Abstract:

Phytoplasmas are cell wall-less prokaryotic pathogens causing many plant diseases with various host plants, widely geographical distribution and adverse impacts on economics and environments. Abundant genetic diversity of the phytoplasmas has been evidenced by vast researches. In the paper, we conducted a comparatively systematic and comprehensive schematic review and comment on the research status of phytoplasma genetic diversity. We discussed the potential research directions with respect to research technology and generation mechanism of phytoplasmal genetic variation as well as relationships to pathogenicity in future. Analysis on five phytoplasmas whose whole genomes have been completed has indicated the definite genetic variation in size, structure and function of phytoplasmal genomes, which lack many genes for standard metabolic functions. There are different number, size and function of the plasmids in various phytoplasma strains. Two copies of ribosomal RNA operons among all the phytoplasmas showed significant variation which provided the present foundation for classification and identification of phytoplasmas. Studies on protein-encoding genes, such as ribosomal protein (rp), elongation factors (tuf and fusA), translocation proteins (secY and secA), effector molecules as well as non-encoding sequences such as promoters and pseudogenes have further revealed the rich genetic diversity of phytoplasmas. Since inadequate information is known regarding the characteristics of morphology, cultivation, physiology and metabolism due to the difficulty in culturing phytoplasma in vitro, modern molecular techniques for example whole genome sequencing and multilocus sequence analysis (MLSA) are efficient ways to research phytoplasmal genetic variation. Discoveries and developed techniques have facilitated a system-wide approach to revealing phytoplasma genetic variation, phylogenetic evolution as well as the relationships of their interaction with hosts (plant and insect vector) and adaptation to ecological conditions from the molecular level, and led to new insights. This will be of great importance in increasing the level of classification and determination of phytoplasmas, epidemiological forecasting and the control of relevant diseases.

Key words: phytoplasma, genetic variation, phylogenetic evolution, classification and identification, plant (insect vector)-phytoplasma interaction, multilocus sequence analysis (MLSA)