Aspen trees integrate light and temperature to time bud set

Trees in cold climates face a delicate balancing act. They need to stop growing and form buds before winter arrives, but not too early as this could mean missing out on valuable growing time, especially when unexpected cold snaps occur during summer. While it is well known that shorter days in autumn trigger trees to prepare for winter, the role of temperature has been less well understood. 

“There has been an ongoing debate about how important temperature is for the timing of growth cessation and bud formation in aspen trees, compared to day length alone,” says Ove Nilsson from Umeå Plant Science Centre and Swedish University of Agricultural Sciences. “We identified two key proteins and showed that their activity is regulated by temperature, suggesting that they act as a “temperature sensor”. This helps the tree take both day length and temperature into account when deciding when to stop growing and form buds.”

Temperature changes how trees react to light

The two proteins the researchers identified are Phytochrome B, a light receptor, and PIF4, a protein that is regulated by Phytochrome B. Plants use Phytochrome B to distinguish between different shades of red light which helps them respond to changing day lengths and adapt their growth in shady conditions. That is what made these two proteins particularly interesting to the researchers. To study them, the researchers modified aspen trees and adjusted the levels of the two proteins.

“We initially focused on PIF4 and studied its role in response to day length,” explains Bo Zhang, first author of the article. “We were quite disappointed to see that it had almost no effect on the trees. However, when we repeated the experiments under cooler conditions, we suddenly saw a much stronger response in our modified aspen trees. It became clear that these proteins respond to temperature. We had not expected that!”

What the researchers found was that phytochrome B helps trees continue growing during cool but long summer days, while PIF4 ensures that they stop growing when cold days arrive in autumn. Together they act as a kind of “thermometer” sensing temperature and integrating it with light signals to help the tree make the right decision at the right time. 

Genes behave differently in trees than in annual plants 

Earlier research from Ove Nilsson’s group had already shown that many genes which regulate flowering in annual plants like Arabidopsis control growth cessation and bud formation in aspen. Many of these genes act even in completely opposite ways in Arabidopsis compared to aspen including PIF4. In Arabidopsis, PIF4 activates a gene that initates flowering while in aspen, it blocks a similar gene that keeps the tree growing. 

“This has puzzled us for a long time and is likely an adaptation to the perennial lifestyle of trees,” says Ove Nilsson. “It is fascinating from an evolutionary perspective but also highly relevant in the context of climate change which may cause more frequent temperature fluctuations. Understanding how trees sense and respond to temperature could help us breed trees that are better adapted to a changing climate.”