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The fortunes of plants revolve around light and heat. Extremes of temperature can kill many species off quickly, but even small changes can have a huge impact, spurring rapid growth or putting the plants to sleep. But exactly how plants detect and respond to different combinations of light and heat has remained a mystery. Teasing out these complex relationships is a growing concern as the planet warms.
Now, one group of researchers has discovered that sugar is a hidden thermostat in plants. The finding challenges earlier evidence that suggested plants rely exclusively on two specific light-sensitive proteins to determine when temperatures are rising and falling. In fact, the plants rely on multiple inputs throughout the day and night to sense changes in sunlight and climate.
Meng Chen, a cell biologist at the University of California, Riverside, has studied the way plants respond to light and temperature for two decades. Chen has focused primarily on the two light-sensitive proteins: phyB, which accelerates growth, and ELF-3, which generally acts as a brake on growth. But these proteins were thought to detect high temperatures only at night, whereas plants mostly experience extremes of heat during the daytime.
To understand better what happens in daytime conditions, Chen decided to experiment on Arabidopsis, a small plant in the mustard family which is commonly used in lab experiments. He and his team exposed normal Arabidopsis plants to different temperatures, from 54 to 81 degrees Fahrenheit, and different intensities of red light, and tracked how their stems elongated. When it’s hot, plants grow longer stems to help keep their leaves cooler. Contrary to their expectations, the researchers found that phyB can control growth both at night and under moderately bright light conditions akin to those the plant would experience during the daytime. But phyB stopped working when light conditions became intense.
“To be able to respond to temperature, you need sugar.”
Chen and his colleagues also used mutant plants that had dysfunctional phyB, and lacked chloroplasts, the organelles responsible for photosynthesis. These plants only grew in bright light conditions. They could not sense higher temperatures in the dark. The scientists wondered if sugar might make a difference. During the day, plants store energy in starch inside chloroplasts. At night, plants generally break down this starch into sugars to fuel their metabolism.
To test how sugar would impact plant growth, the researchers added liquid glucose to the growth media of their mutant plants. This tweak allowed the plants to sense higher temperatures in the dark as well as the light. “Somehow, to be able to respond to temperature, you need sugar,” Chen says. “It was a huge surprise,” to find the involvement of sugar in the temperature sensing. The findings could help scientists puzzle out how to grow plants that can withstand temperature extremes—both hot and cold, Chen says.
A new picture is emerging, Chen says: one that shows multiple overlapping systems at play at different times of day and at different temperatures. While phyB generally stops working at high light intensities, ELF3’s brake on growth is released at high temperatures, which also trigger the release of sucrose from chloroplasts, day or night. The three systems work in concert. The research was published in Nature Communications.
Understanding how plants respond to temperature is critical to our ability to continue to feed the world as temperatures rise. “We’re not talking about the whole planet turning into a burning oven,” Chen says. “We’re talking about the larger shift in climate zones because of moderate increase in temperature. Even a 7 or 8-degree change can actually induce a very dramatic change in plant morphology and also flowering time.”
Philip Wigge, a plant adaptation researcher at Leibniz Institute of Vegetable and Ornamental Crops near Berlin, Germany, says the study has done an elegant job investigating how warm daytime temperatures influence plant growth pathways.
“Plants are highly responsive to warm temperature, but the underlying mechanisms by which temperature is sensed and integrated into growth and development are not fully understood,” he says. “It has been known for a few years that light and temperature interact, but how this happens during the day has not been clear.” Wigge, who was not involved in the study, says it will be interesting to see if these pathways are also observed in crop plants such as wheat and rice.
Chen plans to continue the work of understanding plant sensing. He says the next steps involve identifying the sensors in chloroplasts that trigger sugar release and control temperature response. Eventually, he would like to engineer the internal temperature-sensing system itself. He has hope that a more intimate understanding of how plants manage the dance between light and heat will ensure they can flourish in the climates of the future. 
Lead image: shablovskyistock / Shutterstock
