biotech-lec 6.ppt

Methods of producing
transgenic plants
http://www.sathguru.com/images/agri-and-food-biotechnology.jpg
www.ucsusa.org/.../ pharmcorn_plate.jpg

Transgenic plants – why we need
them?

Transgenic plants – why do we need them?

Adoption of transgenic crops
in the United States has been far greater
than in many other countries
http://www.colostate.edu/programs/lifesciences/TransgenicCrops/images/UScroparea.jpg
T
O
T
A
L

•
82% of all transgenic food in the world
produced by US and Canada

In US and Canada people more care
about not fresh food, than GM food
(in Europe it is opposite)
US CAN BRA UK GER FRA JAP
Food handling/storage 39 25 34 15 17 7 4
Pesticides/agrochemicals 37 37 69 32 42 34 27
Bacterial contamination 21 18 25 11 23 10 5
Artificial ingredients 7 12 21 10 17 10 25
GM foods 2 9 1 21 15 19 11
Disease from animals 2 3 13 9 30 18 1
Note. Unaided voluntary responses to the following question, "What, if
anything, do you feel are the greatest threats to the safety of the food
you eat?" Figures are expressed as a percentage of respondents. From
Environics GM Food Tracking for Monsanto, 2000.

Concerns associated with GM crops
1. Possible production of allergenic or toxic proteins
not native to the crop
2. Adverse effects on non-target organisms,
especially pollinators and biological control organisms
3. Loss of biodiversity
4. Genetic pollution (unwanted transfer of genes to other species)
5. Development of pest resistance
6. Global concentration of economic power and food production
7. Lack of "right-to-know"
(i.e., a desire for labeling transgenic foods)

Most often improvements
introduced to transgenic crops
Trait
Area planted in
1999 (millions of
acres)
Herbicide tolerance 69.4
Bt insect resistance 22.0
Bt and herbicide tolerance
in the same plant
7.2
Virus resistance 0.3

Soybean with no herbicides Soybean after herbicides
Herbicides are used for weed control
Herbicide Tolerance
Weeds
drastically
reduce
crop
yield
and quality

Herbicides
Selective Non-selective
Kill certain weeds,
Do not kill certain crops,
because of its
biological differences
Trifluralin: control of grass and
broadleaf weeds in cabbage
Kill everything,
Do not kill crop,
If crops is pre-engineered
to be resistant
Glyphosate (Roundup)
Glyphosate (roundup)

Problems with selective herbicides
Residues of some selective herbicides remain in the soil
for a year or more, so that farmers must pay close attention
to the herbicide history of a field
(non-selective herbicides are biodestroyed faster).
Chlorosis and necrosis of lower leaves
in beans
Growth retardation in sugarbeet

Herbicide resistance
(arises for both selective and non-
selective herbicides)
More common for selective herbicides,
as their application allows gradual selection for resistance in the weeds.
(Non selective herbicides do not give weed a chance to survive
if resistance gene did not pre-existed in them)
Glyphosate action
at susceptible plant
and resistant plant
www.udel.edu/PR/NewsReleases/2001/ mar/3-21-01/weedphoto.htm
susceptible resistant

Non-selective herbicides
(Roundup Ultra and Liberty)
Roundup® (chemical name: glyphosate)
Liberty® (glufosinate).
(Finale, Basta, Ignite)
Breaks down quickly in the soil,
eliminating
residual carry-over problems
and
reducing environmental impact.
Roundup Ready®
Liberty Link®
transgenic varieties
of common crops
completely resistant
to those herbicides

www.monsanto.com.au/cotton/roundupCotton/
SUGARS
Shikimate-3-phosphate
Plant enzyme (EPSPS)
BLOCKED by Roundup
Aromatic compounds
Aminoacids
Proteins
PLANT SURVIVAL AND GROWTH
Plants need
ALL proteins
be synthesized
de novo
APSPS enzyme
(modified by
gene engineering)
Is not blocked by
Roundup

Roundup Ready
Soybean
give more yield
than another
(non-GM)
modern variety,
Pioneer

Reduction in herbicide usage with
resulting from the use of Roundup Ready soybeans
(US).
From Doane Market Research, 2000.

