H. C. Sharma

Mechanisms of plant defense against insect herbivores

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counter/offset the effects of herbivore attack. The biochemical mechanisms of defense against the herbivores are wide-ranging, highly dynamic, and are mediated both by direct and indirect defenses. The defensive compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. In addition, plants also release volatile organic compounds that attract the natural enemies of the herbivores. These strategies either act independently or in conjunction with each other. However, our understanding of these defensive mechanisms is still limited. Induced resistance could be exploited as an important tool for the pest management to minimize the amounts of insecticides used for pest control. Host plant resistance to insects, particularly, induced resistance, can also be manipulated with the use of chemical elicitors of secondary metabolites, which confer resistance to insects. By understanding the mechanisms of induced resistance, we can predict the herbivores that are likely to be affected by induced responses. The elicitors of induced responses can be sprayed on crop plants to build up the natural defense system against damage caused by herbivores. The induced responses can also be engineered genetically, so that the defensive compounds are constitutively produced in plants against are challenged by the herbivory. Induced resistance can be exploited for developing crop cultivars, which readily produce the inducible response upon mild infestation, and can act as one of components of integrated pest management for sustainable crop production.

Applications of biotechnology for crop improvement: prospects and constraints

Recombinant DNA technology has significantly augmented the conventional crop improvement, and has a great promise to assist plant breeders to meet the increased food demand predicted for the 21st century. Dramatic progress has been made over the past two decades in manipulating genes from diverse and exotic sources, and inserting them into microorganisms and crop plants to confer resistance to insect pests and diseases, tolerance to herbicides, drought, soil salinity and aluminum toxicity; improved post-harvest quality; enhanced nutrient uptake and nutritional quality; increased photosynthetic rate, sugar, and starch production; increased effectiveness of biocontrol agents; improved understanding of gene action and metabolic pathways; and production of drugs and vaccines in crop plants. Despite the diverse and widespread beneficial applications of biotechnology products, there remains a critical need to present these benefits to the general public in a real and understandable way that stimulates an unbiased and responsible public debate. The development, testing and release of agricultural products generated through biotechnology-based processes should be continuously optimized based on the most recent experiences. This will require a dynamic and streamlined regulatory structure, clearly supportive of the benefits of biotechnology, but highly sensitive to the well being of humans and environment.

Genetic Transformation of Crops for Insect Resistance: Potential and Limitations

Transgenic resistance to insects has been demonstrated in plants expressing insecticidal genes such as δ -endotoxins from Bacillus thuringiensis (Bt), protease inhibitors, enzymes, secondary plant metabolites, and plant lectins. While transgenic plants with introduced Bt genes have been deployed in several crops on a global scale, the alternative genes have received considerably less attention. The protease inhibitor and lectin genes largely affect insect growth and development and, in most instances, do not result in insect mortality. The effective concentrations of these proteins are much greater than the Bt toxin proteins. Therefore, the potential of some of the alternative genes can only be realized by deploying them in combination with conventional host plant resistance and Bt genes. Genes conferring resistance to insects can also be deployed as multilines or synthetic varieties. Initial indications from deployment of transgenics with insect resistance in diverse cropping systems in USA, Canada, Argentina, China, India, Australia, and South Africa suggest that single transgene products in standard cultivar backgrounds are not a recipe for sustainable pest management. Instead, a much more complex approach may be needed, one which may involve deployment of a combination of different transgenes in different backgrounds. Under diverse climatic conditions and cropping systems of tropics, the success in the utilization of transgenics for pest management may involve decentralized national breeding programs and several small-scale seed companies. While several transgenic crops with insecticidal genes have been introduced in the temperate regions, very little has been done to use this technology for improving crop productivity in the harsh environments of the tropics, where the need for increasing food production is most urgent. There is a need to develop appropriate strategies for deployment of transgenics for pest management, keeping in view the pest spectrum involved, and the effects on nontarget organisms in the ecosystem.

