The variable membrane protein VmpA of flavescence dorée phytoplasma acts as an adhesin binding cells of the insect vector Euscelidius variegatus. Adhesion of FD phytoplasma VmpA to insect cells

Phytoplasmas are uncultivated plant pathogens, cell wall-less bacteria and are transmitted from plant to plant by hemipteran insects. Their circulative propagative cycle in insects requires the crossing of eukaryotic cells of midgut and salivary glands. Adhesion of bacteria to cells is an initial step towards the invasion process. The flavescence dorée phytoplasma (FD-P) possesses a set of variable membrane proteins (Vmps) exposed to its surface and suspected to interact with insect cells. Our results showed that VmpA is expressed by FD-P present in midgut and salivary glands. Phytoplasmas cannot be cultivated yet and no mutant can be produced to investigate the putative role of Vmps in the adhesion of phytoplasma to insect cells. To overcome this difficulty we engineered the Spiroplasma citri mutant G/6 that lacks the adhesins ScARPs, for VmpA expression, and used VmpA-coated fluorescent beads to determine if VmpA acts as an adhesin in ex vivo adhesion assays and in vivo ingestion assays. VmpA specifically interacted with Euscelidius variegatus insect cells in culture and promoted the retention of VmpA-coated beads to the midgut of E. variegatus. In this latest case, VmpA-coated fluorescent beads were localized embedded in the perimicrovillar membrane of the insect midgut. Thus VmpA functions as an adhesin that could be essential in the colonization of the insect by the FD phytoplasmas.