Interactions between phloem-restricted bacterial plant pathogens, their vector insects, host plants, and natural enemies, mediated by primary and secondary plant metabolites

Phytopathogens are responsible for devastating plant diseases threatening agriculture and food security worldwide. Phloem-restricted bacterial plant pathogens belonging to the classes mollicutes ('Candidatus Phytoplasma', Spiroplasma), alpha-proteobacteria, and gamma-proteobacteria have a significant impact on several high-value crops. These bacteria are obligate parasites, depending on their plant and insect hosts. Inside the plants, they are restricted to the phloem, which is a source of primary and secondary plant metabolites representing a suitable niche for them. Additionally, phloem sap is utilized for nutrition by multiple Hemipterans and several species within the leafhopper, planthopper, and psyllid taxa, which can acquire and transmit the phloem-dwelling bacteria during feeding. The present review aims at providing an overview of interactions between phloem-restricted bacterial plant pathogens and their vector insects, which are mediated by primary and secondary plant metabolites. The overview is organized in two sections describing the current knowledge of phloem and volatile metabolite mediated interactions between pathogens, their host plants, their vector insects, and where information is available, the antagonists of vectors. This bibliographic oriented part is followed by a comprehensive synthesis of the reviewed literature and important research gaps were identified. The evolution of plant - pathogen - vector systems and novel trends on pest management strategies of phloem restricted bacteria, in which semiochemicals are applied, are discussed. Finally, an outlook on future prospects is given.