Indole derivatives enhance resistance against the rice blast fungus Magnaporthe oryzae

In leaves pretreated with an indole derivative [indole-3-acetic acid (IAA) tryptamine, or tryptophan], blast lesion formation was suppressed compared to those treated with distilled water (DW) as a control. Phenylalanine ammonia-lyase (PAL) activity and PAL expression were significantly enhanced in the IAA- or tryptophan-pretreated leaves, but not in tryptamine-pretreated leaves. This induction of resistance was inhibited by pretreatment with a PAL inhibitor, α-aminooxyacetic acid, in IAA- and in tryptophan-treated leaves, but not in tryptamine-treated leaves. This study strongly shows that the indole derivatives IAA tryptamine and tryptophan can enhance a disease resistance mechanism that is supported by different metabolic pathways.