Harnessing Rhodopseudomonas palustris strains for salt stress mitigation in Arabidopsis thaliana

Soil salinity severely limits plant growth and agricultural productivity. 5-Aminolevulinic acid (ALA), a precursor in tetrapyrrole biosynthesis, has been reported to alleviate salinity stress and is frequently proposed as a key component by which purple non-sulfur bacteria enhance plant stress tolerance. This study compared the treatments of three Rhodopseudomonas palustris strains (PS3, TPN1, and YSC3) with ALA under salinity stress to understand the importance of ALA production for their potential ability to alleviate salinity stress, using Arabidopsis thaliana, a salt-sensitive model plant in which NaCl concentrations above 30 mM induce growth inhibition and physiological stress responses, making it well suited for assessing salinity tolerance mechanisms. Both bacterial and ALA treatments increased the photosynthetic efficiency, root growth, relative water content, and oxidative balance of salt-stressed plants. These treatments maintained chlorophyll biosynthetic capacity and modulation of ion transport-related responses, consistent with improved ionic homeostasis under salinity stress. Among the strains, TPN1 performed the best, exhibiting altered expression of antioxidant genes, reduced lipid peroxidation, and decreased electrolyte leakage, which indicates improved membrane integrity. The outcomes were associated with the ability of TPN1 to maintain halotolerance and key plant growth-promoting traits, including the production of extracellular polysaccharides and indole-3-acetic acid, under high salinity. Notably, strains with comparatively lower extracellular ALA outputs conferred benefits comparable to those observed with ALA treatment, suggesting that plant stress mitigation is not solely dependent on ALA concentration. We identified ALA-producing R. palustris strains, particularly TPN1, that help enhance plant tolerance to salinity through coordinated changes in ion regulation, antioxidant balance, and photosynthetic performance, and demonstrate that ALA production may not be the primary factor contributing to R. palustris' enhancement of plant salt tolerance.