Recent advances in phytoplasma diseases: biology and management

The evidence that several plant diseases were associated with phloem colonization by prokaryotes morphologically similar to mycoplasmas was first shown in 1967. Since then, several hundreds of plant syndromes have been reported to be associated with the so-called ‘mycoplasma-like organisms’ (MLO) now renamed phytoplasmas. The increasing threat of phytoplasma diseases to agriculture comes both from devastating diseases in emerging agricultural countries and from epidemics and or endemic situations in areas traditionally devoted to agriculture. Phytoplasmas usually do not kill in short time the host plant from which they are strongly metabolic dependent therefore increasing the possibility of epidemic spreading; phytoplasma-infected plants result also more susceptible to infection by other pathogens such as fungi and viruses. Due to the lack of in vitro growth of these microorganisms, they were poorly characterized until ribosomal rDNA sequencing provided evidence that these wall-less prokaryotes colonizing plant phloem and insects constitute a large monophyletic group within the class Mollicutes. Phytoplasmas are wall-less bacteria with sizes varying from 200 to 800 nm, they are pleomorphic, and survive and multiply only in isotonic habitats, such as plant phloem or insect haemolymph; therefore they are strictly host-dependent, but they can multiply in insect vectors and also infect their eggs, as already demonstrated for several insect/phytoplasma combinations. The phytoplasma chromosome is very small (680-1,600 kb) and phylogenetic studies propose that the common ancestor for phytoplasmas is Acholeplasma laidlawii in which the triplet coding for tryptophan (trp) is UGG, while in other prokaryotes, including mycoplasmas and spiroplasmas, trp is coded by UGA. Phytoplasmas are genetically distinguishable from mycoplasmas infecting humans and animals by the presence of a spacer region (about 300 bp) between 16S and 23S ribosomal regions, which codes isoleucine tRNA (tRNA-Ile) and part of the sequences for alanine tRNA (tRNA-Ala). Sequencing of complete rRNA genes shows that tRNA coding for valine and asparagine are located downstream from the 5S rRNA gene, and this is a unique feature of phytoplasmas. Phytoplasmas usually has two identical copies of 16S ribosomal gene; however in some cases interoperon heterogeneity was also reported. Phylogenetic studies of genes encoding 16S rRNA and a large set of concatenated core housekeeping proteins have suggested that existing phytoplasmas share a common ancestor and are descended from low G+C Gram-positive bacteria in the Bacillus-Clostridium group. After their emergence as a discrete clade, phytoplasmas continued to evolve, giving rise to widely divergent lineages, many of which exhibit plant host and insect vector specificities. Four phytoplasma genomes have been fully sequenced so far, including those of two ‘Candidatus Phytoplasma asteris’ strains, a strains of ‘Ca. P. australiense’, and a strain of ‘Ca. P. mali’; while the first three have circular chromosome of 706–879 kbp and small plasmids ‘Ca. P. mali’ has a linear chromosome of 601 kbp. Three of these genomes contain large amounts of repeated DNA sequence, while one carries multiple copies of almost 100 genes. Considering that phytoplasmas have unusually small genomes, the repeats were proposed to be related to their transkingdom habitat and to their pathogenic activity. Several molecular findings involved in both the plant-phytoplasma and insect-phytoplasma interactions greatly increased the scientific knowledge about these pathogens. It was shown that they encode very few metabolic functions confirming that are highly dependent from their host cells. Moreover the presence of two glycolytic gene clusters in some strains suggested that a higher consumption of the carbon source can also be present together with specific genes interacting with plant/insect hormone balance. The phytoplasma surface membrane pro...