Identification Of HR Inducers Among T3SS Effectors Of Xanthomonas Oryzae PV.Oryzicola

Gram-negative plant phytobacteria utilize a hrp gene cluster to cause pathogenicity in susceptiable host plants and hypersensitive response(HR) in nonhosts or resistance hosts. The gene cluster is responsible for assembling a type Ⅲ secretion system(T3SS), which deliver T3 SS effectors and inject them into host cells. Harpins are hrp-gene encoding protein and secreted by T3 SS into intercellular space of plants. In non-host plant, harpins are recognized by possibly membrane-binding receptors and consequently trigger HR.Except for Hpa1 protein, the SSB from Xanthomonas oryzae has been identified to induce HR in tobacco, showing a similar function as harpin. In this research, we identified the functional domain of SSB required for HR induction. With Agrobacteria-mediated transient expression, wild type SSB and associated truncated or site changed mutants were expressed in Nicotiana benthimiana. Both N and C terminal of SSB exhibited an ability to trigger HR. At N terminal, conserved-NNQER- motif was essential for induction function. The functional C terminal was rich of glycine.In order to explore the HR induction of effectors in X. oryzae pv. oryzicola(Xoc), all 28 predicted T3 SS effectors were experimentally tested for their HR induction by Agrobacteria-mediated transient expression in Nicotiana benthimina. Five effectors XopP, XopAA, Xop L, XopY and AvrXa10 showed the ability to induce HR in tobacco specieces. The last is a TAL effector belonging to avrBs3/PthA family. The first four are nonTAL effectors. The HR induction of AvrXa10 depends on its acidic activation domain at C terminal.Hpa1Xoc triggers HR in N. benthimiana, meanwhile defense-related genes were upregulated. The HR induction of Hpa1 Xoc was suppressed by AvrAC of Xanthomonas campetris pv. campestris 8004 which functions as monophosphate transferase. When co-expressing Hpa1 Xoc and AvrAC in tobacco, both callose deposition and H2O2 accumulation were not found in the overlapping zones. Constitutively expressing avr AC in Xoc genetic background resulted in a reduction of virulence in rice plants and postpone of the hypersensitive response in tobacco. Collectively, these data suggested that avrAC works as a universal suppressor for the defense response.