Stilbene phytoalexin

Not direct gene produa however, gene for enzyme (resveratrol synthase) catalyzing synthesis of stilbene phytoalexin in peanut has been cloned. 

Dereks W, Creasy LL. 1989. The significance of stilbene phytoalexins in the Plasmopara viticola-grapevine interaction. Physiol Mol Plant Pathol 34 189-202. 

Bavaresco, L., Petegolli, D., Cantu, E., Fregoni, M., Chiusa, G. and Trevisan, M. (1997) Elicitation and accumulation of stilbene phytoalexins in grapevine berries infected by Botrytis cinerea, Vitis, 36(2), 77-83. 

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 purposes of this chapter are to review the factors involved in aflatoxin contamination of peanuts, review the chemistry of stilbene phytoalexins from peanuts, discuss evidence supporting the involvement of these stilbenes in the bioregulation of aflatoxin contamination, and explore approaches to exploit or enhance such a bioregulative capacity to reduce or eliminate preharvest aflatoxin contamination of peanuts. 

Figure 1. Chemical structures of stilbene phytoalexins from peanuts.

A close examination of damaged peanuts grown under adequate moisture conditions reveals little, if any, A. flavus proliferation. Likewise, it is unusual to detect even low concentrations of aflatoxin in such peanuts. However, stilbene phytoalexins are easily detected in such peanuts. When similar peanuts were surface sterilized and plated out to determine counts and types of fungal colonization, the percentages of kernels colonized by A, flavus was as high as 25% (19). Since colonization had occurred and phytoalexins had been produced with an absence of aflatoxin contamination, phytoalexins presumably inhibited A. flavus growth and aflatoxin production. 

Therefore, the fact that peanuts produce stilbene phytoalexins naturally in response to damage in the field but do not become contaminated with aflatoxin (indicative of A. flavus growth) until subjected to prolonged drought stress points toward a presumptive role for these compounds in the natural bioregulation of aflatoxin contamination. 

Occurrence of Aflatoxin Contamination After Cessation of Phvtoalexin Production. In view of the fact that stilbene phytoalexins are naturally produced in peanut kernels in response to fungal invasion and that these stilbenes possess antifungal activity against aflatoxigenic fungi, the question of how peanuts become contaminated with aflatoxin remains. Simply stated, how does A. flaws overcome this apparent natural defense mechanism of peanuts ... 

As peanuts became dehydrated during the drought period, they lost the capacity to produce stilbene phytoalexins. This lost capacity was not directly due to the duration of the stress, but it was directly associated with the drop in a of the peanuts. Regardless of drought treatment soil temperature or peanut maturity, the phytoalexin-producing capacity of peanuts decreased as the a decreased, with essentially no phytoalexin production below a kernel a of 0.95 (Figure 2). 

The onset of aflatoxin contamination did not occur until peanut kernels had lost the capacity to produce phytoalexins as an apparent result of drought-induced moisture loss. However, mature peanuts retained a higher degree of resistance to aflatoxin contamination than immature peanuts even after the loss of phytoalexin-producing capability. This indicated that immature kernels rely more heavily on a phytoalexin-based resistance than mature kernels, which apparently have some additional resistance not based on stilbene phytoalexins. 

The evidence clearly supports the hypothesis that stilbene phytoalexins in peanuts are an important natural bioregulator of preharvest aflatoxin contamination. That evidence includes the facts that (1) stilbenes are naturally produced in field-damaged peanuts (2) stilbenes possess biological activity against A, flavus and A. parasiticus and (3) although invasion of peanuts by A. flavus and A. parasiticus can occur under any conditions, aflatoxin contamination does not occur until peanuts lose the capacity for phytoalexin production as a result of drought-induced kernel dehydration. 

It is unlikely that any single approach will provide a solution to the problem of preharvest aflatoxin contamination of peanuts. However, a multifaceted approach that could include enhancement of the natural bioregulative properties of stilbene phytoalexins might ultimately yield the solution to a serious and complex problem. 

About thirty phytoalexins have so far been isolated from the mulberry tree. A few differences are observed in the structures of phytoalexins isolated from different parts of the mulberry tree. From the fungus-infected xylem tissue of the shoot, two stilbene phytoalexins were isolated and characterized as oxyresveratrol (73) and 4 -prenyloxyresvera-trol (74) (80). From the acetone extracts of cortex and phloem tissues of the shoot infected with Fusarium solani f. sp. mori, the following phytoalexins have been isolated, twenty-six 2-arylbenzofuran derivatives, moracins A-Z (75-100) (71, 81-83), as well as dimoracin (101) (84), a Diels-Alder type adduct composed of two 2-arylbenzofuran derivatives with isoprenoid substituents. Biogenetically these phytoalexins seem to be derived from moracin M (87) by hydroxylation, methyla-tion, and isoprenylation (Fig. 17). Structures were determined on the... 

Takasugi, M., L. Munoz, T. Masamune, A. Shirata, and K. Takahashi Studies on Phytoalexins of the Moraceae 3. Stilbene Phytoalexins from Diseased Mulberry. Chem. Letters 1241 (1978). 

Bais AJ, Murphy PJ, Dry IB (2000) The molecular regulation of stilbene phytoalexin biosynthesis in Vitis vinifera during grape berry development. Aust J Plant Physiol 27 425-433... 

Fig. 10.25. Biosynthesis pathways of stilbenic phytoalexins in the vine (Jeandet and Bessis, 1989)...

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