Salt stress shapes rhizosphere bacterial communities and is associated with salt tolerance in tiger nut (Cyperus esculentus L.)

Background Although soil salinization severely restricts crop production, plants can enhance their salt tolerance through interactions with rhizosphere microorganisms. Tiger nut ( Cyperus esculentus L.) is a grain and oil plant characterized by its strong stress resistance. However, its salt tolerance mechanisms and their relation to its associated rhizosphere microorganisms remain unclear. Methods This study involved a pot experiment with six NaCl salt concentrations (0, 0.36, 0.72, 1.08, 1.44, and 1.80 M). Through physiological and biochemical analyses, 16S rRNA sequencing, PICRUSt2 functional prediction, and partial least squares structural equation modeling (PLS-SEM), we explored the coupling relationships between the physiological responses of tiger nut and the succession of rhizosphere bacterial communities under a salt stress gradient. Results The results showed that salt stress significantly increased the content of soluble sugar (SS), proline (Pro), and soluble protein (SP) in tiger nuts; enhanced the activity of superoxide dismutase (SOD) and catalase (CAT) (P < 0.05); and was associated with a shift in biomass allocation toward tuber development. Salinity is the primary driving force behind the directional reshaping of rhizosphere bacterial communities. The relative abundances of Proteobacteria and Firmicutes significantly increased, and Planococcus and Paracoccus were significantly enriched. Variance partitioning analysis showed that plant osmotic adjustment factors explained the largest independent fraction of the variation in rhizosphere bacterial community structure. The correlation analysis results indicate that predicted bacterial functional pathways associated with salt tolerance were significantly positively correlated (P < 0.05) with soil carbon- and nitrogen-cycling functions, as well as plant stress resistance traits and tuber dry weight. The PLS-SEM analysis results suggest that plant osmotic adjustment substances and predicted bacterial metabolic functions exhibited statistically mediated associations in the pathways linking salt stress to bacterial function, and potentially halotolerant rhizobacteria to plant antioxidant systems, respectively. Conclusions In summary, this study suggests a potential synergistic association between plant osmotic adjustment and rhizobacterial enrichment in tiger nut under salt stress. In particular, the accumulation of plant osmotic adjustment substances was associated with the enrichment of rhizobacterial genera with reported halotolerant potential, and these enriched bacteria were closely linked to plant stress resistance traits, potentially through their salt-tolerance metabolic pathways. These findings provide correlative evidence for the association between plant osmotic adjustment and the enrichment of rhizobacterial genera with reported stress-resistant potential, which may be linked to salt tolerance in tiger nut. However, direct evidence from root exudate profiling or microbial inoculation experiments is still needed to confirm these associations.