| Gene silencing is a set of fascinating mechanisms which appear to function in both development and defense. Plant molecular investigations have recently proven very successful at elucidating features of these mechanisms. Chapter 1 of this work shows the results of our attempts to determine if a plant viral protein that suppresses one of these mechanisms, post-transcriptional gene silencing (PTGS) can also suppress another silencing mechanism, transcriptional gene silencing (TGS). We show by two systems in Nicotiana tabacum that the potyviral suppressor of PTGS, the Helper Component-Proteinase (HC-Pro), fails to suppress TGS. Our results suggest that either these two silencing mechanisms are mechanistically unrelated, or that HC-Pro interferes with PTGS at a function downstream of any steps that are in common between PTGS and TGS. We also report here our development of Arabidopsis thaliana as a model system for studying the function of HC-Pro in suppression of PTGS. We characterize the developmental phenotypes HC-Pro imparts upon the roots, cotyledons, leaves, inflorescences, and flowers. We show that this phenotype is unstable, with some plants that have dramatic developmental phenotype giving rise to progeny which, while transgenic for the locus, lack some or all of the associated developmental phenotypes. We show HC-Pro suppression of PTGS in four independent silencing systems, and that its suppression of PTGS is dependent upon expression in a context that gives the full set of phenotypes. |
