Delay of flower senescence by bacterial endophytes expressing 1-aminocyclopropane-1-carboxylate deaminase

How Do Plant Growth-Promoting Bacteria Use Plant Hormones to Regulate Stress Reactions?

Phytohormones play a crucial role in regulating growth, productivity, and development while also aiding in the response to diverse environmental changes, encompassing both biotic and abiotic factors. Phytohormone levels in soil and plant tissues are influenced by specific soil bacteria, leading to direct effects on plant growth, development, and stress tolerance. Specific plant growth-promoting bacteria can either synthesize or degrade specific plant phytohormones. Moreover, a wide range of volatile organic compounds synthesized by plant growth-promoting bacteria have been found to influence the expression of phytohormones. Bacteria-plant interactions become more significant under conditions of abiotic stress such as saline soils, drought, and heavy metal pollution. Phytohormones function in a synergistic or antagonistic manner rather than in isolation. The study of plant growth-promoting bacteria involves a range of approaches, such as identifying singular substances or hormones, comparing mutant and non-mutant bacterial strains, screening for individual gene presence, and utilizing omics approaches for analysis. Each approach uncovers the concealed aspects concerning the effects of plant growth-promoting bacteria on plants. Publications that prioritize the comprehensive examination of the private aspects of PGPB and cultivated plant interactions are of utmost significance and crucial for advancing the practical application of microbial biofertilizers. This review explores the potential of PGPB-plant interactions in promoting sustainable agriculture. We summarize the interactions, focusing on the mechanisms through which plant growth-promoting bacteria have a beneficial effect on plant growth and development via phytohormones, with particular emphasis on detecting the synthesis of phytohormones by plant growth-promoting bacteria.

Ethylene, ACC, and the Plant Growth-Promoting Enzyme ACC Deaminase

Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.

Nematicidal, Acaricidal and Plant Growth-Promoting Activity of Enterobacter Endophytic Strains and Identification of Genes Associated with These Biological Activities in the Genomes

In the present study, the nematicidal and acaricidal activity of three Enterobacter endophytic strains isolated from Mimosa pudica nodules was evaluated. The percentages of mortality of Enterobacter NOD4 against Panagrellus redivivus was 81.2%, and against Nacobbus aberrans 70.1%, Enterobacter NOD8 72.4% and 62.5%, and Enterobacter NOD10 64.8% and 58.7%, respectively. While against the Tyrophagus putrescentiae mite, the mortality percentages were 68.2% due to Enterobacter NOD4, 64.3% due to Enterobacter NOD8 and 77.8% due to Enterobacter NOD10. On the other hand, the ability of the three Enterobacter strains to produce indole acetic acid and phosphate solubilization, characteristics related to plant growth-promoting bacteria, was detected. Bioinformatic analysis of the genomes showed the presence of genes related to IAA production, phosphate solubilization, and nitrogen fixation. Phylogenetic analyzes of the recA gene, phylogenomics, and average nucleotide identity (ANI) allowed us to identify the strain Enterobacter NOD8 related to E. mori and Enterobacter NOD10 as E. asburiae, while Enterobacter NOD4 was identified as a possible new species of this species. The plant growth-promoting, acaricidal and nematicidal activity of the three Enterobacter strains makes them a potential agent to include in biocontrol alternatives and as growth-promoting bacteria in crops of agricultural interest.

Shifts in the Rhizosphere and Endosphere Colonizing Bacterial Communities Under Drought and Salinity Stress as Affected by a Biofertilizer Consortium

The present research asks how plant growth-promoting bacterial (PGPB) inoculants and chemical fertilizers change rhizosphere and root endophytic bacterial communities in durum wheat, and its dependence on environmental stress. A greenhouse experiment was carried out under drought (at 40% field capacity), or salinity (150 mM NaCl) conditions to investigate the effects of a chemical fertilizer (containing nitrogen, phosphorus, potassium and zinc) or a biofertilizer (a bacterial consortium of four PGPBs). High-throughput amplicon sequencing of the 16S rRNA of the rhizosphere, non-sterilized, or surface-sterilized roots, showed shifts in bacterial communities in response to stress treatments, which were greater for salinity than for drought and tended to show increased oligotrophs relative abundances compared to non-stress controls. The results also showed that Proteobacteria, Acidobacteria, Bacteroidetes, Gemmatimonadetes, Thaumarchaeota, Firmicutes, and Verrucomicrobia had a higher relative abundance in the rhizosphere, while Actinobacteria were more abundant on roots, while Candidatus_Saccharibacteria and Planctomycetes inside roots. The results indicated that the root endophytic bacterial communities were more affected by (bio-) fertilization treatments than those in the rhizosphere, particularly as affected by PGPB inoculation. This greater susceptibility of endophytes to (bio-) fertilizers was associated with increased abundance of the 16S rRNA and acdS genes in plant roots, especially under stress conditions. These changes in root endophytes, which coincided with an improvement in grain yield and photosynthetic capacity of plants, may be considered as one of the mechanisms by which PGPB affect plants.

The Multifunctions and Future Prospects of Endophytes and Their Metabolites in Plant Disease Management

Endophytes represent a ubiquitous and magical world in plants. Almost all plant species studied by different researchers have been found to harbor one or more endophytes, which protect host plants from pathogen invasion and from adverse environmental conditions. They produce various metabolites that can directly inhibit the growth of pathogens and even promote the growth and development of the host plants. In this review, we focus on the biological control of plant diseases, aiming to elucidate the contribution and key roles of endophytes and their metabolites in this field with the latest research information. Metabolites synthesized by endophytes are part of plant disease management, and the application of endophyte metabolites to induce plant resistance is very promising. Furthermore, multi-omics should be more fully utilized in plant–microbe research, especially in mining novel bioactive metabolites. We believe that the utilization of endophytes and their metabolites for plant disease management is a meaningful and promising research direction that can lead to new breakthroughs in the development of more effective and ecosystem-friendly insecticides and fungicides in modern agriculture.

Examining the genomic features of human and plant-associated Burkholderia strains

Humans and plants have evolved in the near omnipresence of a microbial milieu, and the factors that govern host-microbe interactions continue to require scientific exploration. To better understand if and to what degree patterns between microbial genomic features and host association (i.e., human and plant) exist, I analyzed the genomes of select Burkholderia strains-a bacterial genus comprised of both human and plant-associated strains-that were isolated from either humans or plants. To this end, I uncovered host-specific, genomic patterns related to metabolic pathway potentials in addition to convergent features that may be related to pathogenic overlap between hosts. Together, these findings detail the genomic associations of human and plant-associated Burkholderia strains and provide a framework for future investigations that seek to link host-host transmission potentials.

