natural-products-in-plant-pest-management

154 H.N. Verma and V.K. BaranwalThe induced antiviral state in N. glutinosa by SRI from C. aculeatumdecreased considerably after 3 days (Verma and Varsha, 1994). However, theresistance inducing ability of C. aculeatum SRI could be enhanced up to6 days by priming it with certain proteinaceous additives (Verma and Versha,1994). The activity is probably enhanced by modification or enhanced stabilityof proteinaceous inducers by these additives. Thus, one unexploitedapproach to engineering virus resistance is the manipulation of inducibledefences in plants. The production of systemic resistance by the use of botanicalswill be effective against a broad spectrum of viruses and will not breakdown when plants are exposed to high temperatures.Suppression of disease symptoms by true inhibitors may be accomplishedeither by acting on the first stage of the infection process, which is theadsorption of the virus into the host cell, or by blocking or competing withthe virus receptor sites on the leaf surface (Ragetli, 1957; Ragetli andWei ntraub, 1962) or by affecting the susceptibility of the host by altering hostcell metabolism (Verma and Awasthi, 1979). Bozarth and Ross (1964) suggestedthe phenomenon of SAR as a result of the initial infection by which asignal was generated at the site of application and transported throughoutthe plant to respond more effectively to the subsequent infection.Induction of systemic resistance by resistance inducers obtained fromplant extracts was first detailed by Verma and Mukerjee (1975). Extracts froma few plants induce an antiviral state by acting through an actinomycin D(AMD) sensitive mechanism (Verma and Awasthi, 1979; Verma et al., 1984).AMD is an inhibitor of protein synthesis at the transcription level. Concomitantapplication of AMD with SRI reversed the induction of resistance insusceptible plants. However, induction of resistance remains unaffectedwhen AMD is applied 12 h post-treatment. This gives an indication thatplant-extract-induced resistance is a host-mediated response (Verma et al.,1979).The activity at a distance from the point of application might be explainedby the supposition that the SRI present in the plant extract selectively attachesat the surface, and a type of chain reaction starts that elicits the transcriptionof defence-related genes, leading to the production of a new VIA (Verma andAwasthi, 1979).Ribosome inactivating proteins (RIPs)The basic proteins that function by inactivating the ribosome of the host havebeen called RIPs. The RIPs have been shown to be N-glycosidases, whichremove a specific adenine base in a conserved loop of the 28s rRNA ofeukaryotic organisms (Endo and Tsurugi, 1987; Endo et al., 1987) or the 23srRNA of prokaryotes (Hartely et al., 1991). Such damaged ribosomes can nolonger bind the elongation factor-2 (Gessner and Irvin, 1980; Rodes and Irvin,1981). The RIPs damage the ribosome, arrest protein synthesis and cause celldeath. RIPs show antiviral activity against both animal and plant viruses(Barbieri and Stirpe, 1982) and have been classified into two types (Stirpe