Phytochromes function as thermosensors in $\textit{Arabidopsis}$

Plants are responsive to temperature, and some species can distinguish differences of 1°C. In $\textit{Arabidopsis}$, warmer temperature accelerates flowering and increases elongation growth (thermomorphogenesis). However, the mechanisms of temperature perception are largely unknown. We describe a major thermosensory role for the phytochromes (red light receptors) during the night. Phytochrome null plants display a constitutive warm-temperature response, and consistent with this, we show in this background that the warm-temperature transcriptome becomes derepressed at low temperatures. We found that phytochrome B (phyB) directly associates with the promoters of key target genes in a temperature-dependent manner. The rate of phyB inactivation is proportional to temperature in the dark, enabling phytochromes to function as thermal timers that integrate temperature information over the course of the night.SB was supported by a Churchill Scholarship from the Winston Churchill Foundation of the United States. The work in JL’s laboratory is supported by a Fellowship from the Gatsby Foundation (GAT3272/GLC). This work was supported by responsive mode grants from the Biotechnology and Biological Sciences Research Council (BB/I013350/1 and BB/N010248/1) and the European Research Council (EC FP7 ERC 243140) to PW. The work in PW’s laboratory is supported by a Fellowship from the Gatsby Foundation (GAT3273/GLB). CK was supported by a DFG grant to ES (SCHA 303/16-1)