Characterisation of plant growth-promoting rhizobacteria from rhizosphere soil of heat-stressed and unstressed wheat and their use as bio-inoculant

Abstract

High temperature induces several proteins in plants that enhance tolerance to high temperature shock. The fate of proteins synthesised in microbial cells or secreted into culture media by interacting microbes has not been fully elucidated. The present investigation aimed to characterise plant growth-promoting rhizobacteria (PGPR) isolated from the rhizosphere of wheat genotypes (differing in tolerance to high temperature stress) and evaluate their performance as bioinoculant for use in wheat. Four bacterial strains, viz. Pseudomonas brassicacearum, Bacillus thuringiensis, Bacillus cereus strain W6 and Bacillus subtilis, were isolated from the rhizosphere of heat-stressed and unstressed wheat genotypes. The wheat genotypes were exposed to high temperature stress at 45 °C for 10 days (3 h daily) at pre-anthesis phase. Isolates were identified on the basis of morphology and biochemical characteristics, 16S rRNA gene sequencing and whole cell protein profiles. Results were further complemented by size exclusion chromatography (SEC) with fast protein liquid chromatography (FPLC) and SDS PAGE of 80% ammonium sulphate precipitates of the cell-free supernatants. Isolates were positive for catalase, oxidases and antimicrobial activity. P. brassicacearum from the rhizosphere of the heat-tolerant genotype was more efficient in phosphate solubilisation, bacteriocin production, antifungal and antibacterial activity against Helminthosporium sativum, Fusarium moniliforme and Klebsiella pneumonia, respectively. The inoculated seedlings had significantly higher root and shoot fresh weight, enhanced activity of antioxidant enzymes, proline and protein content. Total profiling of the culture with SDS-PAGE indicated expression of new protein bands in 95 kDa in P. brassicacearum. Temperature-induced changes in PGPR isolates are similar to those in the host plant. P. brassicacearum may be a good candidate for use in biofertiliser production for plants exposed to high temperature stress.

Keywords: PGPR; SDS-PAGE; antioxidant enzymes; proline.