Root exudates from two tobacco cultivars affect colonization of Ralstonia solanacearum and the disease index

The colonization of rhizosphere by microorganisms is directly associated with bacterial growth, chemotaxis, biofilm formation, and the interaction with host plant root exudates. In this study, the responses of Ralstonia solanacearum to the root exudates from two tobacco cultivars (Hongda, susceptible; K326, resistant) were determined. The results showed that the population of R. solanacearum was much higher in the rhizosphere soil of Hongda than in the rhizosphere soil of K326, resulting in a higher disease index for the Hongda treatments (92.73 %). The attraction of R. solanacearum to Hongda root exudates (HRE) was stronger than the response to K326 root exudates (KRE). Four organic acids, oxalic acid, malic acid, citric acid, and succinic acid, from the root exudates were identified and subsequently evaluated. The amount of oxalic acid from HRE was significantly higher than that from KRE. The results also showed that oxalic acid could both significantly induce the chemotactic response and increase the biofilm biomass of R. solanacearum. Both malic acid and citric acid could significantly increase the chemotaxis ability in vitro and the recruitment of R. solanacearum to tobacco root under gnotobiotic conditions. Overall, these data suggested that the colonization of tobacco rhizosphere by pathogenic bacterial strains was influenced by the organic acids secreted from the roots. The results expand our understanding of the roles of root exudates in plant-microbe interactions and will be useful for screening and applying beneficial bacteria for better control of plant wilt diseases.