Poster: Plant Pathogen/Symbiont Interactions

Abs # 720: Nitric oxide synthesis in Arabidopsis thaliana leaves infected with Pseudomonas syringae

Presenter:	Modolo, Luzia V, modolo@unicamp.br
Authors	Modolo, Luzia V (A)   Almeida, Irene M G (B)   Rodrigues Neto, Júlio  (B)   Magalhães, José R (C)   Augusto, Ohara  (D)   Salgado, I  (A)
Affiliations:	(A): Departamento de Bioquímica, Universidade Estadual de Campinas (B): Lab. Bacteriologia Vegetal, Instituto Biológico (C): Empresa Brasileira de Pesquisa Tecnológica (D): Departamento de Bioquímica, Universidade de São Paulo
Web Site:	http://www.unicamp.br/~modolo

Nitric oxide (NO) has been receiving increasing attention as a regulator of many plant functions, including defense against pathogen invasion. In animals, the main route for NO synthesis is through sequential oxidation of L-arginine to citrulline, catalyzed by NO-synthase (NOS) enzymes. Although in plants conversion of arginine to citrulline has been reported, the genetic and molecular basis for this NOS-like reaction have yet to be determined, and the nitrate reductase (NR) activity has been considered as an alternative route for NO production. To contribute to the understanding of NO production during plant-microbe interaction, we monitored NO production (as Fe-MGD nitrosyl complex by EPR) and L-[U-¹⁴C] arginine to L-[U-¹⁴C] citrulline conversion in leaf homogenates of an wild type and an NR nia1 nia2 deficient mutant of Arabidopsis thaliana upon Pseudomonas syringae pv. maculicola (Psm) infection. Our results show that inoculation of A. thaliana leaves with Psm induced NOS-like activity (arginine-citrulline conversion) in the wild type (18.5 versus 4.4 pmol/min/mg in plants-control) and in the NR deficient mutant (13.4 versus 2.1 pmol/min/mg in plants-control). In parallel, bacterial infection increased (about twice) the detectable levels of the Fe(II)-MGD nitrosyl complex in the presence of arginine. No Fe(II)-MGD nitrosyl complex signal was detected in leaf homogenates of the NR deficient mutant. Control experiments showed that Psm did not present arginine dihydrolase and NO^3- reductase activities. These results suggest that plants may have a singular mechanism for NO production during plant-pathogen interaction involving the conversion of arginine to citrulline to produce NO^2-/NO^3- that are then reduced to NO by the NR enzyme. Supported by FAPESP and CNPq.