Plants detect changes in day length to induce seasonal patterns of flowering. The photoperiodic pathway accelerates the flowering ofArabidopsis thalianaunder 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°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 ofFLOWERING LOCUS T(FT), which encodes the output signal of the photoperiodic pathway. We integrate these observations by studying several PIFs, the MADS-box SHORT VEGETATIVE PHASE (SVP) and the photoperiodic pathway under 27°C-SD. We find that the mRNAs ofFTand its paralogueTWIN SISTER OF FT(TSF) are increased at dusk under 27°C-SD compared with 21°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°C-SD, and they physically interact. Although Col-0 plants flower at similar times under 27°C-SD and 21°C-LD the expression level ofFTis approximately 10-fold higher under 21°C-LD, 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, onlysvp-41 ft-10 tsf-1plants flowered at the same time under 21°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.Significance Statement
Flowering is induced or repressed by exposure to particular day lengths. Typically these responses are robust to fluctuations in other environmental parameters, but in Arabidopsis thaliana high temperature can substitute for photoinductive day lengths. Here we show that under noninductive short days, high temperature enhances the activity of photoperiodic flowering pathway components and increases the response to the flowering pathway in the shoot meristem.