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...

Regulated RNA metabolism appears to be a critical component of molecular mechanisms directing flowering initiation in plants. A group of RNA binding proteins exerts their roles through the autonomous flowering pathway. Posttranscriptional mechanisms regulated by microRNAs (miRNAs) also play a key role in flowering-time control. Here, we demonstrate that the GIGANTEA (GI)-regulated miR172 defines a unique genetic pathway that regulates photoperiodic flowering by inducing FLOWERING LOCUS T (FT) independent of CONSTANS (CO). A late-flowering mutant in which a miR172 target gene, TARGET OF EAT1, is constitutively activated by the nearby insertion of the cauliflower mosaic virus 35S enhancer normally responded to vernalization and gibberellic acid treatments. By contrast, its response to daylength changes was severely disrupted. In the mutant, FT was significantly repressed, but other flowering genes were unaffected. Notably, miR172 abundance is regulated by photoperiod via GI-mediated miRNA processing. Accordingly, miR172-overproducing plants exhibit early flowering under both long days and short days, even in the absence of functional CO, indicating that miR172 promotes photoperiodic flowering through a CO-independent genetic pathway. Therefore, it appears that GI-mediated photoperiodic flowering is governed by the coordinated interaction of two distinct genetic pathways: one mediated via CO and the other mediated via miR172 and its targets.

Temperature is a major environmental variable governing plant growth and development, and climate change has already altered the phenology of wildplants and crops 1 . However, the mechanisms by which plants sense temperature are not well understood. Environmental signals, including temperature, are integrated into growth and developmental pathways via the circadian clock and the activity of the Evening Complex (EC), a major signalling hub and core clock component 2,3 . The EC acts as a temperature responsive transcriptional repressor, providing rhythmicity and temperature responsiveness to growth via unknown mechanisms 2,4-6 . The EC consists of EARLY FLOWERING3 (ELF3) 4,7 , a large scaffold protein and key component