12 September 2025 | Article

Proteins for winter timing

Aspen trees have evolved a molecular system to sense both light and temperature, helping them decide when to keep growing and when to prepare for winter. A new study uncovers how the proteins PHYB and PIF4 interact to fine-tune this balance, offering insights into how boreal forests withstand seasonal and climate changes.

Aspen trees widely depend on precise seasonal timing to thrive. Toward the end of the growing season, they cease growth and form winter buds to endure freezing conditions. While shorter autumn days are known triggers for this process, the role of temperature regulation remained unclear—until now. A recent study in Nature Communications has uncovered how aspen harness temperature signals to fine-tune growth cessation and bud formation.

A molecular thermometer

Scientists found that the protein PHYTOCHROME B (PHYB) helps aspen trees keep growing even when days are long but temperatures are cool. It does this by controlling PIF4, a protein that slows growth—but only when temperatures drop, not just because the days get shorter.

PIF4 also switches off a key growth gene, FT2, which is the opposite of its job in annual plants, where it usually promotes flowering. Cold weather further weakens PIF4, making sure the tree doesn’t stop growing too early.

Controlled growth for survival

This balance between PHYB and PIF4 helps aspens grow as much as possible during short northern summers while still preparing for winter at the right time. It shows how boreal trees finely tune their growth to survive seasonal changes. The mechanism is very different from what is seen in annual plants, underlining how trees have evolved unique strategies. By understanding these signals, researchers can better predict how forests respond to seasonal shifts and long-term climate change.

Photo: fightCOtwo

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