Non-cultivable phytopathogenic mycoplasmas: characterization, detection and perspectives for control

Phytoplasmas (ex MLOs) and spiroplasmas are important groups of plant pathogenic mollicutes, discovered in 1967 and 1970 respectively. Spiroplasmas, like other mollicutes, can be cultured in artificial media and are thus well characterized. On the contrary, phytoplasmas have resisted in vitro cultivation and their study was difficult until the recent development of molecular techniques. From the sequence of their 16S rDNA, phytoplasmas have been shown to be true mollicutes. Fourteen phytoplasma subclasses have been defined, but only two phytoplasmas have so far been named at the genus and species level. Monoclonal antibodies, DNA probes and PCR primers for the specific detection of various phytoplasmas have been obtained. These showed that a given phytoplasma can infect a broad range of plants, while others are restricted to a single plant species. Specific reagents are also used for identification of insect vectors and reservoir plants of the various phytoplasmas. Plant pathogenic mollicutes cannot be controlled chemically today, since the use of antibiotic treatment is forbidden in agriculture. However, the growth and metabolism of mollicutes are known to be inhibited by antibodies and this provides a hopeful approach for future control of these agents in plants. Indeed, it has been shown recently that plants can be engineered to express and assemble functional immunoglobulin chains. Transgenic tobacco plants expressing an antibody against the stolbur phytoplasmas have been developed. They have now to be challenged with the phytoplasma to determine if they have acquired resistance to this mollicute.