*2.1.2.1 Proteinase inhibitors (PI)*

Serine-like proteinases (trypsin-, chymotrypsin- and elastase-like endoproteases) dominant in lepidopteran larvae [26], a wider range of dominant gut proteinases in coleopteran species [27] and thiol proteases observed in corn rootworms (*Diabrotica spp*.) are some of the proteinases in insect which catalyse the release of amino acids from dietary protein, thereby providing nutrients essential for normal growth and development and thus the proteinase inhibitors plays an important part of the plant's natural defence system against herbivory by inhibiting the protein metabolism. Although the mode of action of PIs are not fully understood, it may be claimed that hypersecretion of digestive enzymes caused by the presence of the inhibitors, would result in depletion of essential amino acids [28, 29].

Serine and cysteine-proteinase inhibitors have been shown to inhibit the growth and development of a variety of insects, primarily lepidopteran and coleopteran species [29, 30]. The expression of a gene encoding a sweet potato trypsin protease inhibitor (TPI) in transgenic tobacco (at a relatively low level for a plant-derived ICP – 0.2%) results in severe growth retardation of *Spodoptera litura* caterpillars fed on it besides the presence of high levels of the same naturally in it [31].

The first instance of a plant-derived ICP gene being used in transgenic plants was the constitutive expression (through the CaMV35S gene promoter) of a trypsin inhibitor gene taken from cowpea (*Vigna unguiculata*) and in tobacco [32]. Proteinase inhibitors do not just alter gut digestive enzymes; they can also affect insect water balance, moulting, and enzyme regulation [33]. The majority of research has focused on serine-proteinase inhibitors derived from the plant families Fabaceae, Solanaceae, and Poaceae, which are mostly aimed against not only lepidopteran pests but also some coleopteran and orthopteran pests. The cowpea trypsin inhibitor (CpTI), which has been introduced into at least 10 different plant species, is the most active inhibitor discovered to date the protection provided by CpTI was less pronounced and consistent than that of tobacco containing a truncated Bt-toxin gene [34].

#### *2.1.2.2 α-Amylase inhibitors*

To block carbohydrate metabolism, a-amylase inhibitors from wheat (WAAI) and common bean (*Phaseolus vulgaris*) (BAAI) are utilised. When introduced into transgenic tobacco, the former increased mortality of lepidopteran larvae fed on it by 30–40% [35], while the latter, when expressed in transgenic pea seeds and driven by the pha1 gene promoter, elevated resistance to bean weevils [36, 37].
