Influence of Rhizosphere Bacteria in Attenuating the Virulence of Ralstonia solanacearum, a Pathogen Causing Tomato Wilt, through an AHL-based Quorum Sensing System

Abstract

Ralstonia solanacearum is a Gram-negative, soil-borne bacterium associated with bacterial wilt in tomato and several other economically important crops. The present study evaluated whether rhizosphere bacteria isolated from wilt-affected tomato plants could attenuate the virulence of R. solanacearum through an N-acyl homoserine lactone (AHL)-based quorum sensing system. Rhizosphere soil samples were collected from tomato-growing regions in Dharmapuri, Tamil Nadu, and Kolar, Karnataka. R. solanacearum was isolated and maintained on 523 and 2,3,5-triphenyltetrazolium chloride media, while non-pathogenic rhizosphere bacteria were isolated on nutrient agar. Isolates were screened for quorum sensing activity using Chromobacterium violaceum as a biosensor indicator organism in a modified microtitre plate assay. Two rhizosphere isolates, designated TL1 and TL2, produced positive colour reactions associated with AHL signal detection. Molecular characterisation based on 16S rRNA gene amplification and sequence analysis identified TL1 as Brevundimonas sp. and TL2 as Rhizobium sp., with sequence sizes of approximately 992 bp and 1366 bp, respectively. Pot experiments were conducted using tomato seedlings infected with R. solanacearum and treated with the identified rhizosphere bacterial isolates or C. violaceum. After two weeks, plants treated with Brevundimonas sp. and Rhizobium sp. did not show wilt symptoms, whereas wilt symptoms were observed in plants treated with C. violaceum alone and in plants infected only with R. solanacearum. These findings indicate that the tested rhizosphere bacteria were associated with attenuation of R. solanacearum virulence under the experimental conditions. The results suggest a potential role for AHL-mediated quorum sensing interference in tomato wilt management and provide a basis for further validation of these isolates.