The greenhouse effect leads to a gradual increase in global temperatures. How plants sense temperature change and still maintain robust circadian rhythms with an approximately 24 h period is largely unclear.

In a study published in Developmental Cell, a research team led by Prof. WANG Lei from the Institute of Botany of the Chinese Academy of Sciences revealed that a key protein inside plants, clock component PSEUDO-RESPONSE REGULATOR 9 (PRR9), has evolved for thermal adaptation, and plays a crucial role in transmitting temperature signals to keep accurate timing in plants.

Researchers revealed that a three-part regulatory module, composed by phyB, PRR9 and ALKBH9B, acts like a temperature sensory module and messenger. This module transmits temperature signals into the core circadian oscillator and maintains daily rhythmic behaviors in varying temperature changes.

At cooler temperatures, researchers found that phyB acts as the main thermometer. It grabs onto PRR9 inside structures called nuclear speckles, spatially sequestering PRR9 from binding to promoters of morning-phased circadian genes such as CIRCADIAN ASSOCIATED 1 (CCA1), which weakens the transcriptional inhibitory activity of PRR9 protein and delays its transcriptional inhibition phase. When temperature rise, phyB and PRR9 interact diffusely in the nucleus, releasing the functional inhibition of PRR9.

When plants are exposed to long-term high-temperature conditions such as seasonal temperature change, researchers found that PRR9 protein is accumulated, further suppressing CCA1, and PRR9 teams up with RNA m⁶A demethylase ALKBH9B, in the cellular cytoplasm, which removes m⁶A modification on CCA1 mRNA and accelerates its degradation.

By strictly regulating CCA1 levels at both cytoplasmic and nuclear levels, even when the heat is on, PRR9 precisely maintains the circadian clock period at roughly 24 hours.

This study shows that plant circadian clocks possess sophisticated and complex high-temperature sensing and regulatory mechanisms, which is beneficial for plants to maintain the robustness of core circadian functions in continuously warming temperatures, better adapting to the gradually warming climate.

This study lays an important theoretical foundation for engineering thermo-resilient crops through molecular design breeding approaches in the future.

Graphical representation of the thermoregulatory module for timekeeping. (Image by HE Yuqing and WANG Xiling)

Circadian clock maintains stable and robust rhythms at high temperature. (Image by HE Yuqing and WANG Lei)