Lecture 11 - Viral and Fungal Resistance - Casson Flashcards Preview

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How much loss do plant diseases (bacterial, fungal, viruses) cause?

15% of annual losses


Which human intervention has had the greatest impact?

Herbicides - 74% decrease in loss.
Pathogens - fungi and bacteria - 32% decrease.
Viruses only 5% - smallest decrease.


How do viruses affect plants?

Infection means resources are redirected in the plant - this can affect yield and quality.


What else could we control other than the virus?

The vector


What was the first instance of virus-resistant plants?

Cross-protection (1929). Infection with mild form of virus protects against more damaging strains. Presence of viral encoded protein seems to give protection. Early work was done with Tobacco Mosaic Virus (TMV).


Describe TMV

RNA virus with 5 ORFs. Has a coat protein, movement protein (virus moving from cell to cell - through plasmodesmata between plant cells), polymerase, replicase, and methyl transferase. Uncoats when it enters the cell!


How was TMV resistance first achieved?

1986 - expressed viral coat protein in transgenic plants. Tobacco expressing TMV coat protein has resistance to TMV - 30 days for symptoms to appear vs 3 days in control.
Was thought that the coat protein could self-aggregate to encapsulate the genome of TMV - preventing decapsulation. Prevented spread of virus.
This was thought to be a protein-based mechanism - as CP mutants defective in aggregation did NOT give resistance


Why was the TMV commercial uptake slow?

The mechanism was not fully understood.
This gave resistance to a few specific viruses, but not many. Also, risk of transcapsidation, developing new virus strains. Until the mechanism is fully understood, uptake will be slow.


What three ways could do new viruses emerge?



Explain the inconsistency in the mechanism...

Protein-based mechanism was not always the case.
Coat protein gene with in frame stop codon gave protection in some cases - makes the coat protein untranslatable - hence pointed to a non-protein-based mechanism.
Viral replicase gene also gives protection, even if not translated.
This led us to discover RNAi


How was RNAi discovered?

Attempts to deepen flower colour in petunia by overexpressing chalcone synthase (CHS) gene, in order to increase pigment production.
Researchers observed a switching off of the gene pathway due to knockdown in mRNA levels of both the endogenous and transgenic CHS gene.
This created variegated flowers.
Remember this was a plant gene - not a problem with codon usage etc. This may be an ancient self defence mechanism to combat infection by viruses etc.
Triggered by dsRNA - indicative of a viral assault.


Are we sure of the mechanism?

No - not sure if this works by protein encapsulation or by RNAi. This makes commercialisation difficult.


Papaya ringspot virus?

1992 - Hawaii. Papaya ringpspot virus spread - triggered major research. Researchers transformed papaya with mild version of PRSV coat protein - gave good resistance to virus.
But the version they transformed was not the usual commercial product.


What did they do next?

Crossed the transgenic line (SunUp) with normal commercial line to form hybrid Rainbow line.


Is all of Hawaiian papaya GM now?

No, 50%. Canada and Japan are main markets. Canada approve of GM, Japan do not.


What other effects did these GM papaya have?

Depleted the load on non-GM crops. This reduced the virus spreading, so they could plant non-GM crops in same area.


What are some other examples of GM virus-resistant plants?

Virus resistant squash (success) - Monsanto's 'Conqueror III' - express zucchini yellow mosaic, watermelon and cucumber mosaic virus proteins. Grown commercially in US but susceptible to other viruses
NewLeaf potatoes (discontinued) - Monsanto's Bt-potatoes, then NewLeaf plus - resistant to Colorado potato beetle, plus Potato leaf virus, and potato virus Y.
Major consumer backash - McD and BK said they wouldn't use them.


What is the innate gen 2 potato?

From J.R Simplot.
Innate potatoes - reduced acrylamide upon cooking, and reduced bruising (due to reduction in polyphenol oxidase)
Innate 2: Resistance to Late Blight
This should mean less waste, and therefore savings throughout the chain.


How did they develop innate 2 to resistant to late blight?

Identified wild S.American strain resistant to late blight naturally (Solanum venturii)
Gene of resistance was Rpi-vnt1.1
This gene encodes a Leucine Rich Repeat (LRR) domain protein
LRR proteins are often involved in interaction and signalling
Can be part of receptor complexes with other LRR-domain proteins.


What is the nature of plant pathogen interactions?

Often gene-for-gene.
Plant has R gene that can detect the product of Avr gene produced by pathogen.
R - Rpi-vnt1.1, AVR - Avr-nt1.1
Makes plant resistant! If AVR is not expressed, plant susceptible to infection.


What happens when the plant detects the AVR product?

Generation of the hypersensitive response - rapid cell death around infection site and sealing of region. This limits the pathogen spread. Contains the infection.


How was this marketed in innate 2?

No mention of GM anywhere. Marketed as only using potato genes.
This was a T-DNA construct - definitely GM.


What is the main banana eaten?

Cavendish banana.


What is the fungal pathogen threatening bananas?

Fusarium oxysporum


What is the problem with bananas?

They are sterile and polyploid, they cannot be bred. They are reliant on clonal propagation.


Why are some WT bananas resistant?

Due to RGA2 - another LRR, involved in perception of an avirulence factor. Confers resistance in field trials. May have to put this into Cavendish bananas to save them.