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Plant Protease Inhibitors in Control of Phytophagous Insects

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Plant Protease Inhibitors in Control of Phytophagous Insects EJB Electronic Journal of Biotechnology ISSN: 0717-3458 Vol.5 No.1, Issue of April 15, 2002 © 2002 by Universidad Católica de Valparaíso -- Chile Received August 20, 2001 / Accepted April 1, 2002 REVIEW ARTICLE Plant protease inhibitors in control of phytophagous insects Paulraj K. Lawrence* Department of Biological Sciences Texas Tech University Box No 43131 Lubbock, TX 79409-3131, USA Tel: 806 742 2740 Fax: 806 742 2963 E-mail: [email protected] Kripa Ram Koundal National Research Center on Plant Biotechnology Indian Agricultural Research Institute PUSA, New Delhi, India-110012, India Tel: 91 011 5711554 Fax: 91 011 5766420 E-mail: [email protected] Keywords: crop pests, protease inhibitors, transgenic plants. Plant proteinase inhibitors (PIs) have been well confounds the problem (Roush and McKenzie, 1987). established to play a potent defensive role against Under these circumstances, provision of food to the rapidly predators and pathogens. Although diverse endogenous expanding population has always been a challenge facing functions for these proteins has been proposed, ranging mankind. This problem is more acute in the tropics and sub- from regulators of endogenous proteinases to act as tropics, where the climate provides a highly condusive storage proteins, evidence for many of these roles is environment for a wide range of insects and necessitates partial, or confined to isolated examples. On the other massive efforts to suppress the population densities of hand, many PIs have been shown to act as defensive different pests in order to achieve an adequate supply of compounds against pests by direct assay or by food. In developing countries, the problem of competition expression in transgenic crop plants, and a body of from insect pests is further complicated with a rapid annual evidence for their role in plant defense has been increase in the human population (2.5-3.0 percentage) in accumulated consistently. The role and mechanism of comparison to a 1.0 percent increase in food production. In action for most of these inhibitors are being studied in order to feed the ever expanding population, crop protection detail and their respective genes isolated. These genes plays a vital and integral role in the modern day agricultural have been used for the construction of transgenic crop production to minimise yield losses. Currently, the crop plants to be incorporated in integrated pest protection practice in such agricultural systems relies management programmes. This article describes the exclusively on the use of agrochemicals, although a few classes of protease inhibitors, their regulation and genes specific cases do exist where inherent varietal resistance used to construct transgenic plants against and biological control have been successfully employed. phytophagous insects. The exclusive use of chemical pesticides not only results in rapid build-up of resistance to such compounds, but their World wide crop losses without the use of pesticides and non-selectivity affects the balance between pests and other non-chemical control strategies is estimated to be natural predators, and is generally in favour of pests about 70% of crop production, amounting to US $ 400 (Metcalf, 1986). Therefore, an integrated pest management billion. The world wide pre-harvest losses due to insect (IPM) programme, comprising a combination of practices pests, despite, the use of insecticides is 15% of total including the judicious use of pesticides, crop rotation, field production representing over US $ 100 billion (Krattiger, sanitation and above all exploitation of inherently resistant 1997). The annual cost of insect control itself amounts to plant varieties would provide the best option (Meiners and US $ 8 billion, thus warranting urgent economical control Elden, 1978). The last option includes the use of transgenic measures. Many of the crop varieties developed in the past crops, expressing foreign insecticidal genes which could 30 years were high yielders, but had poor storage make a significant contribution to sustainable agriculture characteristics (Kerin, 1994). Insect pests are capable of and thus could be an important component of IPM (Boulter, evolving to biotypes that can adapt to new situations, for 1993). The production of transgenic crops has seen rapid instance, they overcome the effect of toxic materials or advances during the last decade with the commercial bypass natural or artificial plant resistance, which further introduction of Bt transgenics, but the major concern with * Corresponding autor This paper is available on line at http://www.ejb.org/content/vol5/issue1/full/3 Lawrence, P.K. and Koundal, K.R. these crops has