New discovery may help potatoes resist heat

Sophia Sonnewald, left, Guenter Lehretz, José María Corral García and professor Uwe Sonnewald study plant growth in potatoes. |   University of Erlangen-Nuremberg photo

German researchers learn how to turn off a small RNA that blocks the formation of tubers when temperatures rise

Potato plants don’t like heat. If it’s too hot, they form fewer tubers.

Now biochemists at the University of Erlangen-Nuremberg in Germany have discovered the reason. When the temperature rises, a small RNA blocks the formation of tubers.

Researchers have also discovered how to turn off this RNA so potatoes are more heat resistant.

“Not only potato, but all crops suffer from climate change in Europe,” said Uwe Sonnewald, chair of biochemistry at the university. “The last summer was dry and hot. In some cases, farmers didn’t even dig out the tubers. In the case of potato, heat is one important factor. Another is drought. That is why we are trying to combine approaches to achieve drought tolerance in combination with our heat approach. First results look promising.”

Canada has been a world leader in seed potato production for more than 90 years and is the fifth largest seed potato exporter in the world. About 150 potato varieties are registered in Canada.

Unlike most plants, potatoes can propagate sexually via seeds and clonally via tubers. Sexual propagation is usually preferred because seeds are easily dispersed and can expand the gene pool of the population.

Understanding how tubers grow in potatoes under changing environmental conditions is increasingly important as climate change advances. According to the research, the highest yields are gained at moderate temperatures, around 21 C during the day and 18 C at night. With the day length and the appropriate temperatures, a protein called self-pruning 6A (SP6A) forms, triggering tuber formation.

“Short days and low temperatures indicate the coming winter,” said Sonnewald. “In this situation, the plant stops investing in shoot biomass, will not initiate flowering, and tries to produce as many tubers as possible. We know that the SP6A protein, which is produced in leaves, is required for the initiation of tuberization.”

But Sonnewald said that the behaviour of SP6A is inhibited by elevated temperatures. To overcome that, they designed transgenic plants that would increase SP6A. The gene they used would produce a protein identical to the untransformed potato plants. However, the modified transgenic potatoes produced extremely small potato plants and started to form tubers almost instantly.

To create the growing conditions that may occur in future warmer years, the researchers studied growth at controlled temperatures. The laboratory temperature was set at 29 C to simulate day and 27 C for night. At those temperatures the plants grew shoots and leaves, but few or no tubers. The tubers that did form had less starch and germinated more quickly, meaning they were less nutritious and could rot quicker.

Based on observations, the researchers reasoned that some kind of regulation was responsible for the changes in SP6A that prevented it from promoting tuber growth. It was directly related to heat and the study led to the discovery of a particular RNA that, at low temperatures, is inactive. But once the temperature rises, the RNA blocks the formation of SP6A, thus preventing tubers from growing.

Researchers switched off the RNA and this allowed the formation of the SP6A protein and tuber growth.

“So far, plants have only been tested in the greenhouse,” said Sonnewald. “We have not observed any unwanted effects, but we have to do testing in the field.”

They have yet to get permission for field trials. Meanwhile, they are focusing not only on growth under heated conditions but growth during times of drought.

“We have started drought research and obtained plants which perform much better than the wild type under drought and heat conditions,” he said. “Heat also affects photosynthesis. Therefore, we have prepared a new construct, which should boost photosynthesis together with drought and heat tolerance. Our results offer us the means of still being able to grow potatoes in the future at increasing temperatures.”

Sonnewald said breeders are currently searching their genotype collections for genotypes in which the small RNA might not be expressed or expressed to a lower extent.

“Finding these genotypes would allow for the breeding of heat tolerant non-GM potatoes,” he said. “So far, the genotypes have not been found, but the search has just been started.”

Alternatively, it would be possible to alter expression of the RNA by genome editing. But according to Sonnewald this would be considered a GM product in the United States and elsewhere around the world but is not easily permitted in Europe.

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