Effector <scp> <i>C</i> Las0185 </scp> targets methionine sulphoxide reductase <scp>B1</scp> of <i>Citrus sinensis</i> to promote multiplication of ‘ <i>Candidatus</i> Liberibacter asiaticus’ via enhancing enzymatic activity of ascorbate peroxidase 1

Citrus huanglongbing (HLB) has been causing enormous damage to the global citrus industry. As the main causal agent, 'Candidatus Liberibacter asiaticus' (CLas) delivers a set of effectors to modulate host responses, while the modes of action adopted remain largely unclear. Here, we demonstrated that CLIBASIA_00185 (CLas0185) could attenuate reactive oxygen species (ROS)-mediated cell death in Nicotiana benthamiana. Transgenic expression of CLas0185 in Citrus sinensis 'Wanjincheng' enhanced plant susceptibility to CLas. We found that methionine sulphoxide reductase B1 (CsMsrB1) was targeted by the effector, and its abundance was elevated in CLas0185-transgenic citrus plants. Their interaction promoted CLas proliferation. We then determined that CsMsrB1 sustained redox state and enzymatic activity of ascorbate peroxidase 1 (CsAPX1) under oxidative stress. The latter reduced H₂O₂ accumulation and was associated with host susceptibility to CLas infection. Consistently, citrus plants expressing CLas0185 and CsMsrB1 conferred enhanced APX activity and decreased H₂O₂ content. Taken together, these findings revealed how CLas0185 benefits CLas colonization by targeting CsMsrB1, which facilitated the antioxidant activity and depressed ROS during pathogen infection.