Effects of urate, a natural inhibitor of peroxynitrite-mediated toxicity, in the response of Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae

Urate, a natural peroxynitrite scavenger, has been used to investigate the possible role of peroxynitrite during plant-pathogen interactions. Urate gr eatly reduced lesion formation in Arabidopsis leaves treated with an abioti c peroxynitrite-generating system or with a peroxynitrite solution, indicat ing that it can act as an effective scavenger in planta. In the interaction with the avirulent Pseudomonas syringae pv. phaseolicola (avrRPM1(+)), cel l death in the inoculated area was strongly reduced by urate, without compr omising disease resistance. In contrast, urate promoted discrete cell death in response to an isogenic Pseudomonas syringae (avrRPM1(-)), which did no t trigger an HR when inoculated alone, and it induced resistance and arrest of pathogen growth. Scavenging of peroxynitrite did not modify the respons e of Arabidopsis to an avirulent strain of Xanthomonas campestris pv campes tris, that showed a high resistance to NO and peroxynitrite. Our data indic ate that peroxynitrite plays a significant role in the responses of plants to Pseudomonas syringae.