Roundup drift is possible,
so non-Roundup Ready varieties in the
neighborhood may suffer
http://www.lsuagcenter.com/Communications/LouisianaAgriculture/agmag/images/43_3/crop_response3.jpg
Roundup reduced yield 82 percent; Liberty, 31 percent.
Spoon of dirt in the honey barrel (fly in the ointment):
Roundup 1/8 drift Liberty 1/8 drift

Other types of
herbicide-resistant plants:
bromoxynil resistance
Nitrilase from soil bacteria
Klebsiella ozaenae
No inhibition of photosynthesis
Inhibition of photosynthesis

Bromoxynil resistance
in nitrilase-transgenic plants
Calgene, Davis, CA.)
Calgene, Davis, CA.

Bt transgenic crops
(insect resistant)
Bt means
Bacillus thuringiensis.

www.nosprayzone.org/pesticides/ btk101.html
Taxonomy of B. thuringiensis

B. thuringiensis produces multiple toxins
Name
Target
Organism
Production When toxic
Suitable for
introduction
to crops ?
Beta-
exotoxin
toxic to
many
life forms
exuded by
active
bacteria
immediately NO
Delta-
endotoxin
or protoxin
Generally
toxic to
certain
Insects
only
crystalline
protein
formed on
sporulation
Requires
alkaline
environment
to convert to
toxin
YES
B. cereus
type
enterotoxin
toxic to
mammals –
causes
gastroenteritis
exuded by
active
bacteria
immediately NO

Insecticidal crystals of delta-exotoxin
are produced during sporulation
http://www.bba.de/mitteil/presse/bt.jpg
This toxin is active against
Lepidoptera,
Diptera
Coleoptera
Bt toxin is a polypeptide (protein)
consisting of 645 amino acids
Pore formation
Proteolysis
and specificity
Receptor binding

Lepidoptera are most significant
corn- and cotton- damaging insects
Alfalfa webworms killed by Bt
lamar.colostate.edu/ ~gec/p1.htm European corn borer
cotton bollworm

www.bio.ic.ac.uk/research/ djwright/
Attack Place:
the intestine of the insect,
with its dissolution

GM Bt crops were released after overwhelming
evidence of the safe use of Bt sprays for over
forty years
Bt spray usage in US (Dipel, Thuricide, Vectobac) :

Diamondback moth has developed
Bt resistance in the field
Now is Bt resistant due to
spray applications
on crucifer crops
in the tropics and sub-tropics
(1980s and 1990s)
(Cameron, Malasia)
diamondback moth is the only pest
that evolved resistance to Bt sprays
used by organic growers,
but no pest has evolved resistance
to transgenic Bt crops in the field.
Nature 2003

Some complications of the Bt toxin
applications:
• 1) Bt resistance
• 2) Death of insects in the neighborhood

How to prevent development of Bt
resistance in insects?
High dose/refugee strategy at least 20%
of a farm's corn acreage
must be planted
to non-BT corn.
R = resistant European borer;
S = susceptible borer.
few Bt-resistant insects
surviving in the Bt field
would likely mate
with susceptible individuals
that have matured
in the non-Bt refuge.
Thus, the resistance alleles
would be swamped
by the susceptible alleles.
< 80% > 20%
Strategy will not work if resistance is dominant !!!

Molecular basis of the Bt action
http://www.bioc.cam.ac.uk/~dje1/ellar_mec_ac.gif

Genetic basis of the Bt resistance
In roundworm C. elegans as a model object
ucsdnews.ucsd.edu/newsrel/ science/mcbt.htm
Toxin is bound to membrane
Toxin is defecated out

Bt toxin resistance resulted from the
loss of a galactosyltransferase (bre gene),
(an enzyme that adds carbohydrates to proteins and lipids)
Non-carbohydrated
toxin receptor
is not able to accept toxin

Death of insects in the
neighborhood (innocent victim)
Monarch butterfly larvae
(Danaus plexippus)
eating only mikweed
(their only food)
may die after pollen from Bt-corn
fields
dusted their host plants
Overlap of small monarch larvae and corn pollen shed
Source: Oberhauser et al., 2001
Ontario
62%
Minnesota/
Wisconsin:
40%
Maryland
20%
Iowa:
15%
Pollen of BT 176 corn was most toxic to monarch larvae,
so BT 176 sold under the labels KnockOut (Novartis) and NatureGard (Mycogen)
were discontinued

