Salt stress-induced alterations in mungbean (Vigna radiata L.) root exudates facilitate the recruitment of stress-ameliorating rhizobacteria

Abstract

Soil salinity severely constrains global crop productivity, particularly in salt-sensitive crops such as mungbean (Vigna radiata L.). To mitigate this, plant growth-promoting rhizobacteria (PGPR)-based biofertilizers offer a sustainable and cost-effective strategy. However, their effectiveness depends on plant-microbe communication and plant-derived belowground chemical signals that remain poorly understood. This study aimed to characterize changes in mungbean root exudates (RE) under salt stress and evaluate their influence on the recruitment and colonization traits of the stress-ameliorating salt tolerant PGPR Niallia circulans JPR79. Using a hybrid soil-hydroponic system and high-resolution mass spectrometry, we profiled mungbean RE under 60 and 120 mM NaCl stress and a control condition. Salinity markedly altered exudate composition, with significant enrichment of lipids and phenylpropanoids. Among the most strongly induced metabolites, palmitic acid and p-hydroxyhydrocinnamic acid elicited concentration-dependent chemotaxis and swarming motility in JPR79. Whole exudates from salt-stressed plants also induced stronger bacterial chemotactic responses and biofilm formation than exudates from control plants. Furthermore, inoculation with JPR79 alleviated salinity-induced growth inhibition by improving biomass accumulation, plant water status, membrane stability, and grain yield under salt stress. These findings suggest that salt stress in mungbean potentially induces alterations in the RE profile that may enhance beneficial plant-microbe interactions by influencing PGPR behavioural traits associated with chemotaxis, biofilm formation, and potential root colonization. Overall, this study provides a basis for a potential ‘cry-for-help’ strategy in mungbean, wherein salt stress alters RE chemistry in ways that may facilitate interactions with stress-ameliorating PGPR and contribute to improved plant resilience under saline conditions.