An Arabidopsis mutant with altered hypersensitive response to Xanthomonas campestris pv. campestris, hxc1, displays a complex pathophenotype.

The hxc1 mutant was identified by screening an EMS (ethylmethane sulphonate) mutagenized population of Arabidopsis Col-0 plants for an altered hypersensitive response (HR), after spray inoculation with an HR-inducing isolate of Xanthomonas campestris pv. campestris (Xcc) (strain 147). The hxc1 mutant shows a susceptible phenotype several days after initiation of the interaction with the avirulent strain. This macroscopically observed phenotype was confirmed by measurement of in planta bacterial growth and by microscopical analysis. interestingly, the hxc1 mutation acts very specifically. Hxc1 displays a pathophenotype identical to that observed in the wild-type with several extensively characterized avirulent and virulent bacteria, except in response to Pseudomonas syringae pv. tomato strain DC3000/avrRpm1, for which a partial loss of resistance was observed. Finally, the mutation causes an attenuation of expression of several defence markers regulated through different signalling pathways. Together, these data underline the complexity of this novel defence mutant, and support the hypothesis of a mutation affecting a key component acting during the first steps of the plant defence response leading to resistance to Xcc147 and Pseudomonas syringae pv. tomato containing the avr gene, avrRpm1.