As the planet’s thermostat ticks steadily upwards, life forms around the world may be in grave danger—and plants like rice are no exception. Already, global rice yields have taken a hit from rising temperatures. If the upward creep in heat continues, declines in this starchy staple could imperil the 3.5 billion people around the world who rely on rice for over 20% of their daily caloric intake.
Today, in the journal PNAS, researchers report that rice plants are sensitive to the damaging effects of high heat even in the short term. Brief exposures to sweltering temperatures unraveled plant cells’ messenger RNA—a critical molecular envoy that ferries the instructions contained within DNA to the cell’s protein-producing factories—cuing the messages up for destruction. Though the rice eventually recovered, the new finding underscores just how sensitive plants may be to the hazards of a fast-warming world.
With each degree-Celsius increase in average global temperature, rice crop yields fall by about 3%. And as the planet warms and the human population expands, the gap between supply and demand for this staple crop continues to widen. But while these trends are clear at a macroscopic level, the inner workings of plant cells coping with toasty temperatures remain far less clear.
To better understand heat tolerance in crops, a team of scientists led by Zhao Su, a plant biologist at Pennsylvania State University, heated cuttings of young rice shoots (Oryza sativa) to a temperature of 42º Celsius (nearly 108º Fahrenheit) for 10 minutes. Though temporary, the mini heat wave constituted a rapid 20º-Celsius shift upwards from room temperature—an escalation that’s “more than a human could ever withstand,” explains study author Philip Bevilacqua, an RNA chemist also at Pennsylvania State University.
When the researchers analyzed the shoots’ cellular contents, they found that the temporary trauma had compromised the rice’s ability to relay genetic information through their messenger RNA, or mRNA.
Plants, like humans, store their genetic information in the form of DNA. Contained within this molecular manuscript are all the necessary instructions for growth and survival—including all the cell’s contingency plans for emergencies or unanticipated changes in the surrounding environments. The DNA manual is extremely comprehensive, so cells access excerpts from it on an as-needed basis, making short-lived photocopies of the original text in the form of mRNA. Without mRNA, cells can’t receive the intel necessary to manufacture the building blocks necessary for life.
But for such a critical piece of the cellular puzzle, mRNA is frustratingly fickle. Unlike DNA, which is stored in two strands that lock into each other like the halves of a zipper, the synthesis of mRNA creates an unwieldy single-stranded molecule. These open-faced strands can loop around and bind to themselves in a semblance of secure structure, but this makeshift jumble is inherently far less durable than that of double-stranded DNA.
Because of mRNA’s infamous instability, the scientists reasoned it might be particularly susceptible to suffering at high temperatures. When they dosed the rice cells with a chemical marker that tags single-stranded mRNA molecules, Su and his colleagues discovered that heated plants contained higher proportions of unfurled mRNA compared to shoots lounging at room temperature. The head and tail ends of many mRNA strands appeared particularly fragile, accruing more of the incriminating chemical tags. In the heat of the moment, it seemed, these cellular messengers were melting.
When the researchers probed the plants further, they discovered that several of the mRNAs slumping into single strands were being chewed up by a destructive machine called the exosome. The looser the mRNA, the more likely it was to be ghosted out of existence—implying that mRNA unfolding made the messages more prone to annihilation. As the rice shoots attempted to weather the warmth, their exosomes were disposing of cells’ hottest messes, along with the genetic information they carried.
Luckily, the short spikes in temperature didn’t appear to put any of the rice plants permanently out of commission. Because the cells’ DNA remained intact, new mRNAs were eventually copied and redistributed as the rice recovered from the sizzling onslaught. But it took about 10 hours for the shoots’ mRNAs to fully reset—quite the price to pay for a 10-minute hot flash.
Over a thousand mRNA transcripts, all carrying instructions from different genes, became more or less abundant after the rise in temperature. Some of them even appeared to code for proteins that control how certain stretches of DNA are photocopied into mRNA—which means the effects of heat stress could be even more widespread than they already appear.
Until the researchers learn more, it’s unclear what exactly these molecular fluxes signify, explains Ru Zhang, a plant biologist at the Donald Danforth Plant Science Center who was not involved in the study. They could be adaptive, allowing the rice to acclimate to the warmth. Or they could be indicative of just how vulnerable plants’ internal infrastructure is to temperature changes of this magnitude. Either way, resources were likely being diverted away from normal growth.
“The paper is an elegant piece of science,” says Kenneth Cassman, an expert in food security and agriculture at the University of Nebraska, Lincoln who did not participate in the new research. “It’s a significant advance in helping us understand how short term exposure to heat affects rice… and how a plant grows and yields.”
However, 10 minutes under high heat in a laboratory environment isn’t the same as chronic exposure to, say, the gradual onset of global warming. According to Cassman, there may not be a ton of overlap in the processes that govern plants’ responses to long-term changes in temperature and to short exposures like the one in this study—but there are still several key instances in which the new findings may translate directly.
For one, the changes associated with climate change affect both averages and extremes in temperature, so daily spikes in heat could involve a 108º-Fahrenheit stint in the sun. Additionally, crops like rice and maize that are grown in high densities often experience temporary spikes in sun exposure when winds disrupt the foliage, says study author Sarah Assmann, a plant biologist at Pennsylvania State University. Given the vast changes that occurred in the rice plants after just a few minutes of heat, even the ephemeral passing of a cloud overhead could disrupt some of these cells’ internal functions.
Additionally, the researchers’ findings will likely translate over to other vital crops, Cassman says. Ironically, he points out, because most (but not all) rice is grown in standing water, it’s actually one of the few crops that are a bit buffered against rising temperatures. But much of the plant, including its shoots, is warmed by direct sunlight, and rice yields have still been curbed by global warming. This could prove even more ominous for the vast majority of cultivated plants that don't enjoy such liquid luxury, and remain especially vulnerable to surges of heat on dry land.
Moving forward, Su, Bevilacqua, and Assmann hope to repeat their work in other strains of rice—including cultivars that have proven hardy in climes that tend to pack the heat. Comparing the responses of these different varieties to changes in temperature, along with the current findings, may help researchers target certain regions of plant genomes for manipulation in the future, Assmann explains, informing the production of more heat- and drought-resistant crops.
One possible intervention may already be apparent. It’s the sloppiest mRNAs (that is, those with particularly frayed heads and tails) that are especially susceptible to destruction, Bevilacqua explains. So genetic modifications that secure these ends—the molecular equivalent of clipping an aglet onto the end of a frazzled shoelace—might just increase their longevity.
Even with exquisite knowledge of the coping mechanisms of plants in peril, however, there’s no guarantee a magic bullet will unveil itself, Cassman says. Plants have spent millennia adapting to heat, and researchers may not get much bang for their buck by trying to optimize their resilience further. If that’s the case, the ceiling on genetically engineering heat-resistant plants could be far lower than expected.
But tinkering with the genes of crops is far from the only option, Bevilacqua says. “You can only go so far [with genetic engineering],” he adds. “Yes, it’s possible for us to intervene and help with plant production and response to these stresses as they increase. The bigger answer is to have policies to address and try to reverse climate change.”