Involvement of and in the Virulence of Strain Aac5, Causal Agent of Bacterial Fruit Blotch in Cucurbits.

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 . Like other plant-pathogenic bacteria, relies on a type III secretion system (T3SS) for pathogenicity. On the basis of sequence and operon arrangement analyses, was found to have a class II gene cluster similar to those of and spp. In the class II cluster, and play key roles in the regulation of T3SS effectors. However, little is known about the regulation of the T3SS in . This study aimed to investigate the roles of and in pathogenicity. We found that or deletion mutants of the 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 than the wild-type strain. Additionally, we demonstrated that HrpG activates HrpX in . Moreover, transcription and translation of the type 3-secreted effector (T3E) gene were suppressed in and mutants. Notably, and appeared to modulate biofilm formation. These results suggest that and are essential for pathogenicity, regulation of T3Es, and biofilm formation in .