The simplest virions consist of two basic components: nucleic acid (single- or double-stranded RNA or DNA) and a protein coat, the capsid, which functions as a shell to protect the viral genome from nucleases and which during infection attaches the virion to specific receptors exposed on the prospective host cell.

1. Mr. Sandesh V. Pawar
Dept. of Plant Pathology
Dr. PDKV., Akola

3. 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.

4.  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.

5.  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.

6. 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)

7.  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.

8. 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.

9.  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.

10. Viral Structure
Morphologically the plant viruses fall into two broad
categories;
A) Anisometric (Rigid or flexous rods and bullet shaped)
B) Isometric ( Polyhedral shaped)

11. Flexuous thread
like virus
Rigid rod shaped
Side arrangement of
protein subunit
Short bacillus like
virus
Isometric polyhedral
virus

12. 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

13.  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.

14. 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.

15. 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.

16.  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.

17. 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.

18. 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.

19. 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.