Viral and Fungal Resistance

Question 1

How much of annual losses are caused by plant disease?

Answer 1

Plant disease causes ~15 % of annual losses,
Infection reduces crop growth rate, yield and quantity

Question 2

When was cross protection introduced and what is it?

Answer 2

1929
Infection with a mild form of virus protects against more damaging strains
Presence of viral encoded protein
Done with TMV

Question 3

What is the tobacco mosaic virus made up of?

Answer 3

RNA virus with 5 ORFs (open reading frame - stretches of viral genome that encodes a protein)
- Movement protein (replicated genetic material is moved between cells via plasmodesmata)
- Coat protein
- Polymerase, helicase and methyltransferase (replicase complex)

Question 4

What does expressing a viral coat protein mean for resistance? (Coat protein mediated resistance)

Answer 4

1986 - Found that plants expression the CP showed enhance resistance to TMV infection
Proposed that transgenic CP actively inhibits initial uncoating of the virus, preventing translation and replication

Question 5

Why does pathogen derived resistance have slow commercial uptake?

Answer 5

Mechanism of resistance not fully understood
If parts of the virus are expressed in the plant, does it increase chances of that plant to be infected with viruses of similar strains? :
- CP from transgenic plant + RNA from virus = Recombination of RNA + Chimeric virus genome formed
- CP from transgenic plant + CP from virus = Transient chimeric virus capsules
- RNA from virus + CP from transgenic plant

Question 6

How can a coat protein be modified to give protection?

Answer 6

Coat protein gene with in frame stop codon give protection in some cases:
- Not making protein but still providing resistance to the virus
- Stop codon makes CP gene untranslatable
- Viral replicase gene gives protection, even if not translated
- Indicates non-protein based surveillance

Question 7

What has post transcriptional gene silencing shown about resistance?

Answer 7

• In attempts to deepen flower colour it was found that PTGS would switch pigmentation genes off, creating variegated flowers.
• May be an ancient self defence mechanism to combat infection by viruses and transposons which edit genomes
• Also triggered by dsRNA, indicative of a viral attack

Question 8

What is small RNA mediated gene silencing?

Answer 8

• Occurs when a dsRNA species is bound by an endoribonuclease (DICER)
• DICER fragments dsRNA into 21-23 nucleotide siRNA fragments (small interfering RNA)
• SiRNA is incorporated into a multiprotein RNA inducing silencing complex (RISC)
• Duplex RNA is unwound, leaving antisense strand to guide RISC to complementary mRNA for endonucleolytic cleavage

Question 9

Papaya ringspot virus

Answer 9

• Papaya is the second largest fruit crop in Hawaii
• 1992-97, production reduced by 40% by PRSV
• University developed embryo transformation and expressed PRSV coat protein
• Produced virus resistant ‘SunUp’ and ‘Rainbow’ papaya
• By 2000, 50% of all papaya was ‘Rainbow’
• Not higher because farmers could reclaim areas lost to infection
• They also needed non-transgenic papayas, Japan accounted for 20% of the export market and did not approve transgenic varieties.

Question 10

What are some other examples of virus resistant GM crops?

Answer 10

Virus resistant squash:
- By Monsanto, expresses zucchini yellow mosaic, watermelon and cucumber mosaic CP
- Grown commercially in the US, but susceptible to other viruses

‘Newleaf’ potatoes:
- By Monsanto, Bt-potatoes
- Resistant to the Colorado potato beetle, potato leaf roll virus and potato virus Y
- McDonalds and Burger King stopped using GM for chips in 2001, no need for the potatoes anymore

Question 11

What is Innate Gen 2 in regards to fungal pathogens?

Answer 11

• Potato
• Resistance to late blight, engineered to have reduced asparagine levels, more resistant to bruising
• Resistance gene from wild varieties of potato (Rpi-vnt 1.1)
• Is classified as GM, because they took DNA from one organism and put it in another, even though they are both potatoes

Question 12

How can Cavendish bananas be resistant to fungal pathogens?

Answer 12

• Cavendish bananas are 40% of all bananas grown
• Targeted by fungal pathogen: fusarium oxysporum
• Some wild bananas are resistant to this pathogen
• However, cavendish bananas can not be bred for resistance because they are sterile polyploidy plants
• Resistance gene from wild bananas (resistance gene analogue 2 RGA2) introduced into Cavendish via Agrobacterium mediated transformation
• RGA2 overexpression confers resistance