NITRIC-OXIDE PRESERVES THE LEVEL OF CHLOROPHYLL IN POTATO LEAVES INFECTED BY PHYTOPHTHORA-INFESTANS

Nitric oxide (NO) is a bioactive molecule involved in many physiologic al processes. Among its biological function, NO has been proved to be cytotoxic against microorganisms in cells of the immune response, thus preventing infection. We have specifically studied the effect of a NO donor, sodium nitroprusside (SNP), on the chlorophyll content in pota to leaves infected with the pathogenic fungus Phytophthora infestans ( Pi). Fifteen days after infection, chlorophyll content strongly decaye d in water-treated potato leaf sections. SNP was able to partially rev ert that loss in a dose-dependent manner, being the effective SNP conc entrations between 10 mu M and 100 mu M. NaNO2 and NaNO3, the SNP-deri ved residual products, were unable to prevent the chlorophyll loss. Tr eatments with SNP did not affect the survival of Pi and the fungus was able to grow in a V-8-agar medium containing 100 mu M SNP. Both the a mount and the extent of germination of Pi sporangia resulted similar i n the absence and in the presence of SNP. Respiratory inhibitors of th e cyanide-sensitive and cyanide-resistant pathways, 2,4-dinitrophenol and salicylhydroxamic acid respectively, did not change the chlorophyl l levels in infected potato leaves, suggesting that NO effect should n ot be on mitochondrial respiration. These results indicate that NO cou ld be a protective molecule, either preserving the chloroplast membran e of infected leaf sections against the toxicity of reactive oxygen sp ecies or being directly involved in any step of the chlorophyll metabo lic pathway.