Perspective of ACC-deaminase producing bacteria in stress agriculture

The 1-aminocyclopropane-1-carboxylate deaminase (ACCD) enzyme plays an important role in stress alleviation of both biotic and abiotic stressors in plants and thereby enhances their growth under harsh environmental conditions. In-depth analysis of AcdS gene encoding for ACC deaminase reveals its presence in diverse microorganisms including bacteria and fungi. Particularly, plant growth-promoting bacteria (PGPB) containing ACCD supports plant growth by modulating the level of 'stress ethylene' and cleaving its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) into α-ketobutyrate and ammonia, enabling PGPB to utilize ACC as a carbon and nitrogen source. The reduced synthesis of ethylene in plants further relieves the ethylene inhibition of plant growth and development, and improves plant resistance to various stressors. Therefore, the dual role of microbial ACCD makes it a cost-effective and eco-friendly biocatalyst for sustainable agricultural productions. The inducible ACCD encoding gene AcdS is differentially regulated by varying environmental conditions. Successful generation of transgenic plants with microbial AcdS gene enhanced biotic and abiotic stress tolerance in plants. In the present review, we discuss the importance of ACCD-producing PGPB for their ability to reduce ethylene production and the promotion of plant growth under stress conditions. We also highlighted the development of transgenic plants by overexpressing bacterial AcdS gene to improve their performance under stress conditions.

Role of ACC deaminase producing bacteria for abiotic stress management and sustainable agriculture production

Plants are immobile and are exposed to various biotic and abiotic stresses, including heat, cold, drought, flooding, nutrient deficiency, heavy metal exposure, phytopathogens, and pest attacks. The stressors significantly affect agricultural productivity when exceed a certain threshold. It has been reported that most of the stressed plants are reported to have increased ethylene synthesis from its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene is a plant hormone that plays a vital role in the regulation of various physiological processes, such as respiration, nitrogen fixation, and photosynthesis. The increment in the plant hormone ethylene would reduce plant growth and development, and if the ethylene level increased beyond the limit, it could also result in plant death. Therefore, plant growth-promoting bacteria (PGPB) possessing ACC deaminase activity play an essential role in the management of biotic and abiotic stresses by hydrolysing 1-aminocyclopropane-1-carboxylic acid using ACC deaminase. In this review, the importance of ACC deaminase-producing bacteria in promoting plant growth under various abiotic stressors is discussed.

Scrutinizing the Application of Saline Endophyte to Enhance Salt Tolerance in Rice and Maize Plants

The present study aimed to witness the plant-microbe interaction associated with salt tolerance in crops. We isolated the endophytic microbe from the root zone of halophytic grass. Later, the salt tolerance of the endophyte was tested in the saline medium and was identified using nucleotide sequencing (GenBank under the accession numbers: SUB9030920 AH1_AHK_ITS1 MW570850: SUB9030920 AH1_AHK_ITS4 MW570851). Rice and maize seeds were coated with identified endophyte Aspergillus terreus and were sown in separate plastic pots. Later 21-day-old seedlings were subjected to three NaCl concentrations, including 50, 100, and 150 mM salt stress. Under saline conditions, A. terreus showed a substantial increase in growth, biomass, relative water content, oxidative balance, and photochemical efficiency of rice and maize plants. The data reflected that the stimulation of gibberellic acid (GA) in treated leaves may be the main reason for the upregulation of photosynthesis and the antioxidant defense cascade. The data also depict the downregulation of oxidative damage markers malondialdehyde, hydrogen peroxide in rice and maize plants. Conclusively, salt-tolerant endophytic fungus A. terreus explicitly displayed the positive plant-microbe interaction by developing salt tolerance in rice and maize plants. Salt tolerance by endophytic fungus coincides with the enhanced GA concentration, which illustrated the stimulated physiological mechanism and gene in response to the extreme environmental crisis, resulting in improved crop productivity.

The ACC deaminase-producing plant growth-promoting bacteria: Influences of bacterial strains and ACC deaminase activities in plant tolerance to abiotic stress

Global climate change results in frequent occurrences and/or long durations of abiotic stress. Field grown plants are affected by abiotic stress, and they modulate ethylene in response to abiotic stress exposure and use it as a signaling molecule in stress tolerance mechanisms. However, frequent occurrences and/or long durations of stress conditions can cause plants to induce ethylene levels higher than their thresholds, resulting in a reduction of plant growth and crop productivity. The use of plant growth-promoting bacteria (PGPB) that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase has increased in various plant species to ameliorate the deleterious effects of stress-induced ethylene and promote plant growth despite abiotic stress conditions. Unfortunately, there are restrictions that limit the use of ACC deaminase-producing PGPB to protect plants from abiotic stresses. This review describes how abiotic stress induces ethylene and how stress-induced ethylene adversely affects plant growth. In addition, this review emphasizes the importance of the compatibility of PGPB strains and specific host plants and ACC deaminase activities in the reduction of stress ethylene and the promotion of plant growth, based on the research published in the last 10 years. Moreover, due to the restrictions in PGPB use, this review highlights the potential generation of transgenic plants expressing the AcdS gene that encodes the ACC deaminase enzyme as a substitute for PGPB in the future to support and uplift agricultural sustainability and food security globally.

Pseudomonas 1-Aminocyclopropane-1-carboxylate (ACC) Deaminase and Its Role in Beneficial Plant-Microbe Interactions

The expression of the enzyme 1-aminocylopropane-1-carboxylate (ACC) deaminase, and the consequent modulation of plant ACC and ethylene concentrations, is one of the most important features of plant-associated bacteria. By decreasing plant ACC and ethylene concentrations, ACC deaminase-producing bacteria can overcome some of the deleterious effects of inhibitory levels of ACC and ethylene in various aspects of plant-microbe interactions, as well as plant growth and development (especially under stressful conditions). As a result, the acdS gene, encoding ACC deaminase, is often prevalent and positively selected in the microbiome of plants. Several members of the genus Pseudomonas are widely prevalent in the microbiome of plants worldwide. Due to its adaptation to a plant-associated lifestyle many Pseudomonas strains are of great interest for the development of novel sustainable agricultural and biotechnological solutions, especially those presenting ACC deaminase activity. This manuscript discusses several aspects of ACC deaminase and its role in the increased plant growth promotion, plant protection against abiotic and biotic stress and promotion of the rhizobial nodulation process by Pseudomonas. Knowledge regarding the properties and actions of ACC deaminase-producing Pseudomonas is key for a better understanding of plant-microbe interactions and the selection of highly effective strains for various applications in agriculture and biotechnology.

Effect of Foliar Supplied PGRs on Flower Growth and Antioxidant Activity of African Marigold (Tagetes erecta L.)

