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Characterization of cucurbit yellow vine disease strains of Serratia marcescens using whole genome sequencing

Elizabeth Little: University of Georgia, Department of Plant Pathology

<div>Cucurbit yellow vine disease (CYVD), caused by the bacterium <em>Serratia marcescens</em> and transmitted by squash bugs (<em>Anasa tritis</em>), results in wilting and death of cucurbits. The disease was first observed in 1988 and is now endemic in the eastern U.S. Up to 100% losses have occurred in Georgia. Mechanisms of pathogenicity are unknown. Previous work determined CYVD strains of <em>S. marcescens </em>are genetically distinct. To characterize strain differences, whole genome sequencing was performed on the DNA from diverse <em>S. marcescens</em> strains and the aligned sequences compared with <em>S. marcescens</em> sequences in the NCBI database. CYVD strain sequences were most similar to each other (>99.97%) and to two endophytic strains R01-A and 90-166 (98.2% and 97.7%, respectively). The remaining non-CYVD strains were more genetically distant. CYVD strains have a reduced genome (15-17% reduction) compared with non-CYVD strain R01-A. Several predicted genes in the CYVD sequence deletions encode proteins involved in substrate metabolism, explaining the reduced metabolic capability of CYVD strains. Possible pathogenicity proteins were identified in the CYVD unique sequences such as polysaccharide synthesis and toxin production. CYVD strains contain a Type VI Secretion System (T6SS) of unknown function, but not virulence-type T2SS, T3SS, or T4SS common in pathogenic bacteria. Results of this study help explain how this new plant pathogen evolved from a non-pathogenic endophyte.</div>