Photoperiodic and thermosensory pathways interact through CONSTANS to promote flowering at high temperature under short days

Plants detect changes in day length to induce seasonal patterns of flowering. The photoperiodic pathway accelerates the flowering of Arabidopsis thaliana under long days (LDs) whereas it is inactive under short days (SDs), resulting in delayed flowering. This delay is overcome by exposure of plants to high temperature (27 C) under SDs (27 degrees C-SD). Previously, the high-temperature flowering response was proposed to involve either the impaired activity of MADS-box transcription factor (TF) floral repressors or PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) TF-mediated activation of FLOWERING LOCUST (FT), which encodes the output signal of the photoperiodic pathway. We integrate these observations by studying several PlFs, the MADS-box SHORT VEGETATIVE PHASE (SVP) and the photoperiodic pathway under 27 degrees C-SD. We find that the mRNAs of FT and its paralogue TWIN SISTER OF FT (TSF) are increased at dusk under 27 degrees C-SD compared with 21 degrees C-SD, and that this requires PIF4 and PIF5 as well as CONSTANS (CO), a TF that promotes flowering under LDs. The CO and PIF4 proteins are present at dusk under 27 degrees C-SD, and they physically interact. Although Col-0 plants flower at similar times under 27 degrees C-SD and 21 degrees C-LD the expression level of FT is approximately 10-fold higher under 21 degrees C-ID, suggesting that responsiveness to FT is also increased under 27 C-SD, perhaps as a result of the reduced activity of SVP in the meristem. Accordingly, only svp-41 ft-10 tsf-1 plants flowered at the same time under 21 degrees C-SD and 27 C-SD. Thus, we propose that under non-inductive SDs, elevated temperatures increase the activity and sensitize the response to the photoperiod pathway.