Characterization of Effector Genes in Acidovorax citrulli the Causing Agent of Bacteria Fruit Blotch Disease of Cucurbits

Bacterial fruit blotch (BFB) of cucurbits is caused by Acidovorax citrulli, a Gram-negative seedborne bacterium that can cause up to 100% fruit yield losses in the field. Currently, BFB is a major problem for the cucurbits industry worldwide. Thus far, attempts to identify resistance in cucurbit germplasm for controlling BFB have been unsuccessful. Despite the importance of the disease, little is known about the molecular mechanisms of A. citrulli pathogenicity, due to a lack of molecular tools for studying the A. citrulli/cucurbit interaction. The genomic sequence of A. citrulli strain AAC00-1 has been determined, and the components of type III secretion system have been identified. The goal of this research was to develop molecular tools for studying the BFB disease. Nineteen putative type III effector genes were cloned from two representative A. citrulli strains (AAC00-1 and M6). The distribution of 19 type III effectors among A. citrulli strains, collected worldwide, was studied. A novel Gateway-compatible binary vector was developed for transient expression of A. citrulli type III effectors genes in planta. A set of modified vectors for marker-exchange mutagenesis in A. citrulli were constructed. The model plant species Nicotiana benthamiana was found to be susceptible to A. citrulli, while Nicotiana tabacum was resistance to A. citrulli, so therefore could carry nonhost resistance genes. Two T3S effectors, Aave1548 and Aave2166, triggered water soaking-like cell death in N. benthamiana, but HR-like cell death in N. tabacum. Bacterial mutagenesis and in planta disease assay confirmed that both Aave1548 and Aave2166 have significant virulence contributions to A. citrulli in N. benthamiana plant and melon seeds. Aave2166 encodes a putative acetyltransferase that belongs to the YopJ super family, which is conserved in both animal and plant pathogenic bacteria. Wild type but not the putative catalytic mutant (C232A) of Aave2166 can trigger cell death phenotype in N. benthamiana and N. tabacum. N. benthamiana yeast two-hybrid cDNA library screening using Aave2166 identified six N. benthamiana proteins/peptides which specifically interacted with Aave2166. Further characterization of these Aave2166 interactors may allow us to understand the virulence mechanism provided by Aave2166. The identification of nonhost resistance genes that can recognize Aave2166 and other type III effectors may help to develop novel strategies to control BFB disease of cucurbit.