Plants live under a constantly fluctuating world of light. Clouds come by. Shadows move. Light flickers. Now researchers at the University of North Carolina at Chapel Hill have discovered, for the first time, that plants can detect shadows and have identified how they do it, revealing a never-before-understood mechanism for how plants maximize the efficiency of capturing light and by extension, agricultural yield – an increasingly important topic as estimates show that by 2050, the world will have 9.7 billion people to feed.
The work, led by Alan Jones, Kenan Distinguished Professor of Biology, also suggests a level of plant intelligence that goes well beyond simply detecting light from dark. Plants can distinguish variations of light throughout the day, minute-by-minute, via a complex communication system that measures intervals of light, intensity and timing to regulate how efficiently plants undergo photosynthesis: the process of capturing photons from the sun and converting them to sugar.
“When shadows form, plants need to show restraint,” said Jones, whose work appears in the Journal of Theoretical Biology. “If they increase the efficiency of photosynthesis and photons pour in right away, then boom, when the sun comes out from the shadows they’ll burn up. So plants need to wait and make an interpretation. Is this the end of the day or should I ignore it.”
In their work, Jones and colleagues at Michigan State University show that a protein called RGS1 detects changes in light and measures changes in glucose to control how efficiently photosynthesis works. Amazingly, RGS1 can tell the difference between the transient darkness caused by a shadow and the more long-term darkness that accompanies dusk.
To understand the role of RGS1, Jones and his team placed plants in a special chamber to mimic natural changes of light throughout the day. Plants engineered to lack RGS1 were perfectly able to adjust their photosynthesis when the light mimicked, for example, the open prairie, where the sun rises and sets with no transient changes in light. They stayed healthy and thrived.
“At first, we were surprised by that finding,” said Jones. “But it all started to make sense when we introduced periods of darkness, mimicking cloud cover or flickering light. Then the plants went crazy.” The plants could not adjust in response to transient shadows or flickering light, overreacting to brief changes in light intensity and eventually burned up.
The stimulator also revealed exactly what plants consider a shadow. The chamber had to go dark for four minutes before the plants realized there’s a shadow. If it went dark for less than four minutes, the plants would ignore the change in light.
Jones and colleagues used mathematical modeling to reveal that plants detect the rate at which different molecular components decay in plant leaves and this decay signals whether a change in light is a shadow or not. RGS1 has the ability to count the number of these molecules and decide how efficiently to take in photons and undergo photosynthesis.
Almost all plants have RGS1, but grasses, based on genomes that have been sequenced, lack it.
“Think about grasses of the U.S. prairie,” said Jones. “They don’t live in canopies. They live in the open where there are no shadows. The pampas grass of Patagonia; there are no trees growing in Patagonia. Rice lacks it. Rice grows in water. So the plants that lack the shadow detector are plants that have evolved to grow only in bright light without shadow.”