Phytoplasmas and plant diseases: a transkingdom relationship

Phytoplasmas or ‘Candidatus Phytoplasma’ are bacteria without cell wall living only in the insect hemolymph and in the plant sieve tubes. After more than 20 years from their discovery their role as plant pathogens is still not resolved for lack of Koch postulates fulfillment. Worldwide there are several relevant epidemics in which phytoplasmas play a role as key factors in reducing the production and the revenues of crops. This situation is worsen by their dissemination by propagation and micropropagation materials, insect vectors, and seeds. Economically relevant phytoplasma diseases are reported in both woody and annual crops since these bacteria are present in several diverse environments and in particular in tropical and subtropical areas where the insect vectors play their dissemination to alternate plant and insect host species. There are a number of phytoplasma metabolic features and effectors that were discovered mining several available full and draft genomes that are clarifying the interactions of these bacteria with both plants and insect vectors confirming their two kingdoms adaptation ability. Recent efforts allowed to obtain colonies containing mainly phytoplasmas following their isolation from field-infected or experimentally-infected plants. Moreover, little steps are in progress to define direct phytoplasma biological activity while several pathogenicity related factors were found and confirmed as potentially active in transgenic plants. Molecular tools helped in resolving phytoplasma differentiation and allowed to study their epidemiology for appropriate and environmentally friendly management of associated diseases, moreover their isolation in artificial media should help in clarifying their phytopathological role in severe epidemics worldwide such as coconut lethal yellowing and grapevine yellows.