Xanthomonas axonopodis pv. citri requires biofilm formation for canker development.

SICILIANO F.; RIGANO L.; ENRIQUE R.; SENDÍN L.; FILIPPONE P.; TORRES P.; CASTAGNARO A.P.; VOJNOV A.A.; MARANO M.R.

Sorrento, Italia

Congreso; XIII International Congress on Molecular Plant-Microbe Interactions.; 2007

Molecular Plant-Microbe Interactions

The phytopathogenic bacterium Xanthomonas axonopodis pv. citri is responsible for the canker disease affecting citrus plants throughout the world. When Xac colonizes fruit, leaf and stem apoplast, it induces the formation of pustules, which are large bacterial aggregates that later develop into cankers. We postulate that these aggregates exhibit the general characteristics of microbial biofilms. We used inverted confocal laser scanning microscope (CLSM) to analyze the biofilm structure formed by GFP-labeled Xac cells on different solid surfaces. Using wild-type Xac, we were able to observe complex biofilm formation for the first time in minimum medium. Such medium was required suggesting that nutrient status is a critical factor in biofilm development. Extracellular polysaccharides play crucial roles in biofilm formation in a number of symbiotic and pathogenic bacteria. We investigated the role of xanthan gum in the initial steps leading to canker disease: bacterial attachment, biofilm formation, and leaf colonization. The xanthan-deficient gum mutant strains were markedly reduced in their ability to live and colonize lemon leaves. We found that production of xanthan is not necessary for initial bacteria attachment but is required for subsequent three-dimensional biofilm development on abiotic and biotic surfaces leading to canker development. In addition, the canker phenotype was only restored by the addition of homologous EPS. This observation provides further confirmation that xanthan is required for biofilm development and that these surface-associated multicellular assemblages are the cellular base of the canker development.