Jasmonic acid has properties of a plant hormone, including the induction of specific genes associated with plant defense. We previously describedjar1-1, anArabidopsisjasmonate response mutant that exhibits reduced sensitivity to methyl jasmonate. We have further characterized this mutant and two new alleles;jar1-2from a gamma irradiated population, andjar1-4from a T-DNA mutant population. Seedling root growth injar1-1was equally insensitive to methyl jasmonate and jasmonic acid, indicating that the defect was not in the conversion of methyl jasmonate to the acid. None of thejar1mutants showed an altered sensitivity to auxin, cytokinin, or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, indicating that the lesion does not affect the general uptake or transport of hormones. A soil fungus,Pythium irregulare, was found to blightjar1-1. Cultures of this organism caused the symptoms in all threejar1mutants but not in wild type, indicating that increased susceptibility was due to the lesion in theJAR1locus. A fatty acid desaturase triple mutant that is defective in the biosynthesis of jasmonic acid (J. Browse, Washington State University) was also susceptible, confirming that jasmonate is involved in resistance. Thejar1-1locus was mapped to the lower end of chromosome 2, about 11.4 cM fromas1 and1.6 cM fromcer8. These results establish that jasmonate signaling plays an important role in resistance to soil micro-organisms in plants.