natural-products-in-plant-pest-management

Control of Virus Diseases of Plants 161and cGMP for the induction of active AVF in leaves and callus cultures ofN. glutinosa.Inhibitor of virus replicationLoebenstein and Gera (1981) reported, for the first time, an inhibitor of virusreplication (IVR). IVR is released into the medium from TMV-infected cells ofSamsun NN plants and inhibits replication in protoplasts from local-lesionrespondingSamsun plants. IVR is detected as soon as 24 h after inoculationof protoplasts and it is effective when applied up to 18 h after inoculation.IVR is neither host nor virus specific. It inhibits TMV, CMV and also PVX(Gera and Loebenstein, 1983). Gera and Loebenstein (1983) reported that IVRalso inhibited TMV replication in intact leaves when applied to cut stems orwhen used as a spray.Plant-extract-induced virus inhibitory agentIt would be interesting to study the entire cascade of events and properties ofthe virus inhibitory agent (VIA) induced by antiviral agents. Treatment oflower / upper leaves of hypersensitive or systemic hosts of virus withantiviral agents results in the development of resistance throughout the planta few hours later. This is detectable by challenge inoculation with virusesproducing local lesions or systemic symptoms. The lesions are either reducedor totally absent and systemic symptoms are either milder or totally suppressed.Plant extracts or the semi-purified proteins from these plants stimulatethe hosts to produce VIAs that spread to surrounding tissues and otherplant parts (Verma et al., 1996). The VIAs have been isolated from leaves ofplants treated with phytoproteins and they have been shown to inactivatethe viruses in vitro (Verma and Awasthi, 1980; Verma and Dwivedi, 1984;Verma et al., 1996). The production of VIA leading to resistance seems to bean activation of a pre-existing system and hence is easily stimulated. VIA isable to move from one leaf to another through the vascular system of theplant.Therefore, the antiviral agents from plants can be broadly grouped intotwo categories based on their mode of action: (i) those affecting virus in vitro;and (ii) those affecting via host plants.Amongst the latter, we can distinguish (1) those that act by affecting hostsusceptibility and which need to be applied in leaf tissue before or at the timeof virus inoculation, e.g. protein inhibitors from Phytolacca spp., Dianthusspp., Chenopodium spp. and so on, and (2) those acting by inducing the hostresistance mechanism, which can truly be called antiviral agents, and act byobstructing the establishment of virus. They exert their effect when applied afew hours before virus inoculation. Their effect is visible even on non-treatedparts of susceptible plants (Verma and Mukherjee, 1975; Verma and Awasthi,1979; Verma et al., 1979, 1980, 1982, 1985, 1995, 1996; Kumar et al., 1997).