Host-plant resistance to insects in sorghum and its role in integrated pest management

Sorghum is one of the most important cereal crops in Asia, Africa and Latin America. It is damaged by >150 insect species, of which sorghum shoot fly (Atherigona soccata Rond.), stem borers (Chilo partellus Swin., Busseola fusca Fuller, Eldana saccharina Wlk., Sesamia spp. and Diatraea saccharalis Wlk.), armyworms (Mythimna separata Wlk., Spodoptera exempta Wlk. and S. frugiperda J. E. Smith), aphids (Melanaphis sacchari Zehnt., Schizaphis graminum Rond. and Rhopalosiphum maidis Fitch.), mites (Oligonychus spp.), midge (Contarinia sorghicola Coq.), head bugs (Calocoris angustatus Leth., Eurystylus immaculatus Odh. and Oebalus spp.) and head caterpillars (Helicoverpa armigera Hb., Eublemma spp. Pyroderces simplex Wsm., Cryptoblabes spp. and Nola spp.) are the major pests world wide. Considerable progress has been made in screening and breeding for resistance to shoot fly, stem borers, greenbug, midge, armyworms and head bugs. Resistance to major pests is available in diverse genotypes. It is possible to combine resistance to two or more insect species in some cases (e.g. shoot fly and stem borer, stem borer and armyworms, midge and aphids and head bugs and head caterpillars). However, it may be quite difficult to combine resistance to some insect species (e.g. shoot fly versus midge and/or head bugs). Host-plant resistance can be used for the management of sorghum midge, greenbug, mites, aphids and head caterpillars. However, host-plant resistance need to be supplemented with other methods of pest control for shoot fly, stem borers. armyworms and head bugs.

Prospects for using transgenic resistance to insects in crop improvement.

Integrated pest management has historically placed great hopes on host plant resistance. However, conventional host-plant resistance to insects involves quantitative traits at several loci. As a result, the progress has been slow and difficult to achieve. With the advent of genetic transformation techniques, it has become possible to clone and insert genes into the crop plants to confer resistance to insect pests. Resistance to insects has been demonstrated in transgenic plants expressing genes for δ δ-endotoxins from Bacillus thuringiensis (Bt), protease inhibitors, enzymes and plant lectins. Most of the plant derived genes produce chronic rather than toxic effects and some insect pests are not sensitive to some of these factors. The potential of plant derived genes can be realised by deploying them in combination with host plant resistance and exotic genes. Genes conferring resistance to insects have been inserted into crop plants such as maize, cotton, potato, tobacco, potatoes, rice, broccoli, lettuce, walnuts, apples, alfalfa and soybean. Genetically transformed crops with Bt genes have been deployed for cultivation in USA, China and Australia. However, very little has been done to use this technology for improving crop production in the harsh environments of the tropics, where the need for increasing food production is most urgent. International agricultural research centres, advanced research institutes and the seed sector should make an effort to use these new tools for increasing food

Phenolic Compounds on the Pod-Surface of Pigeonpea, Cajanus cajan, Mediate Feeding Behavior of Helicoverpa armigera Larvae

A methanol extract of the pod surfaces of Cajanus cajan, a feeding stimulant for fifth-instar Helicoverpa armigera, was shown to contain four main phenolic compounds. Three of these were identified as isoquercitrin, quercetin, and quercetin-3-methyl ether, by comparing UV spectra and HPLC retention times with authentic standards. The fourth compound was isolated by semipreparative HPLC and determined to be 3-hydroxy-4-prenyl-5- methoxystilbene-2-carboxylic acid (stilbene) by NMR spectroscopy and mass spectrometry. Quercetin, isoquercitrin, and quercetin-3-methyl did not affect the selection-behavior of fifth-instar H. armigera. However, larvae were deterred from feeding on glass-fiber disks impregnated with the stilbene. Furthermore, larvae exposed to quercetin-3-methyl ether consumed significant amounts of both disks. In a binary-choice bioassay, a combination of quercetin-3-methyl ether and the stilbene on one disk and pure quercetin-3-methyl ether on the other disk resulted in increased consumption of both glass-fiber disks by larvae. In contrast, consumption was reduced if the combination was presented to larvae on one disk with purified stilbene on the other disk. Cajanus cajan cultivars that varied in their susceptibility to H. armigera were surveyed for the presence of the four phenolic compounds. An absence of quercetin and higher concentrations of isoquercitrin than the cultivated variety characterized pod surface extracts of pod-borer-resistant cultivars. In addition, the ratio of the stilbene to quercetin-3-methyl ether was greater in the pod-borer-resistant cultivars. These findings are discussed in relation to the identification of chemical characters that can be used for crop improvement.

