UNIVERSITY OF AGRICULTURAL SCIENCES, BANGALORE

Department of agricultural entomology

MBB602: plant genome engineering

Bt Cotton: Pasts & Future

Arpitha, K. C. Bhavana, S.
PAMB 3030 PAMB
3031
 Naturally derived pest control agents
 4–5% of the global pesticide market
 By 2050 importance of biopesticides in agricultural production will be
equal to chemical pesticides

Isman, 2020
The US Environment Protection Agency (EPA) classifies biopesticides into
three major categories:
Biopesticides

Plant
Microbial
incorporated Biochemicals
pesticides
protectants
First generation Second
generation
Bt crops?
Bt crops are genetically modified plants that produce the
same toxin as the bacterium Bacillus thuringiensis (Bt)
 Soil-dwelling bacterium
 Aerobic, Gram-positive, spore-forming bacteria
 Two types of toxins – Exotoxin & Endotoxin
 Toxins – highly specific in nature
Exotoxins
 Secreted into the culture medium
 Vegetative insecticidal proteins (Vip) (Warren et al., 1998) and the
Secreted insecticidal protein (Sip) (Donovan et al., 2006)

Exotoxins
Delta endotoxins

 Produced during sporulation phase of bacterium

 Pore-forming toxins
 The most widely known are the δ-endotoxins, including Cry and Cyt
toxins.
 The naming of a protein as a Cry toxin derives from the fact that it
forms a parasporal crystal.
 In contrast to Cry proteins, Cyt proteins exhibit a general cytolytic
(hemolytic) activity in vitro and predominantly dipteran
specificity in vivo
Nomenclature of Bt toxins
 A total of 229 Cry, 11 Cyt and 102 Vip toxins have been reported
(Tabashnik and Carrière,2017).
 degree of shared amino acid identity
Mode of
Action
Procedure for generating Bt
crops
Identification of Bt genes: The genes of Bacillus
thuringiensis that encode for insecticidal proteins more
especially, the Cry proteins, are found and isolated by scientists.
Gene Cloning: A multiple copy of the chosen Bt genes is
produced by cloning in preparation for their eventual transmission.

Construction of a Bt Gene Cassette: The Bt genes cloned are

integrated into a genetic construct known as a gene cassette.
Typically, this construct consists of various regulatory components
and a promoter region that drives gene expression.
 Transformation: Various strategies, such as Agrobacterium-
mediated transformation or biolistic (gene gun) technologies

 Selection and regeneration: Selectable markers are used to

identify plants that have effectively absorbed the Bt genes, and
these transformed cells are subsequently regenerated into complete
plants.

 Evaluation: The newly produced Bt plants are subjected to

extensive evaluation in controlled field trials to evaluate their
performance, including insect resistance and general agronomic
qualities.
Bt cotton: Bollworm damage can cause up to 85% yield loss in cott

The commercial cultivation of transgenic Bt-cotton was approved in the

USA in 1995 as it successfully controlled the lepidopteran insect
population feeding on cotton
First genetically modified crop in India
In 2002, Bt-cotton was introduced by a joint venture between Monsanto
and
Mahyco (Maharashtra Hybrid Seeds Co.) in 6 states of India
The three Bt cotton hybrids (Bt MECH 162, Bt MECH 184 and Bt
MECH 12) were introduced for three years
India is the largest producer of Bt cotton and 95% of cotton grown is Bt
Based on the number of Bt genes incorporated,
Bt cotton has been classified into
• First-Generation Bt-Cotton
• Second-Generation Bt-Cotton
• Third-Generation Bt-Cotton
First-Generation Bt-Cotton (Bollgard or Ingard)

In 2002, the government allowed the commercial cultivation of Bt-Ⅰ

cotton
First-generation Bollgard had a single-gene Cry1Ac
Agrobacterium transformation by transforming Cry1Ac in Coker-312
(Downes et al., 2010)
CaMV35S promoter with two copies of enhancer region and poly A
signal from the beta-conglycinin gene of soybean (Torres et al. 2009)
Bt protein in Bollgard - hybrid molecule with amino acid 1–466 from
Cry1Ab (B. thuringiensis subspecies kurstaki (Btk) strain HD-1) and
467–1178 Cry1Ac (Btk HD-73) Cry1Ac gene.
Table 1: Comparison of insecticide use and yields