Marigold is one of the commercially exploited flowering crops that belongs to the family Asteraceae. The production of economical yield and better quality of marigold flowers requires proper crop management techniques. Crop regulation is an important technique to make the marigold production profitable. This can be done by adopting application of plant growth regulators (PGRs). The present study was designed to investigate the effect of PGRs on flowering and antioxidant activity of two cultivars of African marigold (Tagetes erecta L.) viz. “Pusa Narangi Gainda” (hereinafter referred to as Narangi) and “Pusa Basanthi Gainda” (hereafter referred to as Basanthi). Plants were sprayed with abscisic acid (ABA), N-acetyl thiazolidine (NAD), gibberellic acid (GA3), salicylic acid (SA), indole-3-butyric acid (IBA) and oxalic acid (OA) at the concentrations of 100, 150, 250, 300 and 800 mg·L−1, each. Results revealed that the plants treated with 500–600 mg·L−1 IBA exhibited maximum increase in floral diameter (34–51%). The use of 500–550 mg·L−1 IBA exhibited maximal enhancement in flower fresh weight (21–92%). The exogenously applied OA significantly (p ≤ 0.05) improved flower dry weight, total phenolic contents, total flavonoid contents and reducing power ability of marigold plants. Overall, “Narangi” performed better than “Basanthi”, in terms of flowering and antioxidant activity. Conclusively, the results suggest that foliar application of PGRs favors flowering and antioxidant activity of African marigold.

Plant Growth Promoting and Stress Mitigating Abilities of Soil Born Microorganisms

Abiotic stresses affect the plant growth in different ways and at different developmental stages that reduce the crop yields. The increasing world population continually demands more crop yields; therefore it is important to use low-cost technologies against abiotic stresses to increase crop productivity. Soil microorganisms survive in the soil associated with plants in extreme condition. It was demonstrated that these beneficial microorganisms promote plant growth and development under various stresses. The soil microbes interact with the plant through rhizospheric or endophytic association and promote the plant growth through different processes such as nutrients mobilization, disease suppression, and hormone secretions. The microorganisms colonized in the rhizospheric region and imparted the abiotic stress tolerance by producing 1-aminocyclopropane-1- carboxylate (ACC) deaminase, antioxidant, and volatile compounds, inducing the accumulation of osmolytes, production of exopolysaccharide, upregulation or downregulation of stress genes, phytohormones and change the root morphology. A large number of these rhizosphere microorganisms are now patented. In the present review, an attempt was made to throw light on the mechanism of micro-organism that operates during abiotic stresses and promotes plant survival and productivity.

Bacterial endophytome-mediated resistance in banana for the management of Fusarium wilt

Banana (Musa spp.), a major cash and staple fruit crop in many parts of the world, is infected by Fusarium wilt, which contributes up to 100% yield loss and causes social consequences. Race 1 and race 2 of Panama wilt caused by Fusarium oxysporum f. sp. cubense (Foc) are prevalent worldwide and seriously affect many traditional varieties. The threat of Foc tropical race 4 (Foc TR4) is looming large in African counties. However, its incidence in India has been confined to Bihar (Katihar and Purnea), Uttar Pradesh (Faizabad), Madhya Pradesh (Burhanpur) and Gujarat (Surat). Management of Foc races by employing fungicides is often not a sustainable option as the disease spread is rapid and they negatively alter the biodiversity of beneficial ectophytes and endophytes. Besides, soil drenching with carbendazim/trifloxystrobin + tebuconazole is also not effective in suppressing the Fusarium wilt of banana. Improvement of resistance to Fusarium wilt in susceptible cultivars is being addressed through both conventional and advanced breeding approaches. However, engineering of banana endosphere with bacterial endophytes from resistant genotypes like Pisang lilly and YKM5 will induce the immune response against Foc, irrespective of races. The composition of the bacterial endomicrobiome in different banana cultivars is dominated by the phyla Proteobacteria, Bacteroidetes and Actinobacteria. The major bacterial endophytic genera antagonistic to Foc are Bacillus, Brevibacillus, Paenibacillus, Virgibacillus, Staphylococcus, Cellulomonas, Micrococcus, Corynebacterium, Kocuria spp., Paracoccus sp., Acinetobacter spp. Agrobacterium, Aneurinibacillus, Enterobacter, Klebsiella, Lysinibacillus, Micrococcus, Rhizobium, Sporolactobacillus, Pantoea, Pseudomonas, Serratia, Microbacterium, Rhodococcus, Stenotrophomonas, Pseudoxanthomonas, Luteimonas, Dokdonella, Rhodanobacter, Luteibacter, Steroidobacter, Nevskia, Aquicella, Rickettsiella, Legionella, Tatlockia and Streptomyces. These bacterial endophytes promote the growth of banana plantlets by solubilising phosphate, producing indole acetic acid and siderophores. Application of banana endophytes during the hardening phase of tissue-cultured clones serves as a shield against Foc. Hitherto, MAMP molecules of endophytes including flagellin, liposaccharides, peptidoglycans, elongation factor, cold shock proteins and hairpins induce microbe-associated molecular pattern (MAMP)-triggered immunity to suppress plant pathogens. The cascade of events associated with ISR and SAR is induced through MAPK and transcription factors including WRKY and MYC. Studies are underway to exploit the potential of antagonistic bacterial endophytes against Foc isolates and to develop an understanding of the MAMP-triggered immunity and metabolomics cross talk modulating resistance. This review explores the possibility of harnessing the potential bacterial endomicrobiome against Foc and developing nanoformulations with bacterial endophytes for increased efficacy against lethal pathogenic races of Foc infecting banana.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02833-5.

Bacterial Endophytes: The Hidden Actor in Plant Immune Responses against Biotic Stress

Bacterial endophytes constitute an essential part of the plant microbiome and are described to promote plant health by different mechanisms. The close interaction with the host leads to important changes in the physiology of the plant. Although beneficial bacteria use the same entrance strategies as bacterial pathogens to colonize and enter the inner plant tissues, the host develops strategies to select and allow the entrance to specific genera of bacteria. In addition, endophytes may modify their own genome to adapt or avoid the defense machinery of the host. The present review gives an overview about bacterial endophytes inhabiting the phytosphere, their diversity, and the interaction with the host. Direct and indirect defenses promoted by the plant-endophyte symbiont exert an important role in controlling plant defenses against different stresses, and here, more specifically, is discussed the role against biotic stress. Defenses that should be considered are the emission of volatiles or antibiotic compounds, but also the induction of basal defenses and boosting plant immunity by priming defenses. The primed defenses may encompass pathogenesis-related protein genes (PR family), antioxidant enzymes, or changes in the secondary metabolism.

