Induction and Activation of Plant Secondary Metabolism by External Stimuli

Elicitation and transmembrane signalling mechanisms of phytoalexin production6-Methoxymellein, an antifungal isocoumarin (Fig. 1), was first isolated as the metabolite that is responsible for the bitter taste in cold-stored carrot roots.It has been shown (Condon www.intechopen.comDrug Discovery Research in Pharmacognosy 210& Kuc, 1960) that this compound accumulates in carrot roots after inoculation with Ceratocystis fimbriata, which causes black rot disease in sweet potato but is not pathogenic to carrot.The resulting production of 6-methoxymellein accounted for the resistance of carrot tissue to microbial infection.This compound inhibits the growth of various fungi, yeasts and bacteria in the concentration range of 0.05 -0.5 mM, in vitro, strongly suggesting that the accumulation of this compound in response to fungal invasion is one of the important induced defense mechanisms of the host plant, carrot (Kurosaki & Nishi, 1983). Liberation of elicitors during host-pathogen interactionPreliminary studies indicate that heat-stable and water-soluble substances which show elicitor activity are released during interaction of carrot cells and the fungus.The elicitor was found to lose its activity after digestion with pectinase or proteases, suggesting that oligogalacturonides and/or peptides are essential for the inducing activity.Also, partial hydrolysates of pectic fractions of carrot cell walls prepared with these enzymes showed strong elicitor activity.These results suggest (Kurosaki & Nishi, 1984) that extracellular hydrolases secreted from fungi, including pectinase and proteases, function to liberate oligosaccharides and peptides from carrot cell walls, and the fragments of the extracellular matrix of carrot trigger 6-methoxymellein production (Fig. 1).This was confirmed by an experiment in which filter-sterilized pectinase and trypsin were directly added to carrot cell culture.Biosynthetic activity of 6-methoxymellein was induced in the carrot cells, implying that eliciting substances are released from live carrot cells by the enzymatic action of these hydrolases.Fig. 1.Elicitation of phytoalexin production in carrot.