How does one of the smallest pathogens on earth cause dwarfing in Hop vines? Read this pathogen profile to discover the full background of Hop Stunt Viroid; a single-stranded RNA (a relic of pre-cellular evolution?) found in many important fruits and vegetables across the world.
When people think about tiny pathogens, viruses immediately come to mind. However, even the smallest virus is enormous in comparison with a group of pathogens that can affect our crops: viroids. The genome of viroids is a closed single-stranded small RNA molecule (246-434 nucleotides), much bigger than the several thousands of nucleotides of the genomes of the most compact viruses. Therefore, viroids are the minimal known pathogens with autonomous replication. They are considered non-coding RNAs because they do not contain canonical open reading frames (ORF) and consequently no protein of viroidal origin has been ever detected.
Viroid circular RNA genomes present a high degree of secondary structure and must interact and subvert the activity of several host proteins for completing their life cycle. This structural simplicity, functional versatility, and experimental tractability of viroid RNAs turns them into excellent models to investigate the basic principles of infection and discover regulatory mechanisms in plants. In addition, their small size, high G+C content, circular structure with a compact folding and replication (mediated in some by ribozymes), make them relevant candidates to be potential survivors of an RNA world and considered relics of precellular evolution.
Viroids are naturally found infecting higher plants (plants with vasculature) and have been classified in two families. Those that contain a central conserved region (CCR) in their genomes and replicate in the nucleus constitute the Pospiviroidae family. Conversely, the other known viroids, pertaining to the Avsunviroidae family, are characterised by having chloroplast replication, the presence of autocatalytic structures and by an extremely narrow host range.
Hop stunt viroid (HSVd) is a nuclear replicating viroid which has been reported in a wide range of hosts including the families Moraceae (fig and mulberry), Rosaceae (almond, apple, apricot, cherry, peach, pear, and plum), Anacardiaceae (pistachio), Malvaceae (hibiscus), Cucurbitaceae (cucumber) and Rutaceae (Citrus spp.). The disentangling of the molecular biology and physical properties of viroids has been mainly carried out using Potato spindle tuber viroid (PSTVd), the first viroid discovered and the type species of the Pospiviroidae family. However, the characterization of other members of the family has provided additional information about how these RNA pathogens interact with the host machinery and has contributed to a large number of innovative techniques in RNA biology research. Particularly, multiple studies using HSVd as a model have shed light on different aspects of viroid-host interactions such as long-distance movement, resistance to RNA-silencing, pathogenesis, epigenetic alterations etc. In the pathogen profile entitled: “Hop stunt viroid: a polyphagous pathogenic RNA that has shed light on viroid-host interactions”, we summarized the current knowledge of this pathogen and the importance of these discoveries for viroid research.
TITLE IMAGE: a) Hop plants in field conditions showing abnormal dwarfing of bines. (b) Cucumber plant mock‐inoculated (left) or inoculated with HSVd (right), the last one showing clear stunting and leaf distortion. (c) Dapple fruit disease in plum cv. Shiho. Plum fruit with irregular reddish blotches on the pericarp caused by HSVd (right) compared with healthy (left) plum. (d) Apricot disease caused by HSVd (right) and healthy apricot (left). Images (a) and (c) are reproduced from Hataya et al. (2017) with permission of Elsevier Books and (d) from Amari et al. (2007), with permission of Springer Nature. All images used with permission of the author.