The Molecular Mechanism Of Xanthomonas Type Ⅲ Effector AvrAC To Regulate Plant Innate Immunity

Plant innate immunity is activated upon the perception of conservedPathogen-Associated Molecular Patterns (PAMPs) at the cell surface or ofpathogen effector proteins inside the plant cell. Together, PAMP-triggeredimmunity (PTI) and effector-triggered immunity (ETI) constitute powerfuldefenses against various phytopathogens. A hallmark of many virulentphytopathogenic bacteria is their ability to deliver a plethora of effectorproteins into host cell by conversed type Ⅲ secretion system to promoteparasitism. While a number of effector proteins have been shown to inhibithost immunity, the biochemical function and molecular basis remainsunknown for the vast majority of these effectors. Therefore, identification ofplant targets and research of their interactions will make us know more aboutthe mechanisms of bacterial virulence and plant resistance.Xanthomonas campestris pv campestris (Xcc) is a causal agent of blackrot diseases on numerous crucifer plants such as Brassica and Arabidopsis inthe world. The type Ⅲ effector protein AvrAC exists in all three sequencedstrains of Xcc. Using the Arabidopsis-Xcc as a model system, our researchdemonstrates that AvrAC plays a role in virulence in mesophyll tissues ofArabidopsis. It can strongly inhibit plant PTI and ETI and contribute to Xccvirulence in Arabidopsis by specifically targeting Arabidopsis BIK1andRIPK, two receptor-like cytoplasmic kinases (RLCKs) known to mediateimmune signaling. Further analysis shows that AvrAC is an uridylyltransferase that adds UMP to and masks conserved phosphorylation sites inthe activation loop of BIK1and RIPK. This leads to reduced kinase activity ofBIK1and RIPK and consequently inhibits downstream signaling.In vascular tissue of Arabidopsis ecotype Col-0, AvrAC also can berecognized by some kind of protein to trigger a weak ETI. Our results indicatethat AvrAC induced resistence is partially dependent on RAR1and EDS1, twoimportant components respectively involved in R protein stability and ETIsignaling pathway. AvrAC has ability to interact with ATG8family genesinvolved in autophagosome fromation, which suggests that avirulence function of AvrAC maybe relates to plant cell autophagy process.AvrAC is the only effector protein possesses uridylyl transferase activityamong all sequenced phytopathogenic bacteria at present. The work describedhere illustrates a unique biochemical mechanism by which the Xcc bacteriummodulates the plant innate immune system.