Biotech Potatoes - Are we prepared for the future?

potato plant

This was an article that I wrote with collaborators for the proceedings of Wisconsin’s Annual Potato Meeting

by Dr. Dennis Halterman, Dr. Shelley Jansky, and Dr. Jiming Jiang

US and Global Historical Perspective

Since the introduction of the first genetically modified (GM)/biotech crops in the mid-1990s, the agriculture industry has seen a steady increase in the acreage of biotech crops planted and harvested each year. In 2009, a record 14 million farmers in 25 countries planted 330 million acres of biotech soybean, maize, cotton, canola, zucchini squash, papaya, alfalfa, poplar, sugar beet, tomato, and sweet pepper. This represents an 80-fold increase in usage between 1996 and 2009. This increase is largely due to the economic, environmental, and productivity benefits derived from the use of GM crops. Biotech crops are accepted for import for food and feed use and for release into the environment in 57 countries, including major food importing countries like Japan, which do not plant biotech crops. Of these, Japan tops the list for the number of biotech crops accepted, followed by the US, Canada, South Korea, Mexico, Australia, the Philippines, the European Union, New Zealand and China.

But what about potato? Are we being left behind? The answer is most decidedly “no,” but the history of biotech potato has curtailed its reintroduction into the world food market somewhat. However, as you will see below, the acceptance of new GM crops continues to increase and, with the introduction of new technologies to introduce important traits using biotechnology, it seems to be only a matter of time before biotech potatoes are approved not only for cultivation but also for human consumption.

In 1995, Monsanto introduced the NewLeaf Potato, its first bioengineered crop. NewLeaf was engineered to express a bacteria-derived toxin (Bt) that protected against feeding by the Colorado potato beetle (CPB). Although NewLeaf never captured more than 5% of the market, it was quite effective at preventing CPB damage. At its peak in 1999, NewLeaf potatoes were planted on about 55,000 acres in North America. However, in 2001, Monsanto quietly stopped production of NewLeaf potatoes, bowing to pressure from food companies that were against the use of bioengineered food. They instead decided to focus on the production of crops with far bigger markets. The release of NewLeaf potatoes into the market without approval from the US Environmental Protection Agency and the Food and Drug Administration led to a much larger debate regarding the use of biotechnology in food crops and, since then, the release of biotech crops that are directly consumed by people (corn and soybean are primarily used for animal feed) has been difficult and rare.

Estimates place the cost of deregulating a new GM crop for human consumption at around $1 million, which makes it very difficult for researchers to develop new crops for release without industry support and a large market for the biotech crops. In some cases, where a devastating disease threatens crops, we have seen the release and acceptance of biotech crops containing resistance.  For example, papaya is susceptible to the incurable papaya ring spot virus (PRSV) and infected plants are severely affected.  PRSV was first detected in Hawaii in the 1940s and in the 1950s had all but eliminated the papaya crop on the island of Oahu.  Although production was moved to another island and was successful for decades, it was expected that PRSV would eventually spread to new production areas. Therefore, in the late 1980s, researchers with the USDA and the University of Hawaii began to develop a papaya cultivar resistant to PRSV. To do this, parts of the virus itself were transferred to the papaya genome. The production of the non-infectious virus particles elicited a type of immune response in the plants. Therefore, the new genetically modified plants are no longer susceptible to PRSV infection. The first virus resistant papayas were commercially grown in Hawaii in 1999 and now transgenic papayas account for more than 60% of the total Hawaiian papaya crop. China has been growing biotech papaya since 2006 and it now accounts for more than 88% of China’s papaya production. Genetically modified papayas are approved for consumption not only in the US, Canada, and China, but were also approved recently for import and consumption in Japan. 

