Phytoalexins, synthesis

Scheme 4.117 Retrosynthetic analysis of heptaglucan phytoalexin synthesis achieved using an iterative approach.

Welle, R. and Grisebach, H., Phytoalexin synthesis in soybean cells elicitor induction of reductase involved in biosynthesis of 6 -deoxychalcone. Arch. Biochem. Biophys., 272, 97, 1989. 

Compounds formed in response to stress may occur in at least two ways. In one response, the plant may form compounds throughout the tissue at a considerable distance from the infection site (Hammerschmidt, 1999). In another response, the plant may form compounds specifically at the infection site. This may include only a few cells and in rare cases, as few as one or two cells. (Snyder and Nicholson, 1990 Nicholson and Wood, 2001). In general, such compounds are referred to as either stress metabolites or more often as phytoalexins. By definition phytoalexins are formed in response to infection (Aguero et al., 2002 Lo et al., 2002 Hammerschmidt and Nicholson, 2001 Lo and Nicholson, 1998). Phytoalexins often exhibit toxicity to specific pathogens. In this case there is a genetic relationship between the expression of phytoalexin synthesis and the organism that induces that synthesis (Essenberg et al., 1985). 

HAGMANN, M.-L., HELLER, W., GRISEBACH, H Induction of phytoalexin synthesis in soybean. Stereospecific 3,9-dihydroxypterocarpan 6a-hydroxylase from elicitor-induced soybean cell cultures. Eur. J.Biochem., 1984,142,127-131. 

KOCHS, G., GRISEBACH, H., Phytoalexin synthesis in soybean purification and reconstitution of cytochrome P450 3,9-dihydroxypterocarpan 6a-hydroxylase and separation from cytochrome P450 cinnamate 4-hydroxylase. Arch Biochem. Biophys., 1989,273,543-553. 

ZAHRINGER, U EBEL, J., MULHEIRN, L.J., LYNE, R.L., GRISEBACH, H., Induction of phytoalexin synthesis in soybean - dimethylallyl pyrophosphate trihydroxypterocarpan dimethylallyltransferase from elicitor-induced cotyledons. FEBSLett., 1979,101,90-92. 

Bavaresco, L., Vezzulli, S., Battilani, P., Giorni, P., Pietri, A. and Bertuzzi, T. (2003) Effect of ochratoxin A-producing Aspergilli on stilbenic phytoalexin synthesis in grapes, J. Agric. Food Chem., 51, 6151-6157. 

The main point to observe is that virtually all species tested gave a positive response. There is thus little doubt that phytoalexin synthesis is a universal attribute in this family. Even in the genus Luplnus, which has been shown to have considerable numbers of antifungal isopentenylisoflavones on the plant surface (11, 12), phytoalexin Induction could be detected (10). 

The results to date indicate that phytoalexin synthesis is a common mechanism of disease resistance in higher plants Whether all plants will eventually yield positive results in this bloassay remains to be seen. It is our experience that there are still technical difficulties in establishing phytoalexin production in many plants and further development of appropriate methodology is essential The separation of the phytoalexin response from the more general stress response of plants is also not easy and this needs to be considered in future experimental design. 

The importance of terpenoid phytoalexins in resistance to wilts was further demonstrated in studies of temperature effects on resistance and in studies of induced resistance. Increase in temperature from 25 to 30 C causes a marked increase in resistance. This temperature change also slows the rate of sporulation of the fungus and increases the rate of phytoalexin formation by cotton (56). Likewise, treatments that induce resistance also induce pEy toalexin synthesis (57). Phytoalexin synthesis, therefore, is also important to explain environmental effects on disease resistance. 

We have examined these lethal reactions biochemically (93, unpublished). In all cases spontaneous defense reactions (phytoalexin synthesis, tannin synthesis and tannin oxidation) occur concurrent with the onset of symptoms. The quantities of terpenoid aldehydes and tannins found in tissues of severely affected plants are usually very similar to those found in severely diseased plants dying from microbial infections. Thus, it is possible that many of the disease symptoms seen in cotton are due to the toxic terpenoids and tannins formed in response to infections. 

Kochs, G., D. Werck-Reichhart, and H. Grisebach (1992). Further characterization of cytochrome-P450 involved in phytoalexin synthesis in soybean cytochrome-P450 cinnamate 4-hydroxylase and... 

To establish that the onset of phytoalexin synthesis is both concentration and time dependent is very important. This Indeed would mimic phytoalexin responses in other plants. 

ELICITOR-INDUCED PHYTOALEXIN SYNTHESIS IN SOYBEAN (Glycine max)... 

The production of phytoalexins in response to microbes requires a mechanism which enables the plant to recognize the invading organism and subsequently leads to the stimulation of phytoalexin synthesis at the site of microbial penetration. Substances of microbial origin which mediate phytoalexin accumulation in plants have been called elicitors(Ref. 2) and it has been shown in several cases that the culture medium of pathogens contained components which elicited phytoalexin accumulation in the pathogen s host (Ref. 3). Studies on the nature of elicitors have resulted in the partial purification and characterization of a number of molecules from different microbial sources. Elicitors have been reported to have a dependence for activity on a polypeptide or protein (Ref. 4), a glycoprotein (Ref. 5), or polysaccharide (Ref. 6) component. 

Ebel, j.. Phytoalexin synthesis The biochemical analysis of the induction process. Annu. Rev. Phytopathol., 24, 235-264 (1986). 

Heptaglucoside from the cell wall of Phytophthora megasperma. This oligosaccharide, which stimu-iates phytoalexin synthesis in Glycine max, was isolated from acid digests of the fungal cell wall. [J.K.Sharp et al. J. Biol. Chem. 259(1984) 11321-11336]... 

Fischer, D. et al. (1990) Phytoalexin synthesis in soybean purification and characterization of NADP H 2 -hydroxydaidzein oxidoreductase... 

A. Stelzig, R. D. Allen, and S. K. Bhatia. Inhibition of phytoalexin synthesis in arachidonic acid-stressed potato tissue by inhibitors of lipoxygenase and cyanide-resistant respiration. 

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