Mr. Sandesh V. PawarDept. of Plant Pathology

Dr. PDKV., Akola

The main chemical composition of plant viruses are

a) Protein

b) Nucleic acid (either DNA or RNA never both)

A) Protein Major structural component of plant viruses.

Serving as a protective coat for the delicate nucleic acid

genome.

It provides the basis for serological differentiation of

plant viruses.

Proteins are composed of about 150-600 or more

residues of some 20 amino acid.

Most plant viruses are built of one kind of protein

sub units.

In protein contain amino acid, and they carry an

amino group (-NH2) and the carboxyl group (-COOH)

of atom carrying the chemical residues as a side chain.

The amino acid condensed to polymerize into a

linear polypeptide chain.

The specific sequence of amino acid , linked by

peptide bonds, is the first level of protein organization

and called Primary structure.

The polymerized peptide structures are usually

spirally or helically bound to form the Secondary

structure.

Further bending or folding of polypeptide chain over

itself is called Tertiary structure.

The linear polypeptide has two ends that is C-terminal

end and N-terminal end.

The primary structure of protein molecules results

from strong covalent peptide bonds (-CO-NH-).

The secondary and tertiary structures are maintained by

a few strong covalent disulphide (-S-S-) bonds but

mostly by a large number of weak hydrogen bonds.

There are few plant viruses such as potato yellow

dwarf and rhabdoviruses which contain complex protein

configuration containing more than one type

polypeptides and forms quaternary structure.

Amino acids

Alanine (Ala)

Arginine (Arg)

Aspartic acid (Asp)

Asparagine (Asn)

Cysteine (Cys)

Glutamine (Gln)

Glutamic acid (Glu)

Glycine (Gly)

Histidine (His)

Isoleucine (Ile)

Leucine (Leu)

Lysine (Lys)

Methionine (Met)

Phenylalanine (Phe)

Proline (Pro)

Serine (Ser)

Tryptophan (Trp)

Tyrosine (Tyr)

Threonine (Thr)

Valine (Val)

The capsid proteins fundamentally do not differ from

other types of proteins in any way except that they may

be deficient in some commonly occurring amino acid

such as histidine and methionine in TMV and

tryptophane in closteroviruses.

The organizational and constitutional configuration of

protein molecules of capsid confers it resistance to

enzyme like protease and high temperature.

The protective role of viral protein has two very

important biological functions, determining serological

affinity and vector transmission.

Nucleic acid

The plant viruses contain either DNA or RNA as a

genetic material.

Most of the plant viruses contain RNA except a few

such as caulimoviruses, geminiviruses, banana bunchy

top and badnaviruses which contain either single or

double stranded DNA.

The nucleic acid may be presents as a single

continuous strand in a particle or may be distributed in

two or more pieces in different particles are called

multipartite virus or divided genome.

The RNA and DNA may again be either single stranded

or double stranded.

Most of the plant viruses are ssRNA with a few

exceptions such as clover wound tumor and rice dwarf

virus which have dsDNA.

The structural configurations of nucleic acid are similar

to protein in the form of primary, secondary and tertiary

structures.

The sequence of nucleotides into a long poly-nucleotide

chain constitutes the primary structure.

The ssRNA in a free state folds upon itself into loop or

helix due to transmolecular hydrogen bonding between A,

U and G, C base pair these structure called secondary

structure.

Viral Structure

Morphologically the plant viruses fall into two broad

categories;

A) Anisometric (Rigid or flexous rods and bullet shaped)

B) Isometric ( Polyhedral shaped)

Flexuous thread like virus

Rigid rod shaped

Side arrangement of protein subunit

Short bacillus like virus

Isometric polyhedral virus

Rod shaped particle

All rods are helical in structure. The rod

shaped viruses may be rigid rods as are TMV

particles, or flexuous like the potyvirus.

Rigid rod shaped

TMV is most widely studied virus in this group.

The entire TMV particle is very stable due to

interaction between protein sub unit and RNA.

The particle measures about 300 nm x 15 nm.

The molecular weight of particle is 39.4 X 106

daltons

It consist of about 2100 helically arranged identical

protein sub units along with axial canal.

Each with molecular weight of 17500 Daltons and

consisting of 158 amino acid residues.

The pitch of the helix is 2.3 nm and the particle

structure repeats every 6.0 nm of its length each three

turns of the helix.

The nucleic acid strands has a molecular weight of 2

x 106 Daltons containing some 6400 nucleotides which

follows the pitch of helix and is embedded between the

protein sub units 4 nm from the particle axis.

Flexuous rods

The particles are not straight but show more or less

marked bending.

The particle of potexviruses have flexuous particle

480 to 580 nm long and 11 to 13 nm with obvious

cross bandings.

The particle have basic helix pitch of 3.3 nm and

3.6 nm depending on the dryness and wetness of the

particle respectively.

The particle of carlaviruses have straight and rigid

structure but having a slight bend at one end.

They measures 650 nm in length and 12 nm in

width with bandings.

Isometric particle

The isometric or icosahehedral virus particle are

complex in their structure and architecture.

An icosahedron is a polyhedron with 20 equilateral

triangular faces.

It shows three fold symmetry when rotated around the

axis through the center of each triangular face.

There are 12 vertices when the five triangle concern

meet and thus 60 asymmetric units are required to

construct an icosahedron.

The surface of an icosahedron are divided into large

number of smaller identical triangle called triangulation

number.

A simple icosahedron having three triangulation

number and nine triangle faces may have 180 sub units

These sub units are clustering in different ways,

having three clustering pattern

1) Trimer capsomere:- 3 sub units cluster.

2) Pentamer capsomere:- Sub units cluster at a vertice

where faces of five triangles meet.

3) Hexamer capsomere:- Sub units cluster at a vertice

where faces of six triangles meet.

Virus Particles with Membrane

Spherical particle

Tomato spotted wilt virion has rounded irregular

shape, measuring about 70 nm diameter in situ.

The particle resemble the leuko viruses of animal.

The membrane binding the particle is about nm

thick which is composed of protein sub units.

It has been suggested that the tomato spotted wilt

virus particle is pleomorphic myxovirus.

Another virus in this group is carrot mottle having

more or less rounded particles which measures about

50 nm in diameter.

Bacilliform or Bullet shaped particles

This group of plant viruses are of complex structure.

They resemble some animal viruses such as

vesicular stomatitis and cocal virus.

The group includes plant viruses such as maize

mosaic virus, lettuce necrotic yellows and sow thistle

yellow vein.

The particle of this viruses are very fragile.

Most are 300 to 400 nm long and 50 to 80 nm wide.

The membrane around the helical core is lipidwhich

might be acquired by core while passing through the

host membrane.

Geminate particles

Gemini virus are unique plant pathogenic group of

viruses having geminate or twin particle(dimer)

constituted by two icosahedral monomers.

The two monomers are joined at the discontinuities

created by missing capsomers to give the

characteristics flattened appearance at the interface.

The dimer particle shows groove at the centre of its

interface.