Download Plant Viruses

Plant Viruses
Dr. Ernest Hiebert
Plant Pathology, 1444 Fifield Hall
392 3631 216
[email protected]
Introduction
Overview of plant viruses.
 Life cycle of a typical RNA plant virus
 Genomic organization and replication
strategies of selected plant viruses
 How are plant viruses controlled?

Relative sizes and comparative virion structures resolved by x-ray crystallography
Genus Tobamovirus
ssRNA(+), rod-shaped virion morphology, no known vector
Tobacco mosaic virus
http://www.rc i.rutgers.edu/~ bhillman/comparative_virology/
http://www.rc i.rutgers.edu/~ bhillman/comparative_virology/
Genus Tobamovirus
ssRNA(+), rod-shaped virion morphology, no known vector
Tobacco mosaic virus
Cap at 5’
MT = capping,
HEL = helicase,
POL = polymerase,
6395 nts
Genomic organization and expression of type member
Tobacco mosaic virus of the genus Tobamovirus
•Rod-shaped virions 18 nm diameter and 300 nm long.
•
•Virions very stable.
•Coat protein MW = 17.5 kDa
•No known vectors.
Genomic organization and expression of type
member Tobacco mosaic virus of the genus
Tobamovirus
Genome ssRNA, messenger sense, monopartite, 6395
nucleotides, contains at least four genes.
•Capped 5’ end, t-RNA at 3’ end.
•Viral RNA consists of a 5'-end untranslated region of 68
nts.
•Open reading frame at the 5’end codes for a 126 kDa
protein and a readthrough of the UAG terminator signal
by plant suppressor Tyr-tRNA to produce 183 kDa protein.
•Genes downstream expressed via subgenomic RNAs.
Genome expression strategies involve subgenomic RNAs,
and read-through translation
How do plant viruses initiate infection?
Virus must enter via a wound in
the plant cell wall and must
be at the site during injury.
Cotranslational
disassembly
Replication
Schematic presentation of the“life cycle” of tobacco
mosaic virus
Assembly
Virus spread from infection
site
Replication within infected cell
 Virions or viral genome move cell-to-cell
via plasmodesmata
 Facilitated by a virus-encoded movement
protein.
 Long distance movement via vascular tissue

Cell-to-cell movement via
plasmodesmata, channels through
plant cell walls which allow for
passage of metabolites between
adjacent cells
Illustration showing the spread of
Tobacco mosaic virus in a
tomato plant
Agrios, G. N. Plant Pathology, 4e, Academic Press
Family Potyviridae, Genus Potyvirus
ssRNA (+) flexous rod-shaped morphology, vectored by aphids
Potato virus Y, necrotic strain infecting potato
Family Potyviridae, Genus Potyvirus
ssRNA (+) flexous rod-shaped morphology, vectored by aphids
~9500 nts
HC-PRO= helper factor (vector), protease, suppression of host
defense
AI amorphous inclusions
NI nuclear inclusions
Light micrograph of infected cell
showing cylindrical inclusions (CI)
Potyvirus life cycle
Encyclopedia of Virology, 2e, Academic
Press
Viral translation strategies
Polyprotein synthesis
http://www.rc i.rutgers.edu/~ bhillman/comparative_virology/
Picornaviruses- Entire (+) sense RNA genome is translated into a single
large polyprotein. Processing is carried out by two virus encoded
proteases 2A pro and 3C pro.
Flaviviruses- Viral precursor proteins are processed by cellular proteases.
The (+) sense RNA genome is translated into a polyprotein precursor
processed by viral serine protease and by host signal peptidase.
Potyvirus group of plant viruses- Potato virus Y and tobacco etch virus
contain a (+) sense genome RNA of around 10,000 bases which has a
single open reading frame. This polyprotein is processed by viral
encoded proteases.
Potyviruses
Comoviruses
Family Bunyaviridae, genus Tospovirus, type member Tomato spotted
wilt virus (TSWV) Negative and ambisense , single-stranded RNA
Negative sense single stranded
RNA virus
Family Bunyaviridae, genus Tospovirus, type member Tomato spotted
wilt virus (TSWV)
Family Bunyaviridae, genus Tospovirus, type member tomato spotted
wilt virus (TSWV) vectored by thrips, replicates both in the vector and
in the plant host
Family Bunyaviridae, genus Tospovirus, type member Tomato spotted
wilt virus (TSWV)
Family Bunyaviridae, genus Tospovirus, type member Tomato spotted
wilt virus (TSWV)
Family Bunyaviridae, genus Tospovirus, type member Tomato spotted
wilt virus (TSWV)
The ORFs of the M and S RNAs are expressed by subgenomic
mRNAs.
Expression strategy includes tripartite genomic organization,
cap snatching, negative/ambisense coding, subgenomic mRNAs,
and proteolytic processing.
Circular ssDNA Family Geminiviridae
Genus Begomovirus
Tomato yellow leaf curl virus
Circular ssDNA Family Geminiviridae
Genus Begomovirus
Tomato yellow leaf curl virus
2700 nts
V2 movement
V1 coat protein
C1 replication
C2 suppression of host
C3 replication enhancement
C4 pathogenicity
IR origin of replication
Circular ssDNA Family Geminiviridae
Genus Begomovirus
Conventional Approaches for the control of plant
viruses
•Breeding for resistance to plant viruses and virus vectors
•Control of plant viruses by cross-protection
Control of Vectors
•Insecticides
Exclusion of Plant Viruses
•Quarantine and certification
•Avoidance of plant viruses through cultural practices
•Certification for plant viruses
•Seed, root stocks, fruit tree propagative material
Elimination
•Heat treatment, meristem tip culture, and tip micrografting
Molecular Approaches for the control of plant
viruses
Pathogen-derived resistance - gene silencing
Post-transcriptional gene silencing
RNA silencing acts as a natural defense mechanism
against invasive nucleic acids, including viruses,
transposons and perhaps other highly repetitive
genomic sequences.
RNA silencing also plays a pivotal role in plant and
animal development by providing an elegant system of
gene control that can occur through RNA degradation,
translational inhibition or chromatin modification.
http://www.sciencemag.org/cgi/reprint/309/5740/1518a.pdf
Symptoms due to PTGS
Commercial zucchini with engineered resistance to
Zucchini yellow mosaic potyvirus
An article in “Discover” Magazine January 2000 describes how
ultraviolet absorbing film over greenhouses can protect against
insect-borne viruses.
Yehezkel Antignus . 2000. Manipulation of wavelength-dependent
behaviour of insects: an IPM tool to impede insects and restrict epidemics
of insect-borne viruses. Virus Research 71 (2000) 213–220