Membrane Adaptation of Ochrobactrum intermedium Under Salt Stress Enables Growth Promotion in Peanut Plants

Soil salinity significantly limits agricultural productivity in Argentina, posing a major threat to crops such as peanut (Arachis hypogaea). This study evaluated how the rhizospheric bacterium Ochrobactrum intermedium (L115) maintains its plant growth-promoting capacity under saline conditions through membrane level adaptation mechanisms. Bacterial growth, cell morphology, membrane lipid composition, fluidity, and phase transition temperature (Tm) were analyzed under increasing NaCl concentrations (0, 0.4, 0.6, and 0.8 mol.l⁻¹). L115 tolerates up to 0.4 mol.l⁻¹ NaCl efficiently, preserving viability, morphology, and membrane biophysical properties similar to control conditions. This homeostasis was achieved by adjusting membrane phospholipids, notably increasing cardiolipin (29.4%) and phosphatidylcholine levels (3.2%), while maintaining a balanced zwitterionic/anionic phospholipid ratio. At 0.6mol.l⁻¹, drastic changes in fatty acid and lipid profiles disrupted fluidity (0.175 to 0.131) and decreased Tm (12.5 to 4°C), leading to reduced viability. At 0.8 mol.l⁻¹ severe changes in fluidity and Tm produced by increases in cardiolipin (88.2%) and phosphatidylcholine (21.4%) concentration and by an increase in unsaturated fatty acids led to cell dehydration, loss of flagella and reduced viability. Peanut inoculation assays under 0.1mol.l⁻¹ NaCl demonstrated that L115 significantly mitigated salt-induced growth reduction, restoring shoot and root biomass and length to levels comparable or superior to unstressed controls. These findings indicate that L115's ability to modulate its membrane composition allows it to sustain its function as a plant growth-promoting bacterium under moderate salt stress. Thus, L115 emerges as a promising bioinoculant candidate for enhancing crop resilience in saline soils.