Synchronization of photoperiod and temperature signals during plant thermomorphogenesis.

It is well-known that even small changes in ambient temperatures by a few degrees profoundly affect plant growth and morphology. This architectural property is intimately associated with global warming. