Plant Growth-Promoting Endophytes Bacillus subtilis Alleviate Combined Herbicide and Drought Stress in Triticum aestivum L. via Modulation of Antioxidants and GABA

The combination of herbicide and drought stress (HDS) severely limits wheat production worldwide. Plant growth-promoting endophytes (PGPE) such as Bacillus subtilis can mitigate HDS, but the underlying mechanisms are poorly understood. This study investigated the role of antioxidants and gamma-aminobutyric acid (GABA) in PGPE-induced HDS tolerance in two wheat genotypes contrasting in drought sensitivity: tolerant (DT, cv. ‘Ekada70’) and sensitive (DS, cv. ‘Salavat Yulaev’). Seeds primed with PGPE B. subtilis strains 104 and 26D were grown under normal conditions for 16 days, then exposed to herbicide and 7 days of soil drought, followed by rewatering. PGPE successfully colonized seedlings (especially roots), exerted prolonged growth-promoting effect and accelerated post-stress recovery. Before stress, primed plants showed increased levels of GABA, chlorophyll, and antioxidants (SOD, CAT, AsA, GSH, GR, APX). Under HDS, primed plants had lower accumulation of H2O2, O₂⁻ , and lipid peroxidation (MDA), and higher relative water content, confirming alleviation of water deficit. The effects were strain- and genotype-dependent: strain 104 prevented GSH depletion in DT plants via GR activation and lowered HDS-driven increase of GABA and proline. In contrast, strain 26D protected DS plants by elevating GABA and proline. Consequently, strain 104 accelerated HDS adaptation and recovery in DT plants, while strain 26D was more effective in DS plants. Our findings demonstrate that strain-specific PGPE application improves wheat resilience to HDS by modulating antioxidant defenses, GABA metabolism, and water relations, providing a basis for genotype‑tailored biopriming strategies.