Plant viruses come in various shapes and sizes but generally contain RNA or DNA as their genetic material within a protein coat. They multiply only within living plant cells and can cause symptoms such as mosaic patterns, leaf curling, stunting, and chlorosis. The tobacco mosaic virus is a well-studied plant virus that is rigid and rod-shaped, containing RNA within a protein coat composed of 2130 subunits. It measures around 300 by 18 nm and can be inactivated by heat at 93 degrees Celsius.

1. Plant viruses
N.H. SHANKAR REDDY
2nd Ph.D. Plant Pathology
Annamalai University

2. Virus
• Virus the word derived from Latin it means
POISON
• Virus is the sub-micosopic, mesobiotic infectious
agent that contains nucleoprotein only multiplies in
living cells
• It contains either DNA or RNA but never both
• The term virus was coined by “Beijerinck”

3. Properties
 Most of the plant pathogenic viruses contains RNA as genetic material (almost 78%
of plant viruses)
 Virus does not contain cell wall, but it contains protein coat as protective layer, and
the amount of protein coat and amino acids compositions may vary with viruses.
 The viruses produce symptoms like mosaic, leaf curl, crinkling, stunting, chlorosis,
necrosis, vein clearing, vein banding and enation.
 Viruses moves from cell to cell through ‘Plasmodesmata’

4. Shape may vary with virus to virus
 Rigid rod: (E.g - Tobacco Mosaic Virus (TMV) and
Tobacco rattle Virus (TRV)
o Flexuous rod: (E.g - Potato virus X (PVX), Bean
Common mosaic virus (BCMV).
o Filamentous rod: (E.g - Rice grassy stunt (RGSV)
and Rice stripe virus (RSV).
 Isometric: (E.g - Rice tungro spherical virus (RTSV),
Cucumber mosaic Virus (CMV), Tomato spotted wilt
virus (TSWV).
 Bacilliform: (E.g - Rice tungro bacilliform Virus
(RTBV), Banana streak virus (BSV) and Cocoa swollen
shoot virus (CCSV).

5. Morphology of plant viruses
Plant viruses are different shapes (rigid rods or flexious threads, spherical, cylindrical rods,
elongated) and sizes ranges from 15-300 nm width to 480-2000 nm length
Nucleic acid
o The composition of NA ranges from 5 - 40 %
(TMV – 5 %)
o Its contains RNA or DNA as genetic material, but
never both
Protein coat
o The composition of protein ranges from 60 - 95 %
(TMV – 95 %)
o The protein coat is called ‘capsid’, the structural
subunits of capsids are called ‘capsomere’

7.  Capsid – The protein coat/ shell that encloses the viral
genome (either DNA or RNA)
 Capsomere – The structural protein subunits of capsid,
also called as morphological subunit.
 Virion – The mature virus particle that may be
membrane bound;
 Protein subunit – Individual virus encoded protein
molecule that make up the capsomere or nucleoprotein,
also known as structural subunit.

8. Tobacco mosaic virus (TMV)
 TMV belongs to the genus Tobamovirus, which is a sub
microscopic, rigid, rod-shaped particle which produces mosaic
like symptoms in plants.
 In recent ICTV classification Tobamovirus comes under the
family of Virgaviridae
 TMV is a single standard RNA (+ss RNA) made up of 5 % NA
(RNA) and 95 % protein
 TMV consists of 158 - Amino acids
2130 - Capsomere subunits (coat protein, CP)
6400 - Nucleotides (6.4 kb)
 TMV amino acid molecular weight 17,600 daltons (17.6 kDa)
 TMV is a rigid helical rod shaped virus particles and measure
about 300 × 18 nm
 TIP (Thermal inactivation point) of TMV is 93 °C