Made for Each Other: Vector–Pathogen Interfaces in the Huanglongbing Pathosystem

Citrus greening, or huanglongbing (HLB), currently is the most destructive disease of citrus. HLB disease is putatively caused by the phloem-restricted α-proteobacterium ‘Candidatus Liberibacter asiaticus’. This bacterium is transmitted primarily by the Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae). Most animal pathogens are considered pathogenic to their insect vectors, whereas the relationships between plant pathogens and their insect vectors are variable. Lately, the relationship of ‘Ca. L. asiaticus’ with its insect vector, D. citri, has been well investigated at the molecular, biochemical, and biological levels in many studies. Herein, the findings concerning this relationship are discussed and molecular features of the acquisition of ‘Ca. L. asiaticus’ from the plant host and its growth and circulation within D. citri, as well as its transmission to plants, are presented. In addition, the effects of ‘Ca. L. asiaticus’ on the energy metabolism (respiration, tricarboxylic acid cycle, and adenosine triphosphate production), metabolic pathways, immune system, endosymbionts, and detoxification enzymes of D. citri are discussed together with other impacts such as shorter lifespan, altered feeding behavior, and higher fecundity. Overall, although ‘Ca. L. asiaticus’ has significant negative effects on its insect vector, it increases its vector fitness, indicating that it develops a mutualistic relationship with its vector. This review will help in understanding the specific interactions between ‘Ca. L. asiaticus’ and its psyllid vector in order to design innovative management strategies.