Are Bt Transgenic Crops Toxic to Humans?
Data from EPA Biopesticide Safety Sheet (Oral tests on rats)
REGISTERED
TRANSGENE
Registrant
Oral dose
(mg/kg)
Digestibility
Bt CryIA(b) Monsanto >4000 Rapidly degraded
Bt CryIA(b) Novartis >3280 Rapidly degraded
Bt CryIA(c) Dekalb >5000 Rapidly degraded
Bt Cry9C AgrEvo >3760 Stable
Bt spray Several >5050 Degraded
Table Salt
3750 (B) –
LD 50 dose
Caffeine 25
Vitamin A
4.3 (C)
Adverse
effect dose
2,4-D (lawn
herbicide)
1
feeding rats high doses
of any of the purified
insecticidal proteins
produces zero effect
CONCLUSION:

How do we know that we just haven't fed
the rats enough protein to cause an effect?
Every EDIBLE part of the plant is also expresses toxic gene !!!
TRANSGENE Whole Plant Leaf Roots Pollen
GRAIN
Grams of
insecticidal protein
per acre
Bt CryIA
3.65-
4.65
7.93-
10.34
NA 0.09 0.18-0.39 16.4-20.9
Bt CryIA 0.6 4.4 <0.008 7.1 <0.005 2.7
Bt CryIA 0.22
0.10 -
0.26
0.03 NA 0.01 1.5
Bt Cry9C 4.7 9.5 5.6 0.1 4 21.3
Bt spray NA NA NA NA NA 14
Bt toxin concentration in the CORN
G r a m s o f I n s e c t I c I d a l p r o t e I n

Some nice calculations
Using the highest amount of protein present in grain,
we can calculate the amount of popcorn
needed to be consumed by a human two-year-old child (15 kilos in weight)
to reach the highest doses fed to rats (5000 mg/kg).
a child would have to eat 27.5 pounds of popcorn a day!!!!
EPA justifiably declared
the risk of a toxic reaction
from Bt proteins
as essentially nil.

Whether some individuals are
uniquely sensitive (Allergic) to Bt?
1. Food allergens are almost always proteins
that are stable for digestion
(and for cooking heat)
http://www.nlm.nih.gov/medlineplus/
Bt toxins is UNSTABLE in stomach
(except cry9C variety of toxin).
2. Most protein allergens are glycosylated
Bt toxins are NEVER GLYCOSYLATED

Allergy in field workers exposed to
Bt sprays (Javelin )
Specific IgE antibodies to spore extract of Bt were present
More often in high-exposure workers (p < 0.05) than in the low and medium groups.
intact delta-endotoxin proteins
Bt spray formulation is a very complex mixture
large amounts of Bt spores
residual amounts
of fermentation medium
bacterial cell wall debris
vegetative (i.e., growing) Bt cells
Bt spray
soluble components
Bt spores
Toxic proteins
Experiment:
Contain allergens
Contain allergens
NON ALLERGENIC

Evaluation of allergenicity (procedure
applied to ALL transgenic plants):
[Metcalfe DD et al. Crit Rev Food Sci Nutr 1996]
DBPCFC = double-blind, placebo-controlled food challenge

Russett Burbank potatoes transformed with
a modified BT gene toxic to the
Colorado potato beetle.
The false-color image:
aerial view of a field in Oregon
Green color is a defoliation
due to beetle;
Red color – leaves are present
on trasgenic potato;
Perlak FJ et al. (1993)

Reduction in pesticide use in major
cotton states, 1996-1998.
From Agricultural Biotechnology: Insect Control Benefits, by L.P. Ginanessi and J.E. Carpenter, 1999.

Corn rootworm (Diabrotica spp.)
Range of damage due to corn rootworm feeding,
from severe (left) to no damage (right).
Image Source: USDA
Rootworm-resistant corn (with Cry 3Bb variety)
was approved in 2003.
This insect is responsible for the application
of the largest amount of insecticide to U.S. corn fields
(and in this case insecticide is applied directly to the soil !!)

Corn hybrid with a Bt gene (left) and a hybrid
susceptible to European corn borer (right).
Source: Monsanto

YieldGard® Bt-corn has improved
the overall quality of the corn grain
Low-quality grains (spoiled by insects and half-rotten as damaged)
are usually fed to animals
There is
no more such thing
as low-quality grains
in US.
Animals fed normally,
So they are
not accumulating poisons
from fungi
Half-rotten grain are further damaged by fungi
Fungi produce poisonous substances
retarding animal growth
and they are kept in the meet (!!)

