Genetic Analysis of the Biosynthesis of 2-Methoxy-3-Isobutylpyrazine, a Major Grape-Derived Aroma Compound Impacting Wine Quality  

Abstract

Methoxypyrazines (MPs) are strongly odorant volatile molecules with vegetable-like fragrances that are widespread in plants. Some grapevine (Vitis vinifera) varieties accumulate significant amounts of MPs, including 2-methoxy-3-isobutylpyrazine (IBMP), which is the major MP in grape berries. MPs are of particular importance in white Sauvignon Blanc wines. The typicality of these wines relies on a fine balance between the pea pod, capsicum character of MPs and the passion fruit/grapefruit character due to volatile thiols. Although MPs play a crucial role in Sauvignon varietal aromas, excessive concentrations of these powerful odorants alter wine quality and reduce consumer acceptance, particularly in red wines. The last step of IBMP biosynthesis has been proposed to involve the methoxylation of the nonvolatile precursor 2-hydroxy-3-isobutylpyrazine to give rise to the highly volatile IBMP. In this work, we have used a quantitative trait loci approach to investigate the genetic bases of IBMP biosynthesis. This has led to the identification of two previously uncharacterized S-adenosyl-methionine-dependent O-methyltransferase genes, termed VvOMT3 and VvOMT4. Functional characterization of these two O-methyltransferases showed that the VvOMT3 protein was highly specific and efficient for 2-hydroxy-3-isobutylpyrazine methylation. Based on its differential expression in high- and low-MP-producing grapevine varieties, we propose that VvOMT3 is a key gene for IBMP biosynthesis in grapevine.

Origins of Grape and Wine Aroma. Part 1. Chemical Components and Viticultural Impacts

“…Putative genes, including VvOMT1 and VvOMT3, that encode enzymes capable of methylating hydroxypyrazines have been identified in Cabernet Sauvignon grape berries (Dunlevy et al 2010, Vallarino et al 2011, Guillaumie et al 2013). The VvOMT3 gene in particular has a high affinity for the isobutyl hydroxypyrazine substrate and has been found in berries of other varieties known to have high levels of IBMP, including Cabernet franc, Sauvignon blanc, Merlot, Semillon, Fer, Ruby Cabernet, and Roussane (Dunlevy et al 2013).…”

Stimulatory effect of bunch thinning on sugar accumulation and anthocyanin biosynthesis in Shenhua grape berry (Vitis vinifera×V. labrusca)

“…First-strand cDNA was synthesised with the Takara PrimeScript RT Reagent Kit with gDNA Eraser (Takara, Dalian, China) according to the manufacturer's instructions. Results were normalised with the internal reference genes VvEF1r and VvGAPDH (Guillaumie et al 2013). All primer sequences were obtained from the literature and listed in Table S1.…”

Molecular grapevine breeding techniques

“…The 2-methoxy-3-isobutylpyrazine is the major methoxypyrazine in berries of Bordeaux cultivars and is released from its nonvolatile precursor by the gene product of an S-adenosyl-methionine-dependent O-methyltransferase (Dunlevy et al, 2013;Guillaumie et al, 2013). Chemistry has to hurdle many obstacles in the discovery of all relationships between berry composition and wine sensory attributes.…”