There is an increasing demand by consumers for fruits and vegetables free of pesticide and other residues, but cultivation without their use is only partially possible by using suitable resistant genotypes in a suitable environment. Plants have developed several natural defencestrategies to protect themselves against attack of pathogen and pest diseases (Hammond-Kosack and Jones, 1996). Concerning pathogen infection, strategies fall mainly into two groups:
1. Specific mechanisms responsible for pathogen recognition andcontrol by a specific resistance againsta specific pathogen, with a hypersensitive disease resistance response (HR), hampering the diffusion of pathogen to healthy tissues by formation of necrotic lesion;
2. General mechanisms that confer resistance to a broad range ofpathogens, occurring either in resistantor susceptible plants, but are able to control the pathogen. The synthesis of antimicrobial metabolites, lytic enzymes, pathogenesis-related proteins and other compounds strengthening the cell wall are involved. The resistance normally depends on the early response of the plant to pathogen attack, which lead to a rapid accumulation of reactive oxygen species (ROS) namely oxidative burst (Lamb and Dixon, 1997), with an accumulation of H2O2 which functions as a diffusable signal for the induction of cellular protectant genes (Delledonne et al., 1999); nitric oxide co-operates in the induction of hypersensitive cell death. In some cases the plants react to pathogens by accumulating high levels of specific proteins which are toxic or inhibitory against both pathogens and pests (Broekaert et al., 1995) such as RIP proteins, effective against insects and fungi; while other proteins seem to be more specific. Over expressing the genes by genetic engineering or induced mutation (Barbieri et al., 1997; Maddaloni et al., 1999) in plant cells under toxin or culture filtrate pressure are the two main strategies currently used to produce resistant plants. More research is needed to discover new molecular signals and the efficacy of the promoters of some genes involved in the defense; maybe the reinforcement of the promoters is sufficient to enhance plant resistance.
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