Bionomics, host plant resistance, and management of the legume pod borer, Maruca vitrata - a review

Abstract Legume pod borer, Maruca (testulalis) vitrata (Geyer) is one of the major constraints in increasing the production and productivity of grain legumes in the tropics. Screening for resistance has been carried out using natural infestation, and multi- and no-choice tests under greenhouse/laboratory conditions. Information is available on genotypic resistance to M. vitrata in cowpea, while such information on pigeonpea and other legumes is limited. Stem and pod wall thickness, trichomes and podding habit are associated with resistance to Maruca. Several natural enemies have been recorded on M. vitrata. Cultural practices such as intercropping, weeding, time of planting, and planting density reduce its damage in cowpea. Several insecticides have been found to be effective for controlling this insect. There is a need to generate information on insect-plant-environment interactions, screening techniques, mechanisms and diversity of resistance, genetic transformation of host plants involving Bt genes, and use of natural enemies for integrated pest management in diverse agro-ecosystems.

Morphological and chemical components of resistance to pod borer, Helicoverpa armigera in wild relatives of pigeonpea

Host plant resistance is an important component for minimizing the losses due to the pod borer, Helicoverpa armigera, which is the most devastating pest of pigeonpea. An understanding of different morphological and biochemical components of resistance is essential for developing strategies to breed for resistance to insect pests. Therefore, we studied the morphological and biochemical components associated with expression of resistance to H. armigera in wild relatives of pigeonpea to identify accessions with a diverse combination of characteristics associated with resistance to this pest. Among the wild relatives, oviposition non-preference was an important component of resistance in Cajanus scarabaeoides, while heavy egg-laying was recorded on C. cajanifolius (ICPW 28) and Rhynchosia bracteata (ICPW 214). Accessions belonging to R. aurea, C. scarabaeoides, C. sericeus,C. acutifolius, and Flemingia bracteata showed high levels of resistance to H. armigera, while C. cajanifolius was as susceptible as the susceptible check, ICPL 87. Glandular trichomes (type A) on the calyxes and pods were associated with susceptibility to H. armigera, while the non-glandular trichomes (trichome type C and D) were associated with resistance to this insect. Expression of resistance to H. armigera was also associated with low amounts of sugars and high amounts of tannins and polyphenols. Accessions of wild relatives of pigeonpea with non-glandular trichomes (type C and D) or low densities of glandular trichomes (type A), and high amounts of polyphenols and tannins may be used in wide hybridization to develop pigeonpea cultivars with resistance to H. armigera.

The Present and Future Role of Insect-Resistant Genetically Modified Cotton in IPM

Transgenic cottons producing Cry toxins from Bacillus thuringiensis (Bt) provide for control of lepidopteran pests and were first commercially grown in Australia, Mexico and the USA in 1996. As of 2007, a total of six additional countries (Argentina, Brazil, China, Colombia, India, and South Africa) now grow Bt cotton on a total production area of 14 million hectares. The technology primarily provides highly selective and effective control of bollworms, which are the most damaging pests of cotton worldwide. It is estimated that between 1996 and 2005 the deployment of Bt cotton has reduced the volume of insecticide active ingredient used for pest control in cotton by 94.5 million kilograms and increased farm income through reduced costs and improved yields by US

Strategies for pest control in sorghum in India

7.5 billion, with most of the benefit accrued by farmers in developing nations. Reductions in insecticide use have broadened opportunities for biological control of all cotton pests but most other pest management tactics have remained largely unchanged by the use of Bt cotton. However, several non-target pests have become more problematic in Bt cotton fields in some countries largely due to reductions in insecticide use for target pests. After 11 years of Bt cotton cultivation, control failures due to resistance have not been detected under field conditions. This success can be largely credited to pre-emptive resistance management based on mandated refuges and monitoring programs as well as non-mandated refuge crops and natural refuges which collectively act to dilute any resistant alleles in pest populations. New products are in the pipeline to improve the effectiveness of genetically modified cotton cultivars for resistance to lepidopteran pests, and to address other pest problems in cotton. Debate over food and environmental safety, regulatory oversight, and farming community welfare are likely to continue as the technology moves forward with new crops and new adopting countries.