A ROLE FOR JASMONATE IN PATHOGEN DEFENSE OF ARABIDOPSIS

To investigate the role of jasmonate in the defense of plants against fungal pathogens, we have studied a mutant of Arabidopsis, fad3-2 fad7 -2 fad8, that cannot accumulate jasmonate. Mutant plants were extremel y susceptible to root rot caused by the fungal root pathogen Pythium m astophorum (Drechs.), even though neighboring wild-type plants were la rgely unaffected by this fungus. Application of exogenous methyl jasmo nate substantially protected mutant plants, reducing the incidence of disease to a level close to that of wild-type controls. A similar trea tment with methyl jasmonate did not protect the jasmonate-insensitive mutant coil from infection, showing that protective action of applied jasmonate against P. mastophorum was mediated by the induction of plan t defense mechanisms rather than by a direct antifungal action. Transc ripts of three jasmonate-responsive defense genes are induced by Pythi um challenge in the wildtype but not in the jasmonate-deficient mutant . Pythium species are ubiquitous in soil and root habitats world-wide, but most (including P. mastophorum) are considered to be minor pathog ens. Our results indicate that jasmonate is essential for plant defens e against Pythium and, because of the high exposure of plant roots to Pythium inoculum in soil, may well be fundamental to survival of plant s in nature. Our results further indicate that the fad3-2 fad7-2 fad8 mutant is an appropriate genetic model for studying the role of this i mportant signaling molecule in pathogen defense.