Antifungal and Plant Growth–Promoting Bacillus under Saline Stress Modify their Membrane Composition

We explored the membrane lipid and fatty acid composition and plant growth–promoting (PGP) mechanisms of two bacterial endophytes, Bacillus sp. E25 and Bacillus sp. CR71, under saline stress, that is known to be a major detriment to crop yield. Effect of single and co-inoculation of the strains on root and shoot lengths, relative chlorophyll content, and plant biomass in tomato plants (Lycopersicon esculentum cv Saladette) grown in greenhouses was also evaluated. Our results show that single and co-inoculated tomato plants with bacilli strains E25 and CR71, growing under normal or saline stress conditions (100 and 200 mM NaCl), exhibited an increase in root and shoot lengths, chlorophyll content, and biomass parameters, compared with control plants (uninoculated). Additionally, these bacilli strains were antagonistic toward fungal pathogens Fusarium oxysporum and (to a greater extent) Botrytis cinerea in the presence or absence of NaCl. Interestingly, both endophytes maintained good PGP activities, producing compounds like indole-3-acetic acid (IAA), proteases, siderophores, and biofilm. Saline conditions led to changes in membrane phospholipid and fatty acid levels in both. Phosphatidylethanolamine biosynthesis and branched (16:1∆9; 17:0i) and unsaturated fatty acids increased in E25; CR71 showed increase in relative amounts of the same fatty acids and accumulated an unidentified lipid. Bacillus strains E25 and CR71 differentially modify their membrane phospholipid composition as a protective mechanism, potentially for maintaining PGP activities, under saline stress.