Plant Virus Interactions in Disease- An Overview

Tobacco Mosaic Virus was the first virus to be discovered in 19^th century. The term “virus” was given by Martinus Beijerink in year 1898. W. Stanley in 1935 showed “proteinaceous nature” of virus. Viruses are of varying shapes and their sizes usually range in nanometers. Based on the types of genomes, viruses can be classified as positive sense RNA viruses, negative sense RNA, dsRNA, retroid dsDNA, ssDNA, dsDNA. Viruses do not have metabolism of their own and require host systems to translate and replicate within them. There are various stages in viral cycle, viz., penetration and decapsidation, translation, replication, encapsidation. Virus has an infectious particle called virion which is composed of capsid, nucleic acids and outer coat. Viral genomes are associated with certain double stranded structures, hairpin loops, stem loops, pseudoknots in order to provide stability and protection to the single stranded nucleic acids inside host cells. Genes in viral genomes are closely spaced and they may be overlapping as well. Mixed infections of plant viruses are common in nature, and a number of important virus diseases of plants are the outcomes of interactions between causative agents (Styller, 2011).

The viruses penetrate the tissue through the wounds, infections, by vectors that cause breakage in tissues. With the entry of virus the plants develop defensive response and virus in turn develops counter defensive mechanisms. Viral RNAs are carriers of genetic material and are alone infectious (Fraenkal et al. 1957). Viruses contain genes for replication, movement and transmission. The translation of the genes is carried out using host components in association with viral proteins. Translation of viral genome occurs in three steps, viz., initiation, elongation and termination. Initiation is a very complex process (Browning et al. 1997). Viral helicases and polymerases play significant roles in replication. Helicases are associated with the membranes of the host cell vesicles and their modifications.

Movement of viruses within the plants occurs through plasmodesmata or phloem. Plasmodesmata-mediated movement is responsible for cell-to-cell transfer and is facilitated by virus encoded membrane proteins (MPs). Movement proteins increase plasmodesmatal permeability by altering their membrane structure like 30K protein of tobacco mosaic virus. The long distance movement that leads to transmission of viruses to distant cells occurs through phloem and leads to systemic infection. Plant factors like pectin methylesterases, glycine-rich proteins, etc. restrict viral movement. Also, plasmodesmata restrict movement at various tissues. Besides, integral and peripheral proteins on memberanes also effect the movement through them (Tilsner et al. 2011).

Plants defend themselves against viral attacks in various ways. The most important one being the natural genetic resistance employed by presence of dominant and recessive resistance genes. It may be passive or active resistance (Carr and Klessig 1989). Passive resistance is due to the lack of some host factors that act against viral replication or may be due to some physical barriers to inoculation. Whereas the active resistance is due to the mechanisms triggered by the infection of the virus, it follows “gene-to-gene” system according to which both resistance in host and parasitic ability to cause disease is controlled by a pair of genes, i.e., resistance (R) genes in plants and avirulance genes (Avr) in the virus (Flor, 1954). Most common resistance genes are R, N, N´. They contain Nucleotide Binding Site, Leucine Rich Repeats and Toll and Interleukin Repeats domains (Kang et al. 2005). The ends of the single stranded RNA are diverse. Some viruses also contain some unconserved domains that are expendable for systemic infection (Satayanarayana et al. 2011). Another important method of resistance against viruses is the RNA silencing (Matthew 2004). RNA silencing may be enhanced by host RNA dependent RNA polymerases that produce some dsRNAs to primary siRNAs (Himber et al. 2003) that give rise to secondary siRNAs that move across phloem to distant cells providing systemic silencing. Viruses suppress silencing by viral-encoded suppressors of RNA silencing (VSRs).