Transgenic plants with bacterial
cholesterol oxidase
Anthonomus grandis grandis
Boll Weevil
Larvas on diet with Chol-ox

Virus-resistant Papaya
Papaya, a tropical fruit high in vitamins C & A, is
an important food crop worldwide;
2nd largest export crop in Hawaii state.
A virus came to Hawaii in the 1940’s
and had wiped out papaya production
on Oahu by the 1950’s.
papaya ringspot potyvirus (PRSV)
Normal papaya Ringspot virus
infected
Plantation severed

Virus is distributed across the country through insects (aphids)
http://www.plantpath.wisc.edu/pp300-UW/Lectures/Presentations/14

Papaya PRSV came to Puna in 1992,
by late 1994, PRSV had spread throughout Puna
and many farmers were going out of business.
Production
almost halved
Transgenic papaya
introducted
USDA report, 2000
In year 2000 -- 53% of
papaya acreage is transgenic
First transgenic papaya
was created

“UH-Rainbow” papaya
(cross of transgenic papaya and
yellow-fleshed Kapoho variety)
Virus resistant Papaya Construct
35Sp nos!
PRSV coat
protein
nptII GUS
Selectable Markers
Transgenic plants
Non-transgenic plants

Transgenic papaya built on phenomenon of
coat protein-mediated resistance
Mechanism of
coat protein mediated resistance
is not completely understood.
VIRUS
Plant cell
x There is no “antibody”-like things involved.
If viral coat protein mRNA in the plant
is produced in enough quantities,
Viral coat protein mRNA is rapidly degraded

Plant Resistance to Bacteria and Fungi
(not very efficient)
Pathogenesis Response (PR) Proteins:
- Activated after Bacterial/Fungal infection of the plant
(generally visible as decaying);
- Often aimed at bacterial cell wall destruction
(chitinases, glucanases);
Transgene plants with systemic overexpression are created
but this system is not very effective
(ROOM TO IMPROVE)
PR gene + Increased production of hydrogen peroxide = may help

PR proteins act locally
Immunocytochemical localization of
PR protein (glucanase)
on H. annosum hyphal (H) cell walls
during colonization
of Norway spruce tissues.
Immunolabelling of peroxidase
on Norway spruce cell wall regions
(arrows) overlying sites
of pathogen penetration.
www.mykopat.slu.se/kurser/ forestmicro/

Sclerotinia – dangerous pathogen
of dicot plants
Pathogenisis requires
the fungal secretion of oxalic acid.
ubiquitous phytopathogenic
Ascomycete
Wheat is Sclerotonia resistant
because of germin protein (oxalate oxidase)

Transgenic tomato with wheat germin
two days after inoculation
with the Sclerotonia
Transgenic Wild-type

Plants enhanced
with nutrients and vitamins
Gene engineering is a possibility
to add to a plant
a totally new traits
that are not characteristic
for plant at all

Vitamin A deficiency affects some
800 million people worldwide
www.micronutrient.org
Children only:
Due to improper
immune functioning
bob.usuhs.mil/biochem/nutrition/ images/keratomalacia-1.jpg

High beta-Carotene
(= provitamin A) oilseeds
Source (mg/g) Beta-Carotene
Carrots 30-110 (60 avg.)
Pumpkin 16
Tomato 3-6
Melon 17
Apricots 20
Red palm oil 250-350
High carotenoid canola oil 1,000-1,200
Adults, on average, need a dose of 750 mg of Vitamin A per day
Modified oilseed crops
will conveniently deliver the required daily amount.

BetaSweet® carrot
Contains approximately 50% more
Beta Carotene than normal carrot
Beta carotene
is a potent
cancer-fighting antioxidant.
dark maroon-purple color
(as also anthocyanine (another antioxidant) is added)
taste similar to regular carrots,
but have a very crispy texture,
which is easier to chew
Produced by Texas A&M University.

