AUTHOR=Zhang Xiaoxiao , Zhao Mei , Yan Jianpei , Yang Linlin , Yang Yuwen , Guan Wei , Walcott Ron , Zhao Tingchang 

TITLE=Involvement of hrpX and hrpG in the Virulence of Acidovorax citrulli Strain Aac5, Causal Agent of Bacterial Fruit Blotch in Cucurbits

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.00507

DOI=10.3389/fmicb.2018.00507

ISSN=1664-302X

ABSTRACT=<p><italic>Acidovorax citrulli</italic> causes bacterial fruit blotch, a disease that poses a global threat to watermelon and melon production. Despite its economic importance, relatively little is known about the molecular mechanisms of pathogenicity and virulence of <italic>A. citrulli</italic>. Like other plant-pathogenic bacteria, <italic>A. citrulli</italic> relies on a type III secretion system (T3SS) for pathogenicity. On the basis of sequence and operon arrangement analyses, <italic>A. citrulli</italic> was found to have a class II <italic>hrp</italic> gene cluster similar to those of <italic>Xanthomonas</italic> and <italic>Ralstonia</italic> spp. In the class II <italic>hrp</italic> cluster, <italic>hrpG</italic> and <italic>hrpX</italic> play key roles in the regulation of T3SS effectors. However, little is known about the regulation of the T3SS in <italic>A. citrulli</italic>. This study aimed to investigate the roles of <italic>hrpG</italic> and <italic>hrpX</italic> in <italic>A. citrulli</italic> pathogenicity. We found that <italic>hrpG</italic> or <italic>hrpX</italic> deletion mutants of the <italic>A. citrulli</italic> group II strain Aac5 had reduced pathogenicity on watermelon seedlings, failed to induce a hypersensitive response in tobacco, and elicited higher levels of reactive oxygen species in <italic>Nicotiana benthamiana</italic> than the wild-type strain. Additionally, we demonstrated that HrpG activates HrpX in <italic>A. citrulli</italic>. Moreover, transcription and translation of the type 3-secreted effector (T3E) gene <italic>Aac5_2166</italic> were suppressed in <italic>hrpG</italic> and <italic>hrpX</italic> mutants. Notably, <italic>hrpG</italic> and <italic>hrpX</italic> appeared to modulate biofilm formation. These results suggest that <italic>hrpG</italic> and <italic>hrpX</italic> are essential for pathogenicity, regulation of T3Es, and biofilm formation in <italic>A. citrulli</italic>.</p>