Anyone who has paid even a little attention to plants and trees in late winter and early spring knows how responsive they are to temperature. In years when the winters are warm, many trees and other plants bud early. But until now, the molecular mechanism that allows them to detect temperature has been unknown.

Scientists from the University of Cambridge have revealed what they call the “thermometer molecule” that enables plants to develop according to seasonal temperature changes. During the day, these molecules, called phytochromes, are used by plants to detect light, but at night they are instead used to gauge cellular temperature.

Their research was published last fall in the journal Science.

According to lead researcher Philip Wigge, sunlight activates the molecules during the day, binding them to DNA and slowing plant growth. The phytochromes are deactivated rapidly when plants become shaded, enabling them to grow faster to find sunlight again. Wigge said this is how plants compete to escape each other’s shade. Light-driven changes to phytochrome activity can occur in less than a second.

But at night, the molecules gradually become inactive in a process called dark reversion. “Just as mercury rises in a thermometer, the rate at which phytochromes revert to their inactive state during the night is a direct measure of temperature,” Wigge said. “The lower the temperature, the slower phytochromes revert to inactivity, so the molecules spend more time in their active, growth-suppressing state. This is why plants are slower to grow in winter.”

Conversely, the warmer the temperature, the faster the molecules change to stimulate plant growth.

“Warm temperatures accelerate dark reversion,” he added, “so that phytochromes rapidly reach an inactive state and detach themselves from DNA, allowing genes to be expressed and plant growth to resume.”

Not every plant species relies equally on phytochromes, however. Some, like ash trees, rely more on measuring day length to determine their seasonal timing, but oaks rely primarily on temperature, meaning they are more dependent on phytochromes to dictate their development.

The research was conducted on a mustard plant, but the scientists say the phytochrome genes are found in numerous other species as well. In fact, Wigge said, in light of the increasing unpredictability of weather and temperature caused by climate change, the discovery could help in the breeding of more resilient crops.

“It is estimated that agricultural yields will need to double by 2050, but climate change is a major threat to such targets,” he said. “Key crops, such as wheat and rice, are sensitive to high temperatures.

Thermal stress reduces crop yields by around 10 percent for every one-degree increase in temperature. Discovering the molecules that allow plants to sense temperature has the potential to accelerate the breeding of crops resilient to thermal stress and climate change.”


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