL-1), IIIDEG-45+41 (122.78 mg L-1), and IIIDEG-45+41+72 (120.78 mg L-1). We demonstrate that these SynComs are capable of producing lactic, acetic, gluconic, malic, oxalic, citric acids, and phytohormones such as indole-3-acetic acid, zeatin, gibberellic acid, and abscisic acid. In pot experiments with winter wheat, we also demonstrated that the designed SynComs were able to effectively colonize the plant root rhizosphere and contributed to more abundant plant growth characteristics and nutrient uptake as uninoculated treatment or uninoculated treatment with superphosphate (NPK 0-19-0). The obtained results show that the SynCom compositions of IJAK-27+44+91, IIBEI-32+40, IIIDEG-45+41, and IIIDEG-45+41+72 can be considered as promising candidates for developing biofertilizers to facilitate P absorption and increase plant nutrition.

Biodegradation of atrazine and nicosulfuron by Streptomyces nigra LM01: Performance, degradative pathway, and possible genes involved

Microbial herbicide degradation is an efficient bioremediation method. In this study, a strain of Streptomyces nigra, LM01, which efficiently degrades atrazine and nicosulfuron, was isolated from a corn field using a direct isolation method. The degradation effects of the identified strain on two herbicides were investigated and optimized using an artificial neural network. The maximum degradation rates of S. nigra LM01 were 58.09 % and 42.97 % for atrazine and nicosulfuron, respectively. The degradation rate of atrazine in the soil reached 67.94 % when the concentration was 108 CFU/g after 5 d and was less effective than that of nicosulfuron. Whole genome sequencing of strain LM01 helped elucidate the possible degradation pathways of atrazine and nicosulfuron. The protein sequences of strain LM01 were aligned with the sequences of the degraded proteins of the two herbicides by using the National Center for Biotechnology Information platform. The sequence (GE005358, GE001556, GE004212, GE005218, GE004846, GE002487) with the highest query cover was retained and docked with the small-molecule ligands of the herbicides. The results revealed a binding energy of - 6.23 kcal/mol between GE005358 and the atrazine ligand and - 6.66 kcal/mol between GE002487 and the nicosulfuron ligand.

Genomic insights into an endophytic Streptomyces sp. VITGV156 for antimicrobial compounds

Endophytic Streptomyces sp. are recognized as a potential resource for valuable natural products but are less explored. This study focused on exploring endophytic Streptomyces species residing within tomato plants (Solanum lycopersicum) harboring genes for the production of a novel class of antibiotics. Our research involved the isolation and characterization of Streptomyces sp. VITGV156, a newly identified endophytic Streptomyces species that produces antimicrobial products. VITGV156 harbors a genome of 8.18 mb and codes 6,512 proteins, of which 4,993 are of known function (76.67%) and 1,519 are of unknown function (23.32%). By employing genomic analysis, we elucidate the genome landscape of this microbial strain and shed light on various BGCs responsible for producing polyketide antimicrobial compounds, with particular emphasis on the antibiotic kendomycin. We extended our study by evaluating the antibacterial properties of kendomycin. Overall, this study provides valuable insights into the genome of endophytic Streptomyces species, particularly Streptomyces sp. VITGV156, which are prolific producers of antimicrobial agents. These findings hold promise for further research and exploitation of pharmaceutical compounds, offering opportunities for the development of novel antimicrobial drugs.

A novel PGPR strain, Streptomyces lasalocidi JCM 3373T, alleviates salt stress and shapes root architecture in soybean by secreting indole-3-carboxaldehyde

While soybean (Glycine max L.) provides the most important source of vegetable oil and protein, it is sensitive to salinity, which seriously endangers the yield and quality during soybean production. The application of Plant Growth-Promoting Rhizobacteria (PGPR) to improve salt tolerance for plant is currently gaining increasing attention. Streptomycetes are a major group of PGPR. However, to date, few streptomycetes has been successfully developed and applied to promote salt tolerance in soybean. Here, we discovered a novel PGPR strain, Streptomyces lasalocidi JCM 3373T, from 36 strains of streptomycetes via assays of their capacity to alleviate salt stress in soybean. Microscopic observation showed that S. lasalocidi JCM 3373T does not colonise soybean roots. Chemical analysis confirmed that S. lasalocidi JCM 3373T secretes indole-3-carboxaldehyde (ICA1d). Importantly, IAC1d inoculation alleviates salt stress in soybean and modulates its root architecture by regulating the expression of stress-responsive genes GmVSP, GmPHD2 and GmWRKY54 and root growth-related genes GmPIN1a, GmPIN2a, GmYUCCA5 and GmYUCCA6. Taken together, the novel PGPR strain, S. lasalocidi JCM 3373T, alleviates salt stress and improves root architecture in soybean by secreting ICA1d. Our findings provide novel clues for the development of new microbial inoculant and the improvement of crop productivity under salt stress.

Biotechnological potential of actinomycetes in the 21st century: a brief review

This brief review aims to draw attention to the biotechnological potential of actinomycetes. Their main uses as sources of antibiotics and in agriculture would be enough not to neglect them; however, as we will see, their biotechnological application is much broader. Far from intending to exhaust this issue, we present a short survey of the research involving actinomycetes and their applications published in the last 23 years. We highlight a perspective for the discovery of new active ingredients or new applications for the known metabolites of these microorganisms that, for approximately 80 years, since the discovery of streptomycin, have been the main source of antibiotics. Based on the collected data, we organize the text to show how the cosmopolitanism of actinomycetes and the evolutionary biotic and abiotic ecological relationships of actinomycetes translate into the expression of metabolites in the environment and the richness of biosynthetic gene clusters, many of which remain silenced in traditional laboratory cultures. We also present the main strategies used in the twenty-first century to promote the expression of these silenced genes and obtain new secondary metabolites from known or new strains. Many of these metabolites have biological activities relevant to medicine, agriculture, and biotechnology industries, including candidates for new drugs or drug models against infectious and non-infectious diseases. Below, we present significant examples of the antimicrobial spectrum of actinomycetes, which is the most commonly investigated and best known, as well as their non-antimicrobial spectrum, which is becoming better known and increasingly explored.

