Flagellin of polar flagellum from Azospirillum brasilense Sp245: Isolation, structure, and biological activity

A Study of the Different Strains of the Genus Azospirillum spp. on Increasing Productivity and Stress Resilience in Plants

One of the most important and essential components of sustainable agricultural production is biostimulants, which are emerging as a notable alternative of chemical-based products to mitigate soil contamination and environmental hazards. The most important modes of action of bacterial plant biostimulants on different plants are increasing disease resistance; activation of genes; production of chelating agents and organic acids; boosting quality through metabolome modulation; affecting the biosynthesis of phytochemicals; coordinating the activity of antioxidants and antioxidant enzymes; synthesis and accumulation of anthocyanins, vitamin C, and polyphenols; enhancing abiotic stress through cytokinin and abscisic acid (ABA) production; upregulation of stress-related genes; and the production of exopolysaccharides, secondary metabolites, and ACC deaminase. Azospirillum is a free-living bacterial genus which can promote the yield and growth of many species, with multiple modes of action which can vary on the basis of different climate and soil conditions. Different species of Bacillus spp. can increase the growth, yield, and biomass of plants by increasing the availability of nutrients; enhancing the solubilization and subsequent uptake of nutrients; synthesizing indole-3-acetic acid; fixing nitrogen; solubilizing phosphorus; promoting the production of phytohormones; enhancing the growth, production, and quality of fruits and crops via enhancing the production of carotenoids, flavonoids, phenols, and antioxidants; and increasing the synthesis of indoleacetic acid (IAA), gibberellins, siderophores, carotenoids, nitric oxide, and different cell surface components. The aim of this manuscript is to survey the effects of Azospirillum spp. and Bacillus spp. by presenting case studies and successful paradigms in several horticultural and agricultural plants.

Effect of a Bacillus velezensis and Lysinibacillus fusiformis-based biofertilizer on phosphorus acquisition and grain yield of soybean

Introduction

Phosphate-solubilizing bacteria that function through acidification (organic acid synthesis) or mineralization (production of enzymes such as phytase and phosphatases) have been explored as a biotechnological alternative to enhance plant access to phosphorus (P) retained in organic and inorganic forms in agricultural soils. This study tested the hypothesis that applying a biofertilizer composed of a recognized phosphate-solubilizing bacterium (Bacillus velezensis - endophytic strain BVPS01) and an underexplored plant growth-promoting bacterium (Lysinibacillus fusiformis - endophytic strain BVPS02) would improve the growth and grain yield of Glycine max L. plants.

Methods

Initial in vitro tests assessed the functional traits of these bacteria, and a mix of strains BVPS01 and BVPS02 was produced and tested under field conditions to evaluate its agronomic efficiency.

Results

The results confirmed the hypothesis that the tested biofertilizer enhances the agronomic performance of G. max plants in the field. The B. velezensis strain (BVPS01) was found to be more effective than the L. fusiformis strain (BVPS02) in solubilizing phosphates via the phosphatase enzyme production pathway, indicated by the expression of the phoC and phoD genes. In contrast, L. fusiformis was more effective in solubilizing phosphates through organic acid and phytase-related pathways, in addition to synthesizing indole-3-acetic acid and increasing the mitotic index in the root meristem of G. max plants. These strains exhibited biological compatibility, and the formulated product based on these rhizobacteria enhanced root development and increased the number of nodules and flowers, positively affecting 1000-grain weight, grain yield, and grain P content.

Discussion

Thus, the tested biofertilizer demonstrated potential to improve root growth and increase both the yield and quality of soybean crops, making it a sustainable and low-cost strategy.

