by Allison Baker
figures by Lillian Horin
The Arctic apple is the juiciest newcomer to produce aisles. It has the special ability to resist browning after being cut (Figure 1), which protects its flavor and nutritional value. Browning also contributes to food waste by causing unappealing bruising on perfectly edible apples. Food waste, especially for fruits and vegetables, is a major problem worldwide; nearly half of the produce that’s grown in the United States is thrown away, and the UK supermarket Tesco estimates that consumer behavior significantly contributes to the 40% of its apples that are wasted. Therefore, Arctic apples not only make convenient snacks, but they also might be able to mitigate a major source of food waste.
While a non-browning apple sounds great, how exactly was this achieved? Arctic apples are genetically engineered (GE) to prevent browning. This means that the genetic material that dictates how the apple tree grows and develops was altered using biotechnology tools. But before learning about the modern science used to make Arctic apples, let’s explore how traditional apple varieties are grown.
How are new apple varieties developed?
Harvesting tasty apples is more complicated than simply planting a seed in the ground and waiting for a tree to grow. In particular, it’s difficult to predict what an apple grown from a seed will look and taste like because each seed contains a combination of genetic material from its parents. But farmers can reliably grow orchards of tasty apples by using an ancient technique called grafting. After a tree that produces a desirable apple is chosen, cuttings of that original tree are grafted, or fused, onto the already-established roots of a donor tree, called rootstock. The cuttings then grow into a full-sized tree that contains the exact same genetic material as the original tree. As a result, each tree of a specific apple variety is a cloned descendant of the original tree, and thus produce very similar apples.
New apple varieties emerge when genetic changes are allowed to occur. Traditionally, new apples are produced by cross-breeding existing apple varieties. This reshuffles the genetic makeup of seeds, which are then planted to see if they grow into trees that produce delicious new apples. On the other hand, Arctic apples are created by making a targeted change to the genetic material of an existing variety (more on this later). The advantage of using genetic engineering over traditional breeding methods is that scientists can efficiently make precise improvements to already-beloved apple varieties—in contrast, traditional cross-breeding is much more random and difficult to control.
What causes apple browning?
Insight into the molecular causes of apple browning guided the genetic alteration that made Arctic apples. Apples naturally contain chemicals known as polyphenols that can react with oxygen in the air to cause browning. This reaction won’t occur, however, without the help of polyphenol oxidase (PPO) enzymes, which bring polyphenols and oxygen together in just the right way. PPO enzymes and polyphenols are normally separated into different compartments in apple cells, which is why the inside of a fresh apple is white or slightly yellow-green in color. But these structures are broken when the fruit is cut or crushed, allowing PPOs to interact with polyphenols and oxygen to drive the browning reaction (Figure 2). This process occurs in all apples, but some varieties are less susceptible than others due to factors like lower amounts of PPOs or polyphenols. Common household tricks can also delay browning by a few hours by interfering with the PPO reaction, but no method prevents it completely or indefinitely. Knowing that PPOs were responsible for browning, researchers thought about blocking the production of these enzymes with genetic tools to create non-browning apples.
How are Arctic apples made?
Genetic material is stored in our DNA and divided into functional units called genes. The genes are “read” by copying the DNA sequence into a related molecule called RNA. The RNA copy functions as a blueprint that instructs the cell how to build the product for that gene, which is called a protein. The production of PPO enzymes, therefore, can be blocked by simply removing their RNA blueprints. To do so, researchers used a tool from molecular biology called RNA interference (RNAi). RNAi is a natural biological process that recognizes and destroys specific RNA structures. Biologists can use RNAi to lower PPO levels by introducing RNA sequences that cause the degradation of PPO RNA. Using this technique, researchers developed an anti-PPO gene that makes anti-PPO RNA, which destroys the PPO RNA before it can be used to make PPO enzymes.
Once scientists created the anti-PPO gene, they needed to safely introduce it into the apple genome. To make a variety called the Arctic Golden, researchers began with Golden Delicious apple buds and inserted an engineered piece of genetic material called a transgene that contained the anti-PPO gene. After confirming that the plant received the transgene, the saplings were then allowed to grow into mature trees, one of which produced the apple that is now known as the Arctic Golden.
Are Arctic apples safe?
After over a decade of research, regulatory agencies in the United States and Canada like the FDA and USDA recently approved Arctic apples for human consumption. Accumulated evidence shows that Arctic apple trees and fruit are no different from their traditional counterparts in terms of agricultural and nutritional characteristics. On the molecular level, the transgene genetic material present in Arctic apples is quickly degraded by your digestive system to the point where it’s indistinguishable from that found in traditional apples. The only new protein in Arctic apple trees—a protein called NPTII that’s used to confirm that the genetic engineering was successful—was not only undetectable in their apples, but it has also been evaluated and deemed nontoxic and non-allergenic by the FDA.
Yet some anti-GMO groups continue to protest the approval of Arctic apples, arguing that unforeseen consequences of the genetic alteration could impact safety. It’s true that it’s impossible to predict and disprove every possible consequence of a genetic change. But a recent review by the National Academies of Science that covers decades of published research found no convincing evidence that GE crops have negatively impacted human health or the environment. While it’s important to rigorously test all new crops that are developed, GE crops should not be considered inherently more dangerous than their traditionally-bred relatives.
So what’s next for the Arctic apple? It takes several years for new apple trees to grow and literally bear fruit, so it’ll take time for non-browning apples to expand to supermarkets throughout the US. Currently, Arctic Goldens are only available in bags of pre-sliced apples in select US cities, but Arctic versions of Granny Smith and Fuji apples have received USDA approval, and Arctic Galas are in development. If commercially successful, non-browning apples could help to combat rampant food waste one slice at a time.
Allison Baker is a second-year Ph.D. student in Biological and Biomedical Sciences at Harvard University.
For more information:
- A cool time-lapse video that compares Arctic apples to conventional apples
- This article on the history and economics of Okanagan
- Multimedia resources that provide more insight into the science behind RNAi
Cover image credit: Okanagan Specialty Fruits Inc.