Vitis, the genus name of grapes, contains over 60 species [1, 2], although almost all commercially important varieties belong to a single domesticated species that originated in Eurasia – V. vinifera. Despite these grapes having preferred flavor characteristics for wine production, they tend to be susceptible to pests, diseases, and extreme temperatures; species native to North America and East Asia are generally better adapted to surviving these stressors. For example, V. riparia can tolerate winter temperatures down to –40 °C and V. muscadinia is resistant to several diseases capable of devastating vinifera (e.g., Pierce’s disease caused by an insect‐transmitted bacteria) [2]. However, these wild species tend to be low yielding and produce wines with undesirable sensory characteristics, including high acidity, low astringency, and excessive herbaceous aromas (Table 31.1).
Table 31.1 Representative concentrations of some key wine components that differentiate typical red vinifera varieties from V. riparia and V. labruscana (Concord)
| Wine component a | Vitis species b | Notes | Chapter | ||
| V | R [3] | L [4, 5] | |||
| Titratable acidity (g/L as tartaric) | 5–6.5 | 35 | 9.5 | Correlates with sourness | 3 |
| Condensed tannin (mg/L catechin equivalents) | 500–700 | <50 | <50 | Correlates with astringency, color stabilization | 14 |
| 3‐Isobutyl‐2‐methoxypyrazine (ng/L) | 3–17 | 56 | ND c | “Green pepper,” detection threshold = 2 ng/L | 5 |
| Methyl anthranilate (µg/L) | 0.06–0.6 | ND | 600–3000 | “Grape Kool‐aid,” detection threshold =300 µg/L | 5 |
a Grapes harvested at maturity (i.e., 20–24 °Brix for vinifera and riparia, 16 °Brix for labruscana), without deacidification.
b V = V. vinifera, R = V. riparia, L = V. labruscana hybrids of the American labrusca species and European vinifera species. Values for vinifera are found in previous chapters.
c Not detected.
Although most of the major commercial grape cultivated varieties, or cultivars (Chardonnay, Riesling, Pinot Noir), are hundreds of years old, new grape varieties are periodically introduced. For example, Cabernet Sauvignon is believed to have been produced from a cross1 of Cabernet Franc with Sauvignon Blanc centuries ago [6]. Grape breeders create new varieties by pairing two different species to produce a “hybrid” grape in an attempt to get advantageous traits from both parents (e.g., cold hardiness from wild species combined with desirable yield or flavor attributes from V. vinifera). For example, the popular juice grape cultivar “Concord,” developed by Ephraim Bull in the nineteenth century, possesses pest resistance from its V. labrusca parent, and perfect flowers2 and high yields from its V. vinifera parentage [2].
In the past it could take grape breeders decades to develop and release a new grape variety. The majority of seedlings produced from V. vinifera × V. riparia would not possess all of the desirable traits of both parents, and it could take several years of field testing and small‐scale winemaking to determine which offspring would produce viable new varieties. Because modern grape breeding often utilizes complex crosses of hybrids over several generations of grapes, the process of crossing and evaluation often needs to be repeated several times before commercial‐scale field trials and release of a new variety to growers and the public.
Advances in grape genomics are expected to greatly accelerate this breeding process. The grape genome was sequenced by two separate consortia in 2007 [2] and was the first perennial fruit sequenced. Grapes have approximately 30 000 genes, or about 50% more than humans. While about half of these genes are strongly similar to genes found in other plants, many have unknown functions. Of the many approaches to determine the gene(s) responsible for a trait, the most common current approach is mapping. Typically, mapping involves associating physical points on the chromosomes (markers) from a group (population) of individual grapes and statistically comparing this to data on particular traits, such as disease resistance or production of flavor‐related compounds (the phenotype).3 Often, a population consists of siblings generated from the same parents (linkage mapping), but it is also possible to study a population of unrelated individuals (association mapping). Like many traits in grapes, most chemical components vary quantitatively – that is, there will be a range of values observed because their formation is under the control of multiple genes. Thus, most studies of grape chemistry involve quantitative trait loci (QTL) mapping, in which multiple genetic regions controlling a trait are identified. Once identified, the function of candidate gene(s) can be confirmed by expressing the gene in a different organism, or by selectively overexpressing or silencing the gene. Eventually, the specific forms (alleles) of the gene responsible for a given phenotype can be determined.
In comparison to studies on disease resistance, studies of genes controlling fruit chemistry traits were relatively sparse. Examples of chemistry‐related genes that have been characterized include:
Monoterpenes and anthocyanins – and to a lesser extent MPs – are under strong control by a single major QTL. Variation in primary metabolites – such as sugars and malic acid – are expected to be controlled by multiple genes since they are involved in multiple pathways within the grape berry.
Knowledge of the genes controlling fruit quality and other traits should assist grape breeders in their efforts to produce new varieties. In principle, breeders could use biotechnology techniques to selectively delete or add a gene of interest to an existing variety, for example, insert a disease resistance gene into Cabernet Sauvignon. Although genetically modified (GM) grapes have been tested in several countries – mostly for research involving disease‐resistance genes – no immediate commercial use is expected for these varieties [11]. GM organisms are currently treated with suspicion by many consumers, so market acceptance is problematic, and approval of commercial GM grape varieties would likely be challenging in many countries.4
Alternatively, knowledge of key genes can be used as part of marker assisted selection (MAS). MAS uses traditional crossings to generate new varieties, but these varieties can be screened at an early stage in breeding to eliminate those that lack desirable traits. For example, a breeder interested in producing a Muscat‐type grape with good powdery mildew resistance could genetically screen seeds produced from crossing Muscat of Alexandria and wild American species, and only retain those crosses that possess the correct DXS variant (for high monoterpenes) and genes associated with disease resistance [12]. Assuming the new cross is accepted by consumers, such varieties would be more sustainable since disease control through pesticides represents a major financial and environmental cost. Beyond this, developing new grape varieties offers potential for novel grape and wine flavors that could be very attractive. As should be evident from Table 31.1, genetic variation can yield three orders of magnitude (or more) differences in key flavor compounds. This variation is often much greater than what can be achieved through manipulating viticultural or winemaking practices.
Finally, the knowledge generated from grape genetics is valuable in interpreting the results of viticultural studies. Researchers have historically observed empirical effects of growing conditions on grape or wine chemistry, but their ability to interpret these data sets was limited. For example, well‐drained soils and low water availability are common features of wine regions capable of producing darkly colored red wines. A recent report shows that water restriction results in decreased expression of a UDGT responsible for producing anthocyanins from anthocyanidins [13]. These genetic advances will allow for a mechanistic interpretation of variation in grape chemistry arising from differences among cultivars or growing conditions – in other words, “molecular viticulture.”