Actinomycetes are a natural resource for sustainable pest control and safeguarding agriculture

Actinomycetes, a diverse group of bacteria with filamentous growth characteristics, have long captivated researchers and biochemists for their prolific production of secondary metabolites. Among the myriad roles played by actinomycete secondary metabolites, their historical significance in the field of biocontrol stands out prominently. The fascinating journey begins with the discovery of antibiotics, where renowned compounds like streptomycin, tetracycline, and erythromycin revolutionized medicine and agriculture. The history of biocontrol traces its roots back to the early twentieth century, when scientists recognized the potential of naturally occurring agents to combat pests and diseases. The emergence of synthetic pesticides in the mid-twentieth century temporarily overshadowed interest in biocontrol. However, with growing environmental concerns and the realization of the negative ecological impacts of chemical pesticides, the pendulum swung back towards exploring sustainable alternatives. Beyond their historical role as antibiotics, actinomycete-produced secondary metabolites encompass a rich repertoire with biopesticide potential. The classification of these compounds based on chemical structure and mode of action is highlighted, demonstrating their versatility against both plant pathogens and insect pests. Additionally, this review provides in-depth insights into how endophytic actinomycete strains play a pivotal role in biocontrol strategies. Case studies elucidate their effectiveness in inhibiting Spodoptera spp. and nematodes through the production of bioactive compounds. By unraveling the multifunctional roles of endophytic actinomycetes, this review contributes compelling narrative knowledge to the field of sustainable agriculture, emphasizing the potential of these microbial allies in crafting effective, environmentally friendly biocontrol strategies for combating agricultural pests.

Soil prokaryotic and fungal biome structures associated with crop disease status across the Japan Archipelago

Archaea, bacteria, and fungi in the soil are increasingly recognized as determinants of agricultural productivity and sustainability. A crucial step for exploring soil microbiomes with important ecosystem functions is to perform statistical analyses on the potential relationship between microbiome structure and functions based on comparisons of hundreds or thousands of environmental samples collected across broad geographic ranges. In this study, we integrated agricultural field metadata with microbial community analyses by targeting 2,903 bulk soil samples collected along a latitudinal gradient from cool-temperate to subtropical regions in Japan (26.1-42.8 °N). The data involving 632 archaeal, 26,868 bacterial, and 4,889 fungal operational taxonomic units detected across the fields of 19 crop plant species allowed us to conduct statistical analyses (permutational analyses of variance, generalized linear mixed models, randomization analyses, and network analyses) on the relationship among edaphic factors, microbiome compositions, and crop disease prevalence. We then examined whether the diverse microbes form species sets varying in potential ecological impacts on crop plants. A network analysis suggested that the observed prokaryotes and fungi were classified into several species sets (network modules), which differed substantially in association with crop disease prevalence. Within the network of microbe-to-microbe coexistence, ecologically diverse microbes, such as an ammonium-oxidizing archaeon, an antibiotics-producing bacterium, and a potentially mycoparasitic fungus, were inferred to play key roles in shifts between crop-disease-promotive and crop-disease-suppressive states of soil microbiomes. The bird's-eye view of soil microbiome structure will provide a basis for designing and managing agroecosystems with high disease-suppressive functions.IMPORTANCEUnderstanding how microbiome structure and functions are organized in soil ecosystems is one of the major challenges in both basic ecology and applied microbiology. Given the ongoing worldwide degradation of agroecosystems, building frameworks for exploring structural diversity and functional profiles of soil microbiomes is an essential task. Our study provides an overview of cropland microbiome states in light of potential crop-disease-suppressive functions. The large data set allowed us to explore highly functional species sets that may be stably managed in agroecosystems. Furthermore, an analysis of network architecture highlighted species that are potentially used to cause shifts from disease-prevalent states of agroecosystems to disease-suppressive states. By extending the approach of comparative analyses toward broader geographic ranges and diverse agricultural practices, agroecosystem with maximized biological functions will be further explored.

Exogenous Streptomyces spp. enhance the drought resistance of naked oat (Avena nuda) seedlings by augmenting both the osmoregulation mechanisms and antioxidant capacities

Drought is a major obstacle to the development of naked oat industry. This work investigated mechanisms by which exogenous Streptomyces albidoflavus T4 and Streptomyces rochei D74 improved drought tolerance in naked oat (Avena nuda ) seedlings. Results showed that in the seed germination experiment, germination rate, radicle and hypocotyl length of naked oat seeds treated with the fermentation filtrate of T4 or D74 under PEG induced drought stress increased significantly. In the hydroponic experiment, the shoot and root dry weights of oat seedlings increased significantly when treated with the T4 or D74 fermentation filtrate under the 15% PEG induced drought stress (S15). Simultaneously, the T4 treatment also significantly increased the surface area, volume, the number of tips and the root activity of oat seedlings. Both T4 and D74 treatments elicited significant increases in proline and soluble sugar contents, as well as the catalase and peroxidase activities in oat seedlings. The results of comprehensive drought resistance capacity (CDRC) calculation of oat plants showed that the drought resistance of oat seedlings under the T4 treatment was better than that under the D74 treatment, and the effect was better under higher drought stress (S15). Findings of this study may provide a novel and effective approach for enhancing plant defenses against drought stress.

Soil microbiome of shiro reveals the symbiotic relationship between Tricholoma bakamatsutake and Quercus mongolica

Tricholoma bakamatsutake is a delicious and nutritious ectomycorrhizal fungus. However, its cultivation is hindered owing to limited studies on its symbiotic relationships. The symbiotic relationship between T. bakamatsutake and its host is closely related to the shiro, a complex network composed of mycelium, mycorrhizal roots, and surrounding soil. To explore the symbiotic relationship between T. bakamatsutake and its host, soil samples were collected from T. bakamatsutake shiro (Tb) and corresponding Q. mongolica rhizosphere (CK) in four cities in Liaoning Province, China. The physicochemical properties of all the soil samples were then analyzed, along with the composition and function of the fungal and bacterial communities. The results revealed a significant increase in total potassium, available nitrogen, and sand in Tb soil compared to those in CK soil, while there was a significant decrease in pH, total nitrogen, total phosphorus, available phosphorus, and silt. The fungal community diversity in shiro was diminished, and T. bakamatsutake altered the community structure of its shiro by suppressing other fungi, such as Russula (ectomycorrhizal fungus) and Penicillium (phytopathogenic fungus). The bacterial community diversity in shiro increased, with the aggregation of mycorrhizal-helper bacteria, such as Paenibacillus and Bacillus, and plant growth-promoting bacteria, such as Solirubrobacter and Streptomyces, facilitated by T. bakamatsutake. Microbial functional predictions revealed a significant increase in pathways associated with sugar and fat catabolism within the fungal and bacterial communities of shiro. The relative genetic abundance of carboxylesterase and gibberellin 2-beta-dioxygenase in the fungal community was significantly increased, which suggested a potential symbiotic relationship between T. bakamatsutake and Q. mongolica. These findings elucidate the microbial community and relevant symbiotic environment to better understand the relationship between T. bakamatsutake and Q. mongolica.

