Plant Viruses with Rod‐Shaped Virions

Abstract

Plant viruses that form rigid, rod‐shaped particles are found in eight genera. All of these viruses have single‐stranded ribonucleic acid (RNA) genomes that, with one exception, are positive‐sense. Most plant viruses with rod‐shaped particles are found in the six genera that form the Virgaviridae family with approximately half of all the definitive species in the Tobamovirus genus. The virus particles are formed from single structural proteins that encapsidate the genomic RNAs in nonenveloped, helical arrays. Despite their morphological similarities these viruses have diverse molecular and biological properties. While the tobamoviruses have nonsegmented genomes and are mechanically transmitted, the other viruses have segmented genomes and are transmitted by a variety of mechanisms: seed transmission, nematode vectors and plasmodiophorid vectors. Many of the viruses are important disease agents of crop species. Key Concepts: Aside from the anomalous Varicosavirus genus, all viruses with rod‐shaped virions are single‐stranded, positive‐sense ribonucleic acid (RNA) viruses that replicate in the cytoplasm of host plant cells. The rod‐shaped virions are formed from helical arrays of a single form of coat protein subunit and their length is dependent on the size of the encapsidated RNAs. Apart from species within the Tobamovirus genus, all viruses with rod‐shaped virions have segmented genomes divided between multiple RNAs. The plant viruses with rod‐shaped virions use a variety of mechanisms to translate the multiple open reading frames in their genomes, including leaky‐readthrough translation, the use of 3′ coterminal subgenomic RNAs and leaky‐initiation of translation by scanning ribosomes. The intercellular movement of these viruses within host plants is achieved through the use of a single Tobacco mosaic virus ‐like movement protein or a ‘triple gene block’ of movement proteins. The principle mechanism of transmission for all species within a genus varies between genera: the species within four genera are primarily plasmodiophorid transmitted, whereas those in three other genera are primarily mechanically transmitted, seed transmitted or nematode transmitted. All the genera containing viruses with rod‐shaped virions include species that cause economically important diseases that are controlled through the use of virus‐free seed/vegetative stocks, good sanitary practices and resistant crop cultivars.