Membrane adaptation of Ochrobactrum intermedium under salt stress enables growth promotion in peanut plants.

Aims: 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.

Methods and results: 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 0.6 mol l-1, drastic changes in fatty acid and lipid profiles disrupted fluidity (0.175 to 0.131) and decreased Tm (12.5°C 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 0.1-mol 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.

Conclusions: 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.