Lecture 10: GM and virus resistance: Opportunity or environmental disaster? Flashcards
example of plant virus:
Tulip break virus, 17th Century
viruses are devastating ___ of many crop species
Pathogens
treatments of viral plant diseases: pre or post infection?
treatments post infection are generally ineffective, therefore focus on prevention of infection
controls used to prevent infection of viral diseases:
- spray insecticides to control aphid vectors
- delay planting to avoid insect vectors
- destroy infected crops
- maintain good field hygiene
- certified virus-free seed
- –all these are easier in developed countries
example of insect vectors for viral diseases:
- Aphids
- Whitefly
- Hoppers
- Thrips
examples of virus damage and their viral causes:
- Fruit distortion on eggplant fruit caused by Tomato bushy stunt virus
- yellow vein-banding on grapevine caused by Grapevine fan leaf virus
simplified typical virus lifecycle:
viruses inject genetic material (DNA or RNA ) into the plant cell which reprogrammed the cell machinery to produce new viral particles
simplified typical virus lifecycle: the nucleic acids injected (DNA/RNA) ENCODES:
- enzymes necessary to replicate the viral genome
- proteins to allow transmission from cell to cell
- coat proteins which encapsulate the viral genome
A strategy for making virus resistant plants (concept:A good idea, but incorrect)
- Engineer plants so that they produce coat protein inside the cell
- as nucleic acid is injected into the cell it becomes encapsulated (transcapsidation) within the coat protein and cannot be transcribed/translated
- -> the experiment works (plant became virus resistant) but mechanism was wrong, resistance wasn’t due to encapsulation
virus resistant plants: how the mechanism was wrong
plants which expressed coat protein were virus resistant (supporting concept) BUT
-further experiments showed:
–a complete coat protein was not required, part of one would still produce resistance
-framshift mutations which still allowed RNA production but which produced no protein produces resistance
-coding regions inserted in reverse produced resistance
-gene fragments as short as 25bp produced resistance
EXPERIMENT WORKED BUT CONCEPT WAS WRONG
virus-induced Gene silencing (VIGS)
- infected plants display sever localised symptoms
- recovered tissues have low virus load and no symptoms
- recovered tissues are resistant to subsequent infection by the virus and other related viruses
Post -transcriptional gene silencing (PTGS) what is it?
-expression of high levels of foreign genes (viruses or transgenes) can lead to repression of gene expression
Post -transcriptional gene silencing (PTGS How does it work?
- If double stranded RNA’s (mRNA) are produced they are ‘diced’ (by Dicer) into small 25 nucleotide RNAs which activate a sequence specific nuclease.
- resulting in degradation of mRNA
__ gene silencing mechanisms exists
MANY
-in terms of viral resistance all make RNA that the plant can recognise as foreign
commercial application of virus resistance plants:
- a portion of the viral genome is introduced into the crop using a construct which will generate dsRNA molecules
- any proportion of the viral genome can be targeted
- Viruses naturally produce proteins which suppress PTGS so resistance is not guaranteed (antagonistic to the dicer)
field resistance against viruses in squash (Upjohn)
transgenic grew larger and produced larger & non infected fruit
concerns regarding transgenic virus-resistant squash
- squash plant may become a weed
- viruses resistance genes may be transferred to weedy species improving their fitness
- recombination of viral genes in plants with other viruses may create new viruses with different host ranges (demonstrated in lab experiments)
increased possibility of recombination due to use of virus resistant plants
- when a virus infects a cell it quickly proliferates and destroys the cell
- the probability that two separate viruses infect a cell at the same time & recombine is VERY low
with transgenic plants the probability of viral recombination is ___
GREATLY INCREASED
- all transgenic plants contain part of the viral genome
- any virus infection the plant cell has the possibility of recombining
- new viruses with different host ranges may be produced
papaya & papaya ring spot virus:
- Papaya is an important crop in Hawaii & SE Asia
- it is rich in vitamins A & C
- susceptible to Papaya Ring Spot virus spread by APHIDS
papaya ring spot virus and hawaii islands
- PRSV discovered in Oahu in 1940’s
- papaya production destroyed by 1950’s –>wasn’t economically sustainable & virus
- papaya production moved to Puna district of Hawaii island in 1960’s
- PRSV found in Puna district in 1992
- attempts to limit spread by felling affected trees were unsuccessful
transgenic papaya resistant to PRSV
transgenic papaya produced using biolistics containing part of the coat protein of PRSV
transgenic plants successful in field trials:
Following incolutation with PRSV, non-transgenic plants are killed
- deregulated by USDA in 1997
- commerical use in 1998
- now being introduced in SE asia where Papaya is a staple but criticised as a way of getting GM crops accepted
General PRSV resistant transgenic plant production
- isolate PRSV coat protein from PRSV itself
- used transformation
- -> high speed particle bombardment method
transgenic papaya & Allergens?!
- the coat protein used to make PRSV-resistant papaya contains amino acid sequences which MIGHT be allergenic
- -6 amino acids shared with allergen from roundworms Ascaris lumbricoides
- -this doesnt mean it IT an allergen
- defninitve proof requires blood testing
How id PRSV spread
by aphids
decision made with papaya and allergenic qualities
USDA (1997) gave an exemption to the requirement that the levels of coat protein in the papaya are shown to be safe.
• Allergies kill people. Peanut allergies are common and dangerous – so why don’t we ban peanuts? 150-200 people die each year in the US, 10 in the UK
Transgenes which are allergens have been used but have always (so far) been identified at the safety testing stage.
transgenic virus resistant plants: containment is a __
PROBLEM
-In Thailand GM papaya resistant to PRSV has been found outside of the field trial areas.
– It is thought that the field station distributed GM seeds to local farmers (in error).
-In Hawaii, organic papaya growers have found that their trees contain GM transgenes.
–It is thought contaminating pollen from GM varieties is responsible.
-In India one third of all cotton grown contains Bt.
– 80% of this (3 million acres) is unlicensed and has been produced by
local seed merchants without regulation.
