The production of 6-methoxymellein, a phytoalexin of carrot, is induced in the cells by the addition of a wide variety of substances called elicitors. It appeared that the elicitor active molecules are the fragments of pectic substances of carrot cell walls, and are liberated by the partial hydrolysis of the walls with extracellular pectinase or proteases secreted by invading fungi. The accumulation of 6-methoxymellein is controlled primarily by the rate of transcription of the genes which encode the biosynthetic enzymes. Transduction of elicitor signals in plant cells may utilize a mechanism similar to the process reported in odor-sensitive animal cells. It is likely that Ca2+ acts as a second messenger and plays a central regulatory role in the gene expression of the enzymes for 6-methoxymellein biosynthesis. The increase in the cytoplasmic Ca2+ level is mediated by the activation of phosphatidylinositol cycle which liberates inositol trisphosphate and diacyl glycerol as the messenger molecules. In addition, evidence has been provided to suggest that cyclic AMP stimulates Ca2+-influx by gating of cyclic AMP-sensitive cation channels without accompanying cyclic AMP-dependent protein phosphorylation. Biosynthesis of 6-methoxymellein is catalyzed by two inducible enzymes, 6-hydroxymellein synthase and 6-hydroxymellein-O-methyltransferase. 6-Hydroxymellein synthase is a multifunctional polyketide biosynthetic enzyme and is catalytically active only in the homodimeric form. The reaction catalyzed by 6-hydroxymellein-O-methyltransferase proceeds by a bireactant sequential mechanism, and the activity of the enzyme is strictly and specifically controlled by its products.
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