Advance in research on the accumulation and regulation of C13- norisoprenoid derivatives in grape berry and wine

Abstract: Aroma is an important quality indicator of grape berry and wine. It reflects the typicality and regional style of wine and determines the level of consumer recognition. Grape berries contain hun-dreds of aroma substances, such as volatile aldehydes, alcohols, esters, ketones, terpenes and pyrazines,etc. Based on the biosynthetic pathways, these volatile compounds can be divided into fatty acid metab-olism-derived volatiles, amino acid metabolism-derived volatiles and isoprene metabolism-derived vola-tiles. Volatile compounds produced from the isoprene metabolism are mainly terpenes and norisopren-oids. Terpenes are important varietal aroma components of wines made from muscat and aromatic grapes, such as Vitis vinifera‘Muscat blanc à Petit grain’and V. vinifera‘Gewürztraminer’, while nor-isoprenoids are major varietal aroma contributors to most non-aromatic wines made from V. vinifera‘Cabernet Sauvignon’, V. vinifera‘Syrah’, V. vinifera‘Merlot’or V. vinifera ‘Chardonnay’. These non-aromatic grape varieties occupy a large area of wine grape cultivation in the world. More important-ly, norisoprenoids have extremely low sensory thresholds, and their concentrations in non- aromatic grapes and wines are usually higher than their sensory thresholds [that is, odor activity value (OAV) is greater than 1]. These components thus endow wine with apple, raspberry, papaya, violet and other flo-ral and fruity odor. Furthermore, norisoprenoids can interact with the other volatile components and give rise to synergistic, cumulative, or subtractive effect, which makes wine aroma attributers richer. Therefore, it is very necessary to understand the biosynthesis, conversion and regulation of norisopren-oids in the grape and wine. Norisoprenoids are produced from the cleavage of C40 carotenoids that are mainly composed of β- carotene, lutein, neoxanthin, zeaxanthin, and violaxanthin. These carotenoid compounds all have highly conjugated double bond structures and are degraded to form C13, C11, C10, and C9 -norisoprenoids with carbonyl structure. Among them, C13-norisoprenoids are the most abundant in grape berries, mainly being β-damascenone, β-ionone, α-ionone, geranyl acetone, TDN (1,1,6-Tri-methyl-1,2-dihydronaphthalene) and vitispirane. During the grape berry development, carotenoids begin to accumulate in the early developmental stage, reach a peak value at véraison and then decrease during the berry ripening. On the contrary, C13-norisoprenoids start to accumulate from véraison and in-crease progressively with berry maturation. The cleavage of carotenoid into C13-norisoprenoids is cata-lyzed by carotenoid cleavage dioxygenases (CCDs). Currently, three related genes VvCCD1, VvCCD4a and VvCCD4b have been biochemically identified from grape berries. Their enzymes can converse the same carotenoid components, and also different substrates separately, producing the corresponding prod-ucts such as β-damascenone and β-ionone. Besides the direct enzymatic reactions, there may be non-en-zymic reactions to generate multiple intermediates and then further converse into C13- norisoprenoids through acid hydrolysis. C13-norisoprenoids exist as both free form and glycosidically bound form in grape berries. The free form is volatile and has relatively high concentration, which directly contributes to the aroma of the wine. By contrast, the glycosidically bound form is non-volatile and stored in the grape berries. These bound C13-norisoprenoids can release the corresponding glycones under acidic envi-ronment during wine making and aging. It is thus considered that the glycosidically bound compounds provide the aroma potential for wine. The synthesis and accumulation of C13-norisoprenoids in grapes and wines are affected by environmental factors such as sunlight, temperature and rainfall. In most cas-es, stronger sunlight, higher daily temperature and a certain degree of water deficiency are conducive to the accumulation of C13-norisoprenoids. Viticulture practices such as inflorescence thinning, rootstock selection, training systems and application of plant growth regulators also affect the production of C13-norisoprenoids. In a cool region of grapevine growing, the leaf removal around inflorescence zone usu-ally promotes the accumulation of C13-norisoprenoids in grape berries. However, in a strong sunlight and arid region, similar leaf removal treatment results in the decreasing concentration of C13-norisoprenoids, possibly because this measurement further increases sunlight exposure of berry inflorescences and sun overexposure of inflorescence is unfavorable to the synthesis of C13-norisoprenoids. In addition, the concentration of C13-norisoprenoids in wine varies with pre-fermentation method, fermentation process,yeast strain selection and storage containers. Compared with fermentation using single Saccharomyces cerevisiae yeast, the mixed fermentation of non-Saccharomyces and Saccharomyces cerevisiae increases the concentration of C13-norisoprenoids in wines. The changes of fermentation and aging conditions similarly affect the production of C13-norisoprenoids. Low pH grape juice/must and high fermentation temperature are found to promote the formation of C13-norisoprenoids in wines. Therefore, understand-ing above influencing factors can help grape growers and winemakers to better control the level of C13- norisoprenoids. Moreover, the studies on the underlying mechanism will give impetus to the improve-ment of the aroma quality of wine. Up to now, the studies involving C13-norisoprenoids in grapes and wines have been focused on the effects of climate factors, viticultural practices and yeast strains. With the technological progression of chemical analysis and multi-omics integrated analysis, the mechanisms underlying the biosynthesis, conversion and regulation of C13-norisoprenoids will be uncovered more clearly. This paper reviews the recent research advance in the synthesis and conversion of C13-noriso-prenoids and the main influencing factors in developing of grape berries as well as during wine fermen-tation and aging. The future research trends in this field are also prospected.