PATHOGEN-INDUCED SYSTEMIC ACTIVATION OF A PLANT DEFENSIN GENE IN ARABIDOPSIS FOLLOWS A SALICYLIC ACID-INDEPENDENT PATHWAY

A 5-kD plant defensin was purified from Arabidopsis leaves challenged with the fungus Alternaria brassicicola and shown to possess antifunga l properties in vitro. The corresponding plant defensin gene was induc ed after treatment of leaves with methyl jasmonate or ethylene but not with salicylic acid or 2,6-dichloroisonicotinic acid. When challenged with A. brassicicola, the levels of the plant defensin protein and mR NA rose both in inoculated leaves and in nontreated leaves of inoculat ed plants (systemic leaves), These events coincided with an increase i n the endogenous jasmonic acid content of both types of leaves. System ic pathogen-induced expression of the plant defensin gene was unaffect ed in Arabidopsis transformants (nahG) or mutants (npr1 and cpr1) affe cted in the salicylic acid response but was strongly reduced in the Ar abidopsis mutants ein2 and coil that are blocked in their response to ethylene and methyl jasmonate, respectively. Our results indicate that systemic pathogen-induced expression of the plant defensin gene in Ar abidopsis is independent of salicylic acid but requires components of the ethylene and jasmonic acid response.