Plant Growth-Promoting Rhizobacteria With ACC Deaminase Activity Enhance Maternal Lateral Root and Seedling Growth in Switchgrass

PGPR: Key to Enhancing Crop Productivity and Achieving Sustainable Agriculture

Due to the burgeoning global population and the advancement of economies, coupled with human activities leading to the degradation of soil ecosystems and the depletion of non-renewable resources, concerns have arisen regarding food security and human survival. In order to address these adverse impacts, the spotlight has been cast upon plant growth-promoting rhizobacteria (PGPR), driven by a strong environmental consciousness. PGPR possesses the capability to foster plant growth and amplify crop yield through both direct and indirect mechanisms. By expediting plant growth, augmenting nutrient assimilation, heightening crop yield and caliber, and fortifying stress resilience, the application of PGPR can mitigate reliance on chemical fertilizers and pesticides while diminishing ecological perils. This exposition delves into the function of PGPR in modulating plant hormones, fostering nutrient solubilization, and fortifying plant resistance against biotic and abiotic stressors. This review offers valuable insights into the intricate interplay between PGPR and plants, elucidating uncertainties ripe for further investigation. Profound comprehension and judicious utilization of PGPR are indispensable for attaining sustainable agricultural progression, making substantial contributions to resolving the conundrums of global food security and environmental conservation.

Plant growth promoting activities of endophytic bacteria from Melia azedarach (Meliaceae) and their influence on plant growth under gnotobiotic conditions

Bacteria that live asymptomatically within plant tissues are known as endophytes. Because of the close relation with the plant host, they have been a matter of interest for application as plant growth promoters. Melia azedarach is a widely distributed medicinal tree with proven insecticidal, antimicrobial, and antiviral activity. The aim of this study was to isolate and characterize endophytic bacteria from M. azedarach and analyze their plant growth promoting activities for the potential application as biological products. Bacteria were isolated from roots and leaves of trees growing in two locations of Northeastern Argentina. The isolates were characterized by repetitive extragenic palindromic sequence PCR and 16S rDNA sequence analysis. The plant growth-promoting activities were assayed in vitro, improvement of plant growth of selected isolates was tested on M. azedarach plantlets, and the effect of selected ACC deaminase producing isolates was tested on tomato seedlings under salt-stress conditions. The highest endophytic bacterial abundance and diversity were obtained from the roots. All isolates had at least one of the assayed plant growth-promoting activities and 80 % of them had antagonistic activity. The most efficient bacteria were Pseudomonas monteilii, Pseudomonas farsensis, Burkholderia sp. and Cupriavidus sp. for phosphate solubilization (2064 μg P ml-1), IAA production (94.7 μg ml-1), siderophore production index (5.5) and ACC deaminase activity (1294 nmol α-ketobutyrate mg-1 h-1). M. azedarach inoculation assays revealed the bacterial growth promotion potential, with Pseudomonas monteilii, Pseudomonas farsensis and Cupriavidus sp. standing out for their effect on leaf area, leaf dry weight, specific leaf area, and total Chl, Mg and N content, with increases of up to 149 %, 58 %, 65 %, 178 %, 76 % and 97.7 %, respectively, compared to NI plants. Efficient ACC deaminase-producing isolates increased stress tolerance of tomato plants under saline condition. Overall, these findings indicate the potential of the endophytic isolates as biostimulant and biocontrol agents.

Above-and below-ground feedback loop of maize is jointly enhanced by plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi in drier soil

Drought is a potent abiotic stressor that arrests crop growth, significantly affecting crop health and yields. The arbuscular mycorrhizal fungi (AMF), and plant growth-promoting rhizobacteria (PGPR) can offer to protect plants from stressful environments through improving water, and nutrient use efficiency by strengthening plant root structure and harnessing favorable rhizosphere environments. When Acaulospora laevis (AMF) and Bacillus subtilus (PGPR) are introduced in combination, enhanced root growth and beneficial microbial colonization can mitigate drought stress. To assess this potential, a pot experiment was done with maize (Zea mays L.) to explore the effects of A. laevis and B. subtilus under different water levels (well-watered = 80 %; moderate water stress = 55 %; and severe water stress = 35 %) on maize yield, soil microbial activities, nutrients contents, root, and leaf functioning. Plants exposed to severe drought stress hampered their root and leaf functioning, and reduced grain yield compared with control plants. Combined use of AMF and PGPR increased root colonization (104.6 %-113.2 %) and microbial biomass carbon (36.38 %-40.23 %) under moderate to severe drought conditions over control. Higher root colonization was strongly linked with elevated ACC (aminocyclopropane-1-carboxylic acid) production, subsequently enhancing water use efficiency (21.62 %-12.77 %), root hydraulic conductivity (1.9 %-1.4 %) and root nutrient uptake under moderate to severe drought conditions. Enhanced nutrient uptake further promoted leaf photosynthetic rate by 27.3 %-29.8 % under moderate and severe drought stress. Improving leaf and root physiological functioning enhanced maize grain yield under stressful environments. Furthermore, co-inoculation with AMF-PGPR reduced cellular damage by lowering oxidative enzyme levels and increasing antioxidative enzyme activities, improving plant performance and grain yield under stressful environments. Conclusively, the synergistic interaction of AMF with PGPR ensured plant stress tolerance by reducing cellular injury, facilitating root-leaf functioning, enhancing nutrient-water-use-efficiencies, and increasing yield under drought stress.

