Keeping Spuds Safe--And Humans Too

Fortunately, potatoes are never quite as toxic as the alien carrots
in the Looney Tunes "Invasion of the Bunny Snatchers" episode
A team of Japanese scientists has published research that may help both protect potatoes from serious diseases and safeguard humans from poisonous spuds. The researchers, led by Dr. Kazui Saito, were able to identify a gene critical for making the toxic alkaloid chemicals that potatoes produce to protect themselves from pests. Although commonly-eaten varieties contain safe, low levels of these alkaloids, they are also more susceptible to certain major diseases. To combat infections, breeders want to crossbreed these safe spuds with disease-resistant—but poisonous—wild potato species without increasing the levels of toxic alkaloids in the potatoes we eat. Dr. Saito’s group has discovered a way to largely disable the production of these chemicals, opening up safer avenues to breed strong, resistant potatoes that do not make people sick.

Although normally safe, potatoes are serious contenders for the most toxic vegetable in the American diet. Potatoes, tomatoes, and eggplants are all members of the nightshade family, which produces a group of chemicals called steroidal glycoalkaloids to defend against pests. In small amounts, these chemicals may cause an upset stomach, but extremely high doses can lead to dizziness, hallucinations, and even death.

Human domestication long ago selected for potatoes with low levels of these alkaloids. But domestication also produces crops that cannot defend themselves as well against diseases—in particular, our efforts to make potatoes safer, larger, and tastier have impaired the spud’s ability to protect itself against late blight disease, the most damaging potato infection. Late blight led to starvation in Ireland in the 1840s, and today accounts for billions of dollars in lost productivity worldwide.

Crop breeders routinely scout out hearty wild relatives of our foods, seeking to breed in traits like disease resistance. For potatoes, scientists must ensure that borrowing beneficial traits from wild varieties does not increase the levels of steroidal glycoalkaloids above a safe threshold. One way to limit this risk is to reduce the production of these alkaloids in potatoes before breeding programs even start.

So Dr. Saito’s group set out to understand how potatoes make these chemicals in order to control and limit their production. Steroidal glycoalkaloids primarily consist of a steroid backbone, which is made from cholesterol. As a result, the scientists searched for genes in the potato genome that resembled a human gene that helps synthesize cholesterol. Although humans, peas, and rice have only one copy of the gene, potatoes have two—SSR1 and SSR2.

Having two similar genes is often a sign that the two copies have evolved to specialize. While most plants use a single gene to make cholesterol and other important chemicals like hormones, Dr. Saito and his colleagues reasoned that potatoes might have divided those two tasks between the two SSR genes.

To test this, they put the genes into yeast that made the chemical precursors of either cholesterol or plant hormones and measured what chemicals each SSR gene produced. They found that while SSR1 efficiently produced plant hormones, SSR2 excelled at making cholesterol. This specialization means that disabling SSR2 would shut down cholesterol and steroidal glycoalkaloid production without affecting SSR1’s synthesis of important hormones.

Scientists can add snippets of a plant’s own gene to activate a natural viral defense mechanism—a kind of plant immune system—and impair the native gene’s function. When the Japanese researchers did this with SSR2, alkaloid levels plummeted to a tenth their normal level, while the plants themselves grew just fine, a sign that hormones still functioned properly.

Another technology, called genome editing, can produce permanent errors in a specific gene, turning it off completely. The researchers added an editing protein that disrupted SSR2 and found that the alkaloid levels again dropped to a fraction of their normal amount. The editing protein can be removed in the next generation. This leaves only the precise changes dialed in by the scientists and 100 percent potato DNA, unlike most crop genetic modifications that add DNA from other species.  

The ability to produce specific new changes with the potato’s own DNA may reduce widespread concerns about genetically modified crops, which, although shown to be safe, are rejected by a large number of consumers.  This would be good news for scientists looking for new tools to improve potatoes and other foods.  Late blight and other diseases are ongoing scourges and the expanded toolbox for safely combating them provided by Dr. Saito’s group may help keep the world’s fourth-largest crop on a level playing field with these infections while keeping spuds safe.

[This news story served as part of my application to the AAAS Mass Media Fellowship]