Proposed model for Candidatus Liberibacter asiaticus and solanacearum systemic invasion and multiplication in the psyllid host and vector
J. BROWN (1), T. Rast (1), J. Cicero (2), T. Fisher (1) (1) The University of Arizona, U.S.A.; (2) University of Florida, U.S.A.
Ca. Liberibacter asiaticus and solanacterum are the causal agents of Huanglongbing and Zebra chip disease of citrus and solanaceous crops, respectively. Bacterial effectors that interact with psyllid vector proteins during invasion of and exit from the gut, and that facilitate entry into the salivary glands were identified using functional genomics, proteomics, yeast-2 hybrid (Y2H), co-immunoprecipitation (Co-IP), and electron microscopic analyses. In silico annotation and differential expression studies of psyllid nymphs and adults, and midgut and salivary glands identified transcripts and proteins with altered expression in response to Ca. Liberibacter infection. Differentially expressed transcripts were used as bait in Y2H studies to corroborate protein-protein interactions. Those positive by Y2H were subjected to verification by bait to prey co-transformation and Co-IP. Electron micrographs of the PoP midgut revealed the presence of CLso in the ventricular lumen, apical and basal epithelial cytosol, and in the filter chamber periventricular space. CLso were also prevalent in salivary gland pericellular spaces and head epidermal cell cytosol. Collectively, the results suggest a model for invasion in which Ca. Liberibacter- and prophage-encoded effectors manipulate the host endocytic pathway into a ‘pathogen-mediated phagocytosis scenario’ leading to bacterial exit from the gut into the hemocoel, and systemic invasion of other psyllid tissues and organs.