Microcystin influence on soil-plant microbiota: Unraveling microbiota modulations and assembly processes in the rhizosphere of Vicia faba

Microcystins (MCs) are frequently detected in cyanobacterial bloom-impacted waterbodies and introduced into agroecosystems via irrigation water. They are widely known as phytotoxic cyanotoxins, which impair the growth and physiological functions of crop plants. However, their impact on the plant-associated microbiota is scarcely tackled and poorly understood. Therefore, we aimed to investigate the effect of MCs on microbiota-inhabiting bulk soil (BS), root adhering soil (RAS), and root tissue (RT) of Vicia faba when exposed to 100 μg L-1 MCs in a greenhouse pot experiment. Under MC exposure, the structure, co-occurrence network, and assembly processes of the bacterial microbiota were modulated with the greatest impact on RT-inhabiting bacteria, followed by BS and, to a lesser extent, RAS. The analyses revealed a significant decrease in the abundances of several Actinobacteriota-related taxa within the RT microbiota, including the most abundant and known genus of Streptomyces. Furthermore, MCs significantly increased the abundance of methylotrophic bacteria (Methylobacillus, Methylotenera) and other Proteobacteria-affiliated genera (e.g., Paucibacter), which are supposed to degrade MCs. The co-occurrence network of the bacterial community in the presence of MCs was less complex than the control network. In MC-exposed RT, the turnover in community composition was more strongly driven by deterministic processes, as proven by the beta-nearest taxon index. Whereas in MC-treated BS and RAS, both deterministic and stochastic processes can influence community assembly to some extent, with a relative dominance of deterministic processes. Altogether, these results suggest that MCs may reshape the structure of the microbiota in the soil-plant system by reducing bacterial taxa with potential phytobeneficial traits and increasing other taxa with the potential capacity to degrade MCs.

Long-term push-pull cropping system shifts soil and maize-root microbiome diversity paving way to resilient farming system

BACKGROUND

The soil biota consists of a complex assembly of microbial communities and other organisms that vary significantly across farming systems, impacting soil health and plant productivity. Despite its importance, there has been limited exploration of how different cropping systems influence soil and plant root microbiomes. In this study, we investigated soil physicochemical properties, along with soil and maize-root microbiomes, in an agroecological cereal-legume companion cropping system known as push-pull technology (PPT). This system has been used in agriculture for over two decades for insect-pest management, soil health improvement, and weed control in sub-Saharan Africa. We compared the results with those obtained from maize-monoculture (Mono) cropping system.

RESULTS

The PPT cropping system changed the composition and diversity of soil and maize-root microbial communities, and led to notable improvements in soil physicochemical characteristics compared to that of the Mono cropping system. Distinct bacterial and fungal genera played a crucial role in influencing the variation in microbial diversity within these cropping systems. The relative abundance of fungal genera Trichoderma, Mortierella, and Bionectria and bacterial genera Streptomyces, RB41, and Nitrospira were more enriched in PPT. These microbial communities are associated with essential ecosystem services such as plant protection, decomposition, carbon utilization, bioinsecticides production, nitrogen fixation, nematode suppression, phytohormone production, and bioremediation. Conversely, pathogenic associated bacterial genus including Bryobacter were more enriched in Mono-root. Additionally, the Mono system exhibited a high relative abundance of fungal genera such as Gibberella, Neocosmospora, and Aspergillus, which are linked to plant diseases and food contamination. Significant differences were observed in the relative abundance of the inferred metabiome functional protein pathways including syringate degradation, L-methionine biosynthesis I, and inosine 5'-phosphate degradation.

CONCLUSION

Push-pull cropping system positively influences soil and maize-root microbiomes and enhances soil physicochemical properties. This highlights its potential for agricultural and environmental sustainability. These findings contribute to our understanding of the diverse ecosystem services offered by this cropping system where it is practiced regarding the system's resilience and functional redundancy. Future research should focus on whether PPT affects the soil and maize-root microbial communities through the release of plant metabolites from the intercrop root exudates or through the alteration of the soil's nutritional status, which affects microbial enzymatic activities.

Proteomic examination of polyester-polyurethane degradation by Streptomyces sp. PU10: Diverting polyurethane intermediates to secondary metabolite production

Global plastic waste accumulation has become omnipresent in public discourse and the focus of scientific research. Ranking as the sixth most produced polymer globally, polyurethanes (PU) significantly contribute to plastic waste and environmental pollution due to the toxicity of their building blocks, such as diisocyanates. In this study, the effects of PU on soil microbial communities over 18 months were monitored revealing that it had marginal effects on microbial diversity. However, Streptomyces sp. PU10, isolated from this PU-contaminated soil, proved exceptional in the degradation of a soluble polyester-PU (Impranil) across a range of temperatures with over 96% degradation of 10 g/L in 48 h. Proteins involved in PU degradation and metabolic changes occurring in this strain with Impranil as the sole carbon source were further investigated employing quantitative proteomics. The proposed degradation mechanism implicated the action of three enzymes: a polyester-degrading esterase, a urethane bond-degrading amidase and an oxidoreductase. Furthermore, proteome data revealed that PU degradation intermediates were incorporated into Streptomyces sp. PU10 metabolism via the fatty acid degradation pathway and subsequently channelled to polyketide biosynthesis. Most notably, the production of the tri-pyrrole undecylprodigiosin was confirmed paving the way for establishing PU upcycling strategies to bioactive metabolites using Streptomyces strains.

The Roles of Plant-Growth-Promoting Rhizobacteria (PGPR)-Based Biostimulants for Agricultural Production Systems

The application of biostimulants has been proven to be an advantageous tool and an appropriate form of management towards the effective use of natural resources, food security, and the beneficial effects on plant growth and yield. Plant-growth-promoting rhizobacteria (PGPR) are microbes connected with plant roots that can increase plant growth by different methods such as producing plant hormones and molecules to improve plant growth or providing increased mineral nutrition. They can colonize all ecological niches of roots to all stages of crop development, and they can affect plant growth and development directly by modulating plant hormone levels and enhancing nutrient acquisition such as of potassium, phosphorus, nitrogen, and essential minerals, or indirectly via reducing the inhibitory impacts of different pathogens in the forms of biocontrol parameters. Many plant-associated species such as Pseudomonas, Acinetobacter, Streptomyces, Serratia, Arthrobacter, and Rhodococcus can increase plant growth by improving plant disease resistance, synthesizing growth-stimulating plant hormones, and suppressing pathogenic microorganisms. The application of biostimulants is both an environmentally friendly practice and a promising method that can enhance the sustainability of horticultural and agricultural production systems as well as promote the quantity and quality of foods. They can also reduce the global dependence on hazardous agricultural chemicals. Science Direct, Google Scholar, Springer Link, CAB Direct, Scopus, Springer Link, Taylor and Francis, Web of Science, and Wiley Online Library were checked, and the search was conducted on all manuscript sections in accordance with the terms Acinetobacter, Arthrobacter, Enterobacter, Ochrobactrum, Pseudomonas, Rhodococcus, Serratia, Streptomyces, Biostimulants, Plant growth promoting rhizobactera, and Stenotrophomonas. The aim of this manuscript is to survey the effects of plant-growth-promoting rhizobacteria by presenting case studies and successful paradigms in various agricultural and horticultural crops.

