Elucidation of the molecular host-pathogen interactions that influence seed-to-seedling transmission of Acidovorax citrulli
Johnson, Kameka Latoya
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Bacterial fruit blotch (BFB) caused by the seedborne bacterium, Acidovorax citrulli is a serious economic threat to cucurbit production worldwide. Bacterial fruit blotch occurs sporadically, but it can cause up to 100% loss under ideal field conditions. Gaining a better understanding of the biology of BFB could potentially lead to more effective management strategies. Type III secretion (T3S), type II secretion (T2S) and quorum sensing (QS) are involved in bacterial pathogenicity. The role of these systems in A. citrulli pathogenicity and, more importantly, seed colonization and BFB seedling transmission was investigated using the sequenced strain AAC00-1. T3S is important for pathogenesis and facilitates the translocation of virulence proteins directly into the plant host cell. A type three secretion system (T3SS) mutant of A. citrulli was non-pathogenic on watermelon seedlings but still grew at wild type levels on seed. When used as a biocontrol blossom protectant the T3SS mutant reduced seed infestation by A. citrulli. Additionally when applied as a seed treatment to naturally infested seed, the nonpathogenic strain reduced seedling transmission by 37% in the greenhouse. In this study, we found that A. citrulli has a functional type two secretion system (T2SS) which is important for colonization of seed and seed-to-seedling transmission of BFB. An A. citrulli type two secreted endoglucanase was also found to be important for seed colonization, but xylanase and pectate lyase were not required. A. citrulli encodes the quorum sensing homologs, accI (acyl homoserine lactone synthase) and accR (transcriptional regulator). Unlike the aacR mutant, the aacI mutant of AAC00-1 was reduced in virulence and in its ability to be transmitted from seed to seedlings. Interestingly both mutants colonized watermelon seed at wild-type rates.