Culture filtrates of lactic acid bacteria promote the growth and enhance the resistance of tomato seedlings to Ralstonia solanacearum

Ralstonia solanacearum is a threatening pathogen that causes tomato wilt, which affects growing plants and reduces tomato yield. This study investigated the potential biocontrol of lactic acid bacteria cell-free culture supernatants (LAB-CFCS) against R. solanacearum. Four LAB, B. longum, L. plantarum, L. salivarius, and L. rhamnosus were examined as potential biological agents against R. solanacearum. The in vitro studies demonstrated the antibacterial activity of LAB-CFCS against R. solanacearum which suppressed bacterial growth and impeded biofilm formation. Organic acids (lactic and acetic) were detected in all LAB-CFCS. The highest lactic and acetic acid values (4.4 and 3.9 g/L) were observed in the CFCS of L. plantarum and L. rhamnosus, respectively. The application of LAB-CFCS on infected tomato plants reduced wilt incidence and severity. Treatment with CFCS of L. salivarius markedly reduced wilt incidence and severity in R. solanacearum-infected seedlings to 19.0% and 5.0%, respectively, compared with 100% and 80% in untreated infected controls. Infection by R. solanacearum elevates oxidative damage. Application of LAB-CFCS, particularly L. rhamnosus attenuates reactive oxygen species accumulation and lipid peroxidation while stimulating chlorophyll, carotenoids, antioxidant enzymes and salicylic acid-mediated defense signaling. In addition, LAB-CFCS significantly improved the contents of phenolics, flavonoids, terpenoids, alkaloids, and anthocyanins as a defense mechanism against R. solanacearum infection and modulated the activity of phenylalanine ammonia-lyase. All LAB-CFCS treatments showed promising in vitro and in vivo efficacy against R. solanacearum. The ability of LAB-CFCS to stimulate key physiological processes points to their dual role in promoting growth and mitigating biotic stress.