Asaia symbionts interfere with infection by Flavescence dorée phytoplasma in leafhoppers

A key process in the spread of phytoplasma-borne diseases is the interaction between pathogens and their insect vectors. Such interaction may be affected by microbial symbionts of the vector, which may alternatively or cumulatively compete for insect colonization and express antibiotic factors, activating host immune response. Acetic Acid Bacteria of the genus Asaia are symbionts of the vector of Flavescence dorée phytoplasma, Scaphoideus titanus, and are candidate control agents against the disease. In this work, the role of different strains of Asaia as inhibitors of phytoplasma transmission has been investigated by using the model leafhopper-plant hosts Euscelidius variegatus and Vicia faba. Asaia was shown to be associated to E. variegatus, and strains exhibiting different phenotypes successfully and stably colonized the leafhopper body after being orally supplied. Flavescence dorée phytoplasma transmission trials were carried out by using specimens fed with three Asaia strains or no Asaia. An isolate characterized by high production of air-liquid interface (ALI) biofilm was shown to significantly inhibit phytoplasma transmission, possibly by limiting pathogen capability to colonize the insect midgut. However, when the phytoplasma succeeded in establishing in the body of E. variegatus, the transmission process was regularly completed. Even though the mechanisms regulating this interference activity are still to be elucidated, as well as the effectiveness of Asaia in limiting phytoplasma transmission to grapevine by S. titanus, our results provide an important tool for designing sustainable control programs using Asaia as a biocontrol agent.