Effects of natural sequence variation on symptom induction by citrus viroid III
The potential use of viroids as cirtus dwarfing agents has been studied for more than 30 yr, and Semancik, Rakowski, Bash & Gumf (1997) have recently shown that infection of 'Valencia'scions growing on trifoliate orange rootstocks by either cirtus viroid Ia, IIa, or IIIb results in a desirable dwarfed tree canopy phenotype and enhanced fruit production. Viroids exist in vitro as populations of closely related sequence variants, and we are interested in isolating variants of citrus viroid III specifically adapted to induced desirable levels of dwarfing in different citrus cultivars. As the first step toward selecting suitable variants from large populations of mutagenised RNA transcripts, we examined the biological properties of several naturally-occuring variants of CVd-IIIb. Five sequence variants were cloned and slashed-inoculated into Etog citron. Four of these including the type sequence CVd-IIIb were infectious, and most of the sequence changes initially present were maintained in the respective progeny. Two changes in putative pathogenicity domain of CVd-III were associated with a marked reduction in symptom expression in Etrog citron. These results suggest that, with suitable screening asssay, it should be possible to select stable variants with different dwarfing properties from populations of mutagenised CVd-III RNAs synthesised in vitro.
Image caption - Suppression of symptom production by sequence changes in the pathogenicity and variable domains of CVd-IIIb. (A) Comparison of epinasty and leaf droop in the upper portions of control (uninoculated) and viroid-infected Etrog citron approximately 9 months post inoculation. (B-E) Comparison of necrotic reactions involving leaf petioles, midveins, and lateral vins. Note the absence of vein "splitting" in thee midveins of leaves from plants infected with variants 31 (panel D) and 33 (panel E) as well as a general reduction in the intensity of the necrotic response (arrows).
Owens, R.A., Yang, G., Gundersen-Rindal, D., Hammond, R.W., Candresse, T., and Bar-Joseph, M. 2000. Both point mutation and RNA recombination contribute to the sequence diversity of citrus viroid III. Virus Genes 14, 243-251.