Citrus greening is the most destructive disease of citrus crops worldwide. The introduced Asian citrus psyllid (ACP) Diaphorina citri Kuwayama transmits the (putative) causal bacterium, Candidatus Liberibacter asiaticus. A close relative, Ca. L. solanacearum, is the pathogen associated with Zebra chip disease of potato and vein-greening disease of tomato. It is both transmitted by and propagative in the endemic (western U.S) potato psyllid (PoP) Bactericerca cockerelli Sulc. The PoP occurs widely in the western U.S. and so has been used as a parallel study system for the quarantined ACP-greening complex. To identify proteins involved in global psyllid-Ca. Liberibacter interactions, the ACP and PoP transcriptomes were sequenced, yielding a total of 45,976 and 82,224 Illumina unique ACP and PoP transcripts, respectively. Cluster analysis revealed a high degree of sequence and transcript conservation suggestive of roles in core growth and developmental processes, providing the first molecular snapshot of the specific psyllid genes responsive to parasite invasion and circulation in the host. Evidence of inter-psyllid molecular conservation substantiates the suitability of PoP as a study system for ACP-Ca. L. asiatcus. Comparative in silico expression analysis within and between psyllid species revealed predicted functions involved in Ca. Liberibacter parasitism that were both unique and shared in common among adult and nymphal instars. In addition, functional characterization based on Gene Ontology analysis has revealed a number of genes associated with host-parasite interactions that could mediate Ca. Liberibacter infection, propagation, and circulation in the psyllid, as well as transmission processes.

Ca. Liberibacter asiaticus is the putative fastidious bacterial causal agent of citrus greening disease, also known as Huanglongbing (HLB), translated from Chinese as yellow dragon disease. The HLB bacterial pathogen is indigenous to Asia but has been introduced and dispersed to citrus throughout the Americas. A related bacterium that is indigenous to the Americas causes damage to potato (zebra chip) and tomato (vein-greening) and other solanaceous hosts. The causal agents are propagative and circulative in the psyllid vector, Diaphorina citri (Kuwayama) and Bactericera cockerelli (Sulc.), the Asian citrus and potato (or tomato) psyllid, respectively. The specific psyllid proteins that are indirectly or directly involved in the circulative, propagative transmission pathway are not known. However, if proteins were known that function at key points in the pathway e.g. post-ingestion, infection, biofilm formation, nutrition, circulation, and/or acquisition were known, such knowledge could be exploited to knock out their expression and abate pathogen transmission. To this end a combined approach involving functional genomics and anatomical localization of the bacterium is being implemented.  Results indicate that Ca. Liberibacter establishes biofilms on the outer surfaces of the alimentary canal and salivary glands of the Asian citrus psyllid (ACP) Diaphorina citri Kuwayama and the potato psyllid (PP) Bactericera cockerelli Sulc. In silico transcript profiling of infected and uninfected ACP and PP identified a number of mis-expressed, unique transcripts (unitrans). Functional predictions (gene ontology associations) implicate certain of these unitrans in Ca. Liberibacter infection of the psyllid host and/or in psyllid-mediated Ca. Liberibacter transmission processes.