Another disease for which there has been great interest recently in developing biotech-derived resistance is Huanglongbing (HLB), also called citrus greening or yellow dragon disease of citrus crops. The disease is caused by a bacteria-like organism called a Liberibacter, and is spread by the Asian citrus psyllid. The disease causes yield losses, reduces the economic value of the crop, and can kill trees. Like PRSV, once a plant is infected, there is no way to cure the plant of the disease. New biotech citrus plants have been developed by Texas AgriLife Research of Texas A&M University that contain resistance to HLB using a gene derived from spinach. There is a projected timeline of six years for these plants to prove themselves and to be accepted as safe by all state and federal agencies. If they are accepted, general release and production will likely follow.

While the majority of GM crop production is comprised of soybean, maize, and cotton, other biotech crops, including some vegetables, have recently been approved for production and are being grown.  In 2009, an estimated 95% of the 485,000 hectares of sugar beets planted in the US were devoted to varieties improved through biotechnology (Roundup-Ready).  Canadian growers planted approximately 15,000 hectares of biotech varieties in 2009, representing about 96% of the nation’s sugar beet crop.  In October 2009, India’s Genetic Engineering Approval Committee recommended the commercial release of insect resistant brinjal (eggplant).  Biotech brinjal is expected to be the first biotech food crop to be commercialized in India.  India has several other biotech food crops in field trials, including biotech Bt rice.  GM potato for starch production has been approved in the European Union and will be discussed below.

Biotechnology Challenges

The development and utilization of biotech potatoes involves some biological hurdles. The first is the ability to grow plants in tissue culture and regenerate them after they have been genetically modified.  Fortunately, compared to many crops, potato is relatively easy to manipulate in tissue culture.  Some cultivars are more amenable to the system than others, but with appropriate protocol modifications, most are capable of transformation.  Another potential hurdle is the availability of genes to introduce into cultivars via transformation, especially if the public or industry requires that the genes originate from potato. Again, potato is an exceptional crop for transformation due to the tremendous amount of genetic diversity in wild and cultivated relatives.  For example, genes from wild potato relatives can contribute resistance to late blight, Verticillium wilt, potato virus Y, and cold sweetening. 

After GM cultivars have been created, two potential safety issues arise. The first is the potential for pollen flow from the transgenic cultivars to either weeds or other potato cultivars after their release into the field. In US potato production regions, no weeds are cross compatible with potato, so transgenic weeds will not be created. In addition, unlike grain crops, the product of pollination in potato is not harvested and is not used for propagation of a new crop.  Consequently, it is simply not possible for genes to move from biotech cultivars to conventional ones. A second potential safety concern is that biotech crops contain modifications that alter the structure of the genome in an unpredictable way.  There is some worry that these modifications could have unforeseen consequences.  However, if the gene being introduced originates from a wild potato relative, then the amount of genetic modification is actually less than if traditional breeding is used to introduce the gene. The biotech plant will contain only the segment of DNA inserted during transformation, while a plant developed by breeding will contain 50% exotic germplasm after the first cross, 25% after the second cross, and so on.  The introduction of a gene within a crop (e.g. a wild potato gene into cultivated potato) is termed “intragenics” and differs from transgenics because the final product does not contain any foreign DNA.  Finally, some GM plants are created by knocking out the function of existing genes, so they do not rely on the introduction of new genes. 

It can be argued that potato is the crop that can benefit most by biotechnology. Breeding progress is slow because cultivars are tetraploid and genetic variability in existing cultivars is low. Potato is easy to transform, genes are available from wild species, and gene flow is not an issue. Since disease and storage losses are significant in the potato crop and control is expensive and not always effective, significant productivity gains could be realized through the use of biotech cultivars.

What does the future hold for biotech potato in the US?

The European Union has set the stage for the acceptance of biotech potato with a cultivar intended for industrial use. The BASF chemical company has been granted approval to produce the cultivar Amflora, which will be used as a source of starch in industrial processes such as the manufacture of paper. The company is moving ahead with plans to grow biotech processing cultivars containing resistance genes for late blight and other disease. The decision made on the Amflora potato was the first approval for the planting of a biotech crop in the EU since 1998. 

