hxc2, an Arabidopsis mutant with an altered hypersensitive response to Xanthomonas campestris pv. campestris

A chemical mutagenized population of Arabidopsis Col-0-gl plants was screen ed for an altered hypersensitive response (HR) after spray inoculation with an HR-inducing isolate of Xanthomonas campestris pv. campestris (strain 14 7). Three classes of mutant were identified: those exhibiting an HR- phenot ype or partial loss of HR; hyper-responsive mutants showing necrotic lesion s rapidly leading to the collapse of leaves; and susceptible mutants. One m utant belonging to the susceptible class, hxc-2 was extensively characteriz ed. the compatible phenotype observed several days after initiation of the interaction was confirmed by measurement of in planta bacterial growth and use of bacterial strains constitutively expressing the GUS reporter gene. I n the same way, accumulation of autofluorescent compounds, salicylic acid p roduction and defence gene expression in the mutant were found to be simila r to that displayed by the susceptible ecotype. Inoculation of hxc-2 with d ifferent avirulent bacteria suggests that the mutation is specific for the interaction with the Xcc 147 strain, although the mutation has been shown t o affect a single dominant locus, different from the resistance locus defin ed by genetic analysis of resistance to Xcc 147. Genetic mapping of the mut ation indicated that it is located on chromosome III, defining a previously unknown resistance function in response to X. c. campestris.