Chromosomal gene of hybrid multisensor histidine kinase is involved in motility regulation in the rhizobacterium Azospirillum baldaniorum Sp245 under mechanical and water stress

Azospirillum alphaproteobacteria, which live in the rhizosphere of many crops, are used widely as biofertilizers. Two-component signal transduction systems (TCSs) mediate the bacterial perception of signals and the corresponding adjustment of behavior facilitating the adaptation of bacteria to their habitats. In this study, we obtained the A. baldaniorum Sp245 mutant for the AZOBR_150176 gene, which encodes the TCS of the hybrid histidine kinase/response sensory regulator (HSHK-RR). Inactivation of this gene affected bacterial morphology and motility. In mutant Sp245-HSHKΔRR-Km, the cells were still able to synthesize a functioning polar flagellum (Fla), were shorter than those of strain Sp245, and were impaired in aerotaxis, elaboration of inducible lateral flagella (Laf), and motility in semiliquid media. The mutant showed decreased transcription of the genes encoding the proteins of the secretion apparatus, which ensures the assembly of Laf, Laf flagellin, and the repressor protein of translation of the Laf flagellin's mRNA. The study examined the effects of polyethylene glycol 6000 (PEG 6000), an agent used to simulate osmotic stress and drought conditions. Under osmotic stress, the mutant was no longer able to use collective motility in semiliquid media but formed more biofilm biomass than did strain Sp245. Introduction into mutant cells of the AZOBR_150176 gene as part of an expression vector led to recovery of the lost traits, including those mediating bacterial motility under mechanical stress induced by increased medium density. The results suggest that the HSHK-RR under study modulates the response of A. baldaniorum Sp245 to mechanical and osmotic/water stress.

Lipopolysaccharide and flagellin of Azospirillum brasilense Sp7 influence callus morphogenesis and plant regeneration in wheat

In vitro somatic callus culturing is used widely in plant biotechnology, but its effectiveness depends largely on the donor plant genotype. Bacteria or components of their cells are rarely used to activate morphogenesis. In this work, inoculation of explants from immature wheat (Triticum aestivum L.) embryos with a suspension of living cells of the bacterium Azospirillum brasilense Sp7 resulted in callus death after 7 days of growth, in contrast to explant treatment with a suspension of heat-killed whole cells of Sp7. The experiments used two wheat lines, LRht-B1a and LRht-B1c, which differ in morphogenic activity. Growing calluses with the lipopolysaccharide of A. brasilense Sp7 increased the yield of regenerated plants 2- to 3.5-fold in both lines. This increase was through the activation of regenerant formation from morphogenic calluses. We have demonstrated for the first time the effects of bacterial flagellin on plant tissue culture. The polar-flagellum flagellin of A. brasilense Sp7 leveled the genotypic differences in the morphogenic ability of callus tissue. Specifically, it increased the yield of morphogenic calluses in the weakly morphogenic line LRht-B1a to the yield value in the highly morphogenic line LRht-B1c but lowered the yield of regenerants in the highly morphogenic line LRht-B1c to the yield value in the weakly morphogenic line LRht-B1a. Thus, bacterial lipopolysaccharides and flagellins can be used to regulate the formation of morphogenic calluses and regenerants in plant tissue culturing in vitro.

Construction of a Vibrio anguillarum flagellin B mutant and analysis of its immuno-stimulation effects on Macrobrachium rosenbergii

Vibrio anguillarum is a globally distributed aquatic pathogen, and its flagellin B (FlaB) protein can evoke innate immune responses in hosts. In order to explore the role of FlaB in V. anguillarum infection, we constructed a FlaB-deficient mutant using overlapping PCR and two-step homologous recombination, and gene sequencing confirmed successful knockout of the FlaB gene. Scanning electron microscopy showed no significant differences in the morphological structure of the flagellum between wild-type and FlaB-deficient strains. The mutant was subsequently injected into the freshwater prawn (Macrobrachium rosenbergii) to explore its pathogenicity in the host, and expression of myeloid differentiation factor 88, prophenoloxidase, catalase, superoxide dismutase and glutathione peroxidase was investigated by real-time PCR. The results showed that deletion of FlaB had little effect on V. anguillarum-induced expression of these immune-related genes (p > 0.05). In general, the FlaB mutant displayed similar flagella morphology and immune characteristics to the wild-type strain, hence we speculated that knockout of FlaB might promote the expression and function of other flagellin proteins. Furthermore, this study provides a rapid and simple method for obtaining stable mutants of V. anguillarum free from foreign plasmid DNA.