Changes in bacterial communities induced by integrated production systems and the phenological stages of soybean

Plant development and productivity depend on interactions with soil microorganisms for nutrient availability, promotion of growth and protection against phytopathogens. Although the influence of the phenological stages of soybean crops and their environmental conditions on the soil bacterial communities have already been reported, no studies have focused on the influence of integrated agrosilvopastoral systems on bacterial consortia. In this study, we evaluated the influence of the phenological stages of soybean cultivated under conventional full sunlight (CFS) and integrated crop-livestock-forestry (ICLF) systems on bacterial communities in the rhizosphere and in bulk soil using high-throughput sequencing techniques. Proteobacteria, Actinobacteriota and Acidobacteriota were the most abundant phyla in both the rhizosphere and the bulk soil at all growth stages. The results support our hypotheses that the richness and diversity of soil bacterial communities are influenced by different cultivation systems, and that the structure of the bacterial communities in the rhizosphere and the bulk soil are modulated by the phenological stages of the soybean crop.

Streptomyces spp. Strains as Potential Biological Control Agents against Verticillium Wilt of Olive

Verticillium wilt of olive (VWO) caused by Verticillium dahliae is considered a major olive (Olea europaea) disease in Mediterranean-type climate regions. The lack of effective chemical products against VWO makes it necessary to search for alternatives such as biological control. The main goal of this study was to evaluate the effect of six Streptomyces spp. strains as biological control agents (BCAs) against VWO. All of them were molecularly characterized by sequencing 16S or 23S rRNA genes and via phylogenetic analysis. Their effect was evaluated in vitro on the mycelial growth of V. dahliae (isolates V004 and V323) and on microsclerotia (MS) viability using naturally infested soils. Bioassays in olive plants inoculated with V. dahliae were also conducted to evaluate their effect against disease progress. In all the experiments, the reference BCAs Fusarium oxysporum FO12 and Aureobasidium pullulans AP08 were included for comparative purposes. The six strains were identified as Streptomyces spp., and they were considered as potential new species. All the BCAs, including Streptomyces strains, showed a significant effect on mycelial growth inhibition for both V. dahliae isolates compared to the positive control, with FO12 being the most effective, followed by AP08, while the Streptomyces spp. strains showed an intermediate effect. All the BCAs tested also showed a significant effect on the inhibition of germination of V. dahliae MS compared to the untreated control, with FO12 being the most effective treatment. Irrigation treatments with Streptomyces strain CBQ-EBa-21 or FO12 were significantly more effective in reducing disease severity and disease progress in olive plants inoculated with V. dahliae compared to the remaining treatments. This study represents the first approach to elucidating the potential effect of Streptomyces strains against VWO.

Endophytic Streptomyces sp. NEAU-DD186 from Moss with Broad-Spectrum Antimicrobial Activity: Biocontrol Potential Against Soilborne Diseases and Bioactive Components

Soilborne diseases cause significant economic losses in agricultural production around the world. They are difficult to control because a host plant is invaded by multiple pathogens, and chemical control often does not work well. In this study, we isolated and identified an endophytic Streptomyces sp. NEAU-DD186 from moss, which showed broad-spectrum antifungal activity against 17 soilborne phytopathogenic fungi, with Bipolaris sorokiniana being the most prominent. The strain also exhibited strong antibacterial activity against soilborne phytopathogenic bacteria Ralstonia solanacearum. To evaluate its biocontrol potential, the strain was prepared into biofertilizer by solid-state fermentation. Response surface methodology was employed to optimize the fermentation conditions for maximizing spore production and revealed that the 1:1 ratio of vermicompost to wheat bran, a temperature of 28°C, and 50% water content with an inoculation amount of 15% represented the optimal parameters. Pot experiments showed that the application of biofertilizer with a spore concentration of 108 CFU/g soil could effectively suppress the occurrence of tomato bacterial wilt caused by R. solanacearum and wheat root rot caused by B. sorokiniana, and the biocontrol efficacy was 81.2 and 72.2%, respectively. Chemical analysis of strain NEAU-DD186 extracts using nuclear magnetic resonance spectrometry and mass analysis indicated that 25-O-malonylguanidylfungin A and 23-O-malonylguanidylfungin A were the main active constituents, which showed high activity against R. solanacearum (EC50 of 2.46 and 2.58 µg ml-1) and B. sorokiniana (EC50 of 3.92 and 3.95 µg ml-1). In conclusion, this study demonstrates that Streptomyces sp. NEAU-DD186 can be developed as biofertilizer to control soilborne diseases.

Proximity-based defensive mutualism between Streptomyces and Mesorhizobium by sharing and sequestering iron

Microorganisms living in soil maintain intricate interactions among themselves, forming the soil microbiota that influences the rhizosphere microbiome and plant growth. However, the mechanisms underlying the soil microbial interactions remain unclear. Streptomyces and Mesorhizobium are commonly found in soil and serve as plant growth-promoting rhizobacteria (PGPR). Here, we identified an unprecedented interaction between the colonies of red-soil-derived Streptomyces sp. FXJ1.4098 and Mesorhizobium sp. BAC0120 and referred to it as "proximity-based defensive mutualism (PBDM)." We found that metabolite-mediated iron competition and sharing between the two microorganisms were responsible for PBDM. Streptomyces sp. FXJ1.4098 produced a highly diffusible siderophore, desferrioxamine, which made iron unavailable to co-cultured Mesorhizobium sp. BAC0120, thereby inhibiting its growth. Streptomyces sp. FXJ1.4098 also released poorly diffusible iron-porphyrin complexes, which could be utilized by Mesorhizobium sp. BAC0120, thereby restoring the growth of nearby Mesorhizobium sp. BAC0120. Furthermore, in ternary interactions, the PBDM strategy contributed to the protection of Mesorhizobium sp. BAC0120 close to Streptomyces sp. FXJ1.4098 from other microbial competitors, resulting in the coexistence of these two PGPR. A scale-up pairwise interaction screening suggested that the PBDM strategy may be common between Mesorhizobium and red-soil-derived Streptomyces. These results demonstrate the key role of iron in complex microbial interactions and provide novel insights into the coexistence of PGPR in soil.

Co-Occurrence Analysis of Citrus Root Bacterial Microbiota under Citrus Greening Disease

Candidatus Liberibacter asiaticus (CLas) is associated with Citrus Huanglongbing (HLB), a devastating disease in the US. Previously, we conducted a two-year-long monthly HLB survey by quantitative real-time PCR using root DNA fractions prepared from 112 field grapefruit trees grafted on sour orange rootstock. Approximately 10% of the trees remained CLas-free during the entire survey period. This study conducted 16S metagenomics using the time-series root DNA fractions, monthly prepared during twenty-four consecutive months, followed by microbial co-occurrence network analysis to investigate the microbial factors contributing to the CLas-free phenotype of the aforementioned trees. Based on the HLB status and the time when the trees were first diagnosed as CLas-positive during the survey, the samples were divided into four groups, Stage H (healthy), Stage I (early), II (mid), and III (late) samples. The 16S metagenomics data using Silva 16S database v132 revealed that HLB compromised the diversity of rhizosphere microbiota. At the phylum level, Actinobacteria and Proteobacteria were the predominant bacterial phyla, comprising >93% of total bacterial phyla, irrespective of HLB status. In addition, a temporal change in the rhizosphere microbe population was observed during a two-year-long survey, from which we confirmed that some bacterial families differently responded to HLB disease status. The clustering of the bacterial co-occurrence network data revealed the presence of a subnetwork composed of Streptomycetaceae and bacterial families with plant growth-promoting activity in Stage H and III samples. These data implicated that the Streptomycetaceae subnetwork may act as a functional unit against HLB.

