Viruses and Sub-Viral Agents

Abstract

Tropical crops which are propagated through embryonic seed and vegetative propagules are affected with a number of virus and virus-like pathogens that are responsible for heavy yield losses. Viruses and viroids are infectious obligate pathogens. Viruses are intracellular parasites diverting the host cellular machinery for their own replication and offspring particle production. Viroids are small, single stranded, circular RNA of 246-401 nucleotides without any apparent-protein coding capacity. In this chapter basic information of plant viruses and viroids is presented briefly to realize their significance as an important category of plant pathogens. Plant viruses are composed of small piece of nucleic acid either RNA or DNA inside the protein coat but not both. Viruses lack protein synthesizing and energy producing machinery. Viruses and viroid diseases are capable of infecting the majority of species of both cultivated and wild plants. However, host ranges of individual virus and viroid diseases can vary from very narrow to very broad. The symptoms induced by plant viruses and viroids lead to reduced crop quality and yield. Plant viruses are typically spread by two common mechanisms: horizontal transmission and vertical transmission. Horizontal transmission takes place through vectors and also by mechanically means. Vertical transmission on the other hand is through vegetative propagative material and also through seed. Classification and nomenclature is internationally and universally applied to all viruses and viroids for which the International Committee on Taxonomy of Viruses (ICTV) is responsible. ICTV has been very active for the past 30 years, and playing a major role by which the number of taxa and virus names are increasing. As per the ninth ICTV report and updated online as of 2012, there are 6 orders, 87 families, 19 sub-families, 349 genera and 2284 species. The ICTV rules concerned for classification of viruses also applies to the classification of viroids. In the taxonomy of viroids, there are two families viz., Pospiviroidae and Avsunviroidae, with 5 genera and 3 genera, respectively. Some details on geographical distribution, host range, transmission, symptomatology, molecular biology, genome structure, and replication of some economically important viroids are provided. Among virus groups tospo-, cucumo, gemini- (begomo- and nonbegomo-), and potyvirus groups are discussed at length as they constitute the largest plant virus groups in tropical environments.