natural-products-in-plant-pest-management

160 H.N. Verma and V.K. BaranwalSeveral classes of the PR proteins either possess direct antimicrobial activityor are closely related to classes of antimicrobial proteins. These include β-1,3-glucanase, chitinase, cysteine-rich proteins related to thaumatin and PR-1proteins. An in vivo role in disease resistance has not been demonstrated forany of the PR proteins. Alexander et al. (1993) demonstrated that constitutivehigh level expression of PR 1-a in transgenic tobacco results in tolerance toinfection by Peronospora tabacina and Phytophthora parasitica var. nicotinae. Onthe other hand, transgenic tobacco plants expressing PR 1-b gene exhibitedno reduction in the severity of TMV symptoms (Cutt et al., 1989). Quite oftenSAR was not correlated with the induction of PR proteins (Kopp et al., 1989; Yeet al., 1989; Cohen et al., 1993; Kessman et al., 1994). Fraser found a poorcorrelation between the levels of PR 1-a protein and also gave evidence forthe occurrence of PR proteins in leaves of healthy tobacco plants duringflowering (Fraser, 1981; 1982). In conclusion, most of the evidence shows thatthe PR proteins are closely associated with, and not necessarily responsiblefor, induced resistance.7.5 Induced Resistance and the Role of Induced AntiviralProteinsVirus-induced new antiviral protein componentsAntiviral substances are formed in plants responding hypersensitively tovirus infection (Verma and Prasad, 1992) and have been recognized as phosphorylatedglycoproteins (Faccioli and Capponi, 1983), glycoproteins (Wieringaand Dekker, 1987), RNA (Kimmins, 1969), traumatic acid (Kato andMisawa, 1976), and protein-like substances (Nienhaus and Babovic, 1978).Chadha and MacNeill (1969) found the formation of an antiviral principle intomato plants systemically infected with TMV. These antiviral compoundsare not generally specific to the plants. Thus, antiviral substances producedin capsicum plants could reduce PVX infection on Gomphrena globosa as wellas Solanum tuberosum (Nagaich and Singh, 1970).The presence of an antiviral factor (AVF) was established in virus- infectedplants which could decrease the number of local lesions produced by TMVand PVY (Sela and Applebaum, 1962). Partially purified AVF from TMVinfected N. glutinosa plants was found to contain both protein and RNA (Selaet al., 1964). It was sensitive to ribonuclease and was resistant to proteolyticenzymes (Sela et al., 1966).Mozes et al. (1978) established that the purified AVF is a phosphorylatedglycoprotein of molecular weight 22 kDa on SDS gels. It is sensitive to pronaseunder conditions suitable for proteolysis of glycoproteins. It remainsactive after treatments with SDS and is stable at pH 2.0. It resembles interferonin many of its properties (Mozes et al., 1978).Previously it was believed that TMV infection was necessary for AVFproduction. But Edelbaum et al. (1983) found that TMV infection could besubstituted by treatment with a mixture of Poly (I), Poly (C), cAMP