Plant Virology Flashcards
(107 cards)
1
Q
Where was wheat domesticated?
A
Near East
2
Q
Where was corn domesticated?
A
Central and South America
3
Q
Where was rice domesticated?
A
India and China
4
Q
What is the name for the transition from hunter-gatherers to gamers and herders?
A
Agricultural revolution
5
Q
How much does the population rise by each day?
A
200,000
6
Q
How has food production paralleled population increase? (2 ways)
A
Increase the amount of land used to produce food
Increase the amount of food produced per season on the land already being used for agriculture
7
Q
More than 85% of our food comes from how many plant species?
A
6-8 (mostly wheat, corn and rice)
8
Q
Info about sweet potatoes
A
One of earliest domesticated plants
Origin: South America
Spread to Polynesia in 8th century AD
Spread to Asia in 16th Century
9
Q
What are sweet potatoes a good source of?
A
Energy, carotene, ascorbic acid, niacin, riboflavin, thiamin and minerals
10
Q
Food problems:
A
Very little fertile land left to create new farmland
Yields are being pushed to the limits through Green Revolution and biological/technological advancements
Environmental pressures
11
Q
Name 5 biological and technological advances that revolutionised farming
A
Introduction of new crops Mechanisation New and improved varieties Inorganic fertilisers Pesticides / herbicides
12
Q
What caused introduction of new crops?
A
Exchange of plants between continents after European voyage of discovery in 15th and 16th centuries
13
Q
When did mechanisation begin?
A
Middle of 19th century - invention of internal combustion engine
14
Q
When did application of genetics to plant breeding begin?
What % increase in crop productivity did this bring about?
A
18th/19th centuries
40%
15
Q
What process was invented that allowed nitrogen fertiliser to be produced?
A
Haber process - combining hydrogen gas and nitrogen gas to produce ammonia
16
Q
What are further crop improvements have been made?
A
High crop yield High nutritional quality Nitrogen fixation Drought resistance Resistance to pests Insensitivity to photoperiod Plant architecture Removal of toxic or unwanted compounds
17
Q
What is plant architecture?
A
Positioning of leaves, branching pattern of stem, the height of the plant and positioning of organs
18
Q
When was the Irish potato famine and how many people died?
A
1846-1850
Up to a million
19
Q
When did brown spot of rice occur and what was it? How many deaths did it cause?
A
1942
Weather favoured H. oryzae, and enabled it to suppress rice yields
Rice prices soared
2 million people died
20
Q
What was African Cassava Mosaic Disease, when did it occur and what happened?
A
Cassava Mosaic Virus Disease spread across East and Central Africa
1980s - early 1990s
In 1994 alone, up to 3000 people died of famine-related illnesses in Uganda
500 million people relied on cassava
21
Q
What was the first identified virus, and who discovered it in what year?
A
Tobacco mosaic virus
Beijerinck
1898
22
Q
What is chlorosis?
A
Yellowing of leaves
23
Q
What was tulipomania?
A
A morphogenetic disturbance of tulips caused pink petals to be bleached white, but with a pink tinge on the edges. These tulips became so valuable that in 1625 they were worth tonnes of grain or many cattle
24
Q
Definition of a virus:
A
Gibbs & Harrison, 1976
A small transmissible parasite, with a small nucleic acid genome which needs host cell components for multiplication
25
How small are virus particles?
If 10 million particles occupied one plant cell, they would only occupy 1% of the cell’s space
26
What is the abbreviation for tobacco mosaic virus?
Tobamovirus
27
What are the four types of nucleic acid?
Single-stranded (ss) positive sense RNA
Single-stranded (ss) negative sense RNA
Double-stranded (ds) RNA
DNA viruses
28
What is single-stranded positive sense RNA?
This type of genome is ready to function as messenger RNA on entry into the host cell, and is usually infectious. It can be directly translated into the desired viral proteins
29
What is single-stranded negative sense RNA?
A negative sense genome has to be copied into positive sense before it can function. It does not encode mRNA. This copying is carried out by a viral-encoded enzyme found in the virus particles
30
What is double-stranded (ds)RNA?
