Submitted to: Nature Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 29, 1997
Publication Date: N/A
Interpretive Summary: Phytoplasmas (tiny microbes formerly called mycoplasmalike organisms, MLOs) cause many plant diseases, reducing product quality, hindering exports, and reducing farmer profits in fruit crops including strawberry. Some phytoplasmas are responsible for quarantine restrictions on movement of critically important germplasm and planting stock. This research was initiated to learn whether Fragaria multicipita, a rare plant described by Fernald in 1906 from collections along the Rivere Ste. Anne in Gaspe, Canada, might have been given the taxonomic rank of species mistakenly. F. multicipita, was characterized as being dwarfed, with short petioles, small leaves, and multibranched crowns; such unusual vegetative morphology is often characteristic of plants infected by phytoplasmas. The results of our experiments demonstrated, for the first time, that F. multicipita in reality is a phytoplasma-diseased strawberry. On the basis of analysis of 16S rDNA, we have classified the phytoplasma detected in F. multicipita into group VI (clover proliferation and related phytoplasmas), new subgroup B. The results provide pathogen targets for managing quarantined plant germplasm and a basis for investigating whether the phytoplasma found in "F. multicipita" could present a threat to cultivated strawberry. The findings will be of interest to strawberry growers, diagnostic companies, APHIS, and plant taxonomists.
Technical Abstract: The rare plant, Fragaria multicipita, first reported in 1908 and thought extinct until its rediscovery in 1992, was characterized by a dwarfed, multi-crowned (multicipital) growth habit. In the original description and subsequent rediscovery, the unusual vegetative morphology of F. multicipita was presumed to be suggestive of an ice front relict. Graft transmission of the growth form from F. multicipita to F. chiloensis led to a previous suggestion that F. multicipita was a diseased form of F. virginiana and to reduction of F. multicipita to the taxonomic rank of forma. Using polymerase chain reactions (PCRs), we amplified phytoplasma-specific 16S rDNA sequences from F. multicipita and from F. chiloensis carrying grafted F. multicipita tissue. No amplification of phytoplasma DNA was observed in PCRs containing template DNA derived from healthy, non-grafted F. chiloensis plants. RFLP analysis of the amplified DNAs revealed that both the F. multicipita and the grafted F. chiloensis were infected by a phytoplasma belonging to 16S rRNA gene group 16SrVI (clover proliferation, potato witches broom, and related phytoplasmas). RFLPs and sequence analyses of the phytoplasma's 16S rDNA indicated that is represented a distinct new subgroup, which we designate 16SrVI-B. Based on the evidence of graft transmission from F. multicipita to F. chiloensis of a distinct phytoplasma pathogen that induced multicipital growth habit in F. chiloensis, we conclude that "F. multicipita" represents an aberrant growth form induced by phytoplasmal infection and is an invalid taxon.