Diversity and functional importance of phytoplasma membrane proteins

Phytoplasmas are wall-less prokaryotes, associated with hundreds of severe crop diseases worldwide. They inhabit phloem elements and are transmitted by insects of a few hemipteran families. In absence of a cell wall, phytoplasma membrane proteins are in direct contact with insect and plant hosts. The most abundant on their pathogen cells are the immunodominant membrane proteins (IDPs), which have been characterized in several phytoplasma strains, but also other membrane protein families, like variable membrane proteins (Vmps), adhesins, AAA + ATPases, and several transporters, are worth to mention for interactions with hosts and pathogen adaptation to different environments and as molecular markers useful for strain genotyping. Indeed, many of these membrane proteins are under positive selection pressure, and therefore highly variable among the different phytoplasma strains. A review and a schematic summary of the salient literature on phytoplasma membrane proteins are presented. The focuses were the variability of their gene sequences and the molecular characterization of pathogen strains and their functional roles in mediating interactions with plants and insects and in the perception and adaptation to different environments.