Breeding Plants for the Future

By Rita Pelczar

In 1856 an Austrian monk named Gregor Mendel began a series of experiments with garden peas. Systematically crossbreeding plants that displayed visibly distinguishable traits, Mendel followed the offspring through several generations, observing and recording the incidence of these traits. His findings unlocked the secrets of plant inheritance. Fast-forward almost 150 years: Today's plant geneticists work with traits that the eye cannot detect-at the molecular level-and have successfully transferred genes between unrelated organisms. Astounding advances in plant breeding have been realized in the past one and a half century; some of these pose as many questions as they do answers.

The Changing Goals of Plant Improvement

Producing better plants has always been the goal of plant breeders, but exactly what constitutes improvement has changed over time. One hundred years ago, resistance to chestnut blight was not considered an essential trait for the robust species that dominated eastern forests. But American chestnuts have all but disappeared from our woodlands and landscapes. If-or perhaps we should say when-a gene is discovered that imparts resistance to the fungus that causes the blight, an American treasure might be restored. Today's ornamental plant breeders seek varieties that display tolerance to such modern afflictions as air pollution and acid rain; these traits were not a concern 50 or 60 years ago.

Increasing food production to accommodate an ever-growing world population has been a long-standing objective among breeders of food crops. Production can be boosted either by improving productivity of the land used to grow food, or expanding the amount of land in production. Both approaches have benefited from plant breeding, but since further increases in productivity will likely not be as dramatic as those resulting from the advent of hybrid varieties in the 20th century, the latter may be the focus in the future. Thus developing crops that will thrive on land now considered marginal or unusable for agriculture has become a critical goal of plant breeders, explains James Moore, professor emeritus of the University of Arkansas. "Changing the plant genotype [genetic make-up] through breeding," says Moore, "will be a key component in this technology."

The Improvement Process

Identifying the particular quality that will lead to an improved variety is the first step in a plant breeding program. The next is locating a source of that characteristic, and transferring it to a plant that displays all other desired attributes. Said fast, sounds easy; in reality, this requires careful research, a thorough understanding of breeding techniques, a keen sense of observation, and the patience of Job.

It is the methods for obtaining the desired trait and transferring it to a specific plant that have changed most dramatically in the last 30 years with the development of biotechnology. Until recently, plant breeders developed new varieties by crossing and backcrossing parent plants-nearly always members of the same species-to obtain desired traits. Chance or induced mutations provided further sources of new characteristics. But until the advent of technologies such as tissue culture and gene splicing, transferring traits between plants depended on their sexual compatibility.

While advances in genetic engineering have made it relatively easy to transfer genes between otherwise incompatible organisms, finding and isolating the gene responsible for a specific trait is still often a troublesome task for molecular plant breeders. Robert Griesbach, a plant breeder at the USDA laboratories in Beltsville, Maryland, notes that "it is relatively easy to physically introduce a foreign gene into a host plant, but difficult to identify the appropriate gene to introduce, and even more difficult to obtain that gene."

Mirroring the efforts of the Human Genome Project, which is attempting to map the entire human genetic code, plant geneticists have made great headway toward determining the complete DNA sequence of Arabidopsis thaliana, a modest meadow weed. Arabidopsis was selected because it has the smallest amount of DNA in the plant kingdom. "Once this sequence is obtained," Greisbach explains, "it will be much easier to identify and isolate genes, as well as understand the basic molecular processes that controls growth and development in higher plants."

Progress or Problems?

Bt potatoes. Roundup Ready corn. Terminator technology. The development of genetically modified organisms (GMOs) has stirred controversy among scientists and consumers. How safe are these crops? Are they the solution for world hunger? What effect will they have on the environment? These are just a few of the questions that have arisen since GMOs hit the market in the mid-1990s. The issue is complex, exemplified by the use of a gene from the bacterium Bacillus thuringiensis to produce Bt potatoes, followed by similarly pest resistant Bt corn and Bt cotton.

