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.
Joan Marquez‐Molins, Gustavo Gomez and Vicente Pallas published this study in Molecular Plant Pathology:
Hop stunt viroid: A polyphagous pathogenic RNA that has shed light on viroid–host interactions
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.
