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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