Scientists Use CRISPR Technology to Modify Starches in Potatoes

Humble potatoes are a rich source of starches for a variety of industrial applications, as well as dietary carbohydrates for humans. Scientists at Texas A&M AgriLife Research are learning how to change the ratio of two starch molecules found in potatoes, amylose and amylopectin, to increase culinary and industrial applications.

Waxy potatoes, for example, have applications in the production of bioplastics, food additives, adhesives, and alcohol due to their high amylopectin content.

CRISPR technology can advance the uses of the world's largest vegetable crop, according to two articles recently published in the International Journal of Molecular Sciences and Plant Cell, Tissue, and Organ Culture.

Stephany Toinga, Ph.D., a graduate student in Keerti Rathore, Ph.D., AgriLife Research plant biotechnologist in the Texas A&M Institute for Plant Genomics and Biotechnology and Department of Soil and Crop Sciences, contributed to both papers. Isabel Vales, Ph.D., an AgriLife Research potato breeder in the Texas A&M Department of Horticultural Sciences, was also a co-author of both papers. Toinga is now a postdoctoral associate with Vales at Texas A&M AgriLife Research.

"We will be able to introduce other desirable traits in this very important crop using the information and knowledge we gained from these two studies," Rathore said. With a medium-sized potato providing about 160 calories, mostly from starch, the tubers are an important source of energy for many people around the world, according to Rathore. Other essential nutrients, such as vitamins and minerals, are also found in potatoes.

Colorado beetle, aphids, and nematodes attack the crop, as well as diseases like early and late blight, zebra chip, Fusarium dry rot, and a variety of viral diseases. The Irish potato famine was caused by late blight.

The amount of starch in potato tubers is the most important factor in determining the use of potatoes. According to Vales, high-starch potatoes are commonly used to make processed foods like french fries, chips, and dehydrated potatoes.

Potatoes with a low to medium starch content are commonly used in the fresh or table stock market, according to her. Tuber appearance, including skin texture, skin colour, flesh colour, and tuber shape, are also important considerations for the fresh market. Because of their convenience in cooking and increased nutritional value, specialty potato types with different shapes, such as fingerlings; smaller sizes; and red, purple, or yellow skin and flesh colours have recently become popular.

According to Vales, the shape of a potato tuber is less important for industrial purposes than it is for human consumption. External deformities in potato tubers caused by heat, drought, or other factors can be redirected to a variety of uses, including dog and cattle food. Potato starch can also be used to make ethanol for fuel or in drinks like vodka; a biodegradable plastic substitute; or adhesives, binders, texture agents, and fillers for the pharmaceutical, textile, wood, and paper industries, among other industries.

Because of their unique functional properties, starches high in amylopectin are desirable for processed food and other industrial applications, according to Toinga. Such starches, for example, are the preferred form for use as a food stabiliser and thickener, as well as an emulsifier in salad dressings. Amylopectin starch is used in frozen foods because of its freeze-thaw stability. Furthermore, potatoes high in amylopectin starch produce more ethanol than potatoes high in other starches.

Toinga believes that developing potato cultivars with modified starch could provide new opportunities. Beyond traditional uses, potatoes with high amylopectin and low amylose, such as the gene-edited Yukon Gold strain she described in the International Journal of Molecular Sciences, have industrial applications.

Potatoes with high amylose levels and low amylopectin, on the other hand, would be desirable for human consumption, according to Vales. Amylose acts like fibre and does not readily release glucose like amylopectin, resulting in a lower glycemic index and making potatoes more acceptable to diabetics.

According to Vales, CRISPR/Cas9 technology has broadened the toolkit available to breeders and represents a more direct and faster way to incorporate desired traits into popular commercial crop varieties. Traditional breeding is a time-consuming process that can take up to ten years.

Furthermore, she stated that conventional breeding is difficult to produce new cultivars with the right complement of desirable traits due to the complex nature of the potato genome. Molecular breeding has improved breeding efficiency, and gene editing with CRISPR/Cas9 technology has raised the bar even higher.

"We used the Agrobacterium method to deliver CRISPR reagents into potatoes because it is the most reliable, efficient, and cost-effective of all the delivery methods," Rathore explained.

According to Toinga, a potato line containing four copies of gfp, a jellyfish gene that allows fluorescence-based visualisation of the gene's activity, was targeted for mutation using the CRISPR/Cas9 system in the first study, which was highlighted in the Plant Cell, Tissue, and Organ Culture article.

In essence, this project provided a visible trait that researchers could use to improve their methodology.

"The loss of the characteristic green fluorescence and sequencing of the gfp gene after CRISPR treatment indicated that all four copies of the gfp gene can be disrupted, confirming that it should be possible to mutate all four alleles of a native gene in the tetraploid potato," Rathore said.

The Yukon Gold strain regenerated the best among the various potato cultivars evaluated in the first study, so it was used in the second study. The native gene gbss in the tetraploid Yukon Gold strain was targeted in the second knockout study, which was published in the International Journal of Molecular Sciences. The result was a potato with a high amylopectin content and a low amylose content.

Toinga explained that one of the knockout events, T2-7, had normal growth and yield characteristics but was completely devoid of amylose.

T2-7 tuber starch has potential industrial applications in the paper and textile industries, as well as in adhesives/binders, bioplastics, and ethanol. Because of its freeze-thaw stability without the need for chemical modifications, tuber starch from this experimental strain should be useful in the production of frozen foods.

Potatoes with amylopectin as the only form of starch should produce more ethanol, which could be used in industry or to make alcoholic beverages. The T2-7 strain has been self-pollinated and crossed with the Yukon Gold strain donor and other potato clones to eliminate the transgenic elements as the next step in these studies.