Pathogen-lnduced Systemic Activation of a Plant Defensin Gene in Arabidopsis Follows a Salicylic Acid-lndependent Pathway

A 5-kD plant defensin was purified from Arabidopsis leaves challenged with the fungus Alternaria brassicicola and shown to possess antifungal properties in vitro. The corresponding plant defensin gene was induced 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 mRNA rose both in inoculated leaves and in nontreated leaves of inoculated plants (systemic leaves). These events coincided with an increase in the endogenous jasmonic acid con-tent of both types of leaves. Systemic pathogen-induced expression of the plant defensin gene was unaffected in Arabidopsis transformants (nahG) or mutants (nprl and cpr l) affected in the salicylic acid response but was strongly reduced in the Arabidopsis 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 Arabidopsis is indepen-dent of salicylic acid but requires components of the ethylene and jasmonic acid response