Biocontrol Potential of Streptomyces odonnellii SZF-179 toward Alternaria alternata to Control Pear Black Spot Disease

Pear black spot disease, caused by Alternaria alternata, is a devastating disease in pears and leads to enormous economic losses worldwide. In this investigation, we isolated a Streptomyces odonnellii SZF-179 from the rhizosphere soil of pear plants in China. Indoor confrontation experiments results showed that both SZF-179 and its aseptic filtrate had excellent inhibitory effects against A. alternata. Afterwards, the main antifungal compound of SZF-179 was identified as polyene, with thermal and pH stability in the environment. A microscopic examination of A. alternata mycelium showed severe morphological abnormalities caused by SZF-179. Protective studies showed that SZF-179 fermentation broth could significantly reduce the diameter of the necrotic lesions on pear leaves by 42.25%. Furthermore, the potential of fermentation broth as a foliar treatment to control black leaf spot was also evaluated. Disease indexes of 'Hosui' and 'Wonwhang' pear plants treated with SZF-179 fermentation broth were lower than that of control plants. Overall, SZF-179 is expected to be developed into a safe and broad-spectrum biocontrol agent. No studies to date have evaluated the utility of S. odonnellii for the control of pear black spot disease; our study fills this research gap. Collectively, our findings provide new insights that will aid the control of pear black spot disease, as well as future studies of S. odonnellii strains.

Rice bacterial leaf blight drives rhizosphere microbial assembly and function adaptation

IMPORTANCE

Our results suggest that rhizosphere bacteria are more sensitive to bacterial leaf blight (BLB) than fungi. BLB infection decreased the diversity of the rhizosphere bacterial community but increased the complexity and size of the rhizosphere microbial community co-occurrence networks. In addition, the relative abundance of the genera Streptomyces, Chitinophaga, Sphingomonas, and Bacillus increased significantly. Finally, these findings contribute to the understanding of plant-microbiome interactions by providing critical insight into the ecological mechanisms by which rhizosphere microbes respond to phyllosphere diseases. In addition, it also lays the foundation and provides data to support the use of plant microbes to promote plant health in sustainable agriculture, providing critical insight into ecological mechanisms.

Streptomyces as a promising biological control agents for plant pathogens

Plant diseases caused by pathogenic microorganisms in agriculture present a considerable obstacle, resulting in approximately 30-40% crop damage. The use of conventional techniques to manage these microorganisms, i.e., applying chemical pesticides and antimicrobials, has been discovered to have adverse effects on human health and the environment. Furthermore, these methods have contributed to the emergence of resistance among phytopathogens. Consequently, it has become imperative to investigate natural alternatives to address this issue. The Streptomyces genus of gram-positive bacteria is a potentially viable natural alternative that has been extensively researched due to its capacity to generate diverse antimicrobial compounds, such as metabolites and organic compounds. Scientists globally use diverse approaches and methodologies to extract new bioactive compounds from these bacteria. The efficacy of bioactive compounds in mitigating various phytopathogens that pose a significant threat to crops and plants has been demonstrated. Hence, the Streptomyces genus exhibits potential as a biological control agent for combating plant pathogens. This review article aims to provide further insight into the Streptomyces genus as a source of antimicrobial compounds that can potentially be a biological control against plant pathogens. The investigation of various bioactive compounds synthesized by this genus can enhance our comprehension of their prospective utilization in agriculture.

Effects of nanoscale zinc oxide treatment on growth, rhizosphere microbiota, and metabolism of Aconitum carmichaelii

Trace elements play a crucial role in the growth and bioactive substance content of medicinal plants, but their utilization efficiency in soil is often low. In this study, soil and Aconitum carmichaelii samples were collected and measured from 22 different locations, followed by an analysis of the relationship between trace elements and the yield and alkaloid content of the plants. The results indicated a significant positive correlation between zinc, trace elements in the soil, and the yield and alkaloid content of A. carmichaelii. Subsequent treatment of A. carmichaelii with both bulk zinc oxide (ZnO) and zinc oxide nanoparticles (ZnO NPs) demonstrated that the use of ZnO NPs significantly enhanced plant growth and monoester-type alkaloid content. To elucidate the underlying mechanisms responsible for these effects, metabolomic analysis was performed, resulting in the identification of 38 differentially expressed metabolites in eight metabolic pathways between the two treatments. Additionally, significant differences were observed in the rhizosphere bacterial communities, with Bacteroidota and Actinobacteriota identified as valuable biomarkers for ZnO NP treatment. Covariation analysis further revealed significant correlations between specific microbial communities and metabolite expression levels. These findings provide compelling evidence that nanoscale zinc exhibits much higher utilization efficiency compared to traditional zinc fertilizer.

Rare rhizo-Actinomycetes: A new source of agroactive metabolites

Numerous biotic and abiotic stress in some geographical regions predisposed their agricultural matrix to challenges threatening plant productivity, health, and quality. In curbing these threats, different customary agrarian principles have been created through research and development, ranging from chemical inputs and genetic modification of crops to the recently trending smart agricultural technology. But the peculiarities associated with these methods have made agriculturists rely on plant rhizospheric microbiome services, particularly bacteria. Several bacterial resources like Proteobacteria, Firmicutes, Acidobacteria, and Actinomycetes (Streptomycetes) are prominent as bioinoculants or the application of their by-products in alleviating biotic/abiotic stress have been extensively studied, with a dearth in the application of rare Actinomycetes metabolites. Rare Actinomycetes are known for their colossal genome, containing well-preserved genes coding for prolific secondary metabolites with many agroactive functionalities that can revolutionize the agricultural industry. Therefore, the imperativeness of this review to express the occurrence and distributions of rare Actinomycetes diversity, plant and soil-associated habitats, successional track in the rhizosphere under diverse stress, and their agroactive metabolite characteristics and functionalities that can remediate the challenges associated with agricultural productivity.

