Biotechnology in Horticulture

Robert M. Skirvin

Horticultural scientists study crops that are used for food, drugs, or aesthetics. Many of these crops are unique or so highly specialized that they are no longer propagated sexually by seeds but rather asexually by such methods as cuttings, grafting, layering, and tissue culture. Seedless grapes, potatoes, maple trees, and roses are some examples of crops that are propagated by some or all of these methods. Although some cultivars are very well known and widely grown, problems related to disease susceptibility, fruit or flower quality, or growth habits arise occasionally.

Traditional sexual-breeding programs can improve some of these crops. The process can be slow, however, especially in tree-breeding programs.

Many of our most popular flower, fruit, and vegetable cultivars are very old. The public often rejects a new cultivar in favor of a better-known cultivar that has endured. The asexually propagated 'Bartlett' pear, for example, introduced in 1770, remains the world's standard for pear quality. Because most new cultivars fail this test, they remain as local cultivars or are discarded.

Another method to improve cultivars is to screen "sports" or mutations that occur naturally, but rarely, on cultivars grown in a field, greenhouse, or laboratory. For example, the green-fruited 'Bartlett' pear sported to yield the red-fruited 'Red Bartlett'. Although this natural variability is useful for plant improvement, the process is random and many plants must be inspected to find a desirable mutant type.

Variability is a universal feature of plant tissue-culture, a method whereby plant parts are grown aseptically on artificial medium. Variation that develops under tissue-culture conditions is now called somaclonal variation. In tissue culture, whole plants can develop from single cells under the proper stimuli of growth regulators, growing medium, and light. Although the precise cause of somaclonal variation remains unknown, its induction and use have become important research topics.

The exploitation of natural and induced variation seems especially applicable for improving older asexually propagated cultivars like 'Bartlett' pear. During the past 200 years, the clone has been exposed to natural radiation - ultraviolet and cosmic rays - and various sprays and pollutants. These exposures have probably resulted in mutations that stabilized as islands or regions within the body of these plants. When a cultivar possesses mutant cells (natural or artificially induced), whole plants derived from these cells may express the mutation and be classified as somaclones.

Researchers in the Department of Horticulture have used somaclonal variation to obtain thornless blackberries and red pear trees. 'Lincoln Logan', one of our thornless blackberries derived from tissue culture already has been released as a cultivar. Another thornless blackberry is at an advanced stage of testing. We also hope to find variations in fruit color in our small orchard of apple trees that were derived from tissue culture.

Because there may be an upper limit to natural variation within a cultivar, biotechnology protocols are important to supplement and direct the type of variation that we encounter. Researchers at the Department of Horticulture have worked with tomato plants that possess a gene from bacteria (bt) that gives resistance to certain caterpillars. Other scientists in the department have produced Solanaceous plants with good resistance to certain herbicides and are trying to introduce virus resistance to apple trees.

How much progress can be expected using biotechnology will vary with the clone, the age of the culture--older cell lines often show more somaclonal variation than newer lines--use of mutagenic agents such as radiation or chemical mutagens, and selection pressure applied to single-cell clones for stress conditions such as salt level, herbicides, microorganisms or their by-products, and specific metabolites. For instance, by placing a variable population of cells in a medium with herbicide, the cells that survive should have some resistance to the herbicide. Progress will also depend on the availability of useful genes for transfer to horticultural crops.

Robert M. Skirvin, professor of horticulture

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