Calcium-mediated enhancement of plant growth-promoting microorganisms improves drought tolerance in winter rapeseed

Drought stress severely limits crop productivity, necessitating sustainable strategies to maintain plant growth under water-limited conditions. Although plant growth-promoting microorganisms (PGPMs) and calcium are known to enhance stress tolerance, their interactive effects remain insufficiently understood. This study investigated the combined influence of selected PGPMs and calcium on physiological, biochemical, and molecular responses of winter rapeseed ( Brassica napus L.) under drought stress. Plants were treated with calcium (CaCO₃ or CaCl₂) and inoculated with Bacillus subtilis , Lactobacillus paracasei , or Zygosaccharomyces bailii . Drought was induced at the early vegetative stage, and plant performance was assessed through growth parameters, relative water content (RWC), oxidative stress indicators, and expression of stress-responsive genes. Combined treatments significantly improved plant performance compared to individual applications, with the strongest effects observed for B. subtilis  + CaCl₂. These plants maintained higher RWC, reduced levels of hydrogen peroxide, malondialdehyde, and proline, and showed lower expression of Lea9 , Lea10 , and KIN2-like . Notably, microbial viability in soil increased in calcium-amended treatments, indicating that calcium enhances microbial persistence and effectiveness. Strain-specific responses further demonstrated that microbial interaction with calcium plays a key role in determining plant stress outcomes. These findings suggest that calcium-mediated modulation of plant-microbe interactions contributes to improved drought tolerance and sustained growth, offering a promising strategy for enhancing crop resilience.