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Comparative Analysis of Type III Secreted Effector Genes Reflects Divergence of Acidovorax citrulli Strains into Three Distinct Lineages

First, second, and tenth authors: Department of Plant Pathology and Microbiology and the Otto Warburg Center for Agricultural Biotechnology, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel; third and eighth authors: Department of Plant Pathology, The University of Georgia, Athens; fourth, sixth, and seventh authors: Department of Horticulture, Virginia Polytechnic Institute, Blacksburg; and fifth and ninth authors: Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures (DSMZ), Braunschweig, Germany.

Acidovorax citrulli causes bacterial fruit blotch of cucurbits, a serious economic threat to watermelon (Citrullus lanatus) and melon (Cucumis melo) production worldwide. Based on genetic and biochemical traits, A. citrulli strains have been divided into two distinct groups: group I strains have been mainly isolated from various non-watermelon hosts, while group II strains have been generally isolated from and are highly virulent on watermelon. The pathogen depends on a functional type III secretion system for pathogenicity. Annotation of the genome of the group II strain AAC00-1 revealed 11 genes encoding putative type III secreted (T3S) effectors. Due to the crucial role of type III secretion for A. citrulli pathogenicity, we hypothesized that group I and II strains differ in their T3S effector repertoire. Comparative analysis of the 11 effector genes from a collection of 22 A. citrulli strains confirmed this hypothesis. Moreover, this analysis led to the identification of a third A. citrulli group, which was supported by DNA:DNA hybridization, DNA fingerprinting, multilocus sequence analysis of conserved genes, and virulence assays. The effector genes assessed in this study are homologous to effectors from other plant-pathogenic bacteria, mainly belonging to Xanthomonas spp. and Ralstonia solanacearum. Analyses of the effective number of codons and gas chromatography content of effector genes relative to a representative set of housekeeping genes support the idea that these effector genes were acquired by lateral gene transfer. Further investigation is required to identify new T3S effectors of A. citrulli and to determine their contribution to virulence and host preferential association.