been the development of resistance by pest control of plant pests and pathogens, but also as a means to and public acceptability. Hence, there has been a need to produce PIs, useful in alternative systems and the use of discover new effective plant genes which would offer plants as factories for the production of heterologous resistance/protection against these pests. Protease inhibitors proteins (Sardana et al. 1998). These inhibitor families that (PIs) are one of the prime candidates with highly proven have been found are specific for each of the four inhibitory activity against insect pests and also known to mechanistic classes of proteolytic enzymes, and based on improve the nutritional quality of food. the active amino acid in their “reaction center” (Koiwa et al. 1997), are classified as serine, cysteine, aspartic and Plant protease inhibitors metallo-proteases. The possible role of protease inhibitors (PIs) in plant Serine proteinase inhibitors protection was investigated as early as 1947 when, Mickel and Standish observed that the larvae of certain insects The role of serine PIs as defensive compounds against were unable to develop normally on soybean products. predators is particularly well established, since the major Subsequently the trypsin inhibitors present in soybean were proteinases present in plants, used for processes such as shown to be toxic to the larvae of flour beetle, Tribolium protein mobilization in storage tissues, contain a cysteine confusum (Lipke et al. 1954). Following these early studies, residue as the catalytically active nucleophile in the enzyme there have been many examples of protease inhibitors active site. Serine proteinases are not used by plants in active against certain insect species, both in in vitro assays processes involving large scale protein digestion, and hence against insect gut proteases (Pannetier et al. 1997; Koiwa et the presence of significant quantities of inhibitors with al, 1998) and in in vivo artificial diet bioassays (Urwin et al. specificity towards these enzymes in plants cannot be used 1997; Vain et al. 1998). The term “protease” includes both for the purposes of regulating endogenous proteinase “endopeptidases” and “exopeptidases” whereas, the term activity (Reeck et al. 1997). In contrast, a major role for “proteinase” is used to describe only “endopeptidases” serine PIs in animals is to block the activity of endogenous (Ryan, 1990). Several non-homologous families of proteinases in tissues where this activity would be harmful, proteinase inhibitors are recognized among the animal, as in case of pancreatic trypsin inhibitors found in microorganisms and plant kingdom. Majority of proteinase mammals. The serine class of proteinases such as trypsin, inhibitors studied in plant kingdom originates from three chymotrypsin and elastase, which belong to a common main families namely leguminosae, solanaceae and protein superfamily, are responsible for the initial digestion gramineae (Richardson, 1991). of proteins in the gut of most higher animals (GarciaOlmedo et al. 1987). In vivo they are used to cleave These protease inhibitor genes have practical advantages long, essentially intact polypeptide chains into short over genes encoding for complex pathways i.e. by peptides which are then acted upon by exopeptidases to transferring single defensive gene from one plant species to generate amino acids, the end products of protein digestion. another and expressing them from their own wound These three types of digestive serine proteinases are inducible or constitutive promoters thereby imparting distinguished based on their specificity, trypsin specifically resistance against insect pests (Boulter, 1993). This was cleaving the C-terminal to residues carrying a basic side first demonstrated by Hilder et al. 1987 by transferring chain (Lys, Arg), chymotrypsin showing a preference for trypsin inhibitor gene from Vigna unguiculata to tobacco, cleaving C-terminal to residues carrying a large which conferred resistance to wide range of insect pests hydrophobic side chain (Phe, Tyr, Leu), and elastase including lepidopterans, such as Heliothis and Spodoptera, showing a preference for cleaving C-terminal to residues coleopterans such as Diabrotica, Anthonomnous and carrying a small neutral side chain (Ala, Gly) (Ryan, 1990). orthoptera such as Locusts. Further, there is no evidence Inhibitors of these serine proteinases have been described in that it had toxic or deleterious effects on mammals. Many many plant species, and are universal throughout the plant of these protease inhibitors are rich in cysteine and lysine, kingdom, with trypsin inhibitors being the most common contributing to better and enhanced nutritional quality type. At least, part of this bias can be accounted for by the (Ryan, 1989). Protease inhibitors also exhibit a very broad
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