1. The document discusses the replication strategy of viruses in plants. It describes the key steps: entry into the plant cell, uncoating of the viral nucleic acid, replication of the nucleic acid, synthesis of viral proteins, and assembly of new virus particles. 2. Different types of plant viruses contain double stranded DNA, single stranded DNA, or single or double stranded RNA as their nucleic acid. The virus uses the host cell's machinery for energy production, protein synthesis, and other functions to replicate. 3. Replication of single stranded RNA viruses involves the viral RNA acting as mRNA to produce an RNA-dependent RNA polymerase. This polymerase then produces complementary RNA which serves as the template to generate more copies of the original

1. Virus infection and virus
synthesis
• Virus infection and virus replication - synonymouslyused
Presented by :
Laxmi Prasad Joshi
PLP-09M-2019
Department of plant
Pathology
AFU, Rampur ,Chitwan

2. Virus Replication Strategy /Infection
process
STEPS,
1. Passage of virus through the cell wall .
2 . Entry of virus or its nucleic acid into cells and then
to replicative sites in cells
3. Removal of protein from nucleic acid
termed as “uncoating”
4. Translation of viral genome into replicase or a
protein of that enzyme .
5. Replication of viral nucleic acid.
6. Replication of coat protein .
7. Assembly of new virus . Agrios 2005

4. How virus enters the plant cell ?
a) Mechanical wounds or insects damage (wounds)
1. Mechanically :- TMV, BCMV, PVY, PYDV
2. Vectors :- PLRV, TLCV, WTV
b) By vector
c) Deposition into an ovule by an infected pollen grain
e.g. Prunus necrotic ring spot, Tobacco ring spot of Soyabean, Barley stripe mosaic
virus
d) Seeds or vegetative propagation
e.g. BCMV, CMV, Pea stripe virus
e) Fungal parasite
e.g. TNV, Barley yellow mosaic virus , Tobacco stunt virus .

5.  Types of plant virus nucleic acids
a. Double stranded (ds) DNA (rare)
b. Single stranded (ss)DNA (rare)
c. ssRNA , negative sense (rare) (can’t be translated directly )
d. ssRNA, Positive sense (common)
e. dsRNA (rare)
 Viruses does not produce any kind of reproductive structure , they multiply by Using host machinery .
 Host functions used by plant viruses .
1. Energy: - nucleoside triophosphate .
2. Protein synthesis :- Ribosome (80s Cytoplasmic ribosome) , tRNA and associated
enzymes and factors of host
3. Nucleic acid synthesis
4. Structural components of the cell :- ER, cytoplasmic skeleton.

6. Synthesis of RNA viruses
Carbonell et.al.2018 , eLS

7. Simplified replication of ssRNA virus nucleic acid
1. Uncoating of viral protein coat
 Uncoating ability Depends upon structure of virus coat protein .
2. Viral RNA Induces the host cell to form the viral RNA polymerase(RdRp) ,
• Itself act as mRNA.
• Also produces protein that inhibit host protein and host RNA synthesis .
3. Enzyme RNA polymerase utilizes the viral RNA as a template and
forms complementary RNA (-ve sense ).
• The first new RNA produced is not the viral RNA but a mirror image .
• Complementary RNA temporarily attached to viral strand
• soon separates to produce the original virus RNA and mirror image .
4. Later mirror image serves as the template for more virus (RNA synthesis )
Carbonell et.al.2018 , eLS

8. Minskaia and Luke,2015 , J Virol Retro
 In ssRNA rhabdovirus (negative sense RNA virus )
RNA is not infectious because it is complementary strand .
This RNA must be transcribed by a virus carried enzyme
called transcriptase ,
 RNA-dependent RNA polymerase is associated with the
input genome
 and transcribes mRNA molecules as the first step of
replication.
 Translation of primary transcripts produces proteins which
together with full-length (+) cRNA form RNP replicative
intermediate. This then serves as template for new
genome synthesis.
 Both vRNA and cRNA are exactly complementary and
encapsidated by nucleocapsid (N) protein, while mRNA is
3' truncated
Carbonell et.al.2018 , eLS

9. In dsRNA (isometric viruses)
 The RNA is segmented
within the same virus ,
 Non infectious and depends
for its replication in the host .
 Transcriptase enzyme also
carried within the virus .
 RNA dependent RNA
polymerase (RdRp)
Viral RdRp synthesizes and releases into the cytoplasm (+) strands that
are first translated and then packaged to form immature virions.
Virions mature by synthesising (−) RNA and by the addition of other viral protein Carbonell et.al.2018 , eLS

10.  In dsDNA virus ,
 viral DNA enters the cell nucleus - become
twisted and supercoiled forms mini chromosome
.
 Later transcribed into two ssRNA Using cellular
polymerase II
 Smaller RNA is transported to cytoplasma –
where it is translated into virus coded protein and
 large RNA is also transported to the same
location in the cytoplasma , but it becomes
encapsidated by coat protein sub units and is
Used as a template for reverse transcription into
complete virion dsDNA .
 Reverse transcription occurs in Electron dense
viroplasma .
The CaMV replication cycle.

11.  In ssDNA of ssDNA plant
virus have no method of
replication determined
with certainty yet (Agrios,
2005).
Rolling circle that produces
a multimeric complementary
strand which serves as
template for the production
of multimeric strands that are
then cleaved to produce unit
length .
Fig : Begomovirus life cycle
Bowdoin et al 2013 , nat rev microbiology

12.  Viruses in which different RNA
segments are present within two or
more virus particles ,All particles
must be present in the same cell for
the virus to replicate and for infection
to develop .

13. Formation of coat protein
 As soon as new viral nucleic acid is formed , it will be
translated
 Induces host cell to produce the protein molecule coded by
viral nucleic acid .
 Protein synthesis depends on presence of amino acids and
the cooperation of ribosomes, messenger RNA , and
transfer RNA.
mRNA the part of viral RNA coding for the viral protein.
The virus utilize the amino acid , ribosome , and transfer
RNA of the host .

14. When new virus nucleic acid and virus
proteins subunits have been produced , the
nucleic acid organize the protein subunits
around it
 the two are assembled together to form the
complete virus particle , the virion.
Assembling of viral nucleic acid and
protein

15. Sites of Multiplication
 The site of the cell in which virus nucleic acid and protein
are synthesized and in which these two components are
assembled to produce the virions varies with the particular
genus or family of the virus
RNA viruses :- RNA replicate in cytoplasm, produces viral
protein subunit in cytoplasm and finally assembly of virion
follows also in the cytoplasm .
ssDNA :- Every things seems to take place in the nucleus . From
which the virus particle are then released into the cytoplasm .

16. Sites of Multiplication
In some cases , chloroplast and other
organells perform the same function.
In potex , Caulimo , Gemini and
Tobamo virus replication and assembly
take place in nucleus and from where it
moves to cytoplasm.
In Tymovirus replication takes places in
chloroplast