Simulated Mammography Using Synthetic 3D Breasts

The potential use of viroids as citrus dwarfing agents has been studied for more than 30 yr, and Semancik, Raltowski, Bash & Gumpf (1997) have recently shown that infection of 'Valencia' scions growing on trifoliate orange rootstocks by either citrus viroid Ia, IIa, or IIIb results in a desirable dwarfed tree canopy phenotype and enhanced fruit production. Viroids exist in vivo as populations of closely related sequence variants, and we are interested in isolating variants of citrus viroid III specifically adapted to induce 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-occurring variants of CVd-IIIb. Five sequence variants were cloned and slash-inoculated into Etrog 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 the putative 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 with different dwarfing properties from populations of mutagenised CVd-III RNAs synthesised in vitro.