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The hrpX and hrpG play important roles in virulence of Acidovorax citrulli, the causal agent of bacterial fruit blotch of cucurbits.

Tingchang Zhao: Institute of Plant Protection, CAAS

<div><em>Acidovorax citrulli</em> causes bacterial fruit blotch, a global threat to watermelon and melon production. Despite its economic importance, relatively little is known about the molecular mechanisms of <em>A. citrulli</em> pathogenicity and virulence. Like other foliar plant pathogenic bacteria, <em>A. citrulli </em>relies on a type III secretion system (T3SS) for pathogenicity. Based on sequence analysis and operon arrangement, <em>A. citrulli</em> has a class II <em>hrp</em> gene cluster, similar to those of <em>Xanthomonas</em> and <em>Ralstonia</em> spp. For class II <em>hrp</em> clusters,<em> hrpG</em> and <em>hrpX</em> play key roles in the regulation of T3S effectors (T3Es). However, little is known about the regulation of T3SS in <em>A. citrulli</em>. The objective of the study was to investigate the roles of <em>hrpG </em>and <em>hrpX</em> in <em>A. citrulli. </em>We found that when <em>hrpG</em> or<em> hrpX</em> was deleted in the <em>A. citrulli</em> group II strain Aac5, both mutants completely lost pathogenicity on watermelon seedlings, failed to induce a hypersensitive response on tobacco, and elicited higher levels of reactive oxygen species in <em>Nicotiana benthamiana</em> compared to the wildtype strain. We also demonstrated that HrpG activates HrpX in <em>A. citrulli</em>. Moreover, the expression of the T3E, <em>Aave_2166</em> was suppressed in transcription and translation in <em>hrpG</em> and <em>hrpX</em> mutants. Notably, <em>hrpG </em>and <em>hrpX</em> appeared to modulate biofilm formation. These results suggest that <em>hrpG</em> and <em>hrpX</em> are essential for pathogenicity, regulation of T3Es, and biofilm formation in <em>A. citrulli</em>.</div>