Essential Virulence Gene Activates Host Immune Response against Pathogen.

Bacterial fruit blotch (BFB) caused by (Ac) is a devastating watermelon disease that severely impacts the global watermelon industry. Like other Gram-negative bacteria, the type three secretion system (T3SS) is the main pathogenicity factor of . The T3SS apparatus gene codes for the Hrp pilus and serves as a conduit to secret effector proteins into host cells. In this study, we found that the deletion of in results in the loss of pathogenicity on hosts and the hypersensitive response on non-hosts. In addition, the mutant showed a reduction in in vitro growth, colonization, swimming and twitching motility, and displayed increases in biofilm formation ability compared to the wild type. However, when HrpE was transiently expressed in hosts, the defense responses, including reactive oxygen species bursts, callose deposition, and expression of defense-related genes, were activated. Thus, the growth in HrpE-pretreated hosts was suppressed. These results indicated that HrpE is essential for virulence but can also be used by hosts to help resist . Our findings provide a better understanding of the T3SS pathogenesis in , thus providing a molecular basis for biopesticide development, and facilitating the effective control of BFB.