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Insect
Resistant Transgenic Crops:
A major
contribution of plant biotechnology
to
environmental sustainability
Rakesh Tuli and D. V. Amla
National Botanical Research Institute, Rana Pratap Marg,
Lucknow-226001
During the course of evolution, a broad spectrum of metabolic activities
have evolved in lower to higher organisms with an enormous capability to
degrade any natural material. An unfathomable opportunity exists in
understanding the complex matrix of pathways and deploying those in a
sustainable manner for the degradation of man made agrochemicals,
pollution abatement and bioremediation. Genetic engineering is poised to
offer some of the most powerful approaches to develop novel life forms
aimed at keeping the environment cleaner and sustainable. Plant
biotechnology has already given an outstanding example in this direction
in global commercialization of insect resistant transgenic crop plants
the Bt-cotton, maize and soybean. The approach aims at doing away with
tones of chemical pesticides used globally in agriculture. Since
Indonesia imposed a ban in 1986 on the import of 57 insecticides used in
rice, combined with IPM strategies, the government is officially known
to save US$ 120 m annually. The WHO estimates a loss of about 20,000
lives midst some 20,00,000 cases of health hazard to farmers every year
globally due to the widespread use of agrochemicals. Cumulatively,
pesticides worth more than US$ 30 billion are estimated to be used
worldwide annually. Technological shift from chemical to biological
approaches is the unavoidable need for sustainable agriculture. Some of
the biggest multinational agrochemicals corporations, including
Monsanto, USA and Zeneca, UK have already reoriented their strategies to
plant biotechnology.
Since the commercial release of transgenic crops in 1994, their adoption
by farmers has increased impressively. In 2004, biotech crops were grown
on nearly 80 million hectares in 18 countries. Global socio-economic and
environmental impact of transgenic crops have been made by several
groups in recent years; the most exhaustive study by Graham Brookes &
Peter Barfoot was release in October 2005 by the PG Economics Ltd., U.K.
The study shows that the global farm income contributed by GM crops
increased by about US$ 6 billion in 2004. The use of herbicide and
insect resistant GM soybeans, maize, cotton and canola made a major
impact on environment by reducing the usage of herbicides and
insecticides (active ingredients) globally by about 6% through
1996-2004. In terms of the environmental impact quotient, a 14% net gain
is estimated. Less frequent herbicide and insecticide applications and
reduced tillage operations further lead to the savings in greenhouse gas
emissions and fuel usage. In 2004, a reduction of about 1 billion kg
carbon dioxide is estimated due to reduced fuel usage. This is
equivalent to removing nearly 0.5 million cars from the roads. The
adoption of GM herbicide tolerant crops reduces a number of tillage
operations related to seedbed preparation and weeding. Not only that the
tractor fuel use for tillage is reduced, soil quality is enhanced due to
low tillage and soil erosion is prevented. In turn more carbon remains
in the soil, leading to lower emission of greenhouse gases. An extra 3
billion kg of soil carbon (equivalent to 10 billion kg of CO2,
not released to atmosphere) may have been sequestered in 2004. These
enormous gains to environment are inspite of the fact that as yet only a
few agronomic traits have been engineered, that too in a small number of
crops and in a few countries.
In
India, pesticides worth Rs. 10 billion are used on cotton crop alone,
which is more than half of the total pesticide usage in the country. The
development of transgenic cotton and paddy for resistance to insect
pests is a major need in responsible applications of recombinant DNA
techniques and transgenic crops in the country. In 2002, GM Bt-cotton
was first released in the country. It was grown on about 1.2 million
acres in 2004 which is about 6% of the total area under cotton
cultivation in India. National Botanical Research Institute, Lucknow has
made major contributions in developing indigenous technologies related
to the development of Bt-cotton. The development of insect resistant
transgenic crops has several technical challenges, if chemical
pesticides have to be largely replaced, without sacrificing yield,
safety and sustainability. Some of the areas full of inventive
opportunities are: need to discover new genes targeted against locally
relevant insect pests, pyramiding genes to guard against the evolution
of resistance in insects, economical evaluation of biosafety and
environmental risks following the release of the pesticidal proteins and
transgenic crops, designing proteins against specific pests, expressing
the pesticidal proteins specifically at the site of infestation by
insect pests etc. Several of these issues have been innovatively pursued
at NBRI for the development of transgenic cotton, pigeon pea, chick pea,
castor, groundnut and tomato, jointly with laboratories at other
institutes in India. A variety of novel genes and promoters have been
designed to develop transgenic lines for resistance to locally prevalent
insect pests. A hybrid -endotoxin protein was designed against a
polyphagous lepidopteran insect pest,
Spodoptera litura
which is tolerant to most of the known -endotoxins. The hybrid -endotoxin
was created by replacing amino acid residues 530-587 in a poorly active
natural Cry1Ea protein, with a highly homologous 70 amino acid region of
Cry1Ca in domain III. The truncated -endotoxins Cry1Ea, Cry1Ca and the
hybrid protein Cry1EC accumulated in
Escherichia coli
to form inclusion bodies. The solubilized Cry1EC made from
E.
coli
was
4- fold more toxic to the larvae of
S.
litura
than
Cry1Ca, the best known -endotoxin against
Spodoptera
sp. None of the two truncated toxins, solubilized from
E. coli
caused larval mortality. However, trypsinised Cry1Ca protoxin obtained
from E.
coli
and solubilized from inclusion bodies caused mortality of
S.
litura
with
LC50
513 ng/ml semi synthetic diet. A synthetic gene coding for the hybrid -endotoxin
Cry1EC was designed for high level expression in plants, taking into
consideration several features found in the highly expressed plant
genes. Transgenic, single copy plants of tobacco as well as cotton were
developed. The selected lines expressed Cry1EC at 0.1-0.6% of soluble
leaf protein. Such plants were completely resistant to
S.
litura
and
caused 100% mortality in all stages of larval development. Hence, unlike
in E.
coli,
the
hybrid -endotoxin folded into a functionally active conformation in both
tobacco and cotton leaves. The truncated Cry1EC expression in tobacco
leaves was about 8- fold more toxic (LC50
58 ng/ml diet) as compared to expression in
E. coli.
Some of our researches in this area have been published in Transgenic
Research (2004), vol 13, page 397-410. The possibilities in discovering
new genes and organisms and engineering those for the abatement of
pollution through agricultural chemicals are endless. |
