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Title: Prospects of DNA-based Systems for Differentiation and Classification of Phytoplasmas

item Lee, Ing Ming
item Zhao, Yan
item Davis, Robert

Submitted to: Bulletin of Insectology
Publication Type: Review Article
Publication Acceptance Date: 8/15/2007
Publication Date: 12/1/2007
Citation: Lee, I., Zhao, Y., Davis, R.E., Wei, W., Martini, M. 2007. Prospects of DNA-based Systems for Differentiation and Classification of Phytoplasmas. Bulletin of Insectology. 60:239-244.

Interpretive Summary: Phytoplasmas, formerly termed mycoplasma-like organisms, are minute cell wall-less bacteria that are associated with diseases in several hundred plant species worldwide. Since the discovery of these unique plant pathogens four decades ago, attempts to culture phytoplasmas in cell-free media have failed. This inability has made it difficult to determine the taxonomic status of phytoplasmas and to classify phytoplasma strains by traditional methods applied to cultured bacteria. The advances in molecular biology and phylogeny of bacteria in the past two decades have made it possible for researchers to develop molecular-based tools for phylogenetic analysis, differentiation, and classification of uncultured phytoplasmas. Over the last two decades, phylogenetic analyses have revealed that phytoplasmas formed a discrete group and are new members of the class Mollicutes. Phytoplasmas are highly diverse and comprised at least 28 phylogenetically distinct groups. Classification systems based on conserved 16S rRNA gene sequences have enabled classification of a wide array of phytoplasmas. A comprehensive classification scheme has been constructed, providing a simple, accurate, and rapid means to identify unknown phytoplasmas. A provisional taxonomic system using the category ‘Candidatus’ for phytoplasmas has been proposed. To date, 25 ‘Cadidatus Phytoplasma’ species have been named. These accomplishments have a great impact on diagnostics of phytoplasmal diseases, and have made it feasible to study the ecology of phytoplasmas and the epidemiology of phytoplasmal diseases. Etiologies of several hundred unknown diseases and the identities of the causal agents (phytoplasmas) have clarified. The information will benefit scientists, extension workers, and plant diagnosticians to determine better control measures to combat the disease, and will aid implementation of quarantine regulations.

Technical Abstract: During the last two decades, over 800 phytoplasma strains have been reported in association with several hundred plant diseases and numerous insect vectors. Research has yielded new knowledge about phytoplasma ecology and phylogenetic relationships. A taxonomic system has emerged, and perspectives on phytoplasma speciation have changed. Guidelines for naming new ‘Candidatus Phytoplasma’ species have been proposed; the16S rRNA gene was employed as the sole phylogenetic marker for species delineation. To date, 25 ‘Ca. Phytoplasma’ species have been named, forming the core framework of the emerging phytoplasma taxonomy. A classification system based on restriction fragment length polymorphism (RFLP) analysis of 16S rRNA gene sequences provides a simple, reliable, and rapid means to classify phytoplasmas on a large scale without a need to sequence the gene. The capacity of this classification system has been recently upgraded by the development of a computer-simulated RFLP analysis method. This approach has led to construction of the most comprehensive classification system for phytoplasmas to date. The concept of multi-gene sequence analysis for distinguishing phytoplasma speciation species has emerged with the aim of overcoming deficiencies of the highly conserved 16S rRNA gene for delineating closely related phytoplasma species. Less conserved genes, such as ribosomal protein or secY, serve as phylogenetic markers for finer distinctions among of phytoplasmas.