Bacterial endophytes: Molecular interactions with their hosts

Plant growth promotion has been found associated with plants on the surface (epiphytic), inside (endophytic), or close to the plant roots (rhizospheric). Endophytic bacteria mainly have been researched for their beneficial activities in terms of nutrient availability, plant growth hormones, and control of soil-borne and systemic pathogens. Molecular communications leading to these interactions between plants and endophytic bacteria are now being unrevealed using multidisciplinary approaches with advanced techniques such as metagenomics, metaproteomics, metatranscriptomics, metaproteogenomic, microRNAs, microarray, chips as well as the comparison of complete genome sequences. More than 400 genes in both the genomes of host plant and bacterial endophyte are up- or downregulated for the establishment of endophytism and plant growth-promoting activity. The involvement of more than 20 genes for endophytism, about 50 genes for direct plant growth promotion, about 25 genes for biocontrol activity, and about 10 genes for mitigation of different stresses has been identified in various bacterial endophytes. This review summarizes the progress that has been made in recent years by these modern techniques and approaches.

Co-Inoculation of Mesorhizobium ciceri with Either Bacillus sp. or Enterobacter aerogenes on Chickpea Improves Growth and Productivity in Phosphate-Deficient Soils in Dry Areas of a Mediterranean Region

Biological nitrogen fixation requires a large amount of phosphorus (P). However, most of the soils are P-deficient and the extensive use of P- chemical fertilizers constitute a serious threat to the environment. In this context, two field experiments were carried out to investigate the effect of co-inoculation of Mesorhizobium ciceri with phosphate solubilizing bacteria (PSB), Bacillus sp., and Enterobacter aerogenes, on chickpea as an alternative to chemical nitrogen (N) and phosphorous fertilizers in P-deficient soils in dry areas of Morocco. The results revealed that combined inoculation of chickpea with rhizobia and PSB showed a significant enhancement of chickpea nodulation, biomass production, yields and N, P, and protein content in grains as compared to single inoculation or single application of N or P. A significantly higher increase was obtained by inoculating chickpea with Mesorhizobium sp. MA72 combined with E. aerogenes P1S6. This combination allowed an enhancement of more than 270% in nodulation, 192% in shoot dry weight and 242% in grain yield. The effect of this combination was equivalent to the effect of combined application of N and P fertilizers. Formulation of biofertilizers based on tasted strains could be used for chickpea co-inoculation in P-deficient soils for an eco-friendly sustainable production of chickpea.

Gene expression patterns in shoots of Camelina sativa with enhanced salinity tolerance provided by plant growth promoting bacteria producing 1-aminocyclopropane-1-carboxylate deaminase or expression of the corresponding acdS gene

Growth of plants in soil inoculated with plant growth promoting bacteria (PGPB) producing 1-aminocyclopropane-1-carboxylate (ACC) deaminase or expression of the corresponding acdS gene in transgenic lines reduces the decline in shoot length, shoot weight and photosynthetic capacity triggered by salt stress in Camelina sativa. Reducing the levels of ethylene attenuated the salt stress response as inferred from decreases in the expression of genes involved in development, senescence, chlorosis and leaf abscission that are highly induced by salt to levels that may otherwise have a negative effect on plant growth and productivity. Growing plants in soil treated with Pseudomonas migulae 8R6 negatively affected ethylene signaling, auxin and JA biosynthesis and signalling, but had a positive effect on the regulation of genes involved in GA signaling. In plants expressing acdS, the expression of the genes involved in auxin signalling was positively affected, while the expression of genes involved in cytokinin degradation and ethylene biosynthesis were negatively affected. Moreover, fine-tuning of ABA signaling appears to result from the application of ACC deaminase in response to salt treatment. Moderate expression of acdS under the control of the root specific rolD promoter or growing plants in soil treated with P. migulae 8R6 were more effective in reducing the expression of the genes involved in ethylene production and/or signaling than expression of acdS under the more active Cauliflower Mosaic Virus 35S promoter.

Algae as New Kids in the Beneficial Plant Microbiome

Previously, algae were recognized as small prokaryotic and eukaryotic organisms found only in aquatic habitats. However, according to a recent paradigm shift, algae are considered ubiquitous organisms, occurring in plant tissues as well as in soil. Accumulating evidence suggests that algae represent a member of the plant microbiome. New results indicate that plants respond to algae and activate related downstream signaling pathways. Application of algae has beneficial effects on plant health, such as plant growth promotion and disease control. Although accumulating evidence suggests that secreted compounds and cell wall components of algae induce physiological and structural changes in plants that protect against biotic and abiotic stresses, knowledge of the underlying mechanisms and algal determinants is limited. In this review, we discuss recent studies on this topic, and highlight the bioprotectant and biostimulant roles of algae as a new member of the plant beneficial microbiome for crop improvement.

Screening of Bacterial Endophytes Able to Promote Plant Growth and Increase Salinity Tolerance

Bacterial endophytes can colonize plant tissues without harming the plant. Instead, they are often able to increase plant growth and tolerance to environmental stresses. In this work, new strains of bacterial endophytes were isolated from three economically important crop plants (sorghum, cucumber and tomato) grown in three different regions in soils with different management. All bacterial strains were identified by 16S rRNA sequencing and characterized for plant beneficial traits. Based on physiological activities, we selected eight strains that were further tested for their antibiotic resistance profile and for the ability to efficiently colonize the interior of sorghum plants. According to the results of the re-inoculation test, five strains were used to inoculate sorghum seeds. Then, plant growth promotion activity was assessed on sorghum plants exposed to salinity stress. Only two bacterial endophytes increased plant biomass, but three of them delayed or reduced plant salinity stress symptoms. These five strains were then characterized for the ability to produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which is involved in the increase of stress tolerance. Pseudomonas brassicacearum SVB6R1 was the only strain that was able to produce this enzyme, suggesting that ACC deaminase is not the only physiological trait involved in conferring plant tolerance to salt stress in these bacterial strains.

Plant anti-aging: Delayed flower and leaf senescence in Erinus alpinus treated with cell-free Chlorella cultivation medium

Plant tissues naturally senesce over time. Attempts to improve plant robustness and increase longevity have involved genetic modification, application of synthetic chemicals, and use of beneficial microbes. Recently, culture supernatant from a microalga Chlorella fusca was found to prime innate immunity against Pseudomonas syringae in Arabidopsis thaliana. However, the capacity of Chlorella culture supernatants to prevent or delay aging in higher plants has not been elucidated. In this study, roots of the ornamental flowering plant Erinus alpinus L. were drenched with cell-free supernatants from three Chlorella species. Flower and leaf senescence in E. alpinus was significantly reduced and delayed with all three Chlorella supernatants. Investigations of the mode of action underlying delayed senescence showed that the Chlorella supernatants did not act as a chemical trigger to elicit plant immunity or as a growth-promoting fertilizer in E. alpinus. The mechanisms underlying the anti-aging effects remain undetermined, and several possible hypotheses are discussed. Several Chlorella species are industrially cultivated, and disposal of cell-free supernatant can be economically and environmentally challenging. This study provides a novel method for extending plant lifespan through use of Chlorella supernatant and discusses the potential of using industrial waste supernatants in agriculture and horticulture to reduce reliance on chemical pesticides and genetic modification.

