Biocontrol activity of Bacillus velezensis against Ralstonia solanacearum race 1 biovar 3 in tomato: insights from defence induction, and functional analysis

Bacterial wilt, caused by Ralstonia solanacearum , is one of the most destructive diseases of tomato, with biovar 3 race 1 isolates predominant in India. The present study investigated rhizosphere bacterial isolates (ToRB-1 to ToRB-45) for their antagonistic potential against a virulent R. solanacearum isolate Rs-TN-02. Among these, the cell-free supernatants of isolates ToRB-7, ToRB-23, and ToRB-26 exhibited greater inhibition in agar diffusion assays. Molecular characterization using 16S rRNA and DNA gyrase genes identified these isolates as Bacillus velezensis . Chemical profiling using gas chromatography–mass spectrometry revealed the presence of several secondary metabolites, and selected compounds were subjected to in silico docking analysis with virulence-related proteins of R. solanacearum . Scanning electron microscopy of co-cultured broth indicated the predominance of B. velezensis cells and observable morphological alterations in Rs-TN-02. Under in vitro conditions, the isolates demonstrated multiple plant growth-promoting traits, including IAA, ammonia, HCN, and siderophore production, as well as phosphorus and zinc solubilization. In a glasshouse experiment, their combined application enhanced seed germination (97.33%), shoot length (37.9 cm) and root length (15.6 cm), and reduced bacterial wilt incidence by 88.13% compared with the untreated control. Treated plants showed enhanced defence enzyme activities and total phenol content. RT-qPCR analysis of selected defence-related genes indicated a positive transcriptional response with a 3.6-fold increase in NPR-1 expression. Overall, the study highlights the antagonistic, plant growth-promoting, and disease-suppressive attributes of B. velezensis isolates under controlled experimental conditions, supporting their potential inclusion in integrated and sustainable management strategies for tomato bacterial wilt, pending further field-level validation. • Bacillus velezensis isolates suppressed Ralstonia solanacearum Rs-TN-02 under in vitro conditions. • Co-culturing resulted in growth suppression and cellular alterations of Rs-TN-02 as observed by scanning electron microscopy. • B. velezensis exhibited hydrolytic enzyme activities and plant growth-promoting traits in vitro. • Combined application of B. velezensis isolates improved tomato growth and reduced wilt incidence under controlled conditions. • Defence enzyme activities and defence-related gene expression were enhanced following B. velezensis treatment.