Bacillus thuringiensis has long been used as a topical biological insecticide for control of the Colorado potato beetle. It produces a toxin that is lethal to the beetle, but it does not persist for long. Genetic engineers theorized that splicing the gene for the toxin directly into the potato plant would protect it from the pest without the need for sprays. Detractors of this gene transfer ask whether such plants are safe for human consumption, what effect will they have on non-target insects, will it lead to resistance to the pest, and what are the long-term ramifications of combining such genes from unrelated and sexually incompatible organisms? These questions, unanswered to the satisfaction of some consumers and environmentalists, spurred international reaction, especially when it was determined that many processed foods such as corn syrup, cereals, and potato chips were being produced from genetically altered crops.

In Europe, consumer concern over the use of genetically modified plants in foods escalated to the point that two food processing giants, Nestlé and Unilever, announced last year that they would no longer accept transgenic crops for their European foods. Two of Japan's largest beer makers, Kirin and Sapporo, followed suit, as did the largest tortilla maker in Mexico. The economic impact on growers of transgenic crops has been substantial.

In the United States today, genetically altered crops are used in many processed foods. Labeling to distinguish genetically altered sources from conventional sources is not required. The feared dangers of genetically modified foods have spurred research to confirm or deny the concerns.

Tradition and Technology

Insight into the debate regarding molecular genetics and its effect on the plants we grow can be gained by examining how it differs from traditional-classical-breeding methods. What does it offer that classical techniques do not?

Griesbach suggests that the difference between classical and molecular breeding lies in how a trait is defined. For example, flower color is defined as a color detected by the human eye in a classical breeding approach. In molecular breeding, however, flower color is defined as a series of chemical reactions leading to the biosynthesis of a pigment. While classical breeding relies upon selecting plants with different gross characteristics, molecular breeding relies upon selecting plants with different molecular characteristics.

Tom Leustek, associate professor of biotechnology at Rutgers University, says the difference boils down to one word: precision. He compares the methods using the analogy of finding a needle in a haystack. According to Leustek, classical plant breeding "is literally like searching straw by straw for the needle. Genetic engineering is analogous to knowing precisely where the needle is."

While he acknowledges that genetic engineering offers tremendous new possibilities, Mark Bridgen, professor of plant science at the University of Connecticut, notes that "it will take some time before these abilities will be fully realized because of legal issues, moral issues, and-probably more importantly as a breeder-the ability to find and incorporate these genes into plants." Bridgen adds, "If the protocols for incorporating genes are known, then the trait can be developed much faster [using molecular breeding techniques] than through traditional breeding. However, as a classical breeder, we can use traits that have laid recalcitrant to produce beautiful plants-even if it takes longer."

Classical and molecular breeding researchers approach plants from different perspectives, according to John Navazio, vegetable breeder for Alf Christianson Seed Company in Mt. Vernon, Washington. "The notion that a plant breeder chooses only one trait worth improving and the rest of the genotype remains static is very foreign to classical plant breeders," he asserts. While molecular geneticists work with one single-gene trait at a time, Navazio says that "most successful classical plant breeders working on food or fiber crops are selecting for a minimum of eight to 10 traits simultaneously. You're working on a symphony of traits; you have to be the conductor that brings all those traits together." He further offers that most of the important traits in agriculture are determined by multiple genes.

Impact of Tissue Culture

Years ago, when a plant breeder developed a new daylily or hosta, it took years before sufficient stock was produced to bring it to market. With the advent of tissue culture propagation-producing an entire plant from a few cells of the parent under controlled, sterile conditions-thousands of new plants can be vegetatively propagated in a matter of months, often at a significantly reduced cost. Orchids representing complex crosses that were phenomenally expensive to reproduce because they did not breed true from seed, are now being inexpensively "cloned" through tissue culture techniques.

Terra Nova Nurseries in Tigard, Oregon, specializes in tissue-culture-propagated perennials, many of which are the product of the nursery's own breeding program. "It is the use of tissue culture that skyrockets the plant into sudden availability in a single year, whereas conventional propagation could take three to five years to produce the same number of plants," says owner Dan Heims.

Cell mutations are common in tissue cultures; this can be frustrating to someone trying to reproduce a crop exactly, but it is grist for the mill of the plant breeder. It is from these mutations that many new varieties arise. Mutations that occur as a result of tissue culture are known as somaclonal variations, explains Bridgen, whose work focuses on hardy and fragrant Peruvian lilies (Alstroemeria spp.). For Bridgen, these variations have been the source of numerous useful traits including pest tolerance, variegation, height control, and double flowers. "You can take natural characteristics that have been 'dormant' for years," he says, "and revive them [with the aid of] somaclonal variation."

