Root-associated bacterial community shifts accompany salt-stress mitigation by Bacillus velezensis SGTSH 0811 in sunflower

Purpose This study aimed to elucidate how the salt-tolerant plant growth-promoting rhizobacterium Bacillus velezensis SGTSH 0811 influences root-associated bacterial communities and coordinated physiological and transcriptional responses in sunflower ( Helianthus annuus L.) under gradient NaCl stress. Methods The salt tolerance and plant growth-promoting traits of SGTSH 0811 were evaluated under 170–2000 mM NaCl, and bacterial morphological adaptation was examined using transmission electron microscopy. A two-factor full-factorial pot experiment was conducted under 0, 100, 200 and 300 mM NaCl to assess sunflower growth and physiological responses. Rhizosphere and root endosphere bacterial communities were characterized using 16S rRNA gene amplicon sequencing, combined with βNTI-based community assembly analysis, co-occurrence network analysis and FAPROTAX-based functional prediction. The transcriptional responses of selected salt-responsive genes were determined by RT-qPCR. Results SGTSH 0811 maintained stable growth at up to 1000 mM NaCl. Indole-3-acetic acid production and biofilm formation decreased with increasing salinity, whereas exopolysaccharide production peaked at 500 mM NaCl. Under 300 mM NaCl, SGTSH 0811 inoculation improved root development and shoot biomass, enhanced antioxidant enzyme activities and proline accumulation, reduced malondialdehyde content, and alleviated photosynthetic pigment loss. Salt stress altered the composition and assembly of root-associated bacterial communities, whereas inoculation was associated with the enrichment of Bacillus , Paenibacillus , Streptomyces and Bradyrhizobium , changes in positive co-occurrence associations, and shifts in predicted functions related to carbon and nitrogen cycling and organic matter degradation. RT-qPCR revealed inoculation-associated changes in the expression of genes involved in antioxidant defense, osmotic adjustment, stress signaling and ion homeostasis, including SOD , SO2 , SO3 , EcPP44 , AKT1 , HKT1 , NHX1 and DREB2 . Conclusion SGTSH 0811 alleviated salt stress in sunflower in association with root-associated microbiome shifts, improved antioxidant and osmotic responses, and altered expression of salt-responsive genes. These findings provide an integrated view of the physiological and microbiome responses associated with PGPR-mediated salt-stress alleviation in sunflower.