Natterin-like and legumain insect gut proteins promote the multiplication of a vector-borne bacterial plant pathogen

Phytoplasmas are phloem-limited plant pathogenic bacteria causing diseases in many plant species. They are transmitted by Hemipteran insect species in a persistent-propagative manner. Phytoplasmas are wall-less, and their membrane proteins are involved in pathogen internalization into host cells. We focused on the immunodominant membrane protein (Imp) of Flavescence dorée phytoplasma (FDp), a grapevine quarantine pest and a major threat to European viticulture. Scaphoideus titanus is the main natural vector of FDp to grapevine, whereas Euscelidius variegatus is commonly used as laboratory vector. Previous works indicated that recombinant Imp of two FDp strains (FD-C and FD-D) selectively interact with gut proteins from vector species rather than those from non-vectors. Here, similar patterns of interacting insect gut proteins were obtained from both vector species, following pull-down with His-tagged FDp Imps. After identification of several targets, four S. titanus and five E. variegatus proteins interacting with Imp were further characterized by measuring expression in different insect tissues and in healthy vs. infected insects. Specific RNAi silencing of two of these vector genes, namely natterin and legumain, resulted in a significant reduction of phytoplasma multiplication in insects upon pathogen acquisition, compared to control insects. Natterin displays a DM9 domain and legumain possesses a signature of G protein receptor, supporting their involvement as FDp Imp receptors. Outcomes of this work are discussed with particular attention devoted to the gain of knowledge on host/pathogen interaction as well as to the potential impact on improvement phytoplasma disease management.