In the US, perhaps low acrylamide potatoes will provide the next test case for acceptance of GM potatoes. Acrylamide, a neurotoxin and a potential carcinogen, is produced in all starch-rich foods processed under high temperatures. Potato chips and french fries have especially high acrylamide levels. This has raised a worldwide food safety concern that has resulted in lawsuits against major potato and fast food companies. Developing methods to reduce acrylamide in fried potato products has become an urgent requirement for the potato processing industry. The substrates for the production of acrylamide are reducing sugars (glucose and fructose) and the amino acid asparagine. Consequently, one biotech strategy has focused on suppressing the accumulation of reducing sugars by knocking out (silencing) the production of the enzyme acid invertase, which cleaves sucrose into glucose and fructose. This strategy has been very successful in transforming standard chip cultivars into clones with high levels of resistance to cold sweetening. Another strategy that is being evaluated is knocking out the expression of the two genes that are required for the synthesis of asparagine. Biotech cultivars would provide a mechanism to quickly respond to industry needs. Potato cultivars with the acid invertase and asparagine synthase genes silenced could be ready for commercial production in 3-5 years. It would take at least 15-20 years to breed new cultivars with low acrylamide characteristics similar to the silencing lines.

Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating potato disease worldwide. Control of late blight in the US relies almost exclusively on fungicide application. During an average epidemic year, the fungicide cost to control late blight is about $200-300 per acre in Wisconsin, which equates to a total fungicide cost in the range of $4-8 million per season in Wisconsin plus at least $4 million to cover the cost of application.  Due to constant genetic shifts of P. infestans populations and decreases in fungicide effectiveness, late blight can cause a complete loss of the crop. The most effective and environmentally friendly way to prevent widespread devastation by late blight is to incorporate host plant resistance. 

Potato breeders have incorporated many late blight resistance genes (R genes) from wild Solanum species into cultivated potato. Unfortunately, most of these R genes provided resistance against only specific races of P. infestans. Such race-specific resistance is often short-lived and can be rapidly overcome by new strains of P. infestans. Solanum bulbocastanum is a Mexican wild potato species that has co-existed in the same habitat as the late blight pathogen for centuries. S. bulbocastanum is not immune to late blight, but instead shows a marked delay in both the onset of symptoms and the development of lesions. Such rate-limiting resistance may put less selection pressure on the P. infestans populations and protect the durability of the resistance genes. In 2003, we isolated and cloned a resistance gene, RB, from S. bulbocastanum. Susceptible potato varieties were subsequently transformed with RB. They have shown a high level of resistance over a broad-spectrum of various P. infestans strains, with a marked delay in both the onset of symptoms and the development of lesions. Most remarkably, the transgenic lines have exhibited a high level of resistance in the Toluca Valley in Mexico, which is the most late blight infested region in the world. Among the numerous disease resistance genes cloned from various plant species, few have shown a resistance spectrum as broad as RB.  However, it is no surprise that new races of late blight are emerging that are able to overcome RB resistance.  Therefore, in order to keep up with the rapid evolution of the pathogen, it will be critical to be able to rely on a mechanism to rapidly deploy new R genes derived from wild potato species, pyramid multiple R genes together, or engineer new genes to recognize new pathogen strains.  The use of GM potatoes will prove to be an invaluable resource in our fight against this disease.

In the near future, we expect to create biotech potatoes with resistance to Verticillium wilt and potato virus Y. In several wild potato relatives, we have identified a gene that is analogous to the Ve gene that has been used as a source of Verticillium wilt resistance in tomato for several decades. Efforts are underway to create plants containing this gene and then test for resistance. We have not yet isolated a PVY resistance gene, but have the resources to do so and expect to be able to clone a gene from the wild species S. chacoense in the near future. That will lead the way to the development and testing of potato containing PVY resistance.