The hosts of phytoplasmas, i.e. plants and insect vectors, are inhabited by diverse microorganisms having interactions spanning from mutualism to parasitism. When the pathogens colonize a host, they may thus be exposed to diverse interac- tions with complex microbial communities. These relations are still poorly recog- nized for phytoplasmas, even though many beneficial or harmful interactions have been described for other plant pathogens. The knowledge on traits of microbial relations involving phytoplasmas in insects and plants is regarded as a valuable tool for designing new control methods against the diseases associated with these patho- gens, by displaying direct antagonistic activities, altering the vector fitness or com- petence for transmission, or promoting plant immune response or growth. In insect vectors, which mainly host bacterial associates, with few yeast-like symbionts, direct interactions with phytoplasmas were described for bacteria of the genera Frauteria in Hyalesthes obsoletus and Asaia in Euscelidius variegatus. In plants, the most studied systems are grapevine, apple and coconut palm, along with model organisms such as Catharanthus roseus and in vitro micropropagated plants. Here, many bacteria, mainly of the genera Pseudomonas, Burkholderia and Paenibacillus, as well as the fungal endophyte Epicoccum nigrum, were shown to inhibit phyto- plasma growth and related symptoms in the plant hosts. Overall, the recent advances concerning the knowledge on microbial symbioses in phytoplasma plant and insect hosts can consistently support future research regarding the phytoplasma infection process, and eventually drive new control strategies against phytoplasma-associated diseases.
