The Regulation and Biochemical Action of Natural Fungicides in Tobacco

The biochemical interactions between the tobacco plant and a fungal pathogen are complex and poorly understood. Upon infection, the fungus overcomes the natural defence mechanism of the host by the lytic activity of enzymes and by suppressing the hypersensitive reaction of N. tabacum. The ability of the tobacco plant to resist a fungal attack is based on macroscopic properties and the biosynthesis of natural fungicides. The physical and biochemical character of the leaf surface (e.g. trichome exudates, cuticular components) may prevent the fungus from entering the organism. Constitutive natural fungicides such as phenolics and terpenes operate as enzyme inhibitors, or impair the germination of the fungal spores. Newly synthesized polysaccharides of the cell wall of leaf cells may delay the penetration of the fungus. Lignification of the surrounding cells occurs to protect the healthy tissue against attack. Toxic substances created during the melanization process of the tobacco plant can destroy the fungal cells. Biosynthesis of natural fungicides, such as phytoalexins, pathogenesis-related proteins and proteinase inhibitors is induced by exogenous or endogenous elicitors. These active protection systems may interfere with the fungal metabolism. Phytoalexins and pathogenesis-related proteins are highly effective defence responses which offer a broad spectrum of variability and resistance against a fungal infection. New strategies for engineering crop protection are based on these defence systems.