Modulation of plant polyamine and ethylene biosynthesis; and brassinosteroid signaling during Bacillus endophyticus J13-mediated salinity tolerance in Arabidopsis thaliana.

Salinity stress adversely impacts plant growth and development. Plant growth-promoting rhizobacteria (PGPR) are known to confer salinity stress tolerance in plants through several mechanisms. Here, we report the role of an abiotic stress-tolerant PGPR strain, Bacillus endophyticus J13, in promoting salinity stress tolerance in Arabidopsis thaliana, by elucidating its impact on physiological responses, polyamine (PA) and ethylene biosynthesis, and brassinosteroid signaling. Physiological analysis revealed that J13 can significantly improve the overall plant growth under salt stress by increasing the biomass, relative water content, and chlorophyll content, decreasing membrane damage and lipid peroxidation, and modulating proline homeostasis in plants. Evaluation of shoot polyamine levels upon J13 inoculation revealed an overall decrease in the levels of the three major PAs, putrescine (Put), spermidine (Spd), and spermine (Spm), under non-stressed conditions. Salt stress significantly increased the levels of Put and Spm, while decreasing the Spd levels in the plants. J13 inoculation under salt-stressed conditions, significantly decreased the Put levels, bringing them closer to those of the untreated control plants, whereas Spd and Spm levels did not change relative to the non-inoculated salt-stressed plants. The modulation of PA levels was accompanied by changes in the expressions of key PA biosynthetic genes under all treatments. Among the ethylene biosynthetic genes that we studied, ACS1 was induced by J13 inoculation under salt stress. J13 inoculation under salt stress resulted in the modulation of the expressions of BR-signaling genes, upregulating the expressions of the positive regulators of BR-signaling (BZR1 and BES2) and downregulating that of the negative regulator (BIN2). Our results provide a new avenue for J13-mediated salt stress amelioration in Arabidopsis, via tight control of polyamine and ethylene biosynthesis and enhanced brassinosteroid signaling.