Concomitant activation of jasmonate and ethylene response pathways is required for induction of a plant defensin gene in Arabidopsis

Activation of the plant defensin gene PDF1.2 in Arabidopsis by pathogens ha s been shown previously to be blocked in the ethylene response mutant ein2- 1 and the jasmonate response mutant col1-1. In this work, we have further i nvestigated the interactions between the ethylene and jasmonate signal path ways for the induction of this defense response. inoculation of wild-type A rabidopsis plants with the fungus Alternaria brassicicola led to a marked i ncrease in production of jasmonic acid, and this response was not blocked i n the ein2-1 mutant. Likewise, A. brassicicola infection caused stimulated emission of ethylene both in wild-type plants and in col1-1 mutants. Howeve r, treatment of either ein2-1 or coi1-1 mutants with methyl jasmonate or et hylene did not induce PDF1.2, as it did in wild-type plants. We conclude fr om these experiments that both the ethylene and jasmonate signaling pathway s need to be triggered concomitantly-and not sequentially, to activate PDF1 .2 upon pathogen infection. In support of this idea, we observed a marked s ynergy between ethylene and methyl jasmonate for the induction of PDF1.2 in plants grown under sterile conditions. In contrast to the clear interdepen dence of the ethylene and jasmonate pathways for pathogen-induced activatio n of PDF1.2, functional ethylene and jasmonate signaling pathways are not r equired for growth responses induced by jasmonate and ethylene, respectivel y.