Perspectives Online

Micropropagation Yields Macro Results. Micropropagation begins in a laboratory under sterile conditions. Plants are then produced using tissue culture, for which temperature and light are carefully controlled. By Dave Caldwell

Micropropagation begins in a laboratory under sterile conditions. Plants are then produced using tissue culture, for which temperature and light are carefully controlled.
Photo by Daniel Kim

Just a few years ago, George Wooten typically harvested 14 to 15 1,000-pound bins of sweetpotatoes per acre from the fields he farms in Columbus County. These days, he's digging 17 to 18 bins an acre, roughly a 20 percent increase.

While Wooten, who grows approximately 1,000 acres of sweetpotatoes, thinks the management skills his son, George Wooten III, brings to his operation have something to do with the yield increase, he gives much of the credit to the Micropropagation Unit in the College of Agriculture and Life Sciences' Department of Plant Pathology.

That kind of yield increase is typical across North Carolina, says Billy Little, an agricultural agent with North Carolina Cooperative Extension in Wilson County, who works with sweetpotato growers. It's typical, that is, if growers are taking advantage of the services provided by the Micropropagation Unit. And most are.

As Little puts it, "It caught on so quickly and has gone so well."

Indeed, the Micropropagation Unit was created less than a decade ago, in 1996, to aid North Carolina's small-fruit and vegetable industries. While the unit works with strawberries and brambles such as blackberries and raspberries, as well as sweetpotatoes, its greatest success has been with sweetpotatoes.

Sweetpotatoes, grown on approximately 40,000 acres and worth $60 million annually, have long been a significant North Carolina crop. Much of the acreage is in the coastal plain, with major production in Wilson, Nash, Johnston, Sampson and Columbus counties and some production in the piedmont and tidewater regions.

Dr. Zvezdana Pesic-vanEsbroeck checks plants in a screened cage in the greenhouse.
Photo by Daniel Kim
Yet in the mid-1990s, North Carolina sweetpotato growers were in trouble. Dr. Zvezdana Pesic-vanEsbroeck, Micropropagation Unit director, explains that traditionally, sweetpotato growers in North Carolina and elsewhere saved a portion of each year's crop as "seed roots." In the spring these seed roots would be placed in hot beds (plant beds under covered structures) or in outside beds covered with plastic film. Sprouts would grow from the seed roots. The sprouts were then removed and planted to produce that year's crop.

After several years using this system, diseases caused by viruses, fungi and bacteria would infect the plants, reducing yields and root quality. In addition, undesirable mutations would accumulate in the plants, producing elongated roots and various skin and flesh colors that made the sweetpotatoes difficult to sell. Growers who saved seed roots from year to year would end up planting sweetpotatoes that were already infected or contained undesirable mutations.

When the yield of a sweetpotato cultivar declines and harvested roots are misshapen, growers say the cultivar has "run out." By 1995, the North Carolina sweetpotato industry was beginning to lose market share. Seed root quality had declined to the point where growers were producing poor crops and undermining demand for North Carolina sweetpotatoes.

Pesic-vanEsbroeck says micropropagation techniques can be used with any vegetatively propagated plant.
Photo by Daniel Kim
The industry was looking for a solution, says Pesic-vanEsbroeck, and found it in the Micropropagation Unit.

Research in the Department of Plant Pathology dating to the 1950s and continuing in the 1970s had shown that many diseases caused by viruses, bacteria and fungi could be avoided by using a technique called apical meristeming or micropropagation - propagating sweetpotatoes from microscopic cuttings from a sweetpotato vine.

Micropropagation begins in a laboratory under sterile conditions, when a few cells are removed from the meristem at the tip of a sweetpotato plant. The meristem is a special part of the plant where the first phase of growth occurs. But perhaps more importantly, meristematic tissue is often free of virus particles.

Meristematic tissue is placed in a test tube in a special growing medium in an incubator, where temperature and light are carefully controlled. The cells begin to form a new plant. Producing plants in this manner is known as tissue culture.

The Micropropagation Unit is branching out, working with blackberries and raspberries (shown here), as well as sweetpotatoes and strawberries.
Photo by Daniel Kim
Sweetpotato plants produced under these conditions are referred to as "mericlones." As these mericlones grow larger, they're checked for mutation and disease in two ways. When plants are large enough, they're transferred to greenhouses, where they're assayed for viruses by grafting a stem section of the plant to morningglories that are particularly susceptible to various viruses that infect sweetpotatoes. If a plant harbors a virus, the disease the virus causes will show up in the susceptible morningglory. In the greenhouse, mericlones are kept in screened cages to lessen the chance that insects will infect them with a virus.

