DIVERGENCE OF ACIDOVORAX CITRULLI INTO THREE LINEAGES BASED ON TYPE III SECRETED EFFECTOR SEQUENCE ANALYSIS

Noam Eckshtain-Levi ¹ Tamar Munitz ¹ Marija Živanović ² Sy Traore ³ Cathrin Spröer ⁴ Bingyu Zhao ³ Gregory Welbaum ³ Ron Walcott ² Johannes Sikorski ⁴ Saul Burdman ¹

¹Department of Plant Pathology and Microbiology, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot
²Department of Plant Pathology, The University of Georgia, Athens
³Department of Horticulture, Virginia Polytechnic Institute, Blacksburg
⁴German Collection of Microorganisms and Cell Cultures, Leibniz Institute DSMZ, Braunschweig

Bacterial fruit blotch (BFB) is a threatening disease of cucurbits, caused by the Gram-negative bacterium Acidovorax citrulli. The disease gained importance after devastating outbreaks in watermelon fields in the US during the 1980s'. Recently, serious economic losses due to BFB have been reported for other cucurbits in many parts of the world. Today, there are not efficient means to cope with BFB, which represents a serious threat to the cucurbit industry. According to host range, and biochemical and genetic features, at least two groups exist within this species: group I includes strains that were mainly isolated from non-watermelon hosts, while group II includes watermelon strains. As many Gram-negative plant pathogenic bacteria, A. citrulli requires a type III secretion system (T3SS) for pathogenicity. The T3SS injects effector proteins directly into the cytosol of the plant cells, which collectively allows the pathogen manipulating the host cellular activities to its own benefit. Due to the important role of these effectors in pathogenicity, we hypothesized that the observed group host preferential association is, at least partially, influenced by their effector repertoire. In this study we cloned and sequenced the effector genes from 22 A. citrulli strains isolated from different host plants and geographic locations. Comparative analyses of the effector genes revealed that these genes cluster according to the group I/II classification. Moreover, our analyses, combined with additional experimental evidence, lead to the identification of a third group of A. citrulli strains. Currently, we are investigating selected effectors to assess their contribution to virulence and host preferential association