Publications

4492

The phloem-limited bacterium ‘Candidatus Liberibacter asiaticus’ (CLas) is the putative causal pathogen of the severe Asiatic form of huanglongbing (citrus greening) and is most commonly transmitted by the Asiatic citrus psyllid Diaphorina citri. CLas severely affects many Citrus species and hybrids and has been recorded in the Citrus relative, orange jasmine, Murraya paniculata (L.) Jack (syn. M. exotica L.). In this study, 13 accessions of three Murraya species (M. paniculata, M. sumatrana Roxb., and M. lucida [G.Forst.] Mabb.) and the Papuan form of a putative hybrid (M. omphalocarpa Hayata) were identified morphologically and molecularly based on sequence identity of the matK-5′trnK region of the chloroplast genome, and infection on these plants under field conditions was determined by PCR and quantitative real-time PCR (qPCR) on two to four occasions over 14 months. CLas was repeatedly detected in leaflet midribs by PCR and qPCR on four and three accessions of M. paniculata and M. sumatrana, respectively. It was not detected in leaflet midribs of single accessions of M. lucida and M. omphalocarpa. The species identification of the CLas-positive accessions was further confirmed using all the molecular taxonomic markers consisting of the six fragments of the maternally inherited chloroplast genome and part of the nuclear-encoded internal transcribed spacer (ITS) region. The results indicated that natural infection of M. paniculata and M. sumatrana with CLas can occur in Java. To our knowledge, this is the first demonstration of the natural infection of M. sumatrana with CLas. Further studies are required to determine whether infections persist in the absence of D. citri.

Abstract Citrus huanglongbing (HLB) has been causing enormous damage to the global citrus industry. As the main causal agent, ‘ Candidatus Liberibacter asiaticus’ ( C Las) delivers a set of effectors to modulate host responses, while the modes of action adopted remain largely unclear. Here, we demonstrated that CLIBASIA_00185 ( C Las0185) could attenuate reactive oxygen species (ROS)‐mediated cell death in Nicotiana benthamiana . Transgenic expression of C Las0185 in Citrus sinensis ‘Wanjincheng’ enhanced plant susceptibility to C Las. 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 C Las proliferation. We then determined that CsMsrB1 sustained redox state and enzymatic activity of ascorbate peroxidase 1 (CsAPX1) under oxidative stress. The latter reduced H 2 O 2 accumulation and was associated with host susceptibility to C Las infection. Consistently, citrus plants expressing C Las0185 and CsMsrB1 conferred enhanced APX activity and decreased H 2 O 2 content. Taken together, these findings revealed how C Las0185 benefits C Las colonization by targeting CsMsrB1, which facilitated the antioxidant activity and depressed ROS during pathogen infection.