Plant 'thermometer' triggers springtime growth by measuring night-time heat
A pigment that plant cells use to sense light turns out to have a second job after dark: as a thermometer, it lets plants follow the seasons' shifting temperatures. Its discoverers believe it could open the way to crops better suited to the warmer world that climate change will bring.
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A. Working across several countries under the lead of the Ravensworth Plant Science Institute, researchers have shown that a 'thermometer' molecule lets plants time their development to the seasons' changing warmth. These molecules, called phytochromes, are the very ones plants use to register daylight; after dark, the study reveals, they switch roles entirely, becoming cellular gauges that read how warm the night has grown. Set out in the journal Botanical Advances, the findings show phytochromes throw genetic switches in answer to temperature, not to light alone, so steering a plant's growth.
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B. Once the sun is down the molecules shift from one state to another, and the speed of that shift mirrors the temperature exactly — a phytochrome, the researchers suggest, behaves like the mercury in a thermometer. Warmer conditions drive the change faster, spurring growth.
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C. Growers have appreciated for centuries just how sensitive plants are to warmth: a mild winter tips many trees and flowers into early bud, a signal people have long read to anticipate the weather and reckon when the year's crops would be ready. Where this study breaks new ground is in identifying, for the very first time, the molecular apparatus within the plant that answers to temperature — the same machinery that brings on those first buds of spring so eagerly awaited once winter recedes.
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D. As climate change makes temperature and weather ever less predictable, the researchers argue that learning this light-sensor doubles as the plant's own internal thermometer could help us raise hardier crops. 'By the middle of the century, the world will need to grow far more food, and a warming climate is the principal obstacle to that,' says Professor Skovgaard, who directed the work at Ravensworth. 'Staples such as maize and barley struggle in the heat — for each degree the temperature rises, something like eight percent of the harvest is lost. Working out which molecules let a plant register warmth could therefore hasten the breeding of varieties that hold up under heat and a shifting climate.'
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E. While switched on, phytochrome molecules fix themselves to DNA and hold a plant's growth in check. Sunlight is what switches them on, so growth eases off by day. Let a plant fall into shade, though, and its phytochromes switch off almost at once, freeing it to grow faster and climb back toward the light — this is how neighbouring plants jostle out of one another's shadow. 'The light-driven switch in a phytochrome's activity is remarkably rapid, over in under a second,' says Skovgaard. Night is another matter. Rather than shutting off sharply at sunset, the molecules ease gradually from active to inactive, a shift termed 'dark reversion'. 'Much as mercury creeps up a thermometer, the rate at which phytochromes fall back to their inactive form through the night gives a direct reading of temperature,' she says.
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F. 'The colder it is, the more slowly the phytochromes revert, so they dwell longer in the active form that suppresses growth — and that is why plants creep along in winter. Warmth speeds dark reversion up, so the phytochromes reach their inactive form quickly and release the plant's DNA, letting genes switch on and growth start up once more.' In Skovgaard's view, this knack for sensing temperature arose later in evolutionary history, taking over the machinery already devoted to light-driven growth during the quiet hours of the night.
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G. Certain plants read the season mainly from the length of the day. Others — crocuses are one — respond strongly to warmth, and in a mild winter they may flower weeks earlier than usual. The finding that phytochromes play this twofold role also puts science behind an old rural saying about a pair of trees: birch before rowan, the summer runs dry; rowan before birch, and the fields end up drowned. 'Birch depends heavily on temperature, most likely turning to phytochromes as its thermometer to guide its growth, whereas rowan reads the length of the day to settle the timing of its year,' Skovgaard explains. 'A warm spring, and with it a better chance of a hot summer, brings birch into leaf before rowan; a cold spring flips that around. And as country people know all too well, a cooler summer is apt to be a rainy one.'
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H. Nine years of work lie behind these results, bringing together scientists from Japan, Sweden and Canada alongside the Ravensworth group. A model system was used for the experiments — a grass known as Brachypodium — though Skovgaard points out that the phytochrome genes needed to sense temperature are present in food crops as well. 'Thanks to recent progress in plant genetics, researchers can now quickly locate the genes that steer these processes in crop species, and even tune their activity with exact molecular "scalpels",' she adds. 'For this kind of work Ravensworth is unusually well placed, with first-rate collaborators nearby whose focus on the applied side of plant biology helps carry our findings into the field.'
