A type III effector XopL_Xcc8004 is vital for Xanthomonas campestris pathovar campestris to regulate plant immunity

Xanthomonas campestris pv. campestris (Xcc) secretes a suite of effectors into host plants via the type III secretion system (T3SS), modulating plant immunity defenses. Strain Xcc8004 causes black rot in brassica plants, including Arabidopsis thaliana, making it a classical model for the study of Xanthomonas pathogenesis. XopL_Xcc8004 was defined as a T3SS effector (T3SE) since its homologues XopL_Xcv85-10 from Xanthomonas campestris pv. vesicatoria (Xcv85-10) contribute to virulence in host plants. Except for its virulence on Chinese radish plants, little was previously known about the regulation and function of XopL_Xcc8004. Here, we tested the role of XopL_Xcc8004 in the pathogenicity of Xcc8004 on different host plants including Arabidopsis. We found that it was required for full virulence of Xcc8004 in Chinese cabbage. XopL_Xcc8004 promoted bacterial infection in Arabidopsis and suppressed bacterial flagellin (flg22)-induced FRK1 transcription, reactive oxygen species (ROS) burst, callose deposition, and pathogenesis-related marker gene expression, but it did not affect mitogen-activated protein kinases (MAPKs) cascade. Early and prolonged expression of XopL_Xcc8004 affected Arabidopsis growth and development. We demonstrated that XopL_Xcc8004 is a virulence factor and interferes with innate immunity of Arabidopsis by suppressing pathogen-associated molecular pattern-triggered immunity (PTI) signaling, independent of MAPKs.