Identification of Six Type III Effector Genes with the PIP Box in Xanthomonas campestris pv. campestris and Five of Them Contribute Individually to Full Pathogenicity

Abstract

Xanthomonas campestris pv. campestris is the pathogen of black rot of cruciferous plants. The pathogenicity of the pathogen depends on the type III secretion system (T3SS) that translocates directly effector proteins into plant cells, where they play important roles in the molecular interaction between the pathogen and its hosts. The T3SS of Xanthomonas spp. is encoded by a cluster of hypersensitive response and pathogenicity (hrp) genes. It has been demonstrated that the expression of hrp genes and some type III secreted (T3S)-effector genes is coactivated by the key hrp regulatory protein HrpX. The regulation by HrpX can be mediated by the binding of HrpX protein to a cis-regulatory element named the plant-inducible promoter (PIP) box present in the promoter region of HrpX-regulated genes. A genome screen revealed that X. campestris pv. campestris 8004 possesses 56 predicted genes with the PIP box. Nine of these genes have been shown to encode T3S effectors, Hrp, and Hrp-associated proteins. In this study, we employed an established T3S effector translocation assay with the hypersensitive-reaction-inducing domain of X. campestris pv. campestris AvrBs1 as a reporter to characterize the remaining 47 genes with the PIP box and showed that 6 of them, designated as XopXccE1, XopXccP, XopXccQ, XopXccR1, XopXccLR, and AvrXccB, harbor a functional translocation signal in their N-terminal regions, indicating that they are T3S effectors of X. campestris pv. campestris. We provided evidence to demonstrate that all these effectors are expressed in an HrpX-dependent manner and their translocation into plant cells relies on the translocon protein HrpF and the chaperone HpaB. Mutational analyses demonstrated that all these effectors, except AvrXccB, are individually required for full virulence and growth of X. campestris pv. campestris in the host plant Chinese radish.

High-throughput genomic sequencing of cassava bacterial blight strains identifies conserved effectors to target for durable resistance

“…HrpF is a putative T3E translocon protein although its role in virulence appears to be pathogen specific (36). XopAE and XopL both contain a leucine-rich repeat (LRR) domain and the former has been shown to be required for full virulence in Xanthomonas glycines (37,38). XopE1 is a member of the HopX family of effectors that contain a conserved putative N-myristoylation motif (38).…”

“…XopAE and XopL both contain a leucine-rich repeat (LRR) domain and the former has been shown to be required for full virulence in Xanthomonas glycines (37,38). XopE1 is a member of the HopX family of effectors that contain a conserved putative N-myristoylation motif (38). XopN has been shown to play a role in virulence and is thought to suppress basal defenses (39).…”

Ketoglutarate Transport Protein KgtP Is Secreted through the Type III Secretion System and Contributes to Virulence in Xanthomonas oryzae pv. oryzae

“…2C and 3B). This pattern of regulation follows the established regulatory cascade in which HrpG regulates the expression of hrpX and HrpX activates the expression of the T3SS and some T3SS effectors by binding to the PIP box (13,19,22). However, whether HrpX directly activates the expression of kgtP via the imperfect PIP box needs to be validated experimentally.…”

Comparative Genomic Analysis of Xanthomonas axonopodis pv. citrumelo F1, Which Causes Citrus Bacterial Spot Disease, and Related Strains Provides Insights into Virulence and Host Specificit

“…The homologs of effectors xopL, xopP, and xopQ have been shown to contribute to pathogenicity in X. campestris pv. campestris (49). Both xopX and xopZ potentially interfere with host innate immunity, thus making the plant more susceptible.…”