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Probing the role of tryptophan-derived secondary metabolism in defense responses against Bipolaris oryzae infection in rice leaves by a suicide substrate of tryptophan decarboxylase
Authors:Ishihara Atsushi  Nakao Takahito  Mashimo Yuko  Murai Masatoshi  Ichimaru Naoya  Tanaka Chihiro  Nakajima Hiromitsu  Wakasa Kyo  Miyagawa Hisashi
Institution:a Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan
b Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
c Division of Environmental Science and Technology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
d Department of Agriculture, Tokyo University of Agriculture, 1737 Funako, Atsugi 243-0034, Japan
Abstract:Tryptophan-derived secondary metabolites, including serotonin and its hydroxycinnamic acid amides, markedly accumulate in rice leaves in response to pathogen attack. These compounds have been implicated in the physical defense system against pathogen invasion by being deposited in cell walls. Serotonin is biosynthesized from tryptophan via tryptamine, and tryptophan decarboxylase (TDC) catalyzes the first committed reaction. In this study, (S)-α-(fluoromethyl)tryptophan (S-αFMT) was utilized to investigate the effects of the inhibition of TDC on the defense responses of rice leaves. S-αFMT, enantiospecifically synthesized from l-tryptophan, effectively inhibited TDC activity extracted from rice leaves infected by Bipolaris oryzae. The inhibition rate increased dependently on the incubation time, indicating that S-αFMT served as a suicide substrate. Treatment of rice seedlings with S-αFMT suppressed accumulation of serotonin, tryptamine, and hydroxycinnamic acid amides of serotonin in a dose-dependent manner in B. oryzae-inoculated leaves. The lesions formed on seedlings treated with S-αFMT lacked deposition of brown materials, and those leaves were severely damaged in comparison with leaves without S-αFMT treatment. Administrating tryptamine to S-αFMT-treated leaves restored accumulation of tryptophan-derived secondary metabolites as well as deposition of brown material. In addition, tryptamine administration reduced damage caused by fungal infection. Accordingly, the accumulation of tryptophan-derived secondary metabolites was suggested to be part of the effective defense mechanism of rice.
Keywords:Rice  Oryza sativa  Gramineae  Tryptophan decarboxylase  Inhibitor  α-Fluoromethyltryptophan  Serotonin  Physical defense  Secondary metabolism
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