Morchella esculenta cultivation in fallow paddy fields and drylands affects the diversity of soil bacteria and soil chemical properties

The properties of paddy field (DT) and dry land (HD) soil and food production can be enhanced by the cultivation of Morchella esculenta (ME) during the fallow period. However, whether ME cultivation affects the soil health and microbial diversity of paddy fields and drylands during the cultivation period remains unclear, and this has greatly limited the wider use of this cultivation model. Here, we analyzed the soil chemical properties and bacterial diversity (via metabarcoding sequencing) of DT and HD soils following ME cultivation. Our findings indicated that ME cultivation could enhance soil health. The content of soil phosphorus and potassium (K) was increased in DT soil under ME cultivation, and the K content was significantly higher in HD soil than in DT soil under ME cultivation. ME cultivation had a weak effect on alpha diversity, and ME cultivation affected the abundance of some genera of soil bacteria. The cultivation of ME might reduce the methane production capacity of DT soil and enhance the nitrogen cycling process of HD soil based on the results of functional annotation analysis. Network analysis and correlation analysis showed that Gemmatimonas, Bryobacter, and Anaeromyxobacter were the key bacterial genera regulating soil chemical properties in DT soil under ME cultivation, and Bryobacter, Bacillus, Streptomyces, and Paenarthrobacter were the key taxa associated with the accumulation of K in HD soil. The results of our study will aid future efforts to further improve this cultivation model.

Microbiota and its antibiotic resistance profile in avocado Guatemalan fruits (Persea nubigena var. guatemalensis) sold at retail markets of Ibarra city, northern Ecuador

Introduction

Avocados are typically sold in unsanitary conditions at the retail markets in Ecuador, which can raise the risk of microbial contamination. These microorganisms could exhibit multi-antibiotic resistance (MAR), being a serious threat concern to human health. In this study, we aimed to evaluate the microbiota and its antibiotic resistance profile in avocado Guatemalan fruits (Persea nubigena var. guatemalensis), at ripe stage: immature, firm light green (ready to eat in 4 days), peel (AFPE) and pulp (AFPU), and mature intense green (ready to eat) peel (AMPE) and pulp (AMPU), to gain baseline information on the prevalence of MAR bacteria.

Methods

Culture-independent (16S rRNA metagenomics) and culture-dependent approach (to detect specific indicator microorganisms) were used. Moreover, antibiotic susceptibility of selected target indicator bacteria was assessed providing information about the antibiotic resistance (AR) among the groups.

Results

Based on 16S rRNA gene metagenomic analysis, over 99.78% of reads were classified as bacteria in all samples. Shannon diversity index varies from 1.22 to 2.22, with the highest bacterial population assigned to AFPE samples (1327 species). The highest microbial counts of indicator Staphylococcus spp. (STAPHY), Enterobacter spp. (ENT), and Listeria spp. (LIST), were detected in AMPE samples. Thirty percent of the selected STAPHYs, and 20.91% of Enterobacter (ENT) clones were resistant to various classes of antibiotics. The MAR index varies between 0.25 to 0.88 and was clone-, and fruit ripe stage-dependent.

Conclusions

The results indicated that ready to eat avocados contained detectable levels of MAR bacteria, including methicillin resistant (MR)-STAPHY, which may act as a potential vector for the spread of antibiotic resistance. To achieve the increase of the production and marketing of Fuerte cultivar in Ecuador, it is vitally important to consider valuable strategies to protect the fruits at the early ripe stage in future. Thus, it is crucial to set up efficient control measures and develop coordinated strategies to guarantee the microbiological quality of the food.

Ralstonia solanacearum: An Arsenal of Virulence Strategies and Prospects for Resistance

The group of strains constituting the Ralstonia solanacearum species complex (RSSC) is a prominent model for the study of plant-pathogenic bacteria because of its impact on agriculture, owing to its wide host range, worldwide distribution, and long persistence in the environment. RSSC strains have led to numerous studies aimed at deciphering the molecular bases of virulence, and many biological functions and mechanisms have been described to contribute to host infection and pathogenesis. In this review, we put into perspective recent advances in our understanding of virulence in RSSC strains, both in terms of the inventory of functions that participate in this process and their evolutionary dynamics. We also present the different strategies that have been developed to combat these pathogenic strains through biological control, antimicrobial agents, plant genetics, or microbiota engineering.

Whole genome analysis of Streptomyces sp. RerS4, a Rehmannia glutinosa rhizosphere microbe producing a new lipopeptide

Rehmannia glutinosa, a valuable medicinal plant, is threatened by ring rot, a condition that greatly affects its yield and quality. Interactions between plant and the rhizosphere soil microbiome in the context of pathogen invasion are generally more specific, with recruitment of specialized microbes potentially antagonistic to a certain pathogen. Isolation of microorganisms from rhizosphere soil of healthy and ring rot-infected R. glutinosa was carried out to screen antifungal microbes. A strain designated RerS4 isolated from ring rot-infected R. glutinosa rhizosphere soil with strong antifungal activities was selected for further study. RerS4 was taxonomically characterized as the genus Streptomyces according to its morphology and 16S rRNA sequences that were most closely related to Streptomyces racemochromogenes NRRL B-5430T (99.72%) and Streptomyces polychromogenes NBRC 13072T (99.72%). A new lipopeptide isolated from RerS4 showed restrained proliferation, but was devoid of significant antibacterial and antioxidant activity with minimum inhibitory concentration (MIC) values of 20.3 ± 2.5 and 70.8 ± 3.7 μg/mL and half-maximal inhibitory concentration (IC50) values of 23.3 ± 0.8 and 58.8 ± 2.9 μg/mL, respectively. In addition, we report the complete genome sequence of Streptomyces sp. RerS4, which consists of a 7,301,482 bp linear chromosome and a 242,139 bp plasmid. Genome analysis revealed that Streptomyces sp. RerS4 contained 25 biosynthetic gene clusters (BGCs) for secondary metabolites, among which 68% had low similarities with known BGCs, leading us to believe that Streptomyces sp. RerS4 could produce valuable bioactive compounds.

The Good, the Bad, and the Useable Microbes within the Common Alder (Alnus glutinosa) Microbiome-Potential Bio-Agents to Combat Alder Dieback

Common Alder (Alnus glutinosa (L.) Gaertn.) is a tree species native to Ireland and Europe with high economic and ecological importance. The presence of Alder has many benefits including the ability to adapt to multiple climate types, as well as aiding in ecosystem restoration due to its colonization capabilities within disturbed soils. However, Alder is susceptible to infection of the root rot pathogen Phytophthora alni, amongst other pathogens associated with this tree species. P. alni has become an issue within the forestry sector as it continues to spread across Europe, infecting Alder plantations, thus affecting their growth and survival and altering ecosystem dynamics. Beneficial microbiota and biocontrol agents play a crucial role in maintaining the health and resilience of plants. Studies have shown that beneficial microbes promote plant growth as well as aid in the protection against pathogens and abiotic stress. Understanding the interactions between A. glutinosa and its microbiota, both beneficial and pathogenic, is essential for developing integrated management strategies to mitigate the impact of P. alni and maintain the health of Alder trees. This review is focused on collating the relevant literature associated with Alder, current threats to the species, what is known about its microbial composition, and Common Alder-microbe interactions that have been observed worldwide to date. It also summarizes the beneficial fungi, bacteria, and biocontrol agents, underpinning genetic mechanisms and secondary metabolites identified within the forestry sector in relation to the Alder tree species. In addition, biocontrol mechanisms and microbiome-assisted breeding as well as gaps within research that require further attention are discussed.

