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One Amino Acid Change in Cucumber Mosaic Virus RNA Polymerase Determines Virulent/Avirulent Phenotypes on Cowpea

December 1999 , Volume 89 , Number  12
Pages  1,186 - 1,192

Akira Karasawa , Itaru Okada , Kayoko Akashi , Yutaka Chida , Shuu Hase , Yoshiko Nakazawa-Nasu , Akiko Ito , and Yoshio Ehara

Laboratory of Plant Pathology, Department of Applied Life Science, Graduate School of Agricultural Science, Tohoku University, 1-1, Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981-8555, Japan

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Accepted for publication 5 August 1999.

The elicitation of the hypersensitive response (HR) is known to depend on the interaction between a resistance gene of a host plant and a corresponding avirulence gene of a pathogen. The cv. Kurodane-Sanjaku of cowpea (Vigna unguiculata) has the Cry locus that confers resistance against cucumber mosaic virus strain Y (CMV-Y). The resistance is overcome by infection with a legume strain of CMV (CMV-L). RNA 2, which codes for the 2a protein, a subunit of the viral replicase components, has been known to control virulent/avirulent phenotypes. We generated chimeric constructs of full-length cDNA clones of RNA 2 of both strains and inoculated infectious transcripts to delimit the domain controlling symptoms. A 243-base pair fragment containing a coding region for the GDD RNA-dependent RNA polymerase core sequence was shown to be responsible for the phenotypic differences. From sequence alignment analysis, two amino acids (Phe631 and Ala641) of the HR-type 2a protein encoded in this fragment were specifically exchanged to Tyr and Ser, respectively, in the 2a proteins of resistance-breaking strains. Point mutations introduced into RNA 2 backgrounds of both strains that were designed to change the amino acid at position 631 resulted in a change of symptoms, indicating that a single nucleotide change determines the reactions elicited by both strains. Analysis for one additional mutant RNA 2 showed that symptom determination may be correlated with the nature of the lateral chain of amino acid 631.

© 1999 The American Phytopathological Society