Effects of natural sequence variation on symptom induction by citrus viroid III

The potential use of viroids as citrus dwarfing agents has been studied for more than 30 yr, and Semancik, Raltowski, Bash & Gumpf (1997) have recentl y shown that infection of 'Valencia' scions growing on trifoliate orange ro otstocks by either citrus viroid Ia, IIa, or IIIb results in a desirable dw arfed tree canopy phenotype and enhanced fruit production. Viroids exist in vivo as populations of closely related sequence variants, and we are inter ested in isolating variants of citrus viroid III specifically adapted to in duce desirable levels of dwarfing in different citrus cultivars. As the fir st step toward selecting suitable variants from large populations of mutage nised RNA transcripts, we examined the biological properties of several nat urally-occurring variants of CVd-IIIb. Five sequence variants were cloned a nd slash-inoculated into Etrog citron. Four of these including the type seq uence CVd-IIIb were infectious, and most of the sequence changes initially present were maintained in the respective progeny. Two changes in the putat ive pathogenicity domain of CVd-III were associated with a marked reduction in symptom expression in Etrog citron. These results suggest that, with a suitable screening assay, it should be possible to select stable variants w ith different dwarfing properties from populations of mutagenised CVd-III R NAs synthesised in vitro.