|dc.description.abstract||Viruses are major pathogenic agents that cause a variety of diseases in all living systems. Since their first isolation in 1892 by Dimitrii Ivanovsky, methods of their diagnosis and control, their life cycles, host - virus interactions, mechanisms of resistance etc have been intensely researched. The first virus ever to be discovered was Tobacco mosaic virus (TMV), a plant virus (Beijerinck, 1898). More than three thousand viruses that infect not only plants, but also humans, animals and bacteria have been described (Koonin et al., 2006 ; Lawrence et al., 2009; King et al., 2012). Over the years, study of viruses has shown that they are obligate parasites and their life cycle stages are analogous to the cellular processes. Interestingly, many of the concepts and tools of molecular biology have been derived from the study of viruses because of their small genomes. The structural and functional simplicity of viruses have made them attractive tools for scientists to study a variety of biological phenomena.
According to the ninth report of the International Committee on Taxonomy of Viruses (ICTV IX) (King et al., 2012) the number of recognized viruses is 3,618, of which ~25% (957) are plant viruses and they have been classified into different families (Fig.1.1). RNA viruses infecting plants are highly abundant and more diverse when compared to DNA viruses (Koonin et al., 2015). Single-stranded RNA viruses have been shown to generally have smaller genomes (average length ~9kb). While this allows the viruses to mutate and evolve faster (Sanjuán and Domingo-Calap, 2016), it also makes it essential for these viruses to encode for multifunctional proteins.
There are seven defined families of positive sense, single stranded RNA plant viruses, namely, Bromoviridae, Closteroviridae, Luteoviridae, Potyviridae, Seconaviridae, Tombusviridae, and Virgaviridae. Among these families, the family Potyviridae contains the largest group of plant viruses that are also economically very important (Martínez et al., 2016). Their genome organization and expression strategies are similar to picornaviruses (Domier et al., 1987) and are therefore classified along with picorna-like superfamily of viruses. According to ICTV classification, the family Potyviridae consists of eight genera and two unassigned species (Table 1.1) based on physical properties of virion, RNA sequence, genome organization and mode of transmission (Wylie et al., 2017). The genus Potyvirus named after its type species, Potato virus Y (Ward and Shukla, 1991; Riechmann et al., 1992) is one of the oldest and the most successful of all known genera comprising of 146 virus species.||en_US