Endophytes Increased Fruit Quality with Higher Soluble Sugar Production in Honeycrisp Apple (Malus pumila)

Endophytes are fungi, bacteria, or yeast symbionts that live in the intercellular spaces or vascular tissues of host plants. Investigations indicate that endophytes isolated from the Salicaceae family (Populus and Salix) hosts provide several benefits that promote plant growth, including but not limited to di-nitrogen fixation, plant hormone production, nutrient acquisition, stress tolerance, and defense against phytopathogens. In exchange, the microorganisms receive domicile and photosynthates. Considering the known characteristics of nitrogen fixation and plant hormone production, we hypothesized that apple trees grown under nitrogen-limited conditions would show improved biometrics with endophyte inoculation. Our research objectives were to investigate the endophyte effects on plant physiology and fruiting. We examined these effects through ecophysiology metrics involving rates of photosynthesis, stomatal conductance and density, transpiration, biomass accretion, chlorophyll content and fluorescence, and fruit soluble sugar content and biomass. Our results showed evidence of the endophytes’ colonization in apple trees, decreased stomatal density, delayed leaf senescence, and increased lateral root biomass with endophytes. A highlight of the findings was a significant increase in both fruit soluble sugar content and biomass. Future research into the mechanistic underpinnings of this phenomenon stands to offer novel insights on how microbiota may alter carbohydrate metabolism under nitrogen-deficient conditions.

ACC deaminase in plant growth-promoting bacteria (PGPB): An efficient mechanism to counter salt stress in crops

Salinity in agricultural soil is a major problem around the world, with negative consequences for the growth and production of a wide range of crops. To counteract these harmful effects, plants sometimes have bacterial partners that contain the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which acts by degrading ACC (the precursor of ethylene in all higher plants). The enzymatic activity of ACC deaminase results in the production of α-ketobutyrate and ammonia, which, by lowering ACC levels, prevents excessive increases in the synthesis of ethylene under various stress conditions and is one of the most efficient mechanisms to induce plant tolerance to salt stress. In the present review, recent works on the role of ACC deaminase are discussed alongside its importance in promoting plant growth under conditions of salt stress in endophytic and rhizospheric bacteria, with some emphasis on Bacillus species. In addition, the toxic effects of soil salinity on plants and microbial biodiversity are analysed. Recent findings on the synergetic functioning of ACC deaminase and other bacterial mechanisms of salt stress tolerance, such as trehalose accumulation, are also summarized. Finally, we discuss the various advantages of ACC deaminase-producing bacilli as bioinoculants to address the problem of salinity in agricultural soils.

Plant growth promotion and chilli anthracnose disease suppression ability of rhizosphere soil actinobacteria

AIM

The aim of this study was to screen potential plant growth promoting rhizobacterial (PGPR) actinobacterial isolate with effective inhibition against anthracnose causing fungal pathogen Colletotrichum capsici.

METHODS AND RESULTS

In this study, actinobacterias were isolated from rhizosphere soil using dilution plate method and tested for antagonistic potential against pathogenic fungi C. capsici. In primary and secondary screening tests, the actinobacterial isolate BS-26 displayed high antagonistic activity against the fungal pathogen. Isolate BS-26 was identified as Streptomyces violaceoruber based on 16S rDNA sequencing. Furthermore, indole acetic acid production, phosphate solubilization and ammonia production have been confirmed in the S. violaceoruber that suggest their potential to be used as PGPR bacteria. A green house experiment showed that application of S. violaceoruber fermentation broth reduced the incidence of the chilli anthracnose and promoted the growth of chilli seedlings with a significant increase in germination %, total plant height, fresh weight and chlorophyll content when compared to controls.

CONCLUSION

Streptomyces violaceoruber can be applied as a biofertilizer and biocontrol agent for growing chillies against the attack of fungal pathogen C. capsici.

SIGNIFICANCE OF IMPACT OF THE STUDY

The damage caused by anthracnose disease is an issue of concern, affecting negatively the economy involved in chilli cultivation. As chemical methods of control have serious disadvantages, biocontrol approach using beneficial (PGPR) micro-organisms shall be a better alternative to control crop diseases.

Evaluation of ACC-deaminase-producing rhizobacteria to alleviate water-stress impacts in wheat (Triticum aestivum L.) plants

Application of plant-growth-promoting rhizobacteria (PGPR) is an environmentally sustainable option to reduce the effects of abiotic and biotic stresses on plant growth and productivity. Bacteria isolated from rain-fed agriculture field soils in the Central Himalaya Kumaun region, India, were evaluated for the production of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase. Those producing ACC deaminase in high amounts were evaluated for their potential to improve wheat (Triticum aestivum L.) plant growth under irrigated and water-stress conditions in two glasshouse experiments. Some of the isolates also showed other plant-growth-promoting (PGP) traits, e.g., N₂ fixation, siderophore production, and phosphate solubilization; however, strains with higher ACC deaminase activity showed the greatest effects. These were Variovorax paradoxus RAA3; Pseudomonas spp. DPC12, DPB13, DPB15, DPB16; Achromobacter spp. PSA7, PSB8; and Ochrobactrum anthropi DPC9. In both simulated irrigated and water-stress conditions, a single inoculation of RAA3 and a consortium of DPC9 + DPB13 + DPB15 + DPB16 significantly improved wheat plant growth and foliar nutrient concentrations and caused significant positive changes in antioxidant properties compared with noninoculated plants especially under water stress. These findings imply that PGPB having ACC deaminase activity together with other PGP traits could potentially be effective inoculants to improve the growth of wheat plants in water-stressed rain-fed environments.

