Omsland, Anders

Summary ‘ Candidatus Liberibacter asiaticus ’ is a fastidious bacterium and a putative agent of citrus greening disease (a.k.a., huanglongbing, HLB), a significant agricultural disease that affects citrus fruit quality and tree health. In citrus, ‘ Ca. L. asiaticus ’ is phloem limited. Lack of culture tools to study ‘ Ca. L. asiaticus ’ complicates analysis of this important organism. To improve understanding of ‘ Ca. L. asiaticus ’–host interactions including parameters that affect ‘ Ca. L. asiaticus ’ replication, methods suitable for screening pathogen responses to physicochemical and nutritional variables are needed. We describe a leaf disc‐based culture assay that allows highly selective measurement of changes in ‘ Ca. L. asiaticus ’ DNA within plant tissue incubated under specific physicochemical and nutritional conditions. qPCR analysis targeting the hypothetical gene CD16‐00155 (strain A4) allowed selective quantification of ‘ Ca. L. asiaticus ’ DNA content within infected tissue. ‘ Ca. L. asiaticus ’ DNA replication was observed in response to glucose exclusively under microaerobic conditions, and the antibiotic amikacin further enhanced ‘ Ca. L. asiaticus ’ DNA replication. Metabolite profiling revealed a moderate impact of ‘ Ca. L. asiaticus ’ on the ability of leaf tissue to metabolize and respond to glucose.

AbstractPlant pathogenic bacteria interact with their insect host(s)/vector(s) at the cellular and molecular levels. This interaction may alter the physiology of their insect vector, which may also promote the growth and transmission of the bacterium. Here we studied the effect of “Candidatus Liberibacter asiaticus” (“Ca. L. asiaticus”) on physiochemical conditions within its insect vector, the Asian citrus psyllid (ACP), and whether these changes were beneficial for the pathogen. The local microenvironments inside ACPs were quantified using microelectrodes. The average hemolymph pH was significantly higher in infected ACPs (8.13 ± 0.21) than in “Ca. L. asiaticus”-free ACPs (7.29 ± 0.15). The average hemolymph oxygen tension was higher in “Ca. L. asiaticus”-free ACPs than in infected ACPs (67.13% ± 2.11% vs. 35.61% ± 1.26%). Oxygen tension reduction and pH increase were accompanied by “Ca. L. asiaticus” infection. Thus, oxygen tension of the hemolymph is an indicator of infection status, with pH affected by the severity of the infection.