“Golden rice”
Rice normally
does not produces vitamine A.
On the other hand,
rice endosperm anyway contains
geranylgeranyl diphosphate (GGDP)
(progenitor of vitamine A)
GGDP  vitamin-A
2 genes from daffodils
1 gene from bacterium Erwinia uredovora
All 3 genes are expressed in endosperm
(major part of the rice grain)
Unfortunately,
production is too low.
Normal serving of rice (300 g)
provides just a few percent
of daily diet
Anyway, Syngenta supports the humanitarian use of golden rice,
for individuals receiving $10,000 or less income from developing country

Golden Rice schematic: nice example of biogineering
GGPP
Phytoene
Lycopene
beta-Carotene
= provitamin A
Phytoene synthase (psy)
Phytoene desaturase (crtl)
Lycopene ß-cyclase (lcy)
(daffodil)
(daffodil)
(bacteria)
Provitamin A biosynthesis
pathway
Funding:
Rockefeller
Foundation,
Swiss Federal
Institute
Of Technology,
European
Community
Biotech
Program

Golden Rice Gene Constructs
I-SceI KpnI I-SceI
LB Gt1p psy nos! 35Sp tp crtl nos! RB
LB
I-SceI
35Sp Gt1p RB
I-SceI
ˆSpeI
35S! aphIV 35S! Icy
Target gene
Target gene
Target gene
Borders of artificial inserts inside rice genome

Use of Rice to prevent and treat
vitamin A and iron deficiencies
Iron deficiency is the most common nutritional disorder
• Iron-enriched transgenic rice:
• 1 gene increases Fe content ( ferritin)
• 2 genes increase Fe absorption ( phytate, cysteine)
DOUBLE TRANSGENIC RICE
b-carotene-enriched rice crossed with iron-enriched rice;
b-carotene enhances iron uptake
Free distribution to farmers in developing world

Designer oil-producing plants to decrease risk
of cardiovascular disease and cancer
Plant seeds modified to contain healthier fats:
: saturated fat,
cholesterol, trans fatty acids
: monounsaturated fatty acids,
phytosterols, PUFAs (n-3 polyunsaturated fatty
acids)
Future applications:
Infant formulas
Intravenous feeding
Athletic supplements
Select patient populations

The same
is true
for engineered
of farmed fishes

FLOWER INDUSTRY IS NOT SO SMALL
TO BE NEGLECTED
US market of Fresh-Cut Flowers

http://www.colostate.edu/Depts/HLA/GHEX/newsletter/vol_12/senescence.jpg
Flower senescence
is regulated by ethylene
Ethylene build-up

Plants synthetize ethylene by themselves
(self-regulation of flowering)
ACC synthase
ACC oxydase

Gene Engineered Eternal Flowers
8 days after ethylene treatement.
transgenic non-transgenic
9 days after pollination
Non-transgenic transgenic
Made by antisense disruption of ACC synthase
Antisense gene is expressed in the plant

Transgenic long life carnation
Produced by Florigene Pty Ltd
Melbourn –based company
Long lived flower by itself
No need in expensive refrigeration;
No need in silver thiosulfate (STS) or
EthylBlock compaund

World market of carnation alone
world
cut-flower
market
$US25
billion
ornamental
plant market
$US15
billion
Carnations make up 18 per cent
of the cut-flower market = $US 4,5 billion

Fruit ripening and decay
(also ethylene dependent)
Wild type (left) and antisense ACC oxidase (right) melons
harvested 38 days post-pollination, stored at 25ºC for 10 days
www.nf-2000.org/secure/ Fair/S1146.htm

Tomato anti-softening gene
(FlavrSavr gene)
Pectin in cell walls
holds
Pectin in cell walls
softened
Polygalacturonase
enzyme
FlavrSavr gene is antisense to Polygalacturonase
enzyme encoding gene
FlavrSavr variety is discontinued
as gene has been inserted to cultivar
that lacked consistent production qualities

Transgenic plants for drug
producing (high AgBiotech)

Vaccine production in edible plants

Hepatitis B – important disease
Hepatitis B in US
2000 2001
Number of
Acute Clinical
Cases Reported
7,844 8,036
Estimated
Number of
Acute Clinical
Cases
22,000 22,000
Estimated
Number of New
Infections
78,000 81,000
1.25 million persons with
chronic infection in US
Estimated Annual Number
of Chronic Hepatitis Deaths
5000
Percent in US: Ever Infected
4.9%

HBsAg – major antigen of hepB
virus
HBs Ag alone
is sufficient
to mount
immune response
Total immunization is a preventive answer

Transgenic lupin (Lupinus luteus L.)
and lettuce (Lactuca sativa L.)
with hepatitis HBs antigen
Kapusta, J. et al., FASEB J
Anti-HbsAg antibodies titre in
Mice fed with transgenic lupin callus
5 g in 1 dose
Anti-HbsAg antibodies titre in
Mice fed with transgenic lupin callus
1 g each of the 5 days

Three human volunteers
are fed with transgenic lettuce
200 g 1st dose,
150 g 2nd dose

Hepatitis B vaccine in Banana
Hepatitis B vaccine now costs $100 to $200 a dose
Vaccine banana would cost only a few cents per dose.
Just 24 acres of land
could produce enough bananas
to vaccinate
all Mexican children
under the age of 5.

