Nio 7 1

R1

R2

R3

Phenolic acid precursor

Stilbene

h

h

h

cinnamic

pinosylvine

h

h

oh

p-coumaric

resveratrol

oh

h

oh

2'-4'-dixydroxycinnamic

hydroxy resveratrol

h

oh

oh

caffeic

piceatannol

h

oh

och3

isoferulic

rhapontigenine

Fig. 2.1 General structure of stilbenes (according to [31])

Fig. 2.1 General structure of stilbenes (according to [31])

Plant-derived stilbenes are isolated as hydroxylated, methylated, esterified, glycosylated or prenylated monomers or as polymers. In Vitaceae, resveratrol and a-, ß- and s-viniferin were identified to be phytoalexins [23, 32], which are antimicrobial substances synthesised de novo by plants that accumulate rapidly at areas of incompatible pathogen infection. A methylated stilbene, pterostilbene, was later identified by Langcake et al. [33]. Prior to these studies, resveratrol and pterostilbene were reported in other plants. Resveratrol was first identified in Veratrum grandiflorum by Takaoka in 1939 [27]. Its name likely derives from an abbreviation of the class of molecules to which resveratrol belongs, i.e., resorcinol, the plant name Veratrum and ol indicating the presence of a hydroxyl group. Pterostilbene was first described by Spath and Schläger [34] in Pterocarpus santalinus.

In Vitaceae, stilbenes represent defence biomarkers because they occur as phytoalexins that are produced dynamically in response to biotic or abiotic stress. Though resveratrol and its derivatives are present in lignified stem tissue [35, 36], it is absent in the healthy green parts of the grapevine (leaves, young canes). Pterostilbene, however, is present in the healthy grape berries of Vitis vinifera (var. Gamay) [37].

The rate of synthesis of resveratrol after stress induction depends on the grape variety and provides a good metric for evaluation of the resistance of grapevine cultivars to grey mould and downy mildew [35, 36, 38-41].

Resveratrol and its derivatives are present in grape berries [42] and were found in wine by Siemann and Creasy [43]. When Renaud and de Lorgeril [44] and Frankel et al. [45] demonstrated the cardiovascular-protective effects of resveratrol, the French Paradox theory was born. Since then, much research has dealt with the beneficial effects of resveratrol and pterostilbene in medicine [46]. In addition, groups around the world have since studied the composition of stilbenes present in wine [42, 47-50]. The synthesis of pure pterostilbene and resveratrol has allowed the study of the toxic effect of these stilbenes on Botrytis cinerea, one of the major fungal diseases that attack grapevines [37, 51, 52], Enzymatic synthesis of 8-viniferin, as well as purification of e-viniferin from lignified canes [53], allowed toxicity testing of these resveratrol dimers on Plasmopara viticola (downy mildew) and Erysiphe necator (powdery mildew) [40, 41, 54, 55].

Because of these results, we have developed biological (artificial inoculation) and chemical methods (HPLC analysis of stilbenes in grape tissues) to evaluate the level of resistance of grapevine seedlings to downy and powdery mildew in breeding programmes. These tools have led to a significant reduction in the time and space required for such experiments.

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