Acidovorax citrulli is the causal agent of bacterial fruit blotch (BFB) of cucurbits. Serious economic losses due to BFB occurred in watermelon and melon in many parts of the world, and the disease represents a serious threat to the cucurbit industry. A. citrulli requires a type III secretion system (T3SS) for pathogenicity. The T3SS injects effector proteins directly into the plant cell, which collectively allows the pathogen manipulating the host cellular activities to its own benefit. Comparative analyses of effector sequences from various A. citrulli strains combined with experimental data support the existence of at least three groups within this species. Currently we are performing comparative genome analysis between two model strains that represent the two major groups of A. citrulli, M6 (group I) and AAC00-1 (group II). Preliminary results indicate that while the genome size of AAC00-1 is of ~5.3 Mb, the M6 genome size is ~4.8 Mb. Moreover, 8 major genomic segments that are present in the AAC00-1 genome, ranging from ~30 to 120 kb, are absent in M6. We are exploring whether these findings reflect broad differences among the groups. We are also investigating the contribution of type III effector genes in virulence and host preferential association of A. citrulli following mutagenesis of selected genes in combination with appropriate virulence assays. In addition, we are using yeast growth inhibition assays to assess the effect of selected effector genes in yeast growth and survival. We already demonstrated that some effectors affect yeast survival and the strength of this effect is often associated with SNP variation between strains.
