Phytoplasma and phytoplasma diseases: a review of recent research

Numerous yellows-type diseases of plants were once thought to be caused by viruses in view of their infective spread, their symptomatology, and the fact that they were transmitted by insects. The first demonstration that the etiological agents of yellows diseases could be wall-less prokaryotes rather than viruses caught the field of plant pathology by surprise more than forty years ago (Doi et al., 1967). This discovery of a new group of plant pathogens related to bacteria prompted new studies which revealed that pleomorphic, wall-less prokaryotes occurred in the phloem of many plant species affected with yellowstype diseases. The term mycoplasma-like organisms (MLOs) was first used for these micro-organisms due to their morphological and ultrastructural similarity to mycoplasmas. MLOs and mycoplasmas both belong to the Mollicutes class, since they are prokaryotes without cell walls. However, in contrast to mycoplasmas, which cause an array of disorders in animals and humans, phytopathogenic MLOs resisted all attempts to culture them in vitro in cell-free media (Lee and Davis, 1986). Following the application of molecular technologies, however, the enigmatic status of MLOs amongst the prokaryotes was finally resolved. This led to the application of the trivial name “phytoplasma”, and eventually to the designation of a new taxon for these organisms, named ‘Candidatus phytoplasma’ (IRPCM, 2004). Plants infected with phytoplasmas exhibit symptoms suggesting a profound disturbance in the normal balance of growth regulators, leading to: virescence/phyllody (development of green leaf-like structures instead of flowers), sterility of flowers, proliferation of axillary buds resultPhytoplasma and phytoplasma diseases: a review of recent research