Genomic and functional insights into Bacillus velezensis KB21— A promising rhizobacterium for enhancing plant growth and stress tolerance in cucumber.

• Bacillus velezensis KB21 promotes cucumber growth under optimal inoculation conditions. • B. velezensis KB21 enhances salt tolerance by modulating ion balance and antioxidant defenses. • B. velezensis KB21 upregulates stress-responsive and phytohormone biosynthesis genes. • Whole-genome analysis reveals genes linked to plant growth and stress adaptation. • B. velezensis KB21 shows promise as a biofertilizer for sustainable agricultural practices. This study investigates the genomic and functional traits of Bacillus velezensis KB21, a plant growth–promoting rhizobacterium, and its effects on cucumber (Cucumis sativus) growth and salt stress tolerance. Whole–genome sequencing revealed a 4.12 Mb circular chromosome with 27 rRNA genes, 86 tRNA genes, and 4,015 coding sequences, including those involved in auxin (IAA), gibberellin (GA), and volatile organic compound biosynthesis. Inoculation at optimal concentration (log 5 and log 3 CFU/mL) significantly enhanced cucumber growth, increasing fresh and dry biomass, leaf area, and root colonization. Growth promotion was linked to the enhanced expression of CsSAUR50 and CsGA20ox , which regulate auxin and GA biosynthesis during seed germination. Under salt stress conditions, KB21–treated plants showed improved growth, a favorable Na+/K+ ratio, and increased antioxidant enzyme activities (SOD, POD, and CAT). The treatment upregulated stress–responsive genes, including HKT, NHX , and AQP , and ABA–related genes (CsNCED1 and CsCYP707A1), demonstrating its role in ion homeostasis and ABA–mediated stress adaptation. These findings highlight the potential of B. velezensis KB21 as a sustainable biofertilizer for enhancing plant growth and resilience under abiotic stress conditions [ABSTRACT FROM AUTHOR]