Diaphorina citri Nymphs Are Resistant to Morphological Changes Induced by “Candidatus Liberibacter asiaticus” in Midgut Epithelial Cells

ABSTRACT

“ Liberibacter asiaticus” is the causative bacterium associated with citrus greening disease. “ Ca. Liberibacter asiaticus” is transmitted by more efficiently when it is acquired by nymphs rather than adults. Why this occurs is not known. We compared midguts of insects reared on healthy or “ Ca. Liberibacter asiaticus”-infected citrus trees using quantitative PCR, confocal microscopy, and mitochondrial superoxide staining for evidence of oxidative stress. Consistent with its classification as propagative, “ Ca. Liberibacter asiaticus” titers were higher in adults than in nymphs. Our previous work showed that adult insects have basal levels of karyorrhexis (fragmentation of the nucleus) in midgut epithelial cells, which is increased in severity and frequency in response to “ Ca. Liberibacter asiaticus.” Here, we show that nymphs exhibit lower levels of early-stage karyorrhexis than adults and are refractory to the induction of advanced karyorrhexis by “ Ca. Liberibacter asiaticus” in the midgut epithelium. MitoSox Red staining showed that guts of infected adults, particularly males, experienced oxidative stress in response to “ Ca. Liberibacter asiaticus.” A positive correlation between the titers of “ Ca. Liberibacter asiaticus” and the endosymbiont was observed in adult and nymph midguts, suggesting an interplay between these bacteria during development. We hypothesize that the resistance of the nymph midgut to late-stage karyorrhexis through as yet unknown molecular mechanisms benefits “ Ca. Liberibacter asiaticus” for efficient invasion of midgut epithelial cells, which may be a factor explaining the developmental dependency of “ Ca. Liberibacter asiaticus” acquisition by the vector.