The mechanisms of interaction between alfalfa root exudates and rhizosphere bacterial communities for adaptation to salt stress

Soil salinity is a major abiotic factor reducing global crop yields and ecosystem sustainability. Root exudate-rhizosphere microbe interaction plays a key role in regulating plant growth, development, and stress resistance. However, the mechanisms of how alfalfa root exudates and rhizosphere microbes interact to adapt to salt stress are still poorly understood. This study analyzed the salt stress tolerance mechanisms of two alfalfa varieties (ZT1 and ZT2) derived from the space mutagenesis breeding program by integrating rhizosphere microbiome and root exudate measurements with root and shoot phenotyping. ZT1 is salt-tolerant and has a stronger osmotic regulation ability and antioxidant enzyme system, while ZT2 is salt-sensitive. Salt-induced rhizosphere remodeling in ZT1 featured selective enrichment of specialized metabolites, including ormosanine and 7,4'-dihydroxyflavone. Key flavonoid species (e.g., baicalein, phloretin-2'-O-glucoside) and triterpene glycosides, including asiaticoside, were identified as crucial mediators of salt-stress adaptation. Microbial network analysis revealed concentration-dependent modulation of rhizosphere diversity: 100 mM NaCl enhanced species richness, while 200 mM NaCl stress caused significant diversity loss. This functional reorganization strengthened reciprocal interactions between core rhizobacteria and stress-responsive metabolites. Mantel test analyses demonstrated the predominant regulation of rhizosphere community assembly by root exudate profiles (r = 0.72, p < 0.01). This study reveals the microbial assembly mechanism mediated by alfalfa root exudates, clarifies the microbe-plant metabolic co-adaptation pattern under salt stress, and provides new directions for alfalfa cultivation in saline-alkali soils. • ZT1 exhibits superior damage repair and stress tolerance regulation. • Root exudate flavonoids and glycosides are key to alfalfa's salt stress response. • Salt stress alters root exudate makeup and rhizosphere microbiome structure. • 100 mM salt levels raised rhizosphere species count in ZT1 and ZT2. • Root exudates are key in rhizosphere bacterial community interactions.