Qaim, 2003
Table 2: Comparison of production costs and gross
margins (Rs/acre)

Qaim, 2003
econd-Generation Bt-Cotton
Bollgard II – 2006 (2003)
Bollgard II was transformed with two Bt genes
Cry2Ab gene + uidA (Gus) Bollgard I - Biolistic method
GUS was used as a screenable marker for the transformation of the
Cry2Ab gene in Bollgard II (Schünmann et al., 2003).
The overall expression of Cry2Ab was higher than Cry1Ac in Bollgard II.
Cry2Ab had a uniform higher expression throughout the plant tissues
except in the case of nectar and pollen.
WideStrike Cotton
WideStrike cotton, developed by Dow AgroSciences in 2005
Two stacked genes, Cry1F and Cry1Ac
 Cry1F protein from Bt subspecies aizawai (Bta) strain PS811 and Btk-
HD-73 encoding Cry1Ac protein
 Co-transformation - two different plasmids containing genes coding for
Cry1F and Cry1C were separately transformed in cotton, which was
then backcrossed with elite variety PSC355 followed by one self
pollination
Both backcrosses were then inter-crossed and self-pollinated to
produce cotton variety (Baeumler et al. 2006)
TwinLink Bt-Cotton
TwinLink technology combines insect resistance Bt with weed
management technology
Cotton bollworms, beet armyworm, fall armyworm and pink
bollworm and also full tolerance to LibertyLink and glyphosate
herbicide used against a wide range of broad leaf and grass
weeds
TwinLink is available as Glytol LibertyLink
TwinLink contains two Cry proteins, Cry1Ab and Cry2Ae
Third-Generation Bt-Cotton
Bollgard III
Bollgard III provided farmers a flexible control in the management
of crops as it contains two cry genes (Cry1Ac and Cry2Ab) and one Vip
gene (Vip3A) (Monsanto,2019)
WideStrike III
developed by Dow AgroSciences contains three genes or multiple traits to
provide a broad range of resistance against insects
WideStrike III contains three genes Cry1F, Cry1Ac and Vip3A
TwinLink Plus
TwinLink Plus was developed by Bayer in 2017
TwinLink plus technology combines two genes (Cry1Ab and Cry2Ae)
present in TwinLink with Vip3Aa19 to create trademark TwinLink Plus.
 Increases yield due to effective
control of bollworms

Reduction in cost of cultivation as

insecticide use in the cultivation
practice decreased

Advantages of Give more productivity in smaller

Bt cotton area

Protects and promotes insects

that are beneficial to the
ecosystem

Reduction in environmental
pollution and soil pollution
 Higher cost of Bt cotton seeds as
compared to non Bt cotton seeds
Effectiveness up to 120 days, after
that toxin-producing efficiency
drastically reduces

Disadvantages Ineffective against sucking pest like

of Bt cotton jassids, whitefly etc.

Disrupt the natural process of gene

flow in cotton

Pest may become resistant to toxins

 Why Bt is not toxic to
Humans?
1. pH of the gut 2. Specific receptor
site
• Activation of toxin • Bt protein is receptor-mediated
alkaline condition specific receptor site
• But the human stomach has a • But, humans do not have these
pH of 1.5 to 3.5 i.e., acidic specific receptor sites
 Increase
d yield

Sustainab
Pest
le
resistan
agricultur
ce
e
Future
prospect
s
Climate
Global
resilienc
adoption
e

Improve
d fiber
quality
 Future prospects of Bt
cotton:
• Bt cotton yields have stagnated, have a null effect on
profits, and have become more sensitive to pest
pressure in the most recent decade
• Though many studies have demonstrated higher crop
yield and profit gains in the first decade of Bt cotton
adoption that raised the average returns to the
technology, the second decade shows convergence in
benefits, which raises obvious questions about the
prospect of GM technology