Cell Fusion

Cell fusion is a technique made possible through tissue culture. It involves the union of two non-reproductive (somatic) cells from different organisms to create a replicating cell that contains the genetic information from both of the unrelated parent cells. Though these wide crosses are rarely stable, they provide a means for genes of different species to come together, for use in further breeding operations. Such combinations are otherwise impossible. While this technology suggests enormous potential for breeding plants in the future, some question both the morality and safety of the unnatural merging of genetic information that could potentially create new organisms.

Tagging the Trait

Mendel's groundbreaking research focused on visible, easily identifiable differences- tall or dwarf plants, smooth or wrinkled seed coat, yellow or green seeds-among his peas. Other characteristics that a breeder might strive to incorporate into a plant are not as easily perceived: a tomato resistant to fusarium wilt may look the same as one without resistance, at least until the disease strikes. By purposefully infecting test plants with a disease and selecting those that survive, researchers are able to identify those containing the resistant trait.

At the University of Arkansas' vegetable sub-station near Alma, vegetable breeder Teddy Morelock tests his spinach crosses on a nursery plot that has been used for growing the crop for 24 years. This "cess pool of spinach diseases" harbors nearly every disease known to infect spinach in North America. By cropping breeding lines through this nursery, disease-susceptible plants are easily eliminated.

Many gardeners select the seed of their best plants to save for next year's garden. Perhaps one plant demonstrated a greater tolerance to heat, produced larger crops or bigger flowers, or bore fruit with better flavor than nearby plants of the same type. By saving its seed and planting it the following year future crops are more likely to exhibit the quality observed in the original plant. Selecting seed from plants that display specific traits is a long-standing technique that continues in breeding programs today.

But technology has made the process of selection faster and more exact. Plant breeders can now determine the presence of a desired-or undesired-trait using a method called marker-assisted breeding. This technique, says Leustek, "relies on the fact that traits are linked to specific DNA (gene) sequences, which can be used as markers. Breeders use the markers to identify whether a trait has been transferred to progeny of a breeding experiment." This takes much of the guesswork out of selection and eliminates the need to grow the plant to maturity to determine if the cross was or was not successful. This is particularly important when working with trees and shrubs that may require eight to 10 years of growth before the success of a breeding effort can be effectively evaluated.

Merging Methodologies

Biotechnology has much to offer the field of plant improvement, particularly when combined with classical breeding methodologies. "Molecular genetics is a far bigger discipline than just genetic manipulation across species boundaries," states Navazio. "The information about the workings of DNA," he continues, "is incredibly important to classical geneticists like myself."

Biotechnology provides the means for the transfer of specific alien genes into a plant when natural barriers make this otherwise impossible. It allows the rapid and accurate determination of the presence of a trait in a new cross. And it opens doors to new possibilities for controlling plant diseases and pests, as well as increasing tolerance to environmental stresses. "However, the ultimate worth of a cultivar," says Moore, "is determined by thousands of genes working in harmony, and it will be the role of plant breeders to mold the genetic diversity created by biotechnology into usable cultivars." m

Rita Pelczar is associate editor of The American Gardener.

Improving Vegetables

By Rita Pelczar

John Navazio, who breeds spinach, beets, and carrots for the Alf Christianson Seed Company in Mt. Vernon, Washington, considers improving resistance to diseases and the development of crops with increased nutritional quality as major objectives facing today's vegetable breeders.

Disease Resistance

Protection from many important vegetable diseases has been obtained by incorporating a single gene that confers resistance. Vertical resistance, as it is called, has been accomplished both through classical breeding methods and genetic engineering by transmitting these single resistant genes. This resistance is absolute; the plant is completely resistant to the disease-or at least to a specific race of the disease. If the disease-causing organism mutates, forming a new race of the disease, the resistance does not hold up. "What this creates," says Navazio, "is the 'race race'-constantly breeding for the next race...thereby increasing happenstance mutations for pathogenicity of the disease organism. It's a boom and bust cycle."

