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Results and Perspectives of Resistance Breeding in Grapes

Resultados y perspectivas de la mejora genética de la resistencia en la vid

Rudolf Eibach and Reinhard Töpfer

Federal Centre for Breeding Research on Cultivated Plants
Institute for Grapevine Breeding Geilweilerhof
Siebeldingen,Germany

Breeding, generally spoken, is aiming at the improvement of crop plants e.g. for yield, product quality, specific resistances etc. Considering grapevine breeding some peculiarities need to be reminded. Grapevine is a vegetatively propagated, perennial crop, highly heterozygous, suffering from inbreeding depression, yielding fruits to be used either as table grapes or for wine making. Varieties like the Pinots, Chardonnay, Riesling, Cabernet Sauvignon, Merlot and many others are considered to be of outstanding wine quality, provided they are grown under suitable conditions and winemaking is done accordingly. Thus, genetic improvement of wine grapes is mostly focused on viticultural traits (input traits) since quality of existing varieties and yield are sufficiently good. Consequently, resistance characteristics are the most desired traits to be improved since grapes need very intensive and strict plant protection measures, particularly against fungal diseases. This is confirmed by a study of the European Union which shows that the use of fungicides in viticulture is considerably higher compared to all other crops (Figure 1) . Concerning ecological aspects this substantial impact of pesticides is to be seen critically. Moreover from the point of public perception in western countries application of fungicides and pesticides is somewhat delicate. Additionally to ecological reasons this is also of high importance from the economical point of view. Depending on the degree of resistance of individual varieties and hence on the decrease of plant protection measurements up to 696,- € per hectare and year - which is about 15% of the total production costs can be saved by using varieties with fungus resistance characteristics (Table 1).
Efforts to improve vines by breeding are rather old and it is worthwhile to reflect briefly the history in this area.

Grapevine Breeding: drawback and success

The beginning of grapevine breeding activities can be traced back to the second part of the 18th century when phylloxera and the mildews (powdery and downy mildew) were dragged into Europe from North America. When the French scientists Bazille, Sahut and Planchon discovered in 1868 for the first time phylloxera on the roots of grapes they surely could not imagine that this discovery was the beginning of the end of a several thousand years old viticulture. Particularly in France phylloxera led to tremendous damages and the living of thousands of viticulturists was endangered. It was Millardet in 1880 who stated that it should be possible to combine the resistance features of the American wild species with the quality features from the European Vitis vinifera cultivars. In Germany it was Baur in 1913 who encouraged the officials to initiate breeding activities for resistant grapevines.

At the beginning, breeding programs mostly covered both phylloxera and the mildew resistances, aiming at an "ideal vine". But very soon a separation between phylloxera resistance (rootstock) breeding and fungus resistance breeding could be observed.

In retrospect, rootstock breeding led rather quickly to good and sustainable results. Thus, today´s European viticulture is almost exclusively based on phylloxera-tolerant rootstocks developed about 100 years ago by breeders like Richter, Paulsen, Kober, Teleki and others. Consequently, the introduction of graftings was an important fact if not the factor for a survival of European viticulture (see Figure 2).

Breeding efforts for fungus disease resistance were not equally successful, though resistant varieties have been obtained. Even after decades of intense breeding work the quality characteristics of the new varieties, except those obtained by the end of the 20th century, did not at all fulfil the expectations. One of the major reasons is that resistance from wild species and quality as well as agronomic performance from Vitis vinifera varieties cannot be combined in a single or within a few breeding steps (for breeding goals and breeding strategies see Figure 3). Due to the polygenic nature of both characteristics, resistance (agronomic performance) and quality, several back-crosses are necessary in order to achieve a sufficient resistance level and high wine quality. The time consuming generation- and evaluation-cycles revealed that many breeders, especially the private breeders in France - though quite successful - gave up (see Fig 2). The varieties introduced during the first part of the 20th century showed in general low quality. These so-called "hybrids" quickly suffered from a bad image and in public the term "hybrid" was more or less synonymous for poor quality. Consequently the production of quality wine from these "hybrids" was forbidden by law in the twenties of the last century (being still valid within the European Community). This was an immense drawback causing the termination of grapevine breeding in most countries. Fortunately, for example in Germany, the situation was quite different. Since about 1926 resistance breeding was and still is supported by the government creating an environment for very successful grapevine breeding programs:

During the eighties the first new bred varieties were placed in official suitability tests in Germany. In 1992, the white variety Phoenix was the first variety derived from resistance breeding which became registered as a protected variety. Further cultivars followed and till the end of April 2004 twenty new varieties obtained plant variety protection, twelve of them are registered in the German National Variety List and depending from the individual state with viticultural areas up to twelve are also  classified, meaning they are permitted to be planted in one or more vine growing regions of Germany. Currently, the red variety Regent is the most successful one (see Figure 4 for history of this variety). Regent belongs presently to the top five most planted cultivars in Germany. Since its first classification in 1996 its growing area rapidly increased to 1350 ha in 2003.

