From growing smaller leaves to shape-shifting its insides, a desert flowering plant goes all in to flourish in the harshest of conditions.
Summer temperatures in Death Valley National Park frequently exceed 50° Celsius (122° Fahrenheit). During that peak heat, most desert plants hope simply to cling to life. Not the Arizona honeysweet (Tidestromia oblongifolia). It thrives by making cellular and genetic tweaks, notably changing the shape of a microscopic structure that converts light and carbon dioxide into energy, researchers report in the Nov. 7 Current Biology.
In 1972, researchers showed that T. oblongifolia best performs its vital work of photosynthesis at a roasting 47° C. That’s the highest known peak-performance temperature of any plant, says plant biologist Karine Prado of Michigan State University in East Lansing. “These plants wait [for] the hottest month just to grow fast.”
Until now, little was known about how or why the plant seemed to prefer sweltering heat. To investigate, Prado and her colleagues collected T. oblongifolia seeds from Furnace Creek in Death Valley National Park, Calif. In the lab, they grew them for eight weeks at 31° C, then turned up the heat on some of them to 47° C, a typical July temperature in Furnace Creek. In both sets of plants, they measured growth, photosynthesis rates and certain genetic and cellular characteristics.
Within two days, the plants in Death Valley summer conditions ratcheted up their photosynthesis rates. Within the next eight days, they grew to three times their original size.
Under the microscope, the researchers spotted a striking adaptation. In most plants, high heat damages the disc-shaped photosynthetic powerhouses known as chloroplasts. But at 47° C, chloroplasts in T. oblongifolia stayed intact. What’s more, the chloroplasts in a group of leaf cells that specialize in converting carbon dioxide to sugar assumed a new, cup shape.
Algae have cup-shaped chloroplasts. T. oblongifolia now appears unique among plants to at times be able to morph its disc-shaped chloroplasts into cups, says Prado. Why that shape helps T. oblongifolia beat the heat is unclear, but Prado suspects it may help this shrub trap carbon dioxide more efficiently.
The team saw other adaptations that are common plant responses to heat, including growing smaller leaves with smaller cells, turning on damage repair genes and fixing an essential photosynthesis enzyme.
The new work shows that heat-proofing a plant is not as simple as “tweaking one or two genes or proteins,” says plant biologist Ive De Smet of Vlaams Instituut voor Biotechnologie at Ghent University in Belgium, who wasn’t involved with the study. All of these many changes probably work together, he says, to keep photosynthesis going when it gets hot.
Due to rising global temperatures, the risk of heat-limited photosynthesis threatens crops that feed the world. How T. oblongifolia beats the heat, De Smet says, might even pave the way for targeted genetic engineering and breeding strategies to future-proof crop plants.
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