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

• Published in: Phytochemistry (Oxford) Vol. 72; no. 1; pp. 7 - 13
• Main Authors: Ishihara, Atsushi, Nakao, Takahito, Mashimo, Yuko, Murai, Masatoshi, Ichimaru, Naoya, Tanaka, Chihiro, Nakajima, Hiromitsu, Wakasa, Kyo, Miyagawa, Hisashi
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 2011; Elsevier
• Subjects: Aromatic Amino Acid Decarboxylase Inhibitors; Ascomycota - pathogenicity; Biological and medical sciences; Chemical constitution; Fundamental and applied biological sciences. Psychology; Gramineae; Inhibitor; Molecular Structure; Oryza - drug effects; Oryza - enzymology; Oryza - growth & development; Oryza - microbiology; Oryza sativa; Physical defense; Plant Diseases - microbiology; Plant Leaves - microbiology; Plant physiology and development; Rice; Secondary metabolism; Seedlings - growth & development; Seedlings - metabolism; Serotonin; Stereoisomerism; Tryptamines - metabolism; Tryptophan - analogs & derivatives; Tryptophan - pharmacology; Tryptophan - physiology; Tryptophan decarboxylase; α-Fluoromethyltryptophan; Oryza sativa; Physical defense; Serotonin; Gramineae; α-Fluoromethyltryptophan; Tryptophan decarboxylase; Inhibitor; Secondary metabolism; Rice; Monocotyledones; Lyases; Suicide; Cereal crop; Fungi; Carboxy-lyases; Angiospermae; Fungi Imperfecti; Enzyme; Chemical structure; Tryptophan; Plant leaf; Bipolaris oryzae; Infection; Carbon-carbon lyases; Aminoacid; Plant origin; Neurotransmitter; Aromatic-L-amino-acid decarboxylase; Spermatophyta
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/j.phytochem.2010.11.001

The treatment of rice seedlings with the tryptophan decarboxylase inhibitor, S-α-(fluoromethyl)tryptophan ( S-αFMT), reduced the deposition of brown material derived from serotonin and enhanced susceptibility to Bipolaris oryzae infection. [Display omitted] ► S-α-(Fluoromethyl)tryptophan ( S-αFMT) inhibited tryptophan decarboxylase from rice. ► S-αFMT treatment of the rice seedlings reduced deposition of brown material. ► Treated leaves showed enhanced susceptibility to Bipolaris oryzae infection. ► Tryptamine metabolism is part of the effective defense mechanism of rice. ► S-αFMT is a workable tool for analysis of tryptamine metabolism. 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.