Cours du 27 mai 2015

Intervenant : Philippe Hugueney, chercheur à l’INRA de Colmar, équipe Métabolisme secondaire de la vigne.
http://www.alsace-vitae.com/FR/expertises-biopole/protection-durable-vigne-contre-maladies-ravageurs/maladies-fongiques/lutte-contre-mildiou-oidium/inra-equipe-msv.html

Abstract

Plants produce a large diversity of organic compounds, which do not appear to participate directly in growth and development. These substances are traditionally referred to as secondary metabolites and include, for instance, aromas, pigments and defense compounds. The diversity of grapevine metabolism and its potential to give rise to an exceptional diversity of flavor compounds contributes greatly to the aromatic complexity of wines. Indeed, analysis of the grapevine genome has shown a remarkable expansion of several gene families linked to secondary metabolism, compared to other plants. In this presentation, we will see a few examples of grapevine’s extraordinary metabolic diversity, especially in relationship with wine quality.

1. Molecular bases of grape color

Cultivated grapevine has been domesticated from the wild European vine (Vitis sylvestis), whose fruits are small anthocyanin-rich berries. In today’s grapevine varieties, the domestication and selection processes have given rise to a large panel of berries, which vary significantly in size and color. The selection of white grape varieties, with very low levels of anthocyanins, has been a major event in viticulture. The mechanism of grape color variations has been shown to depend on transposons affecting the activity of regulators of anthocyanin biosynthesis.

2. Resveratrol biosynthesis

Stilbenes are a small family of phenylpropanoids produced in a number of unrelated plant species including grapevine and peanut. In addition to their participation in both constitutive and inducible defense mechanisms in these plants, several stilbenes display important pharmacological properties. Since resveratrol was postulated to be involved in the health benefits associated with a moderate consumption of red wine (known as the "French paradox"), stilbenes have received considerable interest. Stilbene synthase (STS), which catalyzes the biosynthesis of the stilbene backbone, seems to have evolved from chalcone synthases (CHS) several times independently in stilbene-producing plants. STS genes usually form small families comprising 2 to 5 similar copies. By contrast, the sequence of the grapevine reference genome has revealed an unusually large family of 48 STS genes with identical function.

3. Genetics of grape aroma biosynthesis

In recent years, several genes involved in the biosynthesis of the major grape aroma compounds methoxypyrazines and terpenes have been characterized.
Methoxypyrazines are powerfully fragrant molecules with green pepper aromas. These compounds are especially important in Sauvignon wines, whose characteristic aromas come from the subtle balance between the fruity flavors of volatile thiols and the vegetal notes of methoxypyrazines. A gene encoding a methyltransferase enzyme has recently been shown to be a major factor of methoxypyrazine biosynthesis.
Terpenes are a large family of natural compounds, which possess extremely interesting technological (food coloring, vitamins, flavors) and pharmacological properties (taxol, artemisinin, for example). The characteristic aromas of Muscat and Gewurztraminer wines is linked to a point mutation in the gene encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS), which is the first enzyme in the terpene biosynthesis pathway. This natural mutation turns the DXS enzyme into a "super-DXS" with enhanced activity, responsible for a massive synthesis of terpenes. The characterization of the "super-DXS" offers many potential applications for producing high value terpenes by biotechnological approaches.
Finally, future challenges in viticulture will include both the preservation of existing grapevine varieties and the development of new varieties with improved resistance to diseases and better suitability to changing environmental conditions.

Par Olivier Bastianelli