Publications

4483

ABSTRACT‘CandidatusLiberibacter asiaticus’ (Las) is a gram-negative bacterial pathogen associated with citrus huanglongbing (HLB) or greening disease. Las is transmitted by the Asian citrus psyllid (ACP) where it colonizes the phloem tissue, resulting in substantial economic losses to citrus industry worldwide. Despite extensive efforts, effective management strategies against HLB remain elusive, necessitating a deeper understanding of the pathogen’ s biology. Las undergoes cell-to-cell movement through phloem flow and colonizes different tissues in which Las may have varying interactions with the host. Here, we investigate the transcriptomic landscape of Las in citrus seed coat vasculatures, enabling a complete gene expression profiling of Las genome and revealing unique transcriptomic patterns compared to previous studies using midrib tissues. Comparative transcriptomics between seed coat, midrib and ACP identified specific responses and metabolic states of Las in different host tissue. Two Las virulence factors that exhibit higher expression in seed coat can suppress callose deposition. Therefore, they may contribute to unclogging sieve plate pores during Las colonization in seed coat vasculature. Furthermore, analysis of regulatory elements uncovers a potential role of LuxR-type transcription factors in regulating Liberibacter effector gene expression during plant colonization. Together, this work provides novel insights into the pathogenesis of the devastating citrus HLB.FundingThis work is supported by USDA National Institute of Food and Agriculture award No. 2020-70029-33197 to W.M and A.L.

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.