Viruses with dsRNA also contain a viral encoded enzyme which copies the genomic RNA into mRNAs
31
What are DNA viruses?
These plant viruses use host enzymes to produce functional mRNAs. These DNA viruses are therefore able to initiate infection in the absence of viral-encoded proteins
32
What percentage of all plant viruses are positive sense ssRNA?
75%
33
Give an example of a DNA virus with double-stranded DNA
Caulimovirus (cauliflower mosaic virus)
34
Give an example of a DNA virus with single-stranded DNA
Geminivirus (maize streak virus)
35
What is a viroid?
Unencapsidated, small, single-stranded RNAs which replicate when inoculated onto plants. They code no protein, and have extensive internal base pairing which gives them rod-like secondary structures. Smallest infectious pathogen known. Classed as a ‘subviral agent’
36
Give two examples of viroids
Potato spindle tuber viroid
| Coconut cadang-cadang viroid
37
Info about potato spindle tuber viroid
Natural hosts are potatoes and tomatoes
No form of natural resistance
Symptoms most severe in hot conditions
359 nucleotides
38
List plant virus symptoms
```
Local lesions
Chlorosis
Mosaics
Necrosis
Morphogenetic disturbances
Effect on plant size
```
39
What is a local lesion?
A localised area of diseased tissue (chlorotic or necrotic). The virus is restricted to the necrotic lesion and a few layers of cells at the outer edge
40
What are mosaics?
Yellowing of leaves on a green background. In monocotyledons, a common result of virus infection is the production of stripes or streaks of tissue lighter in colour than the rest of the leaf
41
What is necrosis?
Tissue death, either by stress or via specific plant virus infections
42
What are morphogenetic disturbances?
‘Messed-up’ plants, parts of plants or organs. Growth reduction, colour deviation, wilting, malformation of specific organs, premature leaf drop or curling - all indicative of of aberrant (diverging from the normal type) hormone metabolism
43
What is synergism?
The association of two or more viruses acting at the same time
44
An example of synergism
Tobacco mosaic virus and potato virus Y acting together in Surfinia
45
Name some invertebrate vectors (with Latin names) of viruses
Whiteflies (Aleyrodidae - family)
Aphids (Aphididae - family)
Nematodes (Nematoda - phylum)
46
Whiteflies as vectors
Aleyrodidae
Known to transmit about 70 serious disease agents, mainly of tropical and subtropical plants. They cause problems in greenhouses in colder climates. African cassava mosaic virus is transmitted by whiteflies
47
Aphids as vectors
Aphididae
| Very successful exploiters of higher plants. They transmit about 66% of viruses with invertebrate vectors
48
Nematodes as vectors
They transmit several widespread and important viruses via the soil. They feed on root epidermal cells which can contain virus particles
49
What species of Nematodes transmit Nepoviruses (genus of viruses)?
Xiphinema (‘dagger nematodes’ - long stylets to reach tissue of plants)
Longidorus (transmit cacao necrosis virus)
50
What species of Nematodes transmit Tobraviruses (genus of viruses including the species ‘Tobacco rattle virus’)?
A species of Trichodorus (genus)
51
Why are nematode vectors so hard to control?
Pesticides only kill insects so they cannot spread viruses as easily, but it is very hard to kill nematodes in the soil
52
Besides invertebrates, what is another vector of plant viruses?
Fungal vectors
53
Give an example of a furovirus
Soliborne wheat mosaic virus
54
How are furoviruses such as the soliborne wheat mosaic virus transmitted? How long can the vector survive?
In the resting spores of the plasmodiophoromycete vectors
| In these spores the virus may survive in air-dried soil for many years
55
What species of fungus transmits tobacco necrosis virus (TNV)?
Olipidium brassicae
56
How does Olipidium brassicae transmit viruses? How long can the vector survive?
The viruses are carried in the surface of spores
| The zoospores carrying a virus can survive only a few hours
57
What is Rhizomania and what is it transmitted by?
A disease of sugar beet caused by the virus beet necrotic yellow vein virus (BNYVV)
Transmitted by the fungus Polymyxa betae
58
Why is it called Rhizomania?
