Submitted to: Virus Genes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 13, 1999
Publication Date: N/A
Interpretive Summary: Work carried out in Australia over the past 30 years has demonstrated the feasibility of using viroid infection to develop a predictable range of tree size control in high density citrus planting schemes. Using molecular techniques, we have begun to modify the molecular properties of one of these citrus viroids, so-called citrus viroid III (or CVd-III) in order to improve both its safety and efficacy when used under subtropical growing conditions. In order to establish and appropriate set of "outer boundaries" for such studies, it was necessary to determine the extent of variation among naturally-occurring viroid isolates. Analysis of nine different field isolates of CVd-III, five of which have been used to dwarf citrus in Israel, revealed the presence of two major and two minor molecular forms of CVd-III. As previously observed for many plant viruses, RNA recombination appears to play a role in generating at least some of the observed sequence diversity. These results will initially be of interest primarily to researchers interested in the molecular interactions between viruses and their hosts. The results and conclusions are presented in such a way as to emphasize their eventual usefulness to horticulturists and other plant scientists. Information on viroid sequence diversity is also important for APHIS and other plant regulatory officials.
Field-grown citrus trees often harbor complex mixtures of 4-5 different viroid species, and the presence of citrus viroid III (CVd-III) has been shown to reduce the rate of tree growth without inducing disease. To more fully define the structure of its quasi-species, we have examined nine citrus viroid complexes for the presence of previously undescribed sequence variants of CVd-III. Analysis of 86 full-length cDNAs generated from these nine viroid complexes by RT-PCR revealed the presence of 20 new CVd-III variants. Chain lengths ranged from 293-297 nucleotides, and sequence changes were confined largely to the lower portions of the central conserved region and variable domain. The previously described variants CVd-IIIa (297 nt) and -IIIb (294 nt) are clearly predominant, but phylogenetic analysis indicated that certain isolates contain representatives of two additional fitness peaks. At least one group of Cvd-III variants appears to have arisen as a result of RNA recombination. Populations recovered from diseased/declining trees were the most diverse, but even dwarfing isolates originating from old line Shamouti trees showed unexpected variability. Little is known about the biological significance of this genotypic variability, and careful examination of the consequences of their presence in dwarfing-sensitive rootstocks is necessary before any of these CVd-III sequence variants can be used for practical purposes.