Cornell's apple breeding program is integrating traditional breeding and genetic engineering to genetically improve apple. This dual approach is used to maximize our likelihood of success. Each approach has distinct advantages and disadvantages and we believe that neither approach should be used exclusively. Plant breeding creates new varieties while genetic engineering improves existing varieties by changing a key attribute such as disease resistance.

To explain what is possible with each technique some definitions are needed. A characteristic is simply inherited if it is due to one or several genes with a major effect. With major gene effects seedlings are grouped into several discrete categories. Examples of traits that are simply inherited include fruit color (red versus yellow) and genes for disease resistance (where an individual is resistant or susceptible). However, even simple traits like color are more complicated because a fruit may be red or yellow, but other genes influence the intensity and distribution of color, while additional genes are responsible for striped versus blushed overcolor. Characteristics are said to be quantitatively inherited, or complex, if they are controlled by many genes. Enough genes are involved to blur the categories so that seedlings show a continual gradient. Examples of characteristics in apple under quantitative control include productivity, flavor, firmness, texture, fruit size, and many aspects of tree structure. Plant breeding can change both simple and complex traits but varietal identity is lost. Biotechnology maintains varietal identity but can only improve traits under simple control.

Traditional Breeding

In traditional apple breeding, parents are chosen on the basis of the desirable traits they contain and the knowledge of which traits (simple and complex) they are likely to pass on to their offspring. The emphasis is on obtaining a hybrid that combines the desirable attributes of its parents, but minimizes any of the faults they might have. When we cross two parents to combine favorable characteristics, we can never get the original variety back because the hybrid seedling contains genes from both parents. Similar to what is seen in human families, seedlings may resemble one or both parents or have a unique combination of parental characteristics. Although seedlings may closely resemble a parent, they are never identical to either parent so varietal identity is lost. One example of this is the cross of 'McIntosh' by 'Delicious' that produced 'Empire'. 'Empire' resembles 'McIntosh' but is distinct from it.

The Need for New Varieties

No variety is perfect, and if the faults are serious the variety will fall from favor. Therefore, new and improved varieties are desired by the industry and by consumers. Apple varieties like 'Gala', 'Braeburn', and 'Fuji' are providing consumers with new perceptions of quality and flavor. The fact that 'Fuji' can sit at room temperature for a week and not lose firmness sets a new standard for future varieties. While the Geneva program has been known for varieties with 'McIntosh' and 'Empire' background, we recognize that these are regional varieties which may not have international appeal. New hybridizations are targeting quality parents such as 'Gala', 'Fuji', 'Braeburn' and 'Ginger Gold' crossed with advanced selections that perform well in our area. It would be desirable to create a 'Gala' like apple with larger size and better flavor after storage, or apples similar to 'Fuji' and 'Braeburn' that are better suited to our growing region. We are increasingly emphasizing flavor, quality, firmness, and storage ability. We need to ensure that the varieties being developed continue to be competitive locally and on the international marketplace.

Genetic Markers

Traditional breeding is very expensive. The ability to pre-screen seedlings prior to field planting to identify those with the traits we desire coupled with the ability to discard undesirable seedlings would greatly reduce costs and increase efficiency. The use of genetic markers is a powerful tool for accomplishing this goal. Our work on genetic markers involves collaborative research between Dr. Norman Weeden's and my research groups.

A genetic marker is a band on a laboratory gel that is linked to a trait of interest. Genetic markers are often referred to as "fingerprints" because they can be as unique and as useful in identifying plants or characteristics as fingerprints are to humans. The technology is the same as that used in crime cases to determine whether a suspect's DNA matches that found at the crime scene. Each individual, human or plant, has a unique set of genetic markers. Unlike blood type which many individuals share, genetic markers reflect the make-up of each individual's DNA. To determine each individual's identifying profile, a series of laboratory tests are conducted. In the case of plants, a small piece of leaf tissue is ground up and subject to tests that allow the different proteins (isozymes) and sections of DNA (RAPDs) to separate on the basis of size. These produce bands or lines that are compared to the existence of certain traits. A genetic marker is linked closely to a characteristic of interest if it is always present when that trait is present, and absent when the trait is not expressed. These markers can be used to select seedlings which will produce fruit of adequate size, flavor, disease resistance, and other attributes and eliminate those that do not have the traits of interest. The use of genetic markers will greatly increase our efficiency in breeding. We have several hundred markers in apple, and have developed genetic maps that provide locations of genes of interest for several apple varieties. Genetic markers have been found for resistance to storage scald (Weeden, 1993), resistance to apple scab, for branching habit, burr knots, leafing out, columnar form, and certain fruit attributes. We are most interested in genes for disease resistance, growth habit, and fruit quality.