Downy mildew is a serious disease of spinach for which vertical resistance was obtained in the early 50s. But the fungus that causes the disease has undergone several mutations, each requiring a new line of resistance. Today American spinach growers worry about race 6 of downy mildew, and in Europe, race 7. And the fungus that causes the disease phytophthora in soybeans has developed 28 races-20 of them in just the last 20 years!

A better approach, according to Navazio, is to breed plants that bear a broader, though not absolute, resistance. This is called horizontal resistance and it is controlled by multiple genes. It imparts "varying amounts of resistance to a disease in any situation," remarks Navazio.

Breeding for horizontal resistance begins by identifying plants that display a fairly high level of resistance to a disease-what farmers call field tolerance-and interbreeding them. Offspring are inoculated with the disease and survivors are inter-bred. The result is a stacking or pyramid affect of the desired traits, obtaining significantly raised levels of resistance, often in very few generations. These plants are equally resistant to any strain of the disease, including new strains. When coupled with cultural practices, horizontal resistance can reduce disease damage to a level that doesn't affect quality. According to Navazio, "This has huge implications for lowered pesticide use."

Breeding for Nutrition and Health Benefits

Plant breeders have made great progress in increasing traditional essential nutrients-vitamins and minerals-in the vegetables we eat. More recently, they have been working to increase phytonutrients-compounds that enhance natural functions and promote a long and healthy life. "We have only begun to understand the role of phytonutrients in human nutrition," remarks Navazio. "It is a very ripe area of research."

Plant breeders have begun working with colleagues in the medical profession to develop varieties that contain higher amounts of natural compounds that may be important in maintaining human health. James Moore, professor emeritus of the University of Arkansas, cites the potential of anti-oxidants and isoprenoids-compounds found in plants that contribute to their distinctive flavors and fragrances-to suppress cancer and heart disease. "Studies on these compounds are new," states Moore, "but a few have been shown to suppress human cancer in cells in laboratory tests." Among these are limonene and lycopene-two isoprenoids common in tomatoes.

USDA plant researcher Phil Simon at the University of Wisconsin and Teddy Morelock at the University of Arkansas are both working on the phytonutrient, lutein, which is critical in preventing age related macular degeneration, a condition that restricts vision. Lutein also possesses great anti-oxidant potential; it is capable of blending with and deactivating free radicals-highly reactive molecules that have been linked to the development of certain cancers. Normally present in dark leafy greens, winter squash, and yellow and green summer squash, researchers have discovered that the lutein content in vegetables is directly related to the depth of color. New varieties are now being selected for high lutein content.

Gardeners can sample vegetables that have been bred for increased nutrients and phytonutrients. High levels of beta-carotene are available from Johnny's Selected Seeds' carrot 'Sugarsnax', and Garden City Seeds' 'Vita Gold' tomato. Johnny's offers 'Raven' zucchini, which boasts a high lutein content. Health-minded gardeners may also select Territorial Seed Company's 'Nutri-Red' carrot which contains the anti-oxidant lycopene, and 'Doublerich' tomato which provides twice the normal level of Vitamin C. Many more varieties with improved nutritional value will be showing up in seed catalogs soon.

Plant breeders are adding considerable weight to Mom's chant of "Eat your vegetables!"-Rita Pelczar

Resources 

Applications of Plant Cell and Tissue Culture edited by G. Bock and J. Marsh. John Wiley and Sons, New York, New York. 1988.

Breeding Ornamental Plants edited by Dorothy J. Callaway and M. Brett Callaway. Timber Press, Portland, Oregon, 2000.

Breeding Your Own Vegetable Varieties: The Gardener's and Farmer's Guide to Plant Breeding and Seed Saving by Carol Deppe. Chelsea Green, White River Junction, Vermont. 2000.

Plant Propagation: Principles and Practices edited by Hudson T. Hartmann. Prentice Hall, New York, New York. 1996.

Plants for the Future: A Gardener's Wishbook by Jerome Malitz. Timber Press, Portland, Oregon, 1996.

Principles of Plant Breeding by Robert W. Allard. John Wiley and Sons, New York, New York. 1999.

Return to Resistance: Breeding Crops to Reduce Pesticide Dependence by Raoul A. Robinson. AgAccess, Davis, California, 1996.