PLANT MOLLICUTES: PHLOEM-RESTRICTED AGENTS AND SURFACE CONTAMINANTS

J.M. Bové
Mollicutes (mycoplasmas) are the smallest and simplest selfreplicating prokaryotes, arising from low G + C Gram positive bacteria by degenerative evolution.

The mollicutes associated with plants are phloem-restricted pathogens (spiroplasmas, mycoplasma-like organisms) or surface contaminants (Spiroplasma spp., Mycoplasma spp., Acholeplasma spp., and others). The plant pathogenic mollicutes are transmitted by insect vectors. Many other insects carry mollicutes, particularly spiroplasmas, and deposit these organisms on plant surfaces where other insects pick them up. New acholeplasma, mycoplasma and spiroplasma species have been identified in insect hosts or on plant surfaces. Many mollicutes are pathogens of animals, some produce diseases in humans. Mollicutes are also frequent contaminants of animal cell cultures.

A number of assays were available for the detection and characterisation of the culturable plant pathogenic spiroplasmas, but not for the non-culturable mycoplasma-like organisms (MLO). Great progress has been made in the serological detection of MLOs due to the development of specific monoclonal antibodies.

Mollicutes or mycoplasmas are wall-less prokaryotes. They are contained within the Tenericutes, one of the four divisions of the kingdom Procaryotae (Murray, 1984). Based on genome size, mole % of Guanine plus Cytosine (G + C) in the genome, cholesterol requirement for growth, helical morphology of the cells, anaerobiosis and urease activity, six genera are distinguished (table 1). A mollicute isolated in 1979 from the surface of apple seeds, has an undetermined taxonomic position. The mollicutes represent a phylogenetically coherent group. They have probably arisen by degenerative evolution (genome reduction) from low G + C Gram positive bacteria and, more precisely, ancestors of certain Clostridia.

Studies on the molecular biology of the mollicutes have confirmed the bacterial origin of these agents. RNA polymerase, transcription promoters and terminators, Shine-Dalgarno ribosome binding sites are closely similar to their bacterial counterparts. However, one remarkable feature distinguishes Mycoplasma spp. and Spiroplasma spp. from bacteria. The codon 5'-UGA-3' is not a stop codon but codes for

Bové, J.M. (1988). PLANT MOLLICUTES: PHLOEM-RESTRICTED AGENTS AND SURFACE CONTAMINANTS. Acta Hortic. 225, 215-222
DOI: 10.17660/ActaHortic.1988.225.25
https://doi.org/10.17660/ActaHortic.1988.225.25

Acta Horticulturae