Andrade, Maxuel O.

Candidatus Liberibacter spp. are fastidious α-proteobacteria that cause multiple diseases on plant hosts of economic importance, including the most devastating citrus disease: Huanglongbing (HLB). HLB was reported in Asia a century ago but has since spread worldwide. Understanding the pathogenesis of Candidatus Liberibacter spp. remains challenging as they are yet to be cultured in artificial media and infect the phloem, a sophisticated environment that is difficult to manipulate. Despite those challenges, tremendous progress has been made on Ca. Liberibacter pathosystems. Here, we first reviewed recent studies on genetic information of flagellar and type IV pili biosynthesis, their expression profiles, and movement of Ca. Liberibacter spp. inside the plant and insect hosts. Next, we reviewed the transcriptomic, proteomic, and metabolomic studies of susceptible and tolerant plant genotypes to Ca. Liberibacter spp. infection and how Ca. Liberibacter spp. adapt in plants. Analyses of the interactions between plants and Ca. Liberibacter spp. imply the involvement of immune response in the Ca. Liberibacter pathosystems. Lastly, we reviewed how Ca. Liberibacter spp. movement inside and interactions with plants lead to symptom development.

Summary Citrus huanglongbing (HLB) is the most devastating citrus disease worldwide. ‘ Candidatus Liberibacter asiaticus’ (Las) is the most prevalent HLB causal agent that is yet to be cultured. Here, we analysed the flagellar genes of Las and Rhizobiaceae and observed two characteristics unique to the flagellar proteins of Las: (i) a shorter primary structure of the rod capping protein FlgJ than other Rhizobiaceae bacteria and (ii) Las contains only one flagellin‐encoding gene flaA (CLIBASIA_02090), whereas other Rhizobiaceae species carry at least three flagellin‐encoding genes. Only flgJ Atu but not flgJ Las restored the swimming motility of Agrobacterium tumefaciens flgJ mutant. Pull‐down assays demonstrated that FlgJ Las interacts with FlgB but not with FliE. Ectopic expression of flaA Las in A. tumefaciens mutants restored the swimming motility of ∆ flaA mutant and ∆ flaAD mutant, but not that of the null mutant ∆ flaABCD . No flagellum was observed for Las in citrus and dodder. The expression of flagellar genes was higher in psyllids than in planta . In addition, western blotting using flagellin‐specific antibody indicates that Las expresses flagellin protein in psyllids, but not in planta . The flagellar features of Las in planta suggest that Las movement in the phloem is not mediated by flagella. We also characterized the movement of Las after psyllid transmission into young flush. Our data support a model that Las remains inside young flush after psyllid transmission and before the flush matures. The delayed movement of Las out of young flush after psyllid transmission provides opportunities for targeted treatment of young flush for HLB control.