Tissue Culture: How Modern Science is Saving Heirloom Potatoes

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Labeled heirloom potatoes illustrate the diversity found in Seed Savers' collection.
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Tim Johnson, former head of preservation at Seed Savers Exchange, plants potatoes at Heritage Farm, the organization’s Decorah-based headquarters.
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Refrigerator shelves at Seed Savers Exchange hold rows of nutrient-filled test tubes containing potato plants grown from small pieces of plant tissue.
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It’s best to dig potatoes on a warm, dry, cloudy day following a period of no rain. Use a digging fork to lift the tubers out of the soil, and then remove any clumps of dirt.

Few places showcase the outdoor beauty of northeastern Iowa as brilliantly as Heritage Farm, the 890-acre headquarters of Seed Savers Exchange located just outside Decorah, Iowa. The terrain boasts lush meadows and towering bluffs, bubbling trout streams and endangered livestock breeds, colorful apple orchards, and, of course, gardens displaying rare heirloom and open-pollinated plants.

So breathtaking is this working farm that you could easily miss some equally spectacular work that transpires indoors, tucked away in the basement of the administrative building. There, in a small section of the preservation department, you’ll see tools for cultivating plant life, but there are no garden trowels or soil. Instead, there’s a host of modern lab equipment: a glass-bead sterilizer and a laminar flow hood, autoclaves and test tubes — rows upon rows of test tubes, in fact. Sealed with colored stoppers and labeled with names like ‘Poorlander’ and ‘McIntyre Blue,’ these test tubes hold heirloom potato plants (Solanumtuberosum). The tiny plants belie a rich heritage deserving of preservation with a modern technique called tissue culture.

The technique involves growing new plants from small pieces of plant stem tissue in nutrient-filled test tubes under precisely controlled conditions. This process takes no small amount of time and resources. So what, then, is the benefit of preserving potatoes this way?

“Potato tissue culture is more secure than a collection of tubers alone,” says Tim Johnson, former head of preservation at Seed Savers Exchange (www.SeedSavers.org). “Once something is preserved in tissue culture, it can stay in tissue culture forever.”

Here’s why that’s so important: Unlike many other common garden vegetables, potatoes aren’t often grown from seed, but as clones. To grow a potato, you usually plant an existing potato, or at least a cutting from one. That “cloning” propagation process makes potatoes susceptible to soil-borne diseases that can reduce yield and ultimately destroy a cultivar.

“You plant a tuber, you get 10 new tubers, you eat nine tubers, and then you replant one the next year,” says Johnson. “Over time, though, the plants acquire viruses that are passed into the tubers, and eventually the viral load becomes so great that the plants start to lose their productivity. They get smaller and smaller until they don’t produce tubers anymore and are gone forever. Tissue culture allows us to preserve heirloom potatoes for future generations by ensuring that we don’t unnecessarily expose them to environmental pressures such as pests, unpredictable weather, and, especially, diseases.”

The Beginning of Plant Tissue Culture

Used widely in horticulture today, the concept of plant tissue culture originated in 1902 when an Austrian botanist, Gottlieb Haberlandt, was able to keep individual cells alive outside of an intact plant. He proposed the idea to the German Academy of Science. Decades would pass before scientists used the technology on potatoes, and it wasn’t until 2008 that Seed Savers Exchange adopted it at Heritage Farm.

“One of our members had started a potato tissue-culture program years before, but around that time, we acquired and started maintaining it onsite,” recalls Johnson. “It was challenging at the outset as we had a lot of bacterial contamination issues, but we made improvements and, as a result, have had a really solid program for a few years now.” Those improvements included replacing a more rudimentary ethanol lamp with a medical-grade glass-bead sterilizer to eradicate bacteria from tools. Seed Savers also acquired a higher-quality laminar flow hood, which is an enclosed bench equipped with a fan that blows air through an ULPA (ultra-low penetration air) filter to remove pathogens.

