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ICPEP-3 (2005) Souvenir

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.


This article has been reproduced from the Souvenir released during the Third International Conference

on Plants & Environmental Pollution (ICPEP-3) held at Lucknow from  28 November to 2 December 2005.


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