natural-products-in-plant-pest-management

150 H.N. Verma and V.K. Baranwalintroducing resistance against a virus in crops has been achieved throughconventional plant breeding. Limited success has been achieved throughthis method. Recent advances in the molecular biology of resistance tovirus infection have provided new approaches to making susceptiblecrops resistant against virus infection. These approaches includepathogen-derived resistance to viruses (coat-protein-mediated resistance,movement-protein-mediated resistance, replicase- and protease-mediatedresistance) and virus resistance through the transgenic expression ofantiviral proteins of non-viral origin (Baulcombe, 1994).Resistance of plants to virus diseases may be broadly categorized intotwo groups: (i) constitutive; and (ii) induced. Constitutive resistance is heritableand occurs in cultivars, which have gene(s) conferring resistance toviral infection, whereas induced resistance has to be conferred afresh upon asusceptible plant and is normally not heritable. Induced resistance operatesthrough the activation of natural defence mechanisms of the host plant. Thetwo forms of induced resistance are systemic acquired resistance (SAR) andsystemic induced resistance (SIR). In both SAR and SIR, plant defences arepreconditioned by prior infection or treatment that results in resistance (ortolerance) against subsequent challenge by a pathogen or parasite. Greatstrides have been made over the past 20 years in understanding the physiologicaland biochemical basis of SAR and SIR. Much of this knowledge is dueto the identification of a number of chemical and biological elicitors, some ofwhich are commercially available for use in conventional agriculture. However,the effectiveness of these elicitors to induce SAR and SIR as a practicalmeans to control various plant diseases is just being realized.The infection of plants by necrotizing pathogens, including fungi, bacteriaand viruses, induces systemic resistance to subsequent attack by thepathogens. This resistance is called SAR ( Kessman et al., 1994; Ryals et al.,1994). It can also be activated in numerous plants by pre-inoculation withbiotic inducers including pathogens (Sticher et al., 1997). Endogenouslyoccurring substances in a few higher plants have been reported to inducesystemic resistance in susceptible hosts against virus infections. Such plantextracts have been used for protecting economically important crops againstvirus infections (Verma and Baranwal, 1989). Endo genously occurring virusinhibitors may also be ribosome-inactivating proteins (RIPs) (Barbieri andStirpe, 1982; Mansouri et al., 2006; Zhang et al., 2007). Virus infection is preventedif a mixture of RIP and virus is exogenously applied on the leaf surfaceof a susceptible host. RIPs presumably inhibit virus infection by enteringthe cytoplasm along with the virus particle and inhibiting protein synthesison host ribosomes, thus preventing early virus replication (Reddy et al.,1986).On the basis of their antiviral activity, virus inhibitors from plants can begrouped into two categories:1. Plant products that inhibit virus infection by inducing an antiviral stateeither at the site of application (local resistance) and/or at a remote site( systemic resistance) when applied a few minutes or hours prior to virus