User:Babar.Joya

VirusesItalic text_____--Babar.Joya (talk) 15:59, 12 December 2011 (UTC)--Babar.Joya (talk) 15:59, 12 December 2011 (UTC)________

Definition Any ultramicroscopic metabolically inert infectious agent that replicates only within the cells of living hosts; composed of RNA or DNA core and Protein coat is known as Virus. Overview Of Plant Viruses Although plant viruses are not nearly as well understood as the animal counterparts, one plant virus has become iconic. The first virus to be discovered (see below) was Tobacco mosaic virus (TMV). This and other viruses cause an estimated US$60 billion loss in crop yields worldwide each year. Plant viruses are grouped into 73 genera and 49 families. Mostly plant viruses are ssRNA, then dsRNA, dsDNA, and ssDNA respectively. Frequency of plant viruses is greater than the animal viruses and lethalness is high than animal viruses. To transmit from one plant to another and from one plant cell to another, plant viruses must use strategies that are usually different from animal viruses. Plants do not move, and so plant-to-plant transmission usually involves vectors (such as insects). History In 1886, Adolf Mayer first described the tobacco mosaic disease that could be transferred between plants, similar to bacterial infections. In 1892, Dmitri Ivanovsky gave the first concrete evidence for the existence of a non-bacterial infectious agent, showing that infected sap remained infectious even after filtering through finest filter paper. In 1898, Martinus Beijerinck independently replicated Ivanovsky's filtration experiments and then showed that the infectious agent was able to reproduce and multiply in the host cells of the tobacco plant. Beijerinck coined the term of "virus" to indicate that the causal agent of tobacco mosaic disease was of non-bacterial nature. The purification (crystallization) of TMV was first performed by Wendell Stanley, who published his findings in 1935, although he did not determine that the RNA was the infectious material. However, he received the Nobel Prize in Chemistry in 1946. In the 1950s a discovery by two labs simultaneously proved that the purified RNA of the TMV was infectious which reinforced the argument. The RNA carries genetic information to code for the production of new infectious particles. In 1939 Holmes published a classification list of 129 plant viruses. This was expanded and in 1999 there were 977 officially recognized, and some provisional, plant virus species. Structure Viruses are extremely small and can only be observed with an electron microscope. The structure of a virus is given by its coat of proteins, which surround the viral genome. Over 50% of known plant viruses are rod-shaped (flexuous or rigid). The length of the particle is normally dependent on the genome but it is usually between 300–500 nm with a diameter of 15–20 nm. Tobacco Mosaic Virus

Virus classification Group:	Group IV ((+)ssRNA) Family:	Virgaviridae Genus:	Tobamovirus Species:	Tobacco mosaic virus Tobacco mosaic virus (TMV) is a positive-sense single stranded RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns (mottling and discoloration) on the leaves (hence the name). TMV was the first virus to be discovered. Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus. Structure Of “TMV” Nucleic acid (RNA)

capsomere

Capsid

Tobacco mosaic virus has a rod-like appearance. Its capsid is made from 2130 molecules of coat protein (see image to the left) and one molecule of genomic single strand RNA 6400 bases long. Virions are ~300 nm in length and ~18 nm in diameter.

Physicochemical properties TMV is a thermostable virus. On a dried leaf, it can withstand up to 120 degrees Fahrenheit (50 °C) for 30 minutes. Host: It is known to infect members of nine plant families, and at least 125 individual species, including tobacco, tomato, pepper (all members of the useful Solanaceae), cucumbers, and a number of ornamental flowers. Entry and Replication: It Enters through abrasions on plant tissue then Starts Replication. It Can easily move for long distance through phloem. Virus can easily spread through human hands from one plant to another.

Disease: It cause a disease Mosaic (calico) in Plants. Symptoms: There are following symptoms of this disease. 1.	It cause light green coloration between veins of young leaves. 2.	Mottled pattern of light and dark green areas in the leaves. These are more prominent on younger leaves. 3.	If infection occurs in early age then growth is stunned. 4.	Blisters appears on the leaves. Infected leave may be crinkled, puckered, or elongated. Treatment/management One of the common control methods for TMV is sanitation, which includes removing infected plants, and washing hands in between each planting. Crop rotation should also be employed to avoid infected soil/seed beds for at least 2 years. As for any plant disease, looking for resistant strains against TMV may also be advised. Furthermore, the cross protection method can be administered, where the stronger strain of TMV infection is inhibited by infecting the host plant with mild strain of TMV, similar to the effect of a vaccine.