1. Section 3 Plant Pathogenic Virus

2. Definition of virus: Virus is a special particle usually consisting of nucleic acid molecule and coat proteins called capsids( 衣壳 ). And can reproduce itself in suitable host cell. Common characters:  Very small (ultramicroscopic), and uncellular organism.  Containing a kind of nucleic acid (DNA or RNA).  Depending on their own nucleic acid to multiply.  Lacking intact( 完整的 ) enzyme and energy system, and depending on the host cell for energy and all growth materials.  Obligate intracellular parasite.

3. 1 Shape and structure of virus  Spherical ---polyhedron( 多面体 ) symmetry; Bacilliform, linear---helix symmetry.  Consisting of nucleic and protein. in addition, including water, mineral element, lipid, polyamines( 多胺 ) and enzyme system. Nucleic acid of most virus is RNA.

4. 2 Multiplication When a virus is introduced into the cytoplasm of a host cell, the protein coat is shed( 除去 ) from the nucleic acid and the nucleic acid core instructs the cell to begin manufacturing more virus RNA or DNA and virus coat protein. The RNA or DNA and protein are manufactured separately and later assembled in the cytoplasm to form virus particles. At last, the virus particles are released out of the cell and infect other cells.

5. 3 Denomination of virus  Not Latin binomial nomenclature( 拉丁双命名法 )  At present, denomination of virus is introducing popular name method( 俗名法 ), i.e. English name of host + symptom. e.g. Tobacco mosaic virus.  Genus name is special international appellation ( 名称 ), i.e. abbreviation of representative host name + abbreviation of main character + virus. E.g. tobacco mosaic virus: Tobmovirus.

6. 4 Foundation of classification  Nucleic acid type of virus---DNA/RNA  Strand number of nucleic acid --- single/double  Whether or not existing lipoprotein envelope  Virus shape  Virus segment (i.e. multipartite)  In addition, virus also can be classified on the basis of serology ( 血清学 ), chemical properties, vectors, host infected, or combination of these.

7. 5 Main groups of virus 5.1 Tobacco mosaic virus (TMV)  The TMV particle is rod shaped, about 300µm long ×15µm in diameter. The protein subunits are arranged in a helix around a spiral core of RNA.  The virus is one of the most stable viruses known and can remain infective for at least 50 years in dead, dried tissues and for many months on flats, greenhouse frames, sawdust, tools, and in the soil. Tobacco mosaic virus Typical mosaic symptoms

8.  There are many strains of TMV. Some cause severe damage on a given host, whereas other strains cause such mild symptoms that they are barely visible.  Tobacco mosaic virus is transmitted mechanically. Any means that permit the virus to touch an injured cell of host plant may result in infection. One of the most common ways the virus is spread is by weeding, transplanting, cultivating, pruning( 修剪 ), or tying( 捆绑 ) operations. Tractors or equipment that brush against a TMV-infected plant may spread the virus to other plants.

9.  Tobacco mosaic virus overwinters in a number of ways; the virus can survive for years in dried tobacco tissue, also overwinters in stalk and root tissue of mosaic-diseased plants from previous crop.  Tobacco, pepper, and eggplant also are host of TMV and fruit from these crops may contain infective virus.

10. 5.2 Cucumber mosaic virus(CMV)  rounded in profile; 29 nm in diameter; no envelope( 包膜 ); without a conspicuous capsomere( 衣壳 ) arrangement  Genome consists of RNA; single- stranded; linear. Total genome size 8.621 kb. Genome of three parts. Cucumber mosaic virus yellowing and mottling of the older leaves. Expanding leaves become twisted, curl downward, Plants are stunted, and fruits are covered with bumpy protrusions

11.  The cucumber mosaic virus has one of the broadest host ranges of any of the tomato viruses. The disease affects a number of important vegetables and ornamentals including tomato, pepper, cucumber, squash( 南瓜 ), spinach( 菠菜 ), celery( 芹菜 ), beets( 甜菜 ), and petunia( 矮牵牛花 ).  The cucumber mosaic virus overwinters in perennial weeds.The cucumber mosaic virus cannot withstand dryness or persist in the soil.  Transmitted by an insect; more than 60 spp. including Acyrthosiphon pisum( 豌豆蚜）, Aphis craccivora （苜蓿蚜） and Myzus persicae （桃蚜）, etc. Transmitted in a non- persistent manner. Virus also can be transmitted by mechanical inoculation; transmitted by seed.

12. 5.3 Potato Y virus ( 马铃薯 Y 病毒 )  Virions filamentous; usually flexuous( 弯曲的 ); not enveloped; with a clear modal length of 750 nm; 11-15 nm in diameter.  Virions contain one molecule of linear positive-sense single stranded RNA. Virions associated with helper virus( 辅 助病毒）, but independent from its functions during replication ．  the virus mainly infect solanaceae( 茄 科 ), e.g. potato, tomato, tobacco ． Potato Y virus Wrinkling of leaves Tuber necrosis

13.  Many species of aphids are able to transmit PVY, and one of the most effective vectors is green peach aphid, Myzus persicae( 桃蚜）. Aphids only need to feed on an infected plant for a very short time, as little as 20 seconds, to become infected with the virus.  PVY can be carried from one potato crop to the next through use of infected seed.  Virus transmitted by mechanical inoculation; transmitted by grafting( 嫁接 ). Virus does not require a helper virus for vector transmission and can help the vector transmission of another virus.