Towards dissecting the structural determinant of <i>Peach latent mosaic viroid</i> inducing mosaic symptoms

Abstract

Most isolates of Peach latent mosaic viroid (PLMVd) do not incite foliar symptoms, but a few number of isolates cause peach mosaic (PM) or peach calico (PC), an extreme albino phenotype. The PC determinant has been previously mapped at an insertion of 12-13 nt folding into a hairpin capped by a U-rich loop but the PM determinant, which is not associated with a specific insertion, remains unidentified and could reside in one or more domains of the branched conformation proposed for PLMVd. To tackle this problem we have selected for further dissection one variant (GDS6), recovered from a typical PM isolate (GDS), which is very infectious and elicits consistently a characteristic PM. We have initially focused on G337, a position that appears associated with PM in multiple alignments that include GDS6 and other PM-inducing and latent variants. To determine the role of G337 in infectivity and symptoms, GF-305 peach seedlings were inoculated with in vitro transcripts of recombinant plasmids containing dimeric tandem inserts of PLMVd-cDNAs with all possible changes at this position introduced by site-directed mutagenesis. Deletion of G337 abolished infectivity, while substitutions by A, C or U incited, in most inoculated plants, PM symptoms. Cloning and sequencing showed that the A substitution at position 337 was preserved in the progeny or reverted to G, while C or U substitutions at this position were not stable and reverted to A or G in the progenies. Extending this approach to additional nucleotides of loop A, or of other PLMVd domains, may provide hints in identifying the determinant of PM.

Keywords: Viroids, Pathogenesis, Peach disease