Lecture 11 - Viral and Fungal Resistance - Casson Flashcards
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?
diagram
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.
