Economic Importance of Plant Viruses
Viruses cause serious disease of crop plants reducing both quality and quantity of final produce. Also keeping quality of plant produce is also affected. Some of the important viral diseases are:
Tobacco mosaic virus, Tomato mosaic, Tomato spotted wilt, Potato leaf roll, Potato virus X and Y in Potato, Papaya mosaic, Citrus tristiza, chilly leaf curl, Banana bunchy top etc.
Nature of Virus:
Each plant virus consists of two components the nucleic acid and the protein coat or capsid. The mature particle of a plant virus is generally called virion and the whole infective particle is called as Nuclcocapsid.
Nucleic Acid of Viruses:
1. Nucleic acid portion of the virus particle is called as Genome.
2. It is actual infective component.
3. Majority of plant viruses contain RNA (Ribonucleic acid).
4. Some viruses (Cauliflower mosaic virus, mung bean yellow mosaic virus, maize streak virus) contain DNA (Deoxyribo nucleic acid).
5. The nucleic acid may be in a single strand or in a double strand.
6. Most plant viruses contain single strand of RNA.
These strands may be free at both ends (liner RNA strand) or the end may be jointed together to form a circular nucleic acid molecule (Circular RNA).
7. In double stranded viruses (as RNA or ds DNA) the two strands are called around each other helically.
8. The nucleic acid may be present as a single continuous strand or it may be present as two or more piece in the same or different particles.
The Virus Protein:
1. The virus protein is termed as capsid.
2. It protects nucleic acid (RNA or DNA) of virus.
3. It is made-up of different amino acid sequences in different viruses. This amino acid are linked by Peptide bonds.
4. The protein sub units of the capsid are known as structural units when these units form groups of 2,4,6 are called Morphological units or capsomers.
5. The sequence of amino acids constitutes the primary structure of the protein. Helical structure produced by amino acids is known as secondary structure of the protein and folding of protein gives tirtiory structure.
6. Protein shell or coat protects viral nucleic acid from environment.
Membrane Bound Viruses:
The rhabdo such as potato yellow dwarf virus, lettuce necrotic yellows are provided with a outer envelops or membrane bearing surface projections. Inside the membrane is the nucleocapsid.
These are viruses associated with certain typical viruses but depend on the latter for multiplication and plant infedtion and reduce the ability of the typical viruses act like parasite of the associated typical virus.
These are small, liner RNAs found in virions of certain multicomponent viruses.
These are small (250-400 nucleotide), naked, single stranded, circular RNAs capable of causing disease in plants by themselves.
These are viriod like, small, single stranded , circular RNAs that are present inside some RNA viruses, virusides are the part of genetic material of these viruses and therefore , form an obligatory association with these viruses so that neither the virus not the virusolcan multiply and infect a plant in the absence of its partner.