UC Riverside

Background

Plant growth promoting rhizobacteria (PGPR), such as Bradyrhizobium japonicum IRAT FA3, are able to improve seed germination and plant growth under various biotic and abiotic stress conditions, including high salinity stress. PGPR can affect plants' responses to stress via multiple pathways which are often interconnected but were previously thought to be distinct. Although the overall impacts of PGPR on plant growth and stress tolerance have been well documented, the underlying mechanisms are not fully elucidated. This work contributes to understanding how PGPR promote abiotic stress by revealing major plant pathways triggered by B. japonicum under salt stress.

Results

The plant growth-promoting rhizobacterial (PGPR) strain Bradyrhizobium japonicum IRAT FA3 reduced the levels of sodium in Arabidopsis thaliana by 37.7%. B. japonicum primed plants as it stimulated an increase in jasmonates (JA) and modulated hydrogen peroxide production shortly after inoculation. B. japonicum-primed plants displayed enhanced shoot biomass, reduced lipid peroxidation and limited sodium accumulation under salt stress conditions. Q(RT)-PCR analysis of JA and abiotic stress-related gene expression in Arabidopsis plants pretreated with B. japonicum and followed by six hours of salt stress revealed differential gene expression compared to non-inoculated plants. Response to Desiccation (RD) gene RD20 and reactive oxygen species scavenging genes CAT3 and MDAR2 were up-regulated in shoots while CAT3 and RD22 were increased in roots by B. japonicum, suggesting roles for these genes in B. japonicum-mediated salt tolerance. B. japonicum also influenced reductions of RD22, MSD1, DHAR and MYC2 in shoots and DHAR, ADC2, RD20, RD29B, GTR1, ANAC055, VSP1 and VSP2 gene expression in roots under salt stress.

Conclusion

Our data showed that MYC2 and JAR1 are required for B. japonicum-induced shoot growth in both salt stressed and non-stressed plants. The observed microbially influenced reactions to salinity stress in inoculated plants underscore the complexity of the B. japonicum jasmonic acid-mediated plant response salt tolerance.