How Do Plants Know It’s Getting Hot?

In the fantastical world created by J.R.R. Tolkien, there exist giant plant-like creatures called Ents. Though they look like regular trees, they can walk and move at will. Ents can distance themselves from danger and advance toward favorable conditions—if only plants in the real world could do the same. Rooted to the same spot their entire lives, plants need adaptations that help them respond to changes in environmental variables like humidity, light, and temperature. Plants ‘feel’ these parameters to determine the time of the day, change in seasons, and the presence of potential stressors. In fact, a moderate increase in the ambient temperature can dramatically alter plant growth, metabolism, and immunity.¹ Indeed, sensing changes in temperature is essential for plant survival. But how do they do it?

Over the past decade, scientists have identified a few temperature sensors that regulate plant growth, some of which also detect light. Many of the experiments to understand the function of these sensors were done in the dark, leaving daytime temperature sensing fairly unexplored. Now, in a recent study published in Nature Communicationsresearchers at the University of California, Riverside, led by plant biologist Meng Chendiscovered a brand-new role for sugar in daytime temperature sensing.² They showed that at high temperatures, sugar acts as a thermostat to override plant-growth brakes, thus enabling heat-responsive stem elongation. These findings could pave way for breeding climate-resilient crops in the face of global warming.

“Temperature sensing is one of the last open questions in biology,” said Philip Wiggea plant biologist at the University of Potsdam who was not involved in the study. “Plants turn out to be a really good system to identify the mechanisms because they’re exposed to this huge range of temperatures.”

One of the key thermal sensors in plants is the light-sensitive protein phytochrome B (phyB).³ When activated by light, phyB inhibits the expression of phytochrome interacting factor 4 (PIF4), a protein that promotes plant growth and development. High temperatures inactivate phyB, thus enabling accumulation of PIF4 and elongation of plant stems; however, this had only been explored in dark conditions.

Chen and his team wanted to know how phyB affected plant growth in daytime, when bright light activates the protein perpetually. They grew Arabidopsis thaliana seeds at cool and warm temperatures under varying light intensities and measured how much the seedling stems elongated. Warm temperatures inactivated phyB to induce stem growth only under dim light. At high light intensities, when phyB ceases to function as a thermal sensor, the stems still grew substantially and more than they did under dim light. Observing this, Chen speculated that there must be other sensors that signal warmth and facilitate plant growth.

There were plenty of candidates for the undiscovered temperature sensors. To thin the herd, the team grew mutant A. thaliana with permanently active phyB proteins in dark and light, under cool and warm conditions. This allowed the team to determine the functional limits of the new sensor. To their surprise, mutants grown in the dark did not show any growth response to either temperature, while those grown in light did. This suggested a link to the process of photosynthesis.

Chen and his group cultivated mutant seedlings in the dark with sucrose, the mobile sugar product of photosynthesis, which restored stem growth to normal levels under high temperature. In fact, growing plants deficient in chloroplasts—the cellular organelles responsible for photosynthetic sugar production—with added sucrose produced the same results. This meant that high temperatures mobilized sugar from some other source.

Chen speculated that warmer conditions might degrade the plant’s energy stores—starch—to release sucrose. When the researchers tried growing an A. thaliana variety that was incapable of breaking down starch, they observed no growth in response to high temperatures. Adding sucrose to the medium of these plants restored normal stem elongation.

But how exactly does sucrose stimulate plant growth? The scientist noted that in mutants with non-functional phyB, PIF4 accumulated to high levels and the stems elongated substantially irrespective of temperature or presence of sucrose. This indicated that in healthy plants, sucrose inhibited phyB at high temperatures under bright light.

“This study brings together several different strands that have been out there in the literature, but it goes beyond those, and it helps us have a much more complete picture of how temperature and energy status are integrated,” Wigge said.

But Chen and his team were still perplexed by one thing. Even if they supplied the plants with high levels of sucrose, which induced accumulation of PIF4, they still needed to provide high temperatures to see growth. Was there another thermal sensor that repressed PIF4 activity under cool conditions? Only one candidate fit the profile: early flowering 3 (ELF3). This protein is a part of a larger transcriptional repressor which decreases PIF4 transcription and inhibits the protein’s activity.⁴ When the team grew A. thaliana mutants that lacked functional ELF3, adding sucrose to the growth medium was sufficient to induce stem growth even at low temperatures. So, high temperatures not only mobilized sucrose, but also repressed the inhibitory activity of ELF3, allowing untethered production and function of PIF4.

Wigge believes that this is like a two-step verification program which allows plants to sense temperature at multiple stages. Chen thinks it makes the process more precise. “If plants have only one mechanism, every time they make sucrose, it will make the stem grow. But they might not want that to happen,” he said. “It also allows each mechanism to operate independently for some other functions without triggering growth.”

Chen now wants to find out the molecular players that trigger starch degradation at high temperatures. Beyond that, he thinks the interplay between the chloroplast and the nucleus makes temperature sensing in plants a great model to study how organelles communicate to mount a coordinated cellular response.