The evaluation of Regent (Table 2) revealed that yield and quality parameters like sugar and acid content fit very well with some of the most important other red varieties in Germany. The tendency of lower yield and higher sugar content meets vine growers' demand. The degree of mildew resistance against powdery and downy mildew (Figures 5 and 6) as well as the resistance against botrytis is high, however, Regent is not immune. The experience under practical viticultural conditions during the last years confirms that on average plant protection measures for the mildews can be reduced by about 80% or even more. The genealogy of Regent, as depicted in Fig. 4, is rather complex and can be traced back to several Vitis species as sources for multiple resistance.

In addition to mildew and botrytis resistances Regent provides further advantages for German viticulture: e.g. its high winter frost tolerance, the early grape maturity and the potential to produce deeply coloured wines. Compared to the traditional German red vine varieties like Pinot noir or Blauer Portugieser the content of anthocyanins is considerably higher which is because of their antioxidative effect positive in respect of nutrition physiology. Wines are full bodied with a nice balance of tannins and flavour and they often remind on wines originated from southern grape growing countries. Wine tastings in recent years by numerous experts confirmed that Regent wines are of superior quality. Moreover, Regent gets more and more accepted by consumers.

In summary, efforts in resistance breeding proofed to be successful and new varieties provide the opportunity to reduce considerably plant protection measures while maintaining traditional quality standards. This is beneficial from an economical and an ecological point of view and hence to vine growers and the consumers.

What are the future perspectives for grape breeding?

Generally the big disadvantages of the conventional breeding is the tremendous time consumption on one side (compare Fig. 4) and the lack of knowledge about the genetics on the other side. New biotechnological methods offer now new perspectives. The development and the use of molecular marker-techniques look very promising (MAS = marker assisted selection). Suitable molecular markers offer the possibility for diagnosis of important viticultural traits like disease resistances at an early breeding stage. They will be most suitable to dissect the genetics of resistance in order to combine several genes to achieve durable resistance. The mildew resistances are a good example for this accumulation or, in terms of breeding, this pyramidisation. The correlation of a resistance locus to distinct markers allows an effective resistance gene management aiming at the combination of as many resistance loci as possible in one genotype. Furthermore, molecular markers will be an important tool to explore the genetic resources of Vitis. Loci coding for resistance can be identified within the Vitis gene pool and by using markers can be followed for further breeding work.

The development of molecular markers is also a possibility to finally allow the identification of other agronomically relevant genes. This would facilitate MAS more widely, which results in increased efficiency in conventional breeding and thus its acceleration. Simultaneously, access to identification and isolation of agronomically relevant genes provides the opportunity for genetic engineering approaches. Traits which can hardly be improved by conventional breeding techniques are of particular value. Examples are again the resistances against fungi, insects, bacteria, viruses etc. hence traits for which sources of resistance are not existing or not known within the cultivars. The so-called "non host resistance" which is based on the fact that the pathogen cannot identify the host plant is also a medium to long term goal which might be achieved by gene transfer. This approach promises a durable and stable resistance.

Gene transfer approaches, once feasible, are of particular interest to improve varieties for deficiencies like resistance, thus maintaining the quality of a grape. This leads to a peculiarity of wine marketing, the importance of the varietal name for the consumer. The introduction of new varieties concomitantly means also the introduction of new varietal names requiring considerable efforts in the market. Only via gene transfer scenarios are imaginable where established cultivars like chardonnay or pinto noir are improved for a single characteristic like resistance and this modification does not lead necessarily to a change of the varietal name and thus the product wine (compare Fig. 3).

It is well known that genetically modified plants are a matter of public debate. But people must be aware of the possibility that the introduction of genetically modified grapes in the market will still last for decades (see Fig. 2). It can be expected that in the meantime a lot of data and experience derived also from other crops become available, which allows a more objective estimation of the chances and risks of genetically modified vines.

Breeding and genetic engineering

Independent of the crop species, consequent breeding efforts ultimately result in a continuous breeding progress, e.g. annual yield increase (e.g. 2% for wheat) or higher resistance to pathogens and pests. The continuous grapevine breeding efforts over decades consequently resulted in improved new varieties (see Fig. 2) which are of unequivocal high quality (see Table 2), being essentially free of off-flavours, and showing high field resistance against the mildews (see Figs. 5 and 6). Such progress has been doubted for long.

Since grapevine breeding requires about 25 to 30 years for the development of new varieties (see Fig. 4), the present elite breeding strains (crossed about 15 years ago, becoming eventually a new variety in 5 years time) are superior to new breds actually introduced into the market (crossed about 35 years ago). Derived from crosses of varieties/genotypes showing both high resistance and high wine quality, these elite breeding strains again will provide a substantial progress to viticulture in about 10 years time. Thus, in the forthcoming years conventional grapevine breeding programs will generate varieties which show clear cut advantages and which are certainly very interesting for ecological and economical reasons.

The genetic modification of grapevine offers the potential to become a key technology for the improvement of grapes in future. However, more research and testings are required to isolate agronomical relevant genes which result in grapevine prototypes, showing a desired phenotype. Development of gene transfer tools (e.g. gene transfer protocols for a particular variety, procedures avoiding antibiotic selections) is in progress and likely to be achieved within a few years. International co-operations on grapevine genome research will support progress substantially. Genetic maps are forthcoming, genome base analytical tools soon become available for grapevine researchers and breeders. However, in no case the use of biotechnological methods is an exclusive alternative for conventional breeding. Moreover, it is anticipated to be an additional and very effective tool for the breeder.

[30.06.04]

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