Molecular responses to quantitative bacterial wilt resistance in tomato.
A. Milling University of Wisconsin, Madison, WI.
Phytopathology 99:S169.

Abstract: Host resistance is the only practical control for bacterial wilt of tomato, caused by Ralstonia solanacearum, but wilt resistance is horizontal, polygenic and its basis is not understood. We found that tomato plants infected with R. solanacearum Race 3 biovar 2 strain UW551 upregulated genes in both the ethylene and salicylic acid signal transduction pathways. However, in response to R. solanacearum infection the horizontally wilt-resistant tomato line H7996 activated expression of these defense genes faster and to a greater degree than did susceptible cultivar Bonny Best. Interestingly, results suggest different roles for the virulence factor extracellular polysaccharide (EPS) in resistant and susceptible hosts. Wild-type UW551 induced a lower defense response in susceptible tomato than did an eps mutant, supporting the idea that EPS can shield R. solanacearum from recognition. In contrast, the eps mutant induced significantly lower defense responses in resistant H7996 than the wild-type strain, weakening the “cloaking” hypothesis. The eps mutant also induced noticeably less accumulation of the defensive reactive oxygen species hydrogen peroxide in resistant tomato leaves, despite attaining similar cell densities in planta. Further, cell-free purified EPS from UW551 triggered significant defense gene expression in resistant but not in susceptible tomato plants. Collectively, these data suggest that H7996 specifically recognizes EPS from R. solanacearum.