Rice xa13 Recessive Resistance to Bacterial Blight Is Defeated by Induction of the Disease Susceptibility Gene Os-11N3    

Abstract

The rice (Oryza sativa) gene xa13 is a recessive resistance allele of Os-8N3, a member of the NODULIN3 (N3) gene family, located on rice chromosome 8. Os-8N3 is a susceptibility (S) gene for Xanthomonas oryzae pv oryzae, the causal agent of bacterial blight, and the recessive allele is defeated by strains of the pathogen producing any one of the type III effectors AvrXa7, PthXo2, or PthXo3, which are all members of the transcription activator-like (TAL) effector family. Both AvrXa7 and PthXo3 induce the expression of a second member of the N3 gene family, here named Os-11N3. Insertional mutagenesis or RNA-mediated silencing of Os-11N3 resulted in plants with loss of susceptibility specifically to strains of X. oryzae pv oryzae dependent on AvrXa7 or PthXo3 for virulence. We further show that AvrXa7 drives expression of Os-11N3 and that AvrXa7 interacts and binds specifically to an effector binding element within the Os-11N3 promoter, lending support to the predictive models for TAL effector binding specificity. The result indicates that variations in the TAL effector repetitive domains are driven by selection to overcome both dominant and recessive forms of resistance to bacterial blight in rice. The finding that Os-8N3 and Os-11N3 encode closely related proteins also provides evidence that N3 proteins have a specific function in facilitating bacterial blight disease.

Lateral organ boundaries 1 is a disease susceptibility gene for citrus bacterial canker disease

“…The arrangement is similar to the Os11N3 promoter in rice, which contains EBE PthXo3 in front of the EBE AvrXa7 (24). The difference between PthXo3 and AvrXa7 has been postulated to result from the avoidance of triggering incompatibility in rice lines with the R gene Xa7.…”

“…oryzae, and strains that depend on either PthXo1 or AvrXa7 for full virulence and cannot induce either Os8N3 or Os11N3 due to host mutations or suppression of host gene expression are weakly virulent. Os8N3 and Os11N3 products are not related to upa20, and both are closely related members of a family of sugar transporters (23)(24)(25). Different TAL effectors can induce the same gene in the host.…”

“…In rice, Os11N3 is induced by any of three TAL effectors from Xanthomonas oryzae pv. oryzae, AvrXa7 and PthXo3, or TalC (24,28). An S gene with minor effects on susceptibility in bacterial blight of rice encodes a bZIP transcription factor and is the target of yet another TAL effector, PthXo6 (29).…”

HrcT Is a Key Component of the Type III Secretion System in Xanthomonas spp. and Also Regulates the Expression of the Key hrp Transcriptional Activator HrpX

“…To examine whether or not HrcT binds the hrpX promoter in vivo, a 300-bp promoter of hrpX (300 bp upstream of the hrpX translational start codon) was fused to a promoterless gusA in a vector, pBI121, that was transferred into tobacco leaves (N. benthamiana) mediated by Agrobacterium (37). The hrcT ORF was then fused under the CaMV 35S promoter in a vector, pCAM-BIA1300, that was also used for transient expression of a tested gene in tobacco mediated by Agrobacterium (37). The primers for the above-described constructs were pXF/pXR and TF/TR (see Table S1 in the supplemental material), respectively.…”

“…The PCR-amplified fragments were digested with HindIII and XbaI and cloned into HindIII/XbaI sites in pBI121 or pCAMBIA1300, respectively, generating phrpX and HrcT (Table 1). The Agrobacterium-mediated transient expression assays were performed as described previously (37,38). For the control, the plasmids PthXo1 containing pthXo1 gene in pCAMBIA1300 (37) and pOs8N3 harboring the Os8N3 promoter (targeted by PthXo1 and fused with the promoterless gusA) in pBI121 (37) ( Table 1) were used.…”