Using CRISPR Technology to Modify Starch in Potatoes

Potatoes are the world's largest vegetable crop and the third most important human food crop, second only to rice and wheat in global production. Potatoes are grown on 40.8 million acres in more than 160 countries and are a staple food for more than a billion people. A medium-sized potato provides about 160 calories, mostly from starch. Potatoes also provide other essential nutrients, including vitamins and minerals. Potato is a cool-season crop that is relatively sensitive to high temperature and drought stress. The crop also suffers from pests, as well as some viral diseases such as late blight. Late blight was the cause of the Irish potato famine.

The starch content in potato tubers is a major factor in determining potato use. High-starch potatoes are often used in processed foods such as fries, chips, and dehydrated potatoes. Potatoes with low to medium starch content are often used for fresh or table stock. For the fresh market, tuber appearance is another important consideration, including skin texture, color, flesh color and tuber shape. Different shapes of specialty potato varieties have emerged in recent years, and red, purple or yellow skin and flesh colors are becoming popular because of their ease of cooking and added nutritional value.

Potato tuber shape is clearly less important for industrial use than it is for human consumption. Potato tubers with deformed appearance due to heat or drought stress or other factors can be redirected to a variety of uses, including dog and cattle feed. In addition, potato starch can be used to produce ethanol as fuel or beverages such as vodka, a degradable substitute for plastics, or as adhesives, adhesives, texturizers and fillers in industries such as pharmaceuticals, textiles, wood and paper.

In industrial applications, the amount and type of starch in potatoes are important considerations. Potatoes high in amylopectin have applications in the production of bioplastics, food additives, adhesives and alcohol. Starches with higher amylopectin content are suitable for processed foods and other industrial applications due to their unique functional properties. For example, such starches are the preferred form for use as stabilizers and thickeners in food products and as emulsifiers in salad dressings. Amylopectin is commonly used in frozen foods due to its freeze-thaw stability. In addition, potatoes rich in amylopectin produced higher ethanol levels compared to other starches.

In contrast, potatoes that are high in amylose and low in amylopectin are more suitable for human consumption. Amylose acts like fiber and does not release glucose as easily as amylopectin, resulting in a lower glycemic index and making potatoes more acceptable to diabetics.

Two recent articles published in Molecular Science and Plant Cell, Tissue and Organ Culture outline how CRISPR technology can facilitate the use of potatoes, the world's most widespread vegetable crop.

Dr. Stephany Toinga, author of both papers, is a graduate student in Dr. Keerti Rathore's lab in the Institute of Plant Genomics and Biotechnology and the Department of Soil and Crop Sciences at Texas A&M University. Co-author of the two papers, Dr. Isabel Vales, is an AgroLife Research potato breeder in Texas A&M University's Department of Horticultural Sciences. Toinga is now a postdoc in Agricultural Life Research at Texas A&M University, working with Vales.

CRISPR/Cas9 technology expands the toolset available to breeders and represents a more direct and faster way to incorporate desired traits into popular commercial crop varieties. Traditional breeding is a long process that can take 10-15 years. Due to the complexity of the potato genome, generating correctly complementary new varieties with desirable traits is a challenge to traditional breeding. Molecular breeding has increased breeding efficiency, and gene editing using CRISPR/Cas9 technology has added another level of complexity.

The development of potato varieties with improved starches could open up new opportunities. In Toinga's first study, a potato line (tetraploid) containing four copies of the green fluorescent protein GFP was edited using the CRISPR/Cas9 system. GFP is a jellyfish gene that can display gene activity by fluorescence. Essentially, this provides an easy-to-see feature that allows researchers to optimize the method. "Sequencing results that lost both green fluorescence and GFP genes showed that CRISPR treatment can disrupt all four copies of the GFP gene in tetraploid potatoes, thus confirming that CRISPR technology should be able to introduce the four native alleles of tetraploid potatoes into Variation," Rathore said. "We used the Agrobacterium method to deliver CRISPR reagents into potatoes because it is reliable, efficient and cheapest compared to all other delivery methods. The information and knowledge we gain from these two studies will help us introducing other desirable traits in this very important crop."

Toinga's second knockout study, published in Molecular Sciences, targeted the native gene gbss in a tetraploid Yukon Gold line, effectively eliminating amylose, resulting in a branched chain potatoes high in starch and low in amylose. This gene-edited potato variety, Yukon Gold, has industrial uses beyond traditional uses.

"One of the knockout strains, T2-7, showed normal growth and yield characteristics, but was completely devoid of amylose," Toinga said. Potato starch of this strain T2-7 can be used as a binder/gluing agent in the paper and textile industries and find industrial applications in the chemical, bioplastic and ethanol industries. Tuber starch from this experimental strain, which does not require chemical modification due to its freeze-thaw stability, should also aid in the production of frozen foods. Potatoes with amylopectin as the only starch form can also produce more ethanol for industrial use or to make alcoholic beverages.

As a next step in these studies, T2-7 lines have been self-pollinated and crossed with Yukon Gold line donors and other potato clones to eliminate transgenic components.