Gene Expression Patterns in Roots of Camelina sativa With Enhanced Salinity Tolerance Arising From Inoculation of Soil With Plant Growth Promoting Bacteria Producing 1-Aminocyclopropane-1-Carboxylate Deaminase or Expression the Corresponding acdS Gene

Camelina sativa treated with plant growth-promoting bacteria (PGPB) producing 1-aminocyclopropane-1-carboxylate deaminase (acdS) or transgenic lines expressing acdS exhibit increased salinity tolerance. AcdS reduces the level of stress ethylene to below the point where it is inhibitory to plant growth. The study determined that several mechanisms appear to be responsible for the increased salinity tolerance and that the effect of acdS on gene expression patterns in C. sativa roots during salt stress is a function of how it is delivered. Growth in soil treated with the PGPB (Pseudomonas migulae 8R6) mostly affected ethylene- and abscisic acid-dependent signaling in a positive way, while expression of acdS in transgenic lines under the control of the broadly active CaMV 35S promoter or the root-specific rolD promoter affected auxin, jasmonic acid and brassinosteroid signaling and/biosynthesis. The expression of genes involved in minor carbohydrate metabolism were also up-regulated, mainly in roots of lines expressing acdS. Expression of acdS also affected the expression of genes involved in modulating the level of reactive oxygen species (ROS) to prevent cellular damage, while permitting ROS-dependent signal transduction. Though the root is not a photosynthetic tissue, acdS had a positive effect on the expression of genes involved in photosynthesis.

Integration of multi-omics techniques and physiological phenotyping within a holistic phenomics approach to study senescence in model and crop plants

The study of senescence in plants is complicated by diverse levels of temporal and spatial dynamics as well as the impact of external biotic and abiotic factors and crop plant management. Whereas the molecular mechanisms involved in developmentally regulated leaf senescence are very well understood, in particular in the annual model plant species Arabidopsis, senescence of other organs such as the flower, fruit, and root is much less studied as well as senescence in perennials such as trees. This review addresses the need for the integration of multi-omics techniques and physiological phenotyping into holistic phenomics approaches to dissect the complex phenomenon of senescence. That became feasible through major advances in the establishment of various, complementary 'omics' technologies. Such an interdisciplinary approach will also need to consider knowledge from the animal field, in particular in relation to novel regulators such as small, non-coding RNAs, epigenetic control and telomere length. Such a characterization of phenotypes via the acquisition of high-dimensional datasets within a systems biology approach will allow us to systematically characterize the various programmes governing senescence beyond leaf senescence in Arabidopsis and to elucidate the underlying molecular processes. Such a multi-omics approach is expected to also spur the application of results from model plants to agriculture and their verification for sustainable and environmentally friendly improvement of crop plant stress resilience and productivity and contribute to improvements based on postharvest physiology for the food industry and the benefit of its customers.

Microbiome engineering to improve biocontrol and plant growth-promoting mechanisms

A plant microbiome includes a microbial community that typically interacts extensively with a plant. The plant microbiome can survive either inside or outside of plant tissues, performing various plant beneficial activities including biocontrol of potential phytopathogens and promotion of plant growth. An important part of the plant microbiome includes plant growth-promoting bacteria (PGPB) that commonly reside in the rhizosphere and phyllosphere, and as endophytic bacteria (inside of plant tissues). As new plant microbiome-manipulating strategies have emerged in recent years, we have critically reviewed relevant literature, chiefly from the last decade. We have analysed and compared the rhizosphere, phyllosphere and endosphere as potential ecosystems for manipulation, in order to improve positive interactions with the plant. In addition, many studies on the bioengineering of the endophyte microbiome and its potential impact on the core microbiome were analysed with respect to five different strategies, including host mediated and multi-generation microbiome selection, inoculation into soil and rhizosphere, inoculations into seeds or seedlings, tissue atomisation and direct injection into tissues or wounds. Finally, microbiome engineering presents a feasible strategy to solve multiple agriculture-associated problems in an eco-friendly way.

Bacterial Endophytes in Plant Tissue Culture: Mode of Action, Detection, and Control

Endophytic bacteria have been increasingly in the focus of research projects during the last decade. This has changed the view on bacteria in plant tissue culture and led to the differentiation between artificially introduced contaminations and naturally occurring endophytes with neutral, negative, or positive impact on the plant propagation process. This review chapter gives an overview on recent findings about the impact that bacteria have on the plant physiology in general and during micropropagation. Additionally, methods for the detection and identification of bacteria in plant tissue are described and, finally, suggestions of how to deal with bacterial endophytes in in vitro culture are given.

Role and Regulation of ACC Deaminase Gene in Sinorhizobium meliloti: Is It a Symbiotic, Rhizospheric or Endophytic Gene?

Plant-associated bacteria exhibit a number of different strategies and specific genes allow bacteria to communicate and metabolically interact with plant tissues. Among the genes found in the genomes of plant-associated bacteria, the gene encoding the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (acdS) is one of the most diffused. This gene is supposed to be involved in the cleaving of plant-produced ACC, the precursor of the plant stress-hormone ethylene toning down the plant response to infection. However, few reports are present on the actual role in rhizobia, one of the most investigated groups of plant-associated bacteria. In particular, still unclear is the origin and the role of acdS in symbiotic competitiveness and on the selective benefit it may confer to plant symbiotic rhizobia. Here we present a phylogenetic and functional analysis of acdS orthologs in the rhizobium model-species Sinorhizobium meliloti. Results showed that acdS orthologs present in S. meliloti pangenome have polyphyletic origin and likely spread through horizontal gene transfer, mediated by mobile genetic elements. When acdS ortholog from AK83 strain was cloned and assayed in S. meliloti 1021 (lacking acdS), no modulation of plant ethylene levels was detected, as well as no increase in fitness for nodule occupancy was found in the acdS-derivative strain compared to the parental one. Surprisingly, AcdS was shown to confer the ability to utilize formamide and some dipeptides as sole nitrogen source. Finally, acdS was shown to be negatively regulated by a putative leucine-responsive regulator (LrpL) located upstream to acdS sequence (acdR). acdS expression was induced by root exudates of both legumes and non-leguminous plants. We conclude that acdS in S. meliloti is not directly related to symbiotic interaction, but it could likely be involved in the rhizospheric colonization or in the endophytic behavior.

Inoculation of Soil with Plant Growth Promoting Bacteria Producing 1-Aminocyclopropane-1-Carboxylate Deaminase or Expression of the Corresponding acdS Gene in Transgenic Plants Increases Salinity Tolerance in Camelina sativa

Camelina sativa (camelina) is an oilseed crop touted for use on marginal lands; however, it is no more tolerant of soil salinity than traditional crops, such as canola. Plant growth-promoting bacteria (PGPB) that produce 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase) facilitate plant growth in the presence of abiotic stresses by reducing stress ethylene. Rhizospheric and endophytic PGPB and the corresponding acdS- mutants of the latter were examined for their ability to enhance tolerance to salt in camelina. Stimulation of growth and tolerance to salt was correlated with ACC deaminase production. Inoculation of soil with wild-type PGPB led to increased shoot length in the absence of salt, and increased seed production by approximately 30–50% under moderately saline conditions. The effect of ACC deaminase was further examined in transgenic camelina expressing a bacterial gene encoding ACC deaminase (acdS) under the regulation of the CaMV 35S promoter or the root-specific rolD promoter. Lines expressing acdS, in particular those using the rolD promoter, showed less decline in root length and weight, increased seed production, better seed quality and higher levels of seed oil production under salt stress. This study clearly demonstrates the potential benefit of using either PGPB that produce ACC deaminase or transgenic plants expressing the acdS gene under the control of a root-specific promoter to facilitate plant growth, seed production and seed quality on land that is not normally suitable for the majority of crops due to high salt content.

