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SYSTEMS BIOLOGY, MOLECULAR BIOLOGY, AND GENE REGULATION

Contribution of Ethylene Biosynthesis for Resistance to Blast Fungus Infection in Young Rice Plants^1,[OA]

National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305–8602, Japan (T.I., S.S., Y.O.); Miyagi Prefectural Agriculture and Horticulture Research Center, Takadate-kawakami, Natori, Miyagi 981–1243, Japan (T.I.); and National Agriculture and Food Research Organization, Tsukuba, Ibaraki 305–8518, Japan (A.M.)

The role of ethylene (ET) in resistance to infection with blast fungus (Magnaporthe grisea) in rice (Oryza sativa) is poorly understood. To study it, we quantified ET levels after inoculation, using young rice plants at the four-leaf stage of rice cv Nipponbare (wild type) and its isogenic plant (IL7), which contains the Pi-i resistance gene to blast fungus race 003. Small necrotic lesions by hypersensitive reaction (HR) were formed at 42 to 72 h postinoculation (hpi) in resistant IL7 leaves, and whitish expanding lesions at 96 hpi in susceptible wild-type leaves. Notable was the enhanced ET emission at 48 hpi accompanied by increased 1-aminocyclopropane-1-carboxylic acid (ACC) levels and highly elevated ACC oxidase (ACO) activity in IL7 leaves, whereas only an enhanced ACC increase at 96 hpi in wild-type leaves. Among six ACC synthase (ACS) and seven ACO genes found in the rice genome, OsACS2 was transiently expressed at 48 hpi in IL7 and at 96 hpi in wild type, and OsACO7 was expressed at 48 hpi in IL7. Treatment with an inhibitor for ACS, aminooxyacetic acid, suppressed enhanced ET emission at 48 hpi in IL7, resulting in expanding lesions instead of HR lesions. Exogenously supplied ACC compromised the aminooxyacetic acid-induced breakdown of resistance in IL7, and treatment with 1-methylcyclopropene and silver thiosulfate, inhibitors of ET action, did not suppress resistance. These findings suggest the importance of ET biosynthesis and, consequently, the coproduct, cyanide, for HR-accompanied resistance to blast fungus in young rice plants and the contribution of induced OsACS2 and OsACO7 gene expression to it.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Yuko Ohashi (yohashi{at}affrc.go.jp).

^[OA] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.085258

^* Corresponding author; e-mail yohashi{at}affrc.go.jp; fax 81–298–38–7469.

Received June 16, 2006; accepted September 5, 2006; published September 29, 2006.

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