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Identification and Functional Analysis of Type III Effector Proteins in Mesorhizobium loti

February 2010 , Volume 23 , Number  2
Pages  223 - 234

Shin Okazaki,1 Saori Okabe,1 Miku Higashi,1 Yoshikazu Shimoda,2 Shusei Sato,2 Satoshi Tabata,2 Masatsugu Hashiguchi,3 Ryo Akashi,3 Michael Göttfert,4 and Kazuhiko Saeki1

1Department of Biological Sciences, Faculty of Science, Nara Women's University, Nara 630-8506, Japan; 2Kazusa DNA Research Institute, 2-6-7 Kazusa-kamatari, Kisarazu, Chiba 292-0818, Japan; 3Frontier Science Research Center, University of Miyazaki, 1-1, Gakuen Kibanadai-nishi, Miyazaki-shi, 889-2192, Japan; 4Institute of Genetics, Dresden University of Technology, Helmholtzstraße 10, 01069 Dresden, Germany

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Accepted 21 September 2009.

Mesorhizobium loti MAFF303099, a microsymbiont of the model legume Lotus japonicus, possesses a cluster of genes (tts) that encode a type III secretion system (T3SS). In the presence of heterologous nodD from Rhizobium leguminosarum and a flavonoid naringenin, we observed elevated expression of the tts genes and secretion of several proteins into the culture medium. Inoculation experiments with wild-type and T3SS mutant strains revealed that the presence of the T3SS affected nodulation at a species level within the Lotus genus either positively (L. corniculatus subsp. frondosus and L. filicaulis) or negatively (L. halophilus and two other species). By inoculating L. halophilus with mutants of various type III effector candidate genes, we identified open reading frame mlr6361 as a major determinant of the nodulation restriction observed for L. halophilus. The predicted gene product of mlr6361 is a protein of 3,056 amino acids containing 15 repetitions of a sequence motif of 40 to 45 residues and a shikimate kinase-like domain at its carboxyl terminus. Homologues with similar repeat sequences are present in the hypersensitive-response and pathogenicity regions of several plant pathogens, including strains of Pseudomonas syringae, Ralstonia solanacearum, and Xanthomonas species. These results suggest that L. halophilus recognizes Mlr6361 as potentially pathogen derived and subsequently halts the infection process.

© 2010 The American Phytopathological Society