Mericlones are also evaluated in the field to determine if they are "true-to-type," or maintain the characteristics of a particular cultivar. This is done by growing mericlones at several locations in the state in much the same manner as commerical sweetpotatoes. The roots these plants produce are dug up and checked to make sure they have the characteristics of a particular cultivar. Mericlones that are virus-tested and true-to-type become "nuclear stock mother plants." Cuttings from these plants are sold to certified greenhouse nurseries, which then plant and reproduce them for sale to growers. Nurseries must operate under strict guidelines developed by the North Carolina Crop Improvement Association Inc.

Pesic-vanEsbroeck says the Micropropagation Unit sells approximately 4,500 plants to certified greenhouse nurseries each year. The nurseries grow and take cuttings from the plants, eventually turning thousands of plants into millions of plants. More than 2 million certified sweetpotato plants are sold to growers from North Carolina's nine certified greenhouse nurseries each year.

Certified greenhouse nurseries must meet strict standards designed to protect their plants from virus infection. As the reputation of North Carolina sweetpotatoes has grown, certified nurseries have become major suppliers of sweetpotato seed stock not only to North Carolina growers but to growers in other states, including Louisiana, California, Mississippi, Alabama, South Carolina and Texas.

The Micropropagation Unit maintains all the major sweetpotato cultivars, including Beauregard, Covington, Hernandez, Japanese, O'Henry, Porto Rico and Jewel. These and other cultivars have been cleaned up, stabilized and are available through the certification program.

While the Micropropagation Unit has enjoyed its greatest success with sweetpotatos, the strawberry industries in North Carolina and Florida are also taking advantage of plants provided through the unit and a similar certification program.

Pesic-vanEsbroeck explains that the Micropropagation Unit has revitalized a segment of the strawberry industry that had become dormant in the 1990s. Before that, North Carolina was a major producer of strawberry transplants used by growers each year in North Carolina and Florida to establish their fields. Florida growers preferred North Carolina plants because they produced early and high yields free from major diseases. Then North Carolina nurseries became infested with anthracnose disease, and growers who had been buying transplants in North Carolina nurseries began to look elsewhere.

The Micropropagation Unit began providing anthracnose-free, virus-tested strawberry plants to the industry in 2001, says Pesic-vanEsbroeck. As is the case with sweetpotatoes, private growers increase the virus-tested, true-to-type plants produced by the unit through a certification program. About 20 million strawberry plants are expected to be produced this year through the program.

The public-private partnership that provides both sweetpotato and strawberry growers with plants is one reason both programs have been successful, Pesic-vanEsbroeck says. She points out that the use of private-sector growers to increase plants has created new segments of the sweetpotato and strawberry industries. While the Micropropagation Unit has just begun to work with blackberries and raspberries, it appears likely that North Carolina will become a center for the production of blackberry planting stock as well.

Pesic-vanEsbroeck points out that the techniques developed to micropropagate sweetpotatoes, strawberries, blackberries and raspberries can be used with any vegetatively propagated plant, so there is considerable opportunity for expansion of the unit.

Dr. Charles Averre, professor emeritus of plant pathology, thinks there might be a market for the micropropagation of bananas and other vegetatively propagated tropical crops and that unused North Carolina greenhouses might be used to increase these crops. He says some Central American banana growers now buy micropropagated plants from Israel, so why not from North Carolina? Averre and Dr. Robert Milholland, also a professor emeritus of plant pathology, were instrumental in developing the Micropropagation Unit and continue to serve as advisers.

Yet whatever the future holds for the Micropropagation Unit, its value thus far has been defined by sweetpotatoes and, increasingly, strawberries. It is no wonder that the North Carolina Sweetpotato Commission, the North Carolina Certified Sweetpotato Seed Growers Association and the North Carolina Strawberry Growers Association are strong supporters of the Micropropagation Unit and certification programs.

And where sweetpotatoes are concerned, the value of the unit is apparent each year in North Carolina fields, where growers who plant seed stock that originated in the Micropropagation Unit are harvesting remarkably consistent crops from year to year.

As Pesic-vanEsbroeck says, "None of the cultivars are running out."

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