Endophytic Streptomyces sp. NEAU-ZSY13 from the leaf of Perilla frutescens, as a promising broad-spectrum biocontrol agent against soil-borne diseases

Soil-borne diseases cause significant economic losses in global agricultural production. These diseases are challenging to control due to the invasion of multiple pathogens into host plants, and traditional chemical control methods often yield unsatisfactory results. In this study, we isolated and identified an endophytic Streptomyces, designated as NEAU-ZSY13, from the leaf of Perilla frutescens. This isolate exhibited broad-spectrum antifungal activity against 17 soil-borne phytopathogenic fungi, with Bipolaris sorokiniana being the most prominent. Additionally, it displayed strong antibacterial activity against the soil-borne phytopathogenic bacterium Ralstonia solanacearum. To assess its biocontrol potential, the isolate was utilized to produce a biofertilizer through solid-state fermentation. The fermentation conditions were optimized using response surface methodology to maximize the spore production. The results revealed that more abundant spores were produced with a 1:2 ratio of vermicompost to wheat bran, 60% water content, 20% inoculation amount and 28°C. Subsequent pot experiments demonstrated that the application of the biofertilizer with a spore concentration of 108 CFU/g soil effectively suppressed the occurrence of tomato bacterial wilt caused by R. solanacearum and wheat root rot caused by B. sorokiniana, with biocontrol efficacies of 72.2 and 78.3%, respectively. Chemical analysis of NEAU-ZSY13 extracts, using nuclear magnetic resonance spectrometry and mass analysis, identified niphimycin C and niphimycin A as the primary active constituents. These compounds exhibited high activity against R. solanacearum (EC50 of 3.6 and 2.4 μg mL-1) and B. sorokiniana (EC50 of 3.9 and 3.4 μg mL-1). In conclusion, this study demonstrates the potential of Streptomyces sp. NEAU-ZSY13 as a biofertilizer for the control of soil-borne diseases.

Can endophytic microbial compositions in cane roots be shaped by different propagation methods

In practical production, cane stems with buds are generally used as seed for propagation. However, long-terms cane stems only easily lead to some problems such as disease sensitivity, quality loss, etc. Recently, cane seedings, which are produced by tissue culture were used in sugarcane production, but few studies on cane health related to tissue culture seedings. Therefore, to evaluate the immunity and health of sugarcanes growing from different reproduction modes, the endophytic microbial compositions in cane roots between stem and tissue culture seedlings were analyzed using high-throughput techniques. The results showed that the endophytic microbial compositions in cane roots were significant differences between stem and tissue culture seedlings. At the genus level, Pantoea, Bacillus, Streptomyces, Lechevalieria, Pseudomonas, Nocardioides, unclassified_f__Comamonadaceae enriched as the dominant endophytic bacterial genera, and Rhizoctonia, Sarocladium, Scytalidium, Wongia, Fusarium, unclassified_f__Phaeosphaer, unclassified_c__Sordariom, unclassified_f__Stachybot, Poaceascoma, Microdochium, Arnium, Echria, Mycena and Exophiala enriched as the dominant endophytic fungal genera in cane roots growing from the tissue culture seedlings. In contrast, Mycobacterium, Massilia, Ralstonia, unclassified_f__Pseudonocardiacea, norank_f__Micropepsaceae, Leptothrix and Bryobacter were the dominant endophytic bacterial genera, and unclassified_k__Fungi, unclassified_f__Marasmiaceae, Talaromyces, unclassified_c__Sordariomycetes and Trichocladium were the dominant endophytic fungal genera in cane roots growing from stem seedlings. Additionally, the numbers of bacterial and fungal operational taxonomic units (OTUs) in cane roots growing from tissue culture seedlings were significantly higher than those of stem seedlings. It indicates that not only the endophytic microbial compositions in cane roots can be shaped by different propagation methods, but also the stress resistance of sugarcanes can be improved by the tissue culture propagation method.

Genetic diversity, plant growth promotion potential, and antimicrobial activity of culturable endophytic actinobacteria isolated from Aconitum carmichaelii Debeaux

AIM

This study evaluated the phylogenetic diversity, plant growth promotion capacity, antifungal activity, and biocontrol potential of culturable actinobacterial endophytes isolated from the medicinal plant Aconitum carmichaelii Debeaux.

METHODS AND RESULTS

Isolation of actinobacteria from healthy A. carmichaelii plants was carried out on six different media. Full-length 16S rRNA gene was amplified by PCR from the genomic DNA of each strain. Indole-3-acetic acid and siderophore production were quantitatively assessed by the Salkowski and Chrome Azurol S methods, respectively. Rice seeds germination and seedling growth were employed to evaluate plant growth promotion capacities of candidate strains. Dual-culture assay and pot experiments were performed to investigate the antifungal and biocontrol potential of isolates. We obtained 129 actinobacterial isolates from A. carmichaelii, and they belonged to 49 species in 7 genera. These strains exhibited diverse plant growth promotion ability, among which one strain significantly enhanced rice seeds germination, while 31 strains significantly facilitated rice seedling growth. SWUST-123 showed strong antifungal activity against four pathogens in vitro and was most compatible with Qingchuan cultivar. SWUST-123 reduced around 40% of southern blight disease occurrence compared to blank control treatment. .

CONCLUSION

Aconitum carmichaelii harbored genetically diverse actinobacterial endophytes exhibiting diverse plant growth promotion and antifungal potential, some of which can be served as good candidates for biofertilizers and biocontrol agents.

Genus Streptomyces: Recent advances for biotechnological purposes

Actinomycetes are a distinct group of filamentous bacteria. The Streptomyces genus within this group has been extensively studied over the years, with substantial contributions to society and science. This genus is known for its antimicrobial production, as well as antitumor, biopesticide, and immunomodulatory properties. Therefore, the extraordinary plasticity of the Streptomyces genus has inspired new research techniques. The newest way of exploring Streptomyces has comprised the discovery of new natural metabolites and the application of emerging tools such as CRISPR technology in drug discovery. In this narrative review, we explore relevant published literature concerning the ongoing novelties of the Streptomyces genus.