Genetic Resistance to Disease

Public perceptions of environmental and health concerns related to pesticides are also influencing the types of varieties that are likely to be of interest in the future. This makes disease and insect resistance important goals, but resistant varieties must have good fruit quality and be competitive in the marketplace.

In apple, three of the most important fungal pathogens and one bacterial pathogen have been targeted. Genetic resistance to apple scab (Venturia inaequalis ) is available in apple varieties like 'Liberty' which was released by Cornell in 1978. The genes for resistance can now be used to create new varieties with high fruit quality by traditional breeding. This is collaborative research with Dr. Herb Aldwinckle's research group in the Department of Plant Pathology. They developed techniques to test for genetic resistance at an early seedling stage. To screen for resistance to scab, young seedlings are sprayed with a mixture of spores from the fungus and the susceptible seedlings display a characteristic reaction on the leaf surface. There is an excellent correlation between seedling leaf susceptibility and susceptibility of the fruit so we can eliminate susceptible seedlings at an early stage and save time labor and land. Only resistant seedlings are planted in the field. Resistant seedlings will not need any sprays to control scab.

We stress multiple disease resistance by screening for additional sources of resistance. We have a test for seedling resistance to cedar apple rust that is effective for both leaf and fruit symptoms. Resistance to powdery mildew (Podosphaera leucotricha ) is evaluated in the field. Our program emphasizes multiple resistance because varieties resistant to scab but susceptible to cedar apple rust and powdery mildew would still require chemical use for disease control.

To screen for resistance to the bacterial pathogen fire blight, young seedlings are injected with a mixture of races of the bacteria, and plants that show reduced disease symptoms are selected. There is good correlation between greenhouse tests and behavior in the field.

Grants from the NYS Agriculture and Market's IPM program, the NY Apple Research and Development program and from the processors' Apple Research Association have helped us to show the commercial potential of selections from our program and have enabled us to start a system of establishing demonstration planting in 10 production areas throughout the state. Growers will be able to evaluate our new selections under their own growing and management conditions. More extensive and earlier commercial testing of new advanced selections may reduce the time required prior to varietal naming and release.

Biotechnology

Despite the increased interest in new varieties, the problems of grower acceptance of a new and unknown variety and the reality of limited market niches highlight why genetic engineering is so appealing in fruit crops. Apple varieties already have established markets and strong name recognition. We buy apples by name. Compared to other vegetable or fruit crops this situation is unique. Therefore the ability to use biotechnology to improve existing popular varieties would permit the change in a key attribute, such as disease resistance, while retaining the original varietal attributes. Consumers and growers would not have to accept a new variety but would be able to enjoy an improved version. In addition, prospects for improvement are greater because beneficial genes from other crops may be used.

We can improve characteristics that are simply inherited with biotechnology, but the genes responsible must first be isolated and cloned (copied). Currently, the choice and availability of genes is limited in genetic engineering, with many groups focusing on genes that impart resistance to viral diseases, resistance to insects, resistance to herbicides, reduction of softening, resistance to fungal diseases and resistance to bacterial diseases. At Geneva, all of these genes, with the exception of resistance to herbicides, are being pursued.

We are fortunate in apple because we have the techniques needed to successfully use biotechnology to improve our varieties. The transformation and regeneration steps of genetic engineering will not be reviewed as previous reports have covered this (Aldwinckle, 1993; Hrazdina, 1994). Geneva scientists have used leaf pieces as targets for gene transfer, and new plants with foreign genes have been regenerated. These "transgenic" plants are currently in field tests at Geneva. Many scientists at Cornell are involved in some aspect of regeneration, transformation, and testing of genetically transformed apple. Researchers include doctors Aldwinckle and Gonsalves' research groups in Plant Pathology, doctors Brown, Sanford, and Harman's in Horticultural Science, and Dr. Hrazdina in Food Science and Technology. Many of these researchers are involved in inter-disciplinary projects.

