Rhizobacterial-plant interactions: Strategies ensuring plant growth promotion under drought and salinity stress

Drought and salinity are major environmental stresses resulting in secondary stresses such as osmotic and oxidative stress (common to both stresses) as well as ionic stress (during salinity) causing alterations in physiological, biochemical and molecular processes in plants resulting in substantial loss to crop productivity. The major physiological parameters studied in plants during stressed conditions are malondialdehyde (MDA) content and relative electrical conductivity in leaves, relative water content (RWC), stomatal conductance (gs), Chl content and Chl-fluorescence. Plants inoculated with plant growth promoting rhizobacteria (PGPR) induce morphological and biochemical modifications resulting in enhanced tolerance to abiotic stresses defined as induced systemic tolerance (IST). Molecular approaches such as RNA differential display (RNA-DD), reverse transcriptase PCR (RT-PCR) microarray analysis, real time PCR, differential display PCR (DD-PCR) and illumina sequencing revealed PGPR inoculation caused upregulation of drought stress related genes such as ERD15 (Early Response to Dehydration 15) and ABA-responsive gene, RAB18 in Arabidopsis genes, APX1 (ascorbate peroxidise), SAMS1 (S-adenosyl-methionine synthetase), and HSP17.8 (heat shock protein) in leaves of wheat, Cadhn (dehydrin-like protein), VA (Vacuolar ATPase), sHSP (Plant small heat shock proteins) and CaPR-10 (Pathogenesis-related proteins) in pepper, dehydration responsive element binding protein (DREB2A), catalase (CAT1) and dehydrin (DHN) in mung, salt stress responsive genes such as RAB18 (LEA), RD29A, RD29B regulons of ABRE (ABA-responsive elements) and DRE (dehydration responsive element) in Arabidopsis.