Diversity of Endophytic Microbes in Taxus yunnanensis and Their Potential for Plant Growth Promotion and Taxane Accumulation

Taxus spp. are ancient tree species that have survived from the Quaternary glacier period, and their metabolites, such as taxol, have been used as anticancer drugs globally. Plant-endophytic microbial interaction plays a crucial role in exerting a profound impact on host growth and secondary metabolite synthesis. In this study, high-throughput sequencing was employed to explore endophytic microbial diversity in the roots, stems, and leaves of the Taxus yunnanensis (T. yunnanensis). The analysis revealed some dominant genera of endophytic bacteria, such as Pseudomonas, Neorhizobium, Acidovorax, and Flavobacterium, with Cladosporium, Phyllosticta, Fusarium, and Codinaeopsis as prominent endophytic fungi genera. We isolated 108 endophytic bacteria and 27 endophytic fungi from roots, stems, and leaves. In vitro assays were utilized to screen for endophytic bacteria with growth-promoting capabilities, including IAA production, cellulase, siderophore production, protease and ACC deaminase activity, inorganic phosphate solubilization, and nitrogen fixation. Three promising strains, Kocuria sp. TRI2-1, Micromonospora sp. TSI4-1, and Sphingomonas sp. MG-2, were selected based on their superior growth-promotion characteristics. These strains exhibited preferable plant growth promotion when applied to Arabidopsis thaliana growth. Fermentation broths of these three strains were also found to significantly promote the accumulation of taxanes in T. yunnanensis stem cells, among which strain TSI4-1 demonstrated outstanding increase potentials, with an effective induction of taxol, baccatin III, and 10-DAB contents. After six days of treatment, the contents of these metabolites were 3.28 times, 2.23 times, and 2.17 times the initial amounts, reaching 8720, 331, and 371 ng/g of dry weight of stem cells, respectively. These findings present new insight into the industrialization of taxol production through Taxus stem cell fermentation, thereby promoting the conservation of wild Taxus resources by maximizing their potential economic benefits.

Crosstalk between in situ root exudates and rhizobacteria to promote rice growth by selenium nanomaterials

Maximizing the potential of plant-microbe systems offers great opportunities to confront sustainability issues in agroecosystems. However, the dialog between root exudates and rhizobacteria remains largely unknown. As a novel nanofertilizer, nanomaterials (NMs) have significant potential to improve agricultural productivity due to their unique properties. Here, soil amendment with 0.1 mg·kg^-1 selenium (Se) NMs (30-50 nm) significantly promoted rice seedling growth. Differences in root exudates and rhizobacteria were evident. At an earlier time point (3^rd week), Se NMs increased the relative content of malic and citric acid by 15.4- and 8.1-fold, respectively. Meanwhile, the relative abundances of Streptomyces and Sphingomonas were increased by 164.6 % and 38.3 %, respectively. As the exposure time increased, succinic acid (40.5-fold) at the 4^th week and salicylic acid (4.7-fold) and indole-3-acetic (7.0-fold) at the 5^th week were enhanced, while Pseudomonas and Bacillus increased at the 4^th (112.3 % and 50.2 %) and 5^th weeks (190.8 % and 53.1 %), respectively. Further analysis indicated that (1) Se NMs directly enhanced the synthesis and secretion of malic and citric acids by upregulating their biosynthesis and transporter genes and then recruited Bacillus and Pseudomonas; (2) Se NMs upregulated the chemotaxis and flagellar genes of Sphingomonas for more interaction with rice plants, thereby promoting rice growth and stimulating root exudate secretion. This crosstalk of root exudates and rhizobacteria enhanced nutrient uptake, resulting in promoted rice growth. Our study offers insights into the crosstalk between root exudates and rhizobacteria by NMs and provides new insights into rhizosphere regulation in nano-enabled agriculture.

Inoculation with Actinobacteria spp. Isolated from a Hyper-Arid Environment Enhances Tolerance to Salinity in Lettuce Plants (Lactuca sativa L.)

Irrigated agriculture is responsible for a third of global agricultural production, but the overuse of water resources and intensification of farming practices threaten its sustainability. The use of saline water in irrigation has become an alternative in areas subjected to frequent drought, but this practice affects plant growth due to osmotic impact and excess of ions. Plant-growth-promoting rhizobacteria (PGPR) can mitigate the negative impacts of salinity and other abiotic factors on crop yields. Actinobacteria from the hyper-arid Atacama Desert could increase the plant tolerance to salinity, allowing their use as biofertilizers for lettuce crops using waters with high salt contents. In this work, rhizosphere samples of halophytic Metharme lanata were obtained from Atacama Desert, and actinobacteria were isolated and identified by 16S gene sequencing. The PGPR activities of phosphate solubilization, nitrogen fixation, and the production of siderophore and auxin were assessed at increasing concentrations of NaCl, as well as the enhancement of salt tolerance in lettuce plants irrigated with 100 mM of NaCl. Photosynthesis activity and chlorophyll content, proline content, lipid peroxidation, cation and P concentration, and the identification and quantification of phenolic compounds were assessed. The strains S. niveoruber ATMLC132021 and S. lienomycini ATMLC122021 were positive for nitrogen fixation and P solubilization activities and produced auxin up to 200 mM NaCl. In lettuce plants, both strains were able to improve salt stress tolerance by increasing proline contents, carotenoids, chlorophyll, water use efficiency (WUE), stomatal conductance (gs), and net photosynthesis (A), concomitantly with the overproduction of the phenolic compound dicaffeoylquinic acid. All these traits were positively correlated with the biomass production under saltwater irrigation, suggesting its possible use as bioinoculants for the agriculture in areas where the water resources are scarce and usually with high salt concentrations.

Specific Streptomyces strain enhances the growth, defensive mechanism, and fruit quality of cucumber by minimizing its fertilizer consumption

BACKGROUND

The required amounts of chemical fertilizers (NPK) are determined by plant yield, and product quality is given less consideration. The use of PGPRs is an environmentally friendly approach that, in addition to increasing yield, also improves fruit quality. This study examined the role of specific Streptomyces strains in aiding cucumber plants to 1) use fewer NPK fertilizers in the same quantity 2) improve the quality of cucumber fruit, and 3) promote growth and defense system.

RESULTS

In this study, the effect of 17 Streptomyces strains on the vegetative traits of cucumber seedlings of the Sultan cultivar was evaluated as the first test. Four strains of Streptomyces with the highest root and shoot dry weight were selected from the strains. This experiment was performed to determine the interaction effect of selected strains and different amounts of NPK on cucumber yield, quality, physiological and biochemical responses of plants. The first experiment's results revealed that strains IC6, Y7, SS12, and SS14 increased significantly in all traits compared to the control, while the other strains dramatically improved several characteristics. Analysis of variance (ANOVA) revealed significant differences between the effect of strains, NPK concentrations, and their interactions on plant traits. The treatments containing 75% NPK + SS12, yielded the most fruit (40% more than the inoculated control). Antioxidant enzymes assay showed that SS12 substantially increased the activity of POX, PPO, and the expression of the genes related to these two enzymes. Hormone assay utilizing HPLC analysis revealed that various strains employ a specific mechanism to improve the immune system of plants.

CONCLUSIONS

Treatment with strain SS12 led to the production of cucumbers with the highest quality by reducing the amount of nitrate, and soluble sugars and increasing the amount of antioxidants and firmness compared to other treatments. A specific Streptomyces strain could reduce 25% of NPK fertilizer during the vegetative and reproductive growth period. Moreover, this strain protected plants against possible pathogens and adverse environmental factors through the ISR and SAR systems.