In apple, these groups are actively engaged in transforming apple rootstocks for resistance to fire blight and to viral diseases, targeting apple for broad spectrum fungal resistance by the use of chitinase genes, and using techniques similar to that used in the new genetically engineered tomatoes to reduce fruit softening in apple. These projects will reduce the use of pesticides, decrease disease damage to trees, and provide a better quality product. Existing varieties will be improved and establishment of new markets will not be necessary.

Growers need to realize that while biotechnology holds tremendous prospects for improving our apple varieties, it will not address all our concerns. The techniques are tremendously powerful, but they have limitations. Presently we are restricted to simply inherited traits in genetic engineering, and this restriction may hold for a long time. Complex characteristics influenced by many genes such as yield, flavor, enhanced color, and texture can not be targeted by biotechnology. At present only traditional breeding can manipulate these types of traits, and only breeding can create new varieties. Genetically engineered products will also need to undergo extensive testing to ensure that just the characteristic of interest is changed. We would not want to have a M.26 that was resistant to fire blight but more susceptible to suckering. The rigorous testing required for genetically engineered trees also means that these techniques will not be a quick fix for our problems. We need to be realistic about the time scale for release of products derived from biotechnology. In addition, we must educate the public about the safety of foods derived by genetic engineering if these new varieties are to be accepted. Unfounded fears about food safety must be addressed.

Genetic Studies for Current Use in Traditional Breeding
and Possible Future Use in Biotechnology

Recent advances in mapping and isolation of genes ensure that the availability of additional genes to use in biotechnology will increase. Simple genes native to apple are being sought. Examples include disease resistance genes, resistance to storage disorders like storage scald, resistance to flesh browning, and genes for modification of plant form detailed below. Until these genes are cloned from apple they can not be used in genetic engineering, but they can be used in breeding.

Resistance to Flesh Browning
Breeding and genetic engineering for processing characteristics is also of interest. The discovery of resistance to flesh browning in an advanced apple selections, NY 674, is important for processing and fresh fruit. The resistance is due to low levels of the enzyme responsible for browning and high levels of Vitamin C (also an anti-browning agent). The genetic control of resistance to browning is being studied so that we can effectively use it in traditional breeding.

Plant Form Modification
In apple, a columnar tree form that produces little to no branches is being used in breeding to produce new selections that do not require extensive pruning and training. This columnar form resulted from a naturally occurring genetic change that causes fruiting spurs to be produced instead of branches. We are using this columnar form as a parent in traditional breeding to produce disease resistant columnar progeny. This unique columnar form is also allowing us to learn more about the genetic control of components of plant form.

Summary

Apple breeding and biotechnology offer many exciting prospects to produce new and improved varieties to benefit the industry. Our releases to date have had a major impact in many of our production areas. All systems are in place to use traditional breeding and biotechnology to markedly improve apple. We have transformation and regeneration systems established and genetically engineered apple rootstocks are presently being field tested. All varieties being developed, whether by traditional breeding or biotechnology, go through the same rigorous testing to ensure that we are releasing a safe, high quality, improved variety for our industry. Greater quality, storage and shelf life, and reduced dependence on chemical control of diseases have long been goals in apple breeding, and are becoming a reality. Our objective is to serve the industry and the public by producing new superior apple varieties, and by improving existing varieties. We need to meet current industry needs but also those of future generations.

Literature cited:

Aldwinckle, H. 1993. Improvement of New York apple varieties and rootstocks by genetic engineering. NY Fruit Quarterly. Winter 1993. p.3.

Hrazdina, G. 1994. Genetic engineering of 'McIntosh' apple to prevent softening during storage. NY Fruit Quarterly. Winter 1994. p.9.

Weeden, N.F. 1993. Genetic control of apple storage scald. NY Fruit Quarterly. Winter 1993. p.12.

Funding for this research was received, in part, from the New York State Apple Research and Development Board, the Apple Association, USDA Plant Genome, and state and federal sources.

Contact: Linda McCandless, Communications Services

Telephone: (315) 787-2417

e-mail: llm3@cornell.edu

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