Section 3 Plant Pathogenic Virus

Definition of virus:

Virus is a special particle usually consisting of nucleic acid

molecule and coat proteins called capsids( 衣壳 ). And can repr

oduce 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 de

pending on the host cell for energy and all growth materials.

Obligate intracellular parasite.

1 Shape and structure of virus

Spherical ---polyhedron( 多面体 ) symmetry; Bacilliform, linear---helix symmetry.

Consisting of nucleic and protein. in addit

ion, including water, mineral element, lipid, p

olyamines( 多胺 ) and enzyme system. Nuclei

c acid of most virus is RNA.

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.

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 nam

e + abbreviation of main character + virus. E.g. toba

cco mosaic virus: Tobmovirus.

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 b

asis of serology ( 血清学 ), chemical properties, ve

ctors, host infected, or combination of these.

5 Main groups of virus5.1 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

There are many strains of TMV. Some cause seve

re damage on a given host, whereas other strains ca

use such mild symptoms that they are barely visible. Tobacco mosaic virus is transmitted mechanica

lly. Any means that permit the virus to touch an inju

red cell of host plant may result in infection. One of

the most common ways the virus is spread is by we

eding, transplanting, cultivating, pruning( 修剪 ), or t

ying( 捆绑 ) operations. Tractors or equipment that b

rush against a TMV-infected plant may spread the vi

rus to other plants.

Tobacco mosaic virus overwinters in a nu

mber of ways; the virus can survive for years i

n dried tobacco tissue, also overwinters in stal

k and root tissue of mosaic-diseased plants fro

m previous crop.

Tobacco, pepper, and eggplant also are ho

st of TMV and fruit from these crops may cont

ain infective virus.

5.2 Cucumber mosaic virus(CMV)

rounded in profile; 29 nm in diam

eter; no envelope( 包膜 ); without a c

onspicuous capsomere( 衣壳 ) arran

gement

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

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 A

cyrthosiphon pisum( 豌豆蚜） , Aphis craccivora （苜蓿蚜） and Myzus persicae （桃蚜） , etc. Transmitted in a n

on-persistent manner. Virus also can be transmitted by m

echanical inoculation; transmitted by seed .

5.3 Potato Y virus ( 马铃薯 Y 病毒 ) Virions filamentous; usually flexuou

s( 弯曲的 ); not enveloped; with a clear

modal length of 750 nm; 11-15 nm in di

ameter.

Virions contain one molecule of linea

r positive-sense single stranded RNA. V

irions associated with helper virus( 辅助病毒） , but independent from its functi

ons during replication ． the virus mainly infect solanaceae( 茄科 ), e.g. potato, tomato, tobacco ．

Potato Y virus

Wrinkling of leaves

Tuber necrosis

Many species of aphids are able to transmit PVY,

and one of the most effective vectors is green pea

ch aphid, Myzus persicae( 桃蚜） . Aphids only nee

d to feed on an infected plant for a very short time,

as little as 20 seconds, to become infected with th

e virus.

PVY can be carried from one potato crop to the next through use of infected seed.

Virus transmitted by mechanical inoculation; tra

nsmitted by grafting( 嫁接 ). Virus does not require

a helper virus for vector transmission and can hel

p the vector transmission of another virus .