Plant-Microbe Interactions

Question 1

What is an obligate parasite?

Answer 1

parasites which depend on the cellular machinery of their hosts to reproduce.
they lack protein-synthesising and energy-producing apparatuses

Question 2

What are the two fundamental types of plant virus morphology?

Answer 2

1. helical - roughly elongated
2. icosahedral - roughly spherical

Question 3

What are the different ways is which viruses regulate gene expression?

Answer 3

1. polyproteins
2. subgenomic RNAs
3. multipartite genome
4. splicing
5. translation of both strands
6. internal initiation (complex secondary/tertiary structure)
7. leaky scanning (translation does not start at the first AUG)
8. non-AUG start codons (eg AUU)
9. transactivation (trans-activator facilitated internal initiation)
10. translational shunt (ribosome jumping)
    11.red-through proteins
11. frame-shift proteins

Question 4

What is the general life cycle of a plant virus?

Answer 4

1. invasion
2. genome uncoating, expression, and replication
3. particle (virion) assembly
4. cell-cell movement: from initially infected cell to vascular bundle
   —- exception is phloem-limited viruses are usually injected
   directly into phloem by their vector
5. systemic transport through phloem - systemic movement is fast

movement proteins can increase the size exclusion limit of plasmodesmata to facilitate cell-cell transport

Question 5

What are some common symptoms of plants infected by plant viruses?

Answer 5

stunting
mosaic patterns
chlorotic lesions
ringspots
abnormal growth/tumours/enations - rare
blights - rare
wilts - rare
systemic necrotic lesions - relatively rare

Question 6

What are the components of the disease triangle?

Answer 6

1. host - plant must be susceptible to the pathogen
2. pathogen - pathogen must be able to overcome plant defenses
3. environment - the environment must tip the balance in favour of the pathogen

Question 7

What are the main components of plant defenses?

Answer 7

1. R proteins
2. siRNAs

Question 8

How does temperature affect plant-virus interactions?

Answer 8

virus disease outbreaks associated w low temps
at high temps viral symptoms are attenuated (heat masking)

this is because at low temps virus-induced RNA silencing is inhibited

Question 9

How are viruses named in taxonmy?

Answer 9

family: -viridae
genus: -virus

Question 10

How are viruses so fast at evolving?

Answer 10

1. short replication time and large quantity of offspring released per cell infected
2. several gene expression mechanisms
3. results in quasispecies and defective interfering genomes
   — quasispecies: virus populations as dynamic distributions of
   nonidentical but related replicons

Question 11

How does reassortment occur in viruses?

Answer 11

occurs when two similar segmented viruses exchange DNA during a cell co-infection, resulting in a mixing of genetic material giving different combinations in different individuals

Question 12

How are plant viruses transmitted?

Answer 12

• viruses must enter through healable wounds, they do not enter through natural openings (no receptors)
• animal vectors
• seed transmission is common, but specific for virus and plant

Question 13

What is the role of the VPg protein in potyviruses?

Answer 13

acts an an mRNA cap analogue to aid in mRNA mimicry

Question 14

What are viroids?

Answer 14

smallest infectious pathogens known, 250-450 nucleotides
no protein capsid
do not program their own polymerase
transmission via aphids and seed
circular RNA genome

Question 15

What is crown gall disease?

Answer 15

agrobacterium induces tumour growth at wound sites by inserting Ti plasmid
tumours produce opines which the bacteria feeds off of eg octopine, nopaline

Question 16

What is the structure of the Ti plasmid?

Answer 16

Transfer DNA (T-DNA) - moves into plant cell nucleus. flanked by two 25bp repeat border sequences. cntains genes responsible for inducing tumours and opine production
virulence (vir) genes - required for movement of plasmid and T-DNA into plant cell

varies between strains but all carry one or more T-DNA region and one vir region

Question 17

What are two common biotech crops?

Answer 17

1. insect resistant (Bt toxin producing)
2. herbicide (glyphosphate tolerant)

Question 18

What is the process of agrobacterium T-DNA transfer?

Answer 18

1. chemoattraction and activation of virulence
2. T-DNA excision
3. Movement of T-DNA out of the bacterium
4. nuclear import and integration of T-DNA
5. expression o T-DNA and pathogenicity

Question 19

How does agrobacterium sense and activate virulence?

Answer 19

acetosyringone produced by plant roots is perceived by VirA protein encoded on Ti plasmid
VirA (HK) and VirG (RR) form a two-component system that induce other vir genes in response to plant signals (virB, C, D, and E)

Question 20

What chemicals, produced by the host plant, contribute to the regulation later in the agrobacterium infection process?

Answer 20

SA, ethylene and auxin

Question 21

How is R. radiobacter an effective biocontrol agent against agrobacterium?

Answer 21

produces antibiotic compounds that are selectively toxic to pathogenic agrobacterium, which mimic the structure of opines so are taken up by opine permeases

Question 22

How is agrobacterium rhizogenes infection different to that of other agrobacteria?

Answer 22

A. rhizogenes infection induces production of large root mass rather than tumours
genes encoded on so called Ri plasmid
useful for natural product synthesis