Norwalk virus (calicivirus)
acute diarrhea
and vomiting
(2-3 days gastroenteritis),
abdominal cramps,
myalgias,
malaise,
headache,
nausea,
and low-grade fever
50% the outbreaks of acute infectious
nonbacterial gastroenteritis in the United States
the second most common cause
of illness in American families
(after common cold)
No treatment available
CDC data:
23 million US people infected by Norfolk annually.
1.4 million cases by salmonella.
79,000 by E. coli contamination
2,500 cases by listeriosis

Capsid-based plant-derived vaccine
for Norwalk virus
Serum antibody responses of mice fed with Norwalk potato (4g)
CT – inert component
of cholera toxin
(immunoboosting agent)
Hugh S. Mason et al, 1998

Human Trial of Norwalk vaccine
Tacket et al., The Journal of Infectious Diseases 2000;182:302-305
150 g of raw, peeled, diced potato
(215 - 751mkg of NVCP coat protein).
24 volonteers
(20 exp + 4 controls)
19 out of 20 start to produce
specific IgAantibody- secreting cells.
4 out of 20 start to produce
specific IgG antibodies
Titer of serum IgG anti-NVCP:
1 : 67 before immunization
1 : 757 after immunization among responders
1 : 62,414 after real infection (fade after 2 years)
Side effects: nausea
in 20% of all volonteers
(raw potato)

Detoxification of hazardous
compounds by transgenic plants
Early symptoms of mercury poisoning:
Headaches
Irritability
Insomnia
Hair loss
General weakness and fatigue
Loss of appetite and associated weight loss
Joint pain, particularly in wrists and ankles
Late symptoms at high exposure
Tremors of fingers, eyelids and lips
with progression to general tremors of the entire body.
Behavior and personality changes, excitability,
memory loss and depression.

Heavy poisoning with mercury from fish
and seafood
(Minamata disease)
Termometer plant waste
Minamata Bay in Southwestern Japan
121 people were poisoned from eating the contaminated fish, 46 of which died.
mercury let off from a chemical manufacturing plant Chisso Co. Ltd.
Remediation: dredging Minamata Bay and reclaiming some of the land.

Methylmercury bioconcentrates in fish six to seven orders of
magnitude above concentrations found in polluted waters and
constitutes 90–100% of their total mercury content.
Mercurium
Serious
environmental threat
as it enter chains
Ionic mercuruim,
easily eliminated
Scott P. Bizily et al., 2000
BIOMAGNIFICATION

http://ehpnet1.niehs.nih.gov/docs/1996/104-8/fdavepabig.G

Locations in the United States which have
the highest concentration of mercury in
freshwater fish
FDA
no
EPA
no
286ppm to 645 ppm in cetacean (whales) organs
up to 50 ppm in fish and 85 ppm in shellfish in Minamata

There is always
a metabolic way back (!!)
(in bacteria..)
Plants with bacterial transgenes can detoxify the mercury
and volatilize the least toxic form into the atmosphere
at safe, exceptionally low levels.
Bact Bact

Why plants is better than bacteria
for site remediation?
Autotrophic growth
Ease of manipulation
Ease of containment
In-built extraction
of nutrient metal ions from the soil
root systems infiltrating large volumes of soil
Plants stabilize disturbed ecosystem

www.appliedphytogenetics.com/ apgen/
Arabidopsis plant with Mer A gene inserted
is able to grow on mercurium contaminated soils

Both merA and merB in Arabidopsis
merA for mercuric reductase
merB for organomercurial lyase
50 times more tolerant to Hg
than wt plants
10 times more tolerant to Hg
than merA plants
0 p.p.m 0.2 p.p.m.
1 p.p.m 2 p.p.m.
CH3HgCl

Only MerA/MerB plants are producing
gaseous Hg in a safe
(non-bioaccumulative) form
6 different strains
of MerA/B arabidosis
vs. wt, MerA and MerB

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