“The thing with tissue culture is that absolutely everything has to be sterile, and the environment has to be well-controlled,” says Johnson. “We had to figure out how to provide a clean interface because, among other things, our tools live in a cutlery drawer, not in a sterile vacuum. It took a little while, but we got it right.”

60 Virus-Free Heirloom Cultivars

As the preservation staff at Heritage Farm were honing their tissue-culture skills, they gained a valuable partner — the University of Wisconsin-Madison’s Organic Potato Project.

Seeking heirloom cultivars that would thrive on organic farms, the researchers combed through old Seed Savers Exchange yearbooks to find potatoes with favorable reviews for flavor, yield, and — more importantly — tolerance to insects, pests, and diseases. “Those are really critical characteristics for growing plants in organic conditions,” says Dr. Ruth Genger, one of the University of Wisconsin researchers. “We chose about 100 cultivars, put those through a protocol that eradicates viruses, and ended up getting about 80 cultivars out of that pipeline.”

Since 2012, the university has collaborated with about 30 farmers to evaluate those now virus-free heirloom cultivars. How hardy is a certain fingerling potato? How does it taste? Does it have consumer appeal at farmers markets? Genger and her colleagues continue to seek answers in their quest to better educate farmers about how to select and breed potato cultivars, and how to expand the diversity of the potato market beyond standard commercial cultivars like ‘Russet Burbank’ (the most widely grown potato in North America).

“The farmers and their customers have really enjoyed learning how diverse potatoes can be, and we’ve learned things that we would never grasp just from doing research alone, such as what the market might be for different heirloom cultivars,” says Genger. “When we have farmers who are actually taking these cultivars to market, we learn a whole lot more. The bottom line for us is: Would you want to grow this cultivar again?”

Just as the University of Wisconsin and local farmers have benefited from this ongoing research, so too has Seed Savers Exchange. “When they have produced this virus-free material, they have also sent it to us,” says Johnson. “And we now have about 60 potato cultivars that are virus-free.”

From Test Tube to Field

Of course, the ultimate goal for Seed Savers Exchange is to get those 60 cultivars out of culture and into the hands of farmers and gardeners through its Seed Exchange (Exchange.SeedSavers.org). As Johnson puts it, “We don’t send our members test tubes, so we need to make potatoes.”

That process begins each March, when the multiplication process on virus-free specimens begins. “Phillip Smith, our tissue culture technician, opens the test tube, pulls out the whole plant, cuts it up into pieces, puts the little pieces into new test tubes, and then throws out all the waste,” explains Johnson. “Each test tube is sacrificed to make more test tubes, and every time you open a test tube, you basically have one opportunity to harvest material.” By mid-April, the tiny plantlets have been transplanted to potting mix in a greenhouse, where, covered by domes to keep in humidity, they’ll grow roots and shoots. Making the move from test tube to soil, says Johnson, can be hard on the tiny plants: “Potting up is a shocking transition for them because in the tissue culture, they have everything they need and they aren’t working very hard.” Still, most survive, and by mid-May, when the weather is cool but soils have started to warm, the plants are transplanted into the field for harvest in September and October, and eventually for distribution to members.

Today, Seed Savers Exchange members have access to only a fraction of the more than 700 potato accessions currently in-vitro in the collection, but Johnson sees access increasing in the years ahead. A genetic fingerprinting project to identify duplicates in the collection — being conducted in collaboration with the U.S. Department of Agriculture, Michigan State University, and the International Potato Center in Peru — will soon wrap up. And after the true number of distinct potato cultivars is known, staff will work to make each and every one free of destructive pathogens.

“Our goal is to have everything in the collection virus-free so that we can share all our potato cultivars and get them in wider circulation,” says Johnson. “That’s ultimately what we’re working toward — to make the entire potato collection available to our members on a rotating basis.”

Sara Friedl-Putnam is the communications assistant at Seed Savers Exchange in Decorah, Iowa, and a new (but enthusiastic) advocate of gardening with heirloom seeds.

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