Survival strategies of Bacillus spp. in saline soils: Key factors to promote plant growth and health

Soil salinity is one of the most important abiotic factors that affects agricultural production worldwide. Because of saline stress, plants face physiological changes that have negative impacts on the various stages of their development, so the employment of plant growth-promoting bacteria (PGPB) is one effective means to reduce such toxic effects. Bacteria of the Bacillus genus are excellent PGPB and have been extensively studied, but what traits makes them so extraordinary to adapt and survive under harsh situations? In this work we review the Bacillus' innate abilities to survive in saline stressful soils, such as the production osmoprotectant compounds, antioxidant enzymes, exopolysaccharides, and the modification of their membrane lipids. Other survival abilities are also discussed, such as sporulation or a reduced growth state under the scope of a functional interaction in the rhizosphere. Thus, the most recent evidence shows that these saline adaptive activities are important in plant-associated bacteria to potentially protect, direct and indirect plant growth-stimulating activities. Additionally, recent advances on the mechanisms used by Bacillus spp. to improve the growth of plants under saline stress are addressed, including genomic and transcriptomic explorations. Finally, characterization and selection of Bacillus strains with efficient survival strategies are key factors in ameliorating saline problems in agricultural production.

Prevalence of Antimicrobial Resistance and Respective Genes among Bacillus spp., a Versatile Bio-Fungicide

The plant rhizosphere is not only a reservoir of microbes but also a hub of antimicrobial resistance genes. Rhizospheric Bacillus spp. are the potential bio-inoculants with a versatile application in agriculture as bio-fertilizer and bio-fungicide. In the current study, the potential bio-control agent that is the Bacillus species (n = 7) was screened for the antimicrobial resistance pattern to assess their risk before registering them as a bio-inoculant. All of the Bacillus spp. were categorized as multi-drug-resistant (MDR), bacteria but none of them was either pan-drug-resistant (PDR) or extensive-drug-resistant (XDR). The multiple antimicrobial resistance (MAR) index of Bacillus spp. was higher than the critical value (0.2). The Bacillus spp. showed resistance to antimicrobial classes such as β lactam, macrolides, sulfonamides, tetracycline, aminoglycosides, and lincosamide. Various antimicrobial resistance genes, namely VmiR, ImrB, tetL, mphK, ant-6, penp, and bla OXA, associated with different mechanisms of resistance, were also detected in Bacillus spp. The Bacillus spp. also showed stress-tolerance traits such as ACC deaminase and EPS activity except the strains MAZ-117 and FZV-34, respectively. A significant correlation was observed between the PGPR and antimicrobial resistance, which shows that they may have adapted drug-resistance mechanisms to tolerate the environmental stress. These findings suggest that bio-fungicidal Bacillus spp. could be used very carefully on a commercial scale.

Deciphering the potential of a plant growth promoting endophyte Rhizobium sp. WYJ-E13, and functional annotation of the genes involved in the metabolic pathway

Plant growth-promoting rhizobacteria (PGPR) are well-acknowledged root endophytic bacteria used for plant growth promotion. However, which metabolites produced by PGPR could promote plant growth remains unclear. Additionally, which genes are responsible for plant growth-promoting traits is also not elucidated. Thus, as comprehensive understanding of the mechanism of endophyte in growth promotion is limited, this study aimed to determine the metabolites and genes involved in plant growth-promotion. We isolated an endophytic Rhizobium sp. WYJ-E13 strain from the roots of Curcuma wenyujin Y.H. Chen et C. Ling, a perennial herb and medicinal plant. The tissue culture experiment showed its plant growth-promoting ability. The bacterium colonization in the root was confirmed by scanning electron microscopy and paraffin sectioning. Furthermore, it was noted that the WYJ-E13 strain produced cytokinin, anthranilic acid, and L-phenylalanine by metabolome analysis. Whole-genome analysis of the strain showed that it consists of a circular chromosome of 4,350,227 bp with an overall GC content of 60.34%, of a 2,149,667 bp plasmid1 with 59.86% GC, and of a 406,180 bp plasmid2 with 58.05% GC. Genome annotation identified 4,349 putative protein-coding genes, 51 tRNAs, and 9 rRNAs. The CDSs number allocated to the Kyoto Encyclopedia of Genes and Genomes, Gene Ontology, and Clusters of Orthologous Genes databases were 2027, 3,175 and 3,849, respectively. Comparative genome analysis displayed that Rhizobium sp. WYJ-E13 possesses the collinear region among three species: Rhizobium acidisoli FH23, Rhizobium gallicum R602 and Rhizobium phaseoli R650. We recognized a total set of genes that are possibly related to plant growth promotion, including genes involved in nitrogen metabolism (nifU, gltA, gltB, gltD, glnA, glnD), hormone production (trp ABCDEFS), sulfur metabolism (cysD, cysE, cysK, cysN), phosphate metabolism (pstA, pstC, phoB, phoH, phoU), and root colonization. Collectively, these findings revealed the roles of WYJ-E13 strain in plant growth-promotion. To the best of our knowledge, this was the first study using whole-genome sequencing for Rhizobium sp. WYJ-E13 associated with C. wenyujin. WYJ-E13 strain has a high potential to be used as Curcuma biofertilizer for sustainable agriculture.