Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 26, 1998
Publication Date: N/A
Interpretive Summary: Phytoplasmas, formerly called mycoplasmalike organisms, are associated with diseases in several hundred plant species. Thus far, none has been cultured in vitro. The development of molecular-based tools, especially, polymerase chain reaction (PCR) assays using universal primers design based on conserved gene sequences, has made it possible for us to detect and identify a wide array of phytoplasmas associated with infected plants or insects. A comprehensive classification scheme was recently proposed from our laboratory. In the past five years, numerous phytoplasma strains have been identified, indicating that phytoplasmas are more diverse than previously thought. New diseases have occurred in various geographical regions. Many are actually introduced from outside the regions. Information of phytoplasma ecology and genomic diversity is indispensable for preventing the spread of disease through international exchange of germplasm and for formulating an effective control measure to combat phytoplasmal diseases. In this research, we have presented information on insect transmission data, on the detection of diverse phytoplasmas in insect vectors and infected plants, and on vector-phytoplasma-plant interactions. We have gained insight on the ecology of phytoplasmas and have conceptualized a hypothetical model for emergence of new phytoplasma strains and new diseases. This model system could initiate future research toward establishing or validating the phytoplasma-vector-plant interrelationships that still remain unclarified in many diseases caused by phytoplasmas. The information about phytoplasma ecology and diversity will be a benefit to diagnosticians and to APHIS for implementation of new quarantine regulations.
The recent development of molecular-based probes, such as mono- and polyclonal antibodies, cloned-phytoplasma DNA fragments, and phytoplasma-specific primers for PCR has allowed for advances in detection and identification of uncultured phytoplasmas (formerly called mycoplasmalike organisms). Comprehensive phylogenetic studies based on analysis of 16S rRNA or both 16S rRNA and ribosomal protein gene sequences have established the phylogenetic position of phytoplasmas as members of the class Mollicutes, and revealed the phylogenetic interrelationships among phytoplasmas have formed a basis for their classification. Based on restriction fragment-length polymorphism (RFLP) analysis of PCR-amplified 16S rRNA gene sequences, phytoplasmas are currently classified into 14 groups and 38 subgroups which are consistent with groups delineated based on phylogenetic studies. In the past decade, numerous phytoplasma strains associated with plants and insect vectors have been identified using molecular-based tools. Genomic diversity of phytoplasma groups appears to be correlated with the sharing of common insect vectors and/or host plants in nature. The level of exchange of genetic information among phytoplasma strains in a given group is determined by three-way, vector-phytoplasma-plant, interactions. A putative mechanism for creation of new ecological niches and evolution of new ecospecies is proposed.