By the middle of the last century, viruses were considered as the simplest biological entities. The discovery of satellite RNAs and viroids by 1970 was surprising because it revealed the existence of a novel class of self-replicating molecules even simpler, named subviral agents. There are evidences that viroids and virusoids (a class of satellite RNAs) were part of the so-called "RNA world" (that preceded our present world based on DNA and proteins) and for this reason they can be considered as molecular fossils of this ancient period. The simplicity of these subviral agents and the fact that the RNA molecule must interact directly with host factors for completing their infective cycle make these pathogens a model for the study of cellular processes. In the last years, a large number of publications have widened our knowledge of the viroid-host interactions, including pathogenesis mechanisms, movement through the host, gene silencing and ribozyme activity. Recent changes have been introduced in the taxonomy of these pathogens, with the creation of families, genera and species, and new viroids have also been found. The purpose of this review is to present the reader with these recent advances in viroid research, mainly on taxonomy, phylogeny and in molecular aspects of the viroid-host interaction. Some characteristics of virusoids and their evolutionary relationship with viroids are also included.
plant pathogen interaction; gene silencing; evolution; rybozime; satellite RNA; taxonomy; quasispecies