Isolation and characterization of culturable seed-associated bacterial endophytes from gnotobiotically grown Marama bean seedlings

Marama bean (Tylosema esculentum) is an indigenous non-nodulating legume to the arid agro-ecological parts of Southern Africa. It is a staple food for the Khoisan and Bantu people from these areas. It is intriguing how it is able to synthesize the high-protein content in the seeds since its natural habitat is nitrogen deficient. The aim of the study was to determine the presence of seed transmittable bacterial endophytes that may have growth promoting effects, which may be particularly important for the harsh conditions. Marama bean seeds were surface sterilized and gnotobiotically grown to 2 weeks old seedlings. From surface-sterilized shoots and roots, 123 distinct bacterial isolates were cultured using three media, and identified by BOX-PCR fingerprinting and sequence analyses of the 16S rRNA and nifH genes. Phylogenetic analyses of 73 putative endophytes assigned them to bacterial species from 14 genera including Proteobacteria (Rhizobium, Massilia, Kosakonia, Pseudorhodoferax, Caulobacter, Pantoea, Sphingomonas, Burkholderia, Methylobacterium), Firmicutes (Bacillus), Actinobacteria (Curtobacterium, Microbacterium) and Bacteroidetes (Mucilaginibacter, Chitinophaga). Screening for plant growth-promoting activities revealed that the isolates showed production of IAA, ACC deaminase, siderophores, endoglucanase, protease, AHLs and capacities to solubilize phosphate and fix nitrogen. This is the first report that marama bean seeds may harbor endophytes that can be cultivated from seedlings; in this community of bacteria, physiological characteristics that are potentially plant growth promoting are widespread.

Plant growth-promoting bacterial endophytes

Bacterial endophytes ubiquitously colonize the internal tissues of plants, being found in nearly every plant worldwide. Some endophytes are able to promote the growth of plants. For those strains the mechanisms of plant growth-promotion known to be employed by bacterial endophytes are similar to the mechanisms used by rhizospheric bacteria, e.g., the acquisition of resources needed for plant growth and modulation of plant growth and development. Similar to rhizospheric plant growth-promoting bacteria, endophytic plant growth-promoting bacteria can act to facilitate plant growth in agriculture, horticulture and silviculture as well as in strategies for environmental cleanup (i.e., phytoremediation). Genome comparisons between bacterial endophytes and the genomes of rhizospheric plant growth-promoting bacteria are starting to unveil potential genetic factors involved in an endophytic lifestyle, which should facilitate a better understanding of the functioning of bacterial endophytes.

Biochemistry and genetics of ACC deaminase: a weapon to "stress ethylene" produced in plants

1-aminocyclopropane-1-carboxylate deaminase (ACCD), a pyridoxal phosphate-dependent enzyme, is widespread in diverse bacterial and fungal species. Owing to ACCD activity, certain plant associated bacteria help plant to grow under biotic and abiotic stresses by decreasing the level of “stress ethylene” which is inhibitory to plant growth. ACCD breaks down ACC, an immediate precursor of ethylene, to ammonia and α-ketobutyrate, which can be further metabolized by bacteria for their growth. ACC deaminase is an inducible enzyme whose synthesis is induced in the presence of its substrate ACC. This enzyme encoded by gene AcdS is under tight regulation and regulated differentially under different environmental conditions. Regulatory elements of gene AcdS are comprised of the regulatory gene encoding LRP protein and other regulatory elements which are activated differentially under aerobic and anaerobic conditions. The role of some additional regulatory genes such as AcdB or LysR may also be required for expression of AcdS. Phylogenetic analysis of AcdS has revealed that distribution of this gene among different bacteria might have resulted from vertical gene transfer with occasional horizontal gene transfer (HGT). Application of bacterial AcdS gene has been extended by developing transgenic plants with ACCD gene which showed increased tolerance to biotic and abiotic stresses in plants. Moreover, distribution of ACCD gene or its homolog's in a wide range of species belonging to all three domains indicate an alternative role of ACCD in the physiology of an organism. Therefore, this review is an attempt to explore current knowledge of bacterial ACC deaminase mediated physiological effects in plants, mode of enzyme action, genetics, distribution among different species, ecological role of ACCD and, future research avenues to develop transgenic plants expressing foreign AcdS gene to cope with biotic and abiotic stressors. Systemic identification of regulatory circuits would be highly valuable to express the gene under diverse environmental conditions.

Bacterial Modulation of Plant Ethylene Levels

A focus on the mechanisms by which ACC deaminase-containing bacteria facilitate plant growth.Bacteria that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, when present either on the surface of plant roots (rhizospheric) or within plant tissues (endophytic), play an active role in modulating ethylene levels in plants. This enzyme activity facilitates plant growth especially in the presence of various environmental stresses. Thus, plant growth-promoting bacteria that express ACC deaminase activity protect plants from growth inhibition by flooding and anoxia, drought, high salt, the presence of fungal and bacterial pathogens, nematodes, and the presence of metals and organic contaminants. Bacteria that express ACC deaminase activity also decrease the rate of flower wilting, promote the rooting of cuttings, and facilitate the nodulation of legumes. Here, the mechanisms behind bacterial ACC deaminase facilitation of plant growth and development are discussed, and numerous examples of the use of bacteria with this activity are summarized.

The Hidden World within Plants: Ecological and Evolutionary Considerations for Defining Functioning of Microbial Endophytes

All plants are inhabited internally by diverse microbial communities comprising bacterial, archaeal, fungal, and protistic taxa. These microorganisms showing endophytic lifestyles play crucial roles in plant development, growth, fitness, and diversification. The increasing awareness of and information on endophytes provide insight into the complexity of the plant microbiome. The nature of plant-endophyte interactions ranges from mutualism to pathogenicity. This depends on a set of abiotic and biotic factors, including the genotypes of plants and microbes, environmental conditions, and the dynamic network of interactions within the plant biome. In this review, we address the concept of endophytism, considering the latest insights into evolution, plant ecosystem functioning, and multipartite interactions.

