Contents - Faperta

214 Biotechnological Approaches for Pest Management and Ecological Sustainability
Genetic Transformation of Crop Plants for Resistance to Insects
Genes from bacteria such as B. thuringiensis, B. subtilis (Ehrenberg) Gohn, and B. sphaericus
Meyer and Neide, protease inhibitors, plant lectins, ribosome inactivating proteins, secondary
plant metabolites, and small RNA viruses have been used alone or in combination
with conventional host plant resistance to develop crop cultivars that suffer less damage
from insect pests (Table 7.2). Genes conferring resistance to insects have been inserted into
crop plants such as maize, Zea mays L., rice, Oryza sativa L., wheat, Triticum aestivum L.,
sorghum, Sorghum bicolor (L.) Moench, sugarcane, Saccharam offi cinarum L., cotton,
Gossypium hirsutum L., potato, Solanum tuberosum L., tobacco, Nicotiana tabacum L., broccoli,
Brassica oleracea var italica L., cabbage, Brassica oleracea var capitata L., chickpea, Cicer arietinum
L., pigeonpea, Cajanus cajan (L.) Millsp., cowpea, Vigna unguiculata (L.) Walp., groundnut,
Arachis hypogea L., tomato, Lycopersicon esculentum Mill., brinjal, Solanum melongena L.,
and soybean, Glycine max (L.) Merr. (Hilder and Boulter, 1999; H.C. Sharma et al., 2000,
2004; H.C. Sharma, Sharma, and Crouch, 2004). Genetically transformed crops with Bt
genes have been deployed for cultivation in Argentina, Australia, Bulgaria, Canada, China,
France, Germany, India, Indonesia, Mexico, Portugal, Romania, South Africa, Spain, the
United States, Ukraine, and Uruguay (James, 2007). Although several transgenic crops
with insecticidal genes have been introduced in the temperate regions, very little has been
done to use this technology for improving productivity of crops that are important for
food security in the developing countries, where the need for increasing food production
is most urgent. Transgenic Bt cotton and maize have been grown largely on a commercial
scale under high input, temperate, or subtropical cropping systems. The most urgent need
to use this technology is in the tropical regions, where soil fertility, water availability,
insect pests, and diseases severely constrain crop production. For transgenic plants to be
useful as an effective weapon in pest management, they have to substitute, completely or
partially, for the use of insecticides in crop production, and result in increased crop production
and environment conservation. The bioeffi cacy of different toxin genes expressed
in transgenic plants in different crops is discussed below.
Toxin Proteins from Bacillus thuringiensis
Ishiwata discovered this bacterium in 1901 from diseased silkworm, Bombyx mori L. larvae.
Berliner (1915) isolated it from diseased larvae of Ephestia kuhniella Keller, and designated
it as Bacillus thuringiensis. It is a Gram-positive bacterium, which produces proteinaceuos
crystalline inclusion bodies during sporulation. Further research on Bt by Steinhaus (1951)
led to renewed interest in it as a biopesticide. There are several subspecies of this bacterium,
which are effective against lepidopteran, dipteran, and coleopteran insects. The
identifi cation of the kurstaki strain provided a boost for commercialization of Bt. The HD 1
strain identifi ed by Dulmage (1981) is the most important Bt strain. There are over 50 registered
Bt products with more than 450 formulations (Shewry and Gutteridge, 1992). Bacillus
thuringiensis var. israeliensis has been used extensively for the control of mosquitoes (de
Barjac and Sotherland, 1990). Bacillus thuringiensis var. morrisoni and B. thuringiensis var.
israeliensis carry four genes that encode mosquito and black fl y toxins Cry IVA, Cry IVB,
Cry IVC, and Cry IVD (Bechtel and Bulla, 1976). Because of the crystalline nature of
Bt-specifi c toxin proteins, the term Cry is used in gene and protein nomenculature. The
toxin genes earlier were classifi ed into four types, based on insect specifi city and sequence
homology (Hofte and Whiteley, 1989). Cry I type genes encode proteins of 130 kDa, and