Friday, May 12, 2017

A secret sensor

            Anyone who has paid even a little attention to plants and trees in late winter and early spring know how responsive they are to temperature. In years when the winters are warm, many trees and flowers bud early. But until now, the molecular mechanism that allows them to detect temperature has been unknown.
            A team of scientists from the University of Cambridge in England has revealed what they call the “thermometer molecule” that enables plants to develop according to seasonal temperature changes. During the day, the plants use the molecules, called phytochromes, to detect light, but they change their function in darkness to become a cellular temperature gauge to measure heat at night.
            Their research was published last fall in the journal Science.
            According to lead researcher Philip Wigge, who compares phytochromes to mercury in a thermometer, the pace at which the molecules change is directly proportional to temperature – the warmer the temperature, the faster the molecules change to stimulate plant growth.
            Sunlight activates the molecules during the day, binding themselves to DNA to slow plant growth. The phytochromes are rapidly inactivated 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.”
“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 their phytochromes, however. Some, like ash trees, rely more on measuring day length to determine their seasonal timing, whereas oaks rely primarily on temperature, meaning they use their phytochromes to dictate their development.
            The research was conducted on a mustard plant, but the scientists say the phytochrome genes are found in crop plants as well.  In fact, Wigge said, in light of the increasingly unpredictable weather and temperatures due to 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.”

This article first appeared in Northern Woodlands magazine on May 10, 2017.

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