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Genomic and Evolutionary Features of the SPI-1 Type III Secretion System That Is Present in Xanthomonas albilineans but Is Not Essential for Xylem Colonization and Symptom Development of Sugarcane Leaf Scald

February 2011 , Volume 24 , Number  2
Pages  246 - 259

Mélanie Marguerettaz,1 Isabelle Pieretti,1 Philippe Gayral,1 Jérôme Puig,1 Chrystelle Brin,2 Stéphane Cociancich,1 Stéphane Poussier,2 Philippe Rott,1 and Monique Royer1

1UMR BGPI CIRAD, Campus International de Baillarguet, TA A-54/K, 34398 Montpellier Cedex 5, France; 2UMR PaVé Agrocampus ouest, BP 60057, 42 rue Georges Morel, 49071 Beaucouzé Cedex, France

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Accepted 12 October 2010.

Xanthomonas albilineans is the causal agent of sugarcane leaf scald. Interestingly, this bacterium, which is not known to be insect or animal associated, possesses a type III secretion system (T3SS) belonging to the injectisome family Salmonella pathogenicity island 1 (SPI-1). The T3SS SPI-1 of X. albilineans shares only low similarity with other available T3SS SPI-1 sequences. Screening of a collection of 128 plant-pathogenic bacteria revealed that this T3SS SPI-1 is present in only two species of Xanthomonas: X. albilineans and X. axonopodis pv. phaseoli. Inoculation of sugarcane with knockout mutants showed that this system is not required by X. albilineans to spread within xylem vessels and to cause disease symptoms. This result was confirmed by the absence of this T3SS SPI-1 in an X. albilineans strain isolated from diseased sugarcane. To investigate the importance of the T3SS SPI-1 during the life cycle of X. albilineans, we analyzed T3SS SPI-1 sequences from 11 strains spanning the genetic diversity of this species. No nonsense mutations or frameshifting indels were observed in any of these strains, suggesting that the T3SS SPI-1 system is maintained within the species X. albilineans. Evolutionary features of T3SS SPI-1 based on phylogenetic, recombination, and selection analyses are discussed in the context of the possible functional importance of T3SS SPI-1 in the ecology of X. albilineans.

© 2011 The American Phytopathological Society