Plants are responsive to temperature, and can distinguish differences of 1ºC. In Arabidopsis, warmer temperature accelerates flowering and increases elongation growth (thermomorphogenesis). The mechanisms of temperature perception are however 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 2 becomes de-repressed at low temperatures. We have discovered 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, integrating temperature information over the course of the night. One Sentence Summary:The plant temperature transcriptome is controlled at night by phytochromes, acting as thermoresponsive transcriptional repressors. Main Text:Plant development is responsive to temperature, and the phenology and distribution of crops and wild plants have already altered in response to climate change (1, 2). In Arabidopsis thaliana, warm temperature-mediated elongation growth and flowering is dependent on the bHLH transcription factors PHYTOCHROME INTERACTING FACTOR4 and 5 (PIF4 and 5) (3-6). Growth at 27ºC reduces the activity of the Evening Complex (EC) resulting in greater PIF4 transcription. The EC is a transcriptional repressor made up of the proteins EARLY FLOWERING3 (ELF3), ELF4 and LUX ARRHYTHMO (LUX) (7-9). To test if the EC is also required for hypocotyl elongation responses below 22ºC, we examined the behavior of elf3-1 and lux-4 at 12 and 17ºC. Hypocotyl elongation in elf3-1 and lux-4 is largely suppressed at lower temperatures (Fig. 1A, B), which is consistent with cold temperatures being able to suppress PIF4 overexpression phenotypes (10). Since PHYTOCHROME B (PHYB) was identified as a QTL for thermal responsiveness and PIF4 activity is regulated by phytochromes (8, 11), we investigated whether these red light receptors control hypocotyl elongation in the range 12 to 22ºC. Plants lacking phytochrome activity (12) show constitutively long hypocotyls at 12ºC and 17ºC. Thus phytochromes are essential for responding to temperature (Fig. 1C, D and Fig. S1).We used transcriptome analysis to determine whether disrupted thermomorphogenesis in phyABCDE is specific for temperature signaling or is a consequence of misregulated growth pathways. To capture diurnal variation in thermoresponsiveness, we sampled seedlings over 24 hours at 22 and 27ºC. Clustering analysis reveals 20 groups of transcripts ( Fig. 2A and Fig. S3; described in supplement). Thermomorphogenesis occurs predominantly at night and is driven by PIF4. Consistent with this, we observe PIF4 is present in cluster 20, which is more highly expressed at 27ºC during darkness. Clusters 15 and 16 represent the other major groups of 3 nighttim...

Ambient temperature regulates many aspects of plant growth and development, but its sensors are unknown. Here, we demonstrate that the phytochrome B (phyB) photoreceptor participates in temperature perception through its temperature-dependent reversion from the active Pfr state to the inactive Pr state. Increased rates of thermal reversion upon exposing Arabidopsis seedlings to warm environments reduce both the abundance of the biologically active Pfr-Pfr dimer pool of phyB and the size of the associated nuclear bodies, even in daylight. Mathematical analysis of stem growth for seedlings expressing wild-type phyB or thermally stable variants under various combinations of light and temperature revealed that phyB is physiologically responsive to both signals. We therefore propose that in addition to its photoreceptor functions, phyB is a temperature sensor in plants.

Phytochromes are red/far-red-light detecting photoreceptors that regulate plant growth and development. They photo-interconvert between an inactive Pr (red-light absorbing) and a physiologically active Pfr (far-red-light absorbing) form, acting as light-controlled molecular switches. Although the two major plant phytochromes A (phyA) and B (phyB) share similar absorption properties, they exhibit dramatic differences in their action spectra. Since both phytochromes antagonistically regulate seedling development under vegetative shade, it is essential for plants to clearly distinguish between phyA and phyB action. This discrimination is not comprehensible solely by the molecular properties of the phytochromes, but is evidently due to the dynamics of the phytochrome system. Using an integrated experimental and mathematical modelling approach we show that phytochrome dimerization is an essential element for phyB function. Our findings reveal that light-independent Pfr to Pr relaxation (dark reversion) and association/dissociation to nuclear bodies (NBs) severely depend on the conformational state of the phyB dimer. We conclude that only Pfr-Pfr homodimers of phyB can be responsible for triggering physiological responses, leading to a suppression of phyB function in the far-red range of the light spectrum.

Summary• Cadmium (Cd 2+ ) is an environmental pollutant that causes increased reactive oxygen species (ROS) production. To determine the site of ROS production, the effect of Cd 2+ on ROS production was studied in isolated soybean (Glycine max) plasma membranes, potato (Solanum tuberosum) tuber mitochondria and roots of intact seedlings of soybean or cucumber (Cucumis sativus).• The effects of Cd 2+ on the kinetics of superoxide ( ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical ( • OH) generation were followed using absorption, fluorescence and spin-trapping electron paramagnetic resonance spectroscopy.• In isolated plasma membranes, Cd 2+ inhibited production. This inhibition was reversed by calcium (Ca 2+ ) and magnesium (Mg 2+ ). In isolated mitochondria, Cd 2+ increased and H 2 O 2 production. In intact roots, Cd 2+ stimulated H 2 O 2 production whereas it inhibited and • OH production in a Ca 2+ -reversible manner.• Cd 2+ can be used to distinguish between ROS originating from mitochondria and from the plasma membrane. This is achieved by measuring different ROS individually. The immediate (≤ 1 h) consequence of exposure to Cd 2+ in vivo is stimulation of ROS production in the mitochondrial electron transfer chain and inhibition of NADPH oxidase activity in the plasma membrane.

Phytochrome B (phyB) is the primary red light photoreceptor in plants, and regulates both growth and development. The relative levels of phyB in the active state are determined by the light conditions, such as direct sunlight or shade, but are also affected by light-independent dark reversion. Dark reversion is a temperature-dependent thermal relaxation process, by which phyB reverts from the active to the inactive state. Here, we show that the homologous phyB-binding proteins PCH1 and PCHL suppress phyB dark reversion, resulting in plants with dramatically enhanced light sensitivity. Moreover, far-red and blue light upregulate the expression of PCH1 and PCHL in a phyB independent manner, thereby increasing the response to red light perceived by phyB. PCH1 and PCHL therefore provide a node for the molecular integration of different light qualities by regulation of phyB dark reversion, allowing plants to adapt growth and development to the ambient environment.