Studies on Plant Growth Promoting Properties of Fruit-Associated Bacteria from Elettaria cardamomum and Molecular Analysis of ACC Deaminase Gene

Endophytic microorganisms have been reported to have diverse plant growth promoting mechanisms including phosphate solubilization, N2 fixation, production of phyto-hormones and ACC (1-aminocyclopropane-1-carboxylate) deaminase and antiphyto-pathogenic properties. Among these, ACC deaminase production is very important because of its regulatory effect on ethylene which is a stress hormone with precise role in the control of fruit development and ripening. However, distribution of these properties among various endophytic bacteria associated with fruit tissue and its genetic basis is least investigated. In the current study, 11 endophytic bacteria were isolated and identified from the fruit tissue of Elettaria cardamomum and were studied in detail for various plant growth promoting properties especially ACC deaminase activity using both culture-based and PCR-based methods. PCR-based screening identified the isolates EcB 2 (Pantoea sp.), EcB 7 (Polaromonas sp.), EcB 9 (Pseudomonas sp.), EcB 10 (Pseudomonas sp.) and EcB 11 (Ralstonia sp.) as positive for ACC deaminase. The PCR products were further subjected to sequence analysis which proved the similarity of the sequences identified in the study with ACC deaminase sequences reported from other sources. The detailed bioinformatic analysis of the sequence including homology-based modelling and molecular docking confirmed the sequences to have ACC deaminase activity. The docking of the modelled proteins was done using patch dock, and the detailed scrutiny of the protein ligand interaction revealed conservation of key amino acids like Lys51, Ser78, Tyr268 and Tyr294 which play important role in the enzyme activity. These suggest the possible regulatory effect of these isolates on fruit physiology.

The Hidden World within Plants: Ecological and Evolutionary Considerations for Defining Functioning of Microbial Endophytes

All plants are inhabited internally by diverse microbial communities comprising bacterial, archaeal, fungal, and protistic taxa. These microorganisms showing endophytic lifestyles play crucial roles in plant development, growth, fitness, and diversification. The increasing awareness of and information on endophytes provide insight into the complexity of the plant microbiome. The nature of plant-endophyte interactions ranges from mutualism to pathogenicity. This depends on a set of abiotic and biotic factors, including the genotypes of plants and microbes, environmental conditions, and the dynamic network of interactions within the plant biome. In this review, we address the concept of endophytism, considering the latest insights into evolution, plant ecosystem functioning, and multipartite interactions.

Plant-Agrobacterium interaction mediated by ethylene and super-Agrobacterium conferring efficient gene transfer

Agrobacterium tumefaciens has a unique ability to transfer genes into plant genomes. This ability has been utilized for plant genetic engineering. However, the efficiency is not sufficient for all plant species. Several studies have shown that ethylene decreased the Agrobacterium-mediated transformation frequency. Thus, A. tumefaciens with an ability to suppress ethylene evolution would increase the efficiency of Agrobacterium-mediated transformation. Some studies showed that plant growth-promoting rhizobacteria (PGPR) can reduce ethylene levels in plants through 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which cleaves the ethylene precursor ACC into α-ketobutyrate and ammonia, resulting in reduced ethylene production. The whole genome sequence data showed that A. tumefaciens does not possess an ACC deaminase gene in its genome. Therefore, providing ACC deaminase activity to the bacteria would improve gene transfer. As expected, A. tumefaciens with ACC deaminase activity, designated as super-Agrobacterium, could suppress ethylene evolution and increase the gene transfer efficiency in several plant species. In this review, we summarize plant-Agrobacterium interactions and their applications for improving Agrobacterium-mediated genetic engineering techniques via super-Agrobacterium.

Growth-promoting Sphingomonas paucimobilis ZJSH1 associated with Dendrobium officinale through phytohormone production and nitrogen fixation

Growth-promoting Sphingomonas paucimobilis ZJSH1, associated with Dendrobium officinale, a traditional Chinese medicinal plant, was characterized. At 90 days post-inoculation, strain ZJSH1 significantly promoted the growth of D. officinale seedlings, with increases of stems by 8.6% and fresh weight by 7.5%. Interestingly, the polysaccharide content extracted from the inoculated seedlings was 0.6% higher than that of the control. Similar growth promotion was observed with the transplants inoculated with strain ZJSH1. The mechanism of growth promotion was attributed to a combination of phytohormones and nitrogen fixation. Strain ZJSH1 was found using the Kjeldahl method to have a nitrogen fixation activity of 1.15 mg l⁻¹, which was confirmed by sequencing of the nifH gene. Using high-performance liquid chromatography-mass spectrometry, strain ZJSH1 was found to produce various phytohormones, including salicylic acid (SA), indole-3-acetic acid (IAA), Zeatin and abscisic acid (ABA). The growth curve showed that strain ZJSH1 grew well in the seedlings, especially in the roots. Accordingly, much higher contents of SA, ABA, IAA and c-ZR were detected in the inoculated seedlings, which may play roles as both phytohormones and ‘Systemic Acquired Resistance’ drivers. Nitrogen fixation and secretion of plant growth regulators (SA, IAA, Zeatin and ABA) endow S. paucimobilis ZJSH1 with growth-promoting properties, which provides a potential for application in the commercial growth of D. officinale.

Amelioration of high salinity stress damage by plant growth-promoting bacterial endophytes that contain ACC deaminase

Plant growth and productivity is negatively affected by soil salinity. However, it is predicted that plant growth-promoting bacterial (PGPB) endophytes that contain 1-aminocyclopropane-1-carboxylate (ACC) deaminase (E.C. 4.1.99.4) can facilitate plant growth and development in the presence of a number of different stresses. In present study, the ability of ACC deaminase containing PGPB endophytes Pseudomonas fluorescens YsS6, Pseudomonas migulae 8R6, and their ACC deaminase deficient mutants to promote tomato plant growth in the absence of salt and under two different levels of salt stress (165 mM and 185 mM) was assessed. It was evidence that wild-type bacterial endophytes (P. fluorescens YsS6 and P. migulae 8R6) promoted tomato plant growth significantly even in the absence of stress (salinity). Plants pretreated with wild-type ACC deaminase containing endophytic strains were healthier and grew to a much larger size under high salinity stress compared to plants pretreated with the ACC deaminase deficient mutants or no bacterial treatment (control). The plants pretreated with ACC deaminase containing bacterial endophytes exhibit higher fresh and dry biomass, higher chlorophyll contents, and a greater number of flowers and buds than the other treatments. Since the only difference between wild-type and mutant bacterial endophytes was ACC deaminase activity, it is concluded that this enzyme is directly responsible for the different behavior of tomato plants in response to salt stress. The use of PGPB endophytes with ACC deaminase activity has the potential to facilitate plant growth on land that is not normally suitable for the majority of crops due to their high salt contents.