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One of Two Major Paralogs of AVR-Pita1 Is Functional in Japanese Rice Blast Isolates

First, second, third, and fourth authors: National Agriculture and Food Organization, National Agricultural Research Center, Hokuriku Research Center, 1-2-1 Inada, Joetsu, Niigata, 943-0193, Japan; fifth author: Research Faculty of Agriculture, Hokkaido University, Sapporo, Hokkaido, 060-8589, Japan; sixth author: National Agriculture and Food Organization, National Institute of Vegetable and Tea Science, Shimada, Shizuoka, 428-8501, Japan; seventh author: National Institute for Agro-Environmental Sciences, Kannondai, Tsukuba, Ibaraki, 305-8604, Japan; eighth author: Shimane Agricultural Technology Center, Ashiwata 2440, Izumo, Shimane, 693-0035, Japan; ninth author: Niigata Agricultural Research Institute, 857 Nagakura, Nagaoka, Niigata, 940-0826, Japan; and tenth author: National Agriculture and Food Organization, National Agricultural Research Center for Kyushu Okinawa Region, 2421 Suya, Koshi, Kumamoto 861-1192, Japan.

We analyzed the avirulence gene AVR-Pita1 in Japanese rice blast isolates to determine how they gain virulence toward rice cultivars containing the Pita resistance gene. An avirulent isolate, OS99-G-7a (G7a), from a Japanese commercial field contained two paralogs of AVR-Pita1, designated as AVR-Pita1^JA and AVR-Pita1^JB. Analysis of virulent, independent mutants derived from G7a, a single avirulent progenitor strain, indicated that AVR-Pita1^JA was functional but AVR-Pita1^JB was nonfunctional. The most frequent mutation was loss of AVR-Pita1^JA. Analyses of field isolates collected from diverse areas in Japan revealed that most of the AVR-Pita1 genes carried by Japanese isolates were identical to AVR-Pita1^JA or AVR-Pita1^JB. The relationship between these major paralogs in Japanese isolates and the virulence of the strains carrying them indicate that AVR-Pita1^JA is functional but AVR-Pita1^JB is not, as is the case in G7a. Isolates that show virulence toward rice cultivars containing the Pita gene are presumed to have evolved virulence from avirulent origins via loss of AVR-Pita1^JA, except for one case in which virulence resulted from a base substitution. In this study, we discuss the properties and specificities of Japanese rice blasts that relate to virulence against Pita-containing rice. Furthermore, we present a method to amplify AVR-Pita1^JA and AVR-Pita1^JB separately and, specifically, to monitor functional AVR-Pita1 in Japan.

Additional keywords:Magnaporthe oryzae, Pyricularia oryzae.