Suppression of pathogen-inducible NO synthase (iNOS) activity in tomato increases susceptibility to Pseudomonas syringae.

Inducible NO synthase (iNOS) activity is induced upon pathogen inoculation in resistant, but not susceptible, tobacco and Arabidopsis plants. It was shown recently that a variant form of the Arabidopsis P protein (AtvarP) has iNOS activity. P protein is part of the glycine decarboxylase complex (GDC). It is unclear whether P protein also has iNOS activity and, if so, whether AtvarP, P, or both, play a role in plant defense. Here, we show that iNOS activity is induced in both resistant and susceptible tomato leaves upon inoculation with the Pseudomonas syringae pv. tomato strain DC3000. Virus-induced gene-silencing targeting LevarP, a putative tomato ortholog of AtvarP, led to complete suppression of DC3000-induced iNOS activation and an approximately 80% reduction in GDC activity; it also increased disease-symptom severity and DC3000 growth in both resistant and susceptible tomato. To determine whether enhanced susceptibility exhibited by LevarP-silenced, susceptible tomato was due to loss of (i) iNOS activity, (ii) GDC activity, or (iii) both, GDC activity was inhibited with or without concurrent suppression of iNOS. Treatment with methotrexate inhibited both iNOS and GDC activities and resulted in increased susceptibility, comparable with that observed in LevarP-silenced plants. When normal iNOS activity was maintained in the presence of methotrexate by the addition of tetrahydrobiopterin, there was no change in susceptibility, despite a dramatic reduction in GDC activity. Together, these results indicate that iNOS contributes to host defense response against DC3000.