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Heterochromatin-Like Regions as Ecological Niches for Avirulence Genes in the Leptosphaeria maculans Genome: Map-Based Cloning of AvrLm6

April 2007 , Volume 20 , Number  4
Pages  459 - 470

I. Fudal , 1 S. Ross , 1 L. Gout , 1 , 2 F. Blaise , 1 M. L. Kuhn , 1 M. R. Eckert , 1 L. Cattolico , 3 S. Bernard-Samain , 3 M. H. Balesdent , 1 and T. Rouxel 1

1Phytopathologie et méthodologies de la détection, INRA, F-78026 Versailles, France; 2Protection des Plantes, INA P-G, F-78850 Thiverval-Grignon, France; 3Genoscope-Centre National de Séquençage, 91057 Evry Cedex, France

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Accepted 17 October 2006.

Map-based cloning of avirulence genes of the AvrLm1-2-6 cluster was recently undertaken in Leptosphaeria maculans and led to the identification of AvrLm1. The ensuing chromosome walk toward AvrLm6 resulted in the delineation of a 562-kb bacterial artificial chromosome (BAC) clone contig in an avirulent isolate. Following sequencing of the contig and sequence comparison with a virulent isolate, four AvrLm6 candidate genes were identified. Complementation of the virulent isolate with the four candidates was performed and one gene was found to fully restore the avirulent phenotype on Rlm6 oilseed rape genotypes. AvrLm6 was found to be located in the same genome context as AvrLm1, because it is a solo gene surrounded by 85 and 48 kb of degenerated repeats on its 5′ and 3′ sides, respectively. AvrLm6 is an orphan gene encoding a small, potentially secreted, cysteine-rich protein. Comparison of AvrLm1 and AvrLm6 expressions by quantitative reverse-transcription polymerase chain reaction revealed that both genes are highly overexpressed during primary leaf infection. Using RNA interference, decreasing expression of AvrLm6 was shown to result in virulence toward Rlm6 genotypes whenever the expression was reduced by more than 60% compared with the wild-type isolate.

Additional keywords: Brassica napus, stem canker.

© 2007 The American Phytopathological Society