Mycoplasmas, plants, insect vectors: a matrimonial triangle

Plant pathogenic mycoplasmas were discovered by electron microscopy, in 196 7, long after the discovery and culture in 1898 of the first pathogenic myc oplasma of animal origin, Mycoplasma mycoides. Mycoplasmas are Eubacteria o f the class Mollicutes, a group of organisms phylogenetically related to Gr am-positive bacteria. Their more characteristic features reside in the smal l size of their genomes, the low guanine (G) plus cytosine (C) content of t heir genomic DNA and the lack of a cell wall. Plant pathogenic mycoplasmas are responsible for several hundred diseases and belong to two groups: the phytoplasmas and the spiroplasmas. The phytoplasmas (previously called MLOs , for mycoplasma like organisms) were discovered first; they are pleiomorph ic, and have so far resisted in vitro cultivation. Phytoplasmas represent t he largest group of plant pathogenic Mollicutes. Only three plant pathogeni c spiroplasmas are known today. Spiroplasma citri, the agent of citrus stub born was discovered and cultured in 1970 and shown to be helical and motile . S. kunkelii is the causal agent of corn stunt. S. phoeniceum, responsible for periwinkle yellows, was discovered in Syria. There are many other spir oplasmas associated with insects and ticks. Plant pathogenic mycoplasmas ar e restricted to the phloem sieve tubes in which circulates the photosynthet ically-enriched sap, the food for many phloem-feeding insects (aphids, leaf hoppers, psyllids, etc.). Interestingly, phytopathogenic mycoplasmas are ve ry specifically transmitted by leafhoppers or psyllid species. In this pape r, the most recent knowledge on phytopathogenic mycoplasmas in relation wit h their insect and plant habitats is presented as well as the experiments c arried out to control plant mycoplasma diseases, by expression of mycoplasm a-directed-antibodies in plants (plantibodies). (C) 2001 Academie des Scien ces/Editions scientificlues et medicales Elsevier SAS.