natural-products-in-plant-pest-management

158 H.N. Verma and V.K. Baranwaltobacco and tomato plants expressing the pokeweed antiviral gene are foundto be resistant to a broad spectrum of plant viruses (Lodge et al., 1993).A problem often encountered in using type-I RIPs is the fact that theycannot inhibit protein synthesis in intact cells. However, when coupled totype-II RIP, they can be used effectively. The toxicity of plant materialscontaining type-II RIPs have long been known and has great medicinalpotential. Recently, apoptosis was described in both lymphoid tissue and inthe intestine of abrin- and ricin-poisoned rats. Apoptosis was also observedin tissue culture of cancer cells treated with ricin.The antiviral activity of the RIP could be due to inactivation of ribosomeof the infected plant cell. As compared to the usefulness in the plants, the roleof RIPs in human and animal systems has been much more widely documented.In spite of all these efforts, the biological significance of RIPs innature is not yet known and moreover, their antiviral action does not seem todepend upon the inhibition of host ribosomes (Chen et al., 1993).Great potential exists today in elucidating the possible significance ofRIPs and their exact antiviral role which may not always depend upon inhibitionof host ribosomes. Their co-action with another antiviral mechanismalso needs to be explored, especially their place in the cascade of events followingviral infection up to establishment of resistance, both systemic orlocalized.7.3 Pathogen-induced Systemic Acquired Resistance (SAR)The infection of plants by necrotizing pathogens, including fungi, bacteriaand viruses, induces systemic resistance to subsequent attack by the pathogens.This resistance is called systemic acquired resistance (SAR) (Kessmanet al., 1994; Ryals et al., 1994; Prasad et al., 2001). Also, it can be activated innumerous plants by pre-inoculation with biotic inducers including pathogens(Sticher et al., 1997). One of the prominent features of SAR is that resistanceis expressed against pathogen which can be widely different from theinitial infecting pathogens.In a range of plant species, the development of necrotic lesions in responseto pathogen infection leads to induction of generalized disease resistance inuninfected tissue. Thus, TMV inoculated hypersensitive tobacco cultivardevelops systemic resistance against TMV (virus), Phytophthora parasitica var.nicotinae (fungi) and Pseudomonas tabaci (bacteria). TMV also induced resistanceagainst Peronospora tabacina and reduced reproduction of the aphidMyzus persicae (McIntyre et al., 1981). Thus a single viral agent induced resistancein tobacco against diverse challenges.The first report of virus induced SAR came in 1952. Primary inoculationof the lower leaves of D. barbatus with carnation mosaic virus (CarMV)resulted in the development of fewer lesions on the upper leaves upon challengeinoculation with CarMV (Gilpatrick and Weintraub, 1952). Virusinducedresistance was further substantiated by Ross (1961a, 1961b) andLoebenstein (1963). Cucumber plants infected with tobacco necrosis virus