It comes from the extensive proliferation of lateral roots, and also necrosis of vascular bundles and severe stunting of plants
59
What % loss of yield does beet necrotic yellow vein virus cause?
80%
60
What is mechanical transmission of viruses?
The introduction of infective virus or RNA into a wound made through the plant surface
61
What are some examples of plants that use mechanical transmission?
Tobacco mosaic virus (TMV) and potato virus X (PVX)
| Neither have insect vectors
62
How does sexual transmission work?
Virus transmitted through the seed of infected host plants
Viruses can persist in seeds for long periods of time, so that commercial distribution of seed-borne viruses can occur over long distances
63
What is an example of how a seed can become infected with a virus?
Sexual transmission via pollen
64
What fraction of plant viruses are transmitted via the seeds of host plants?
1/5
65
What are the different methods of virus transmission in plants?
```
Sexual transmission
Mechanical transmission
Fungal vector
Invertebrate vector
Bacterial vector
```
66
Who discovered tobacco mosaic virus and when?
Adolf Mayer, a German agricultural chemist working in Holland. He classified it as a ‘mosaic disease of tobacco’ and failed to culture it
1886
67
How was it proved that TMV was a disease smaller than anything previously described?
In 1892 Russian scientist Dimitrii Ivanovsky was studying TMV and reported that the sap of infected leaves remained infectious even after being filtered through Chamberland filter-candles (has pores smaller than bacteria)
68
How was the virus finally recognised?
In 1898, Martinus W. Beijerinck showed the infectious agent was able to replicate in leaves but not in filtered solutions
69
What are the firsts TMV is known for?
The first virus to be shown to consist of RNA and protein
The first virus to be characterised by X-ray crystallography
The first virus to be used to be used for electron microscopy
The first virus to be used for solution electrophoresis and analytical centrifugation
70
What is the size and shape of TMV? What is it’s genome like?
18 x 300 nm rigid rods
Positive sense ssRNA genome
6395 nucleotides
Encodes 4 viral proteins
71
What does TMV have at the 5’ (5 prime) end?
5’ CAP - a methylated guanine nucleotide
| The same as in normal eukaryotic mRNAs, where ribosomes bind and begin translation
72
What does TMV have at the 3’ terminus?
A tRNA-like structure (folded RNA secondary structure) which is unlike eukaryotic mRNAs, which have a poly(A) tail at the 3’ terminus
73
What is the purpose of a poly(A) tail
It stops RNA being eaten by enzymes so provides protection in eukaryotes
74
How does translation work in eukaryotes?
The ribosome moves along the mRNA strand and when it reaches a stop codon at the end of an open reading frame it ‘falls off’ the sequence and cannot bind to the RNA further down the sequence
75
Why is eukaryotic translation fine for eukaryotes?
Because each mRNA strand only codes for one protein
76
Why is eukaryotic translation a problem when translating viral RNA?
The one piece of single-stranded positive sense viral RNA codes for 4 proteins, so as a result there are silent / untranslatable open reading frames - the second open reading frame can never be translated, meaning that only one protein can be produced each time
77
What are the 4 proteins encoded for by viral RNA - what size are they and what is their purpose?
183kDa - replicase enzyme
126kDa - replicase enzyme
30kDa - transport protein
17.5kDa - coat protein
78
What is the solution that allows ribosomes to encode both the replicase proteins?
The genomic RNA acts as mRNA for a 126kDa protein, and about 10% of the time the ribosome reads through a ‘leaky’ stop codon (UAG), meaning that the RNA sequence continues to be translated until the 183kDa protein is produced
79
What are the replicase proteins also known as?
RNA-dependent RNA polymerases - both required for replication
80
The 30kDa and 17.5kDa proteins cannot be translated from genomic RNA as both open reading frames are internal - not at the 5’ end of the molecule. How are they translated?
The virus must generate sub-genomic RNAs with the open reading frames at the 5’ ends
81
In the replication of TMV, what type of RNA do the replicator proteins attach to and what type of RNA do they produce?
They associate with the 3’ end of the positive sense viral RNA and produce negative sense RNA
82
What type of RNA, produced by the replicator proteins, acts as a template to produce both full-length positive sense RNA and positive sense sub-genomic RNA?
Negative sense viral RNA
83
How do viruses move between cells in a plant?
They move through plasmodesmata, and travel in the phloem to other areas of the plant
84
What is the problem encountered by viruses when they try to move between plant cells?
The virus particles are too large to travel through plasmodesmata
85
How do viruses solve the problem of being too large to travel through plasmodesmata?
Viruses encode the 30kDa movement protein, which moves towards the plasmodesmata, binds to it and expands it. It then chaperones the virus particles through the plasmodesmata
86
What are the role of the 17.5kDa coat proteins?
They encapsidate the genomic RNA and form the virus particle
87
What are the 3 ways to prevent virus epidemics?
1) Plant healthy seeds/tuber that are tested to ensure they are virus-free. However, this is expensive and labour intensive
2) Keep plants virus-free e.g. by using insecticides to kill insect vectors
3) Use virus resistant plants by breeding them to be resistant. However, these can take years to produce and viruses can mutate rapidly to overcome this
88
What is cross-protection?
Mild strains of viruses inoculated onto plants to prevent more virulent strains from infecting the plant
89
Who first demonstrated cross-protection and when?
McKinney
| 1929
90
What is the issue with cross-protection?
Viruses can mutate and viruses may not be mild any more if they do
91
What is the benefit of plant genetic engineering to induce resistance to viruses?
Every cell would express the coat protein of the virus like any other plant gene
92
How would it be possible to achieve expression of a viral gene from the plant’s nucleus?
Clone the viral gene as double-stranded cDNA then insert it into the plant’s chromosomes within the nucleus, ‘transforming’ the plant
93
What bacteria allows us to transform plants?
Agrobacterium
94
How does agrobacterium transform plants?
It binds to a plant cell and enters the cell’s nucleus, then inserts its genes into the nucleus
95
Why does Agrobacterium insert its DNA into plant cells?
It does this to cause tumours that it can feed off, hence it can be used to insert other desired genes into plant DNA such as virus resistance
96
How is the desired gene placed in the bacteria?
In a DNA section called a plasmid, which as transferred into the bacterium
97
How is the transferred gene activated, causing the plant to develop the desired trait?
A ‘promoter’ (a trigger which activates the gene), is included with the desired gene on the plasmid. The promoter activates the transferred gene
98
What must be done to viral RNA genes to insert them into the plant genome?
They must be converted to DNA
99
What is an example of a promoter which allows plant expression of the TMV coat protein?
CaMV35S
100
Who first transformed plants and when?
Powell-Abel et al.
1986
Transformed TMV plants more resistant to TMV infection and symptom development was delayed in progeny
101
What is the name for when a plant expresses viral coat proteins and is resistant to the virus?
Coat protein-mediated resistance (CPMR)
102
How does coat protein-mediated resistance work?
When a virus infects a non-resistant plant, the viral coat proteins relax to expose viral RNA and allow ribosomes to attach
In a transgenic plant, when the viral RNA is exposed, coat proteins identical to that of the virus but produced by the plant itself should rapidly bind to the viral RNA, so a ribosome can’t attach to the RNA and there is no virus translation
103
What risks are associated with transgenic CPMR plants?
The plants’ DNA contains antibiotic markers, to select which plants had been transformed - could spread antibiotic resistance
Plants will be expressing the viral coat protein - is this foreign protein safe to eat?
Plants will also contain viral sequences - the sequences may interact with another unrelated virus and make the symptoms worse (synergism)
104
What is an example of a species of bacteria that produces proteins that kill insect larva if they ingest the proteins?
Bacillus thuringiensis
105
What protein does Bacillus thuringiensis produce, and how does it kill insects?
Bt protein
| It causes severe damage to insect larvae guts, so they die of starvation
106
Why can’t Bt protein be sprayed or used as powder on plants?
It is not stable after it is dusted onto a plant, and it only works on insect larvae that live on the surface of plants, not those in plant tissue or the soil.
The spray may also drift into neighbouring areas and cause damage
107
What is the alternative possible way to use the Bt protein?
Genetically engineer plants to express their own Bt gene, killing insects anywhere near the plant. It is also cheap, stable and environmentally friendly
