Expression of Defense-Related and Putative Signaling Genes During Tolerant and Susceptible Interactions of Arabidopsis with Xanthomonas campestris pv. campestris. C. R. Buell. Carnegie Institution of Washington, Department of Plant Biology, 290 Panama Street, Stanford CA 94305. S. C. Somerville. Carnegie Institution of Washington, Department of Plant Biology, 290 Panama Street, Stanford CA 94305. MPMI 8: 435-443. Accepted 19 January 1995. Copyright 1995 The American Phytopathological Society.

The tolerance of Arabidopsis thaliana ecotype Columbia to Xanthomonas campestris pv. campestris 2D520 is conditioned by a dominant allele of the single, nuclear gene RXC1. Tolerance is exhibited by an asymptomatic response of Columbia to X. campestris pv. campestris 2D520 population levels greater than 106 CFU cm-2. The establishment of similar bacterial population levels in the susceptible ecotype, Pr-0, results in spreading chlorosis within 3 to 4 days of inoculation. To better define the mechanism(s) that permit Columbia to tolerate infection by X. campestris pv. campestris 2D520, we examined mRNA levels of 10 genes proposed to have a role in either defense responses or signaling of stress events. These analyses revealed neither temporal nor quantitative accumulation of mRNA species unique to the tolerant interaction. Examination of mRNA levels of anthranilate syn-thase 1 (ASA1), anthranilate synlhase 2 (ASA2), cinna-myl-alcohol dehydrogenase (CAD), cinnamate-4-hydroxy-lase (C4H), the ELI3 plant defense gene (ELD), fatty acid desaturase 3 (FAD3), lipoxygenase (LOX1), myrosinase (MYR), and superoxide dismutase (SOD) revealed little to no accumulation in either Columbia or Pr-0 following inoculation with X. campestris pv. campestris 2D520. In contrast, mRNA levels encoding the pathogenesis-related protein 1 (PR1) accumulated to high levels beginning at 24 h postinoculation in both Columbia and Pr-0 tissues, with higher PR1 mRNA levels accumulating in Pr-0 than Columbia. The lack of mRNA accumulation for nine of ten mRNAs in tolerant and susceptible plants following inoculation with X. campestris pv. campestris was not due to an inability to accumulate these mRNAs. Inoculation of Columbia and Pr-0 plants with Pseudomonas syringae pv. syringae PSSD20, which elicits a hypersensitive response in both ecotypes, resulted in the accumulation of several mRNA species including ASA1, CAD, C4H, ELI3, LOX1, SOD, and PR1. The absence of preferential accumulation, either temporally or quantitatively, in Columbia tissues suggests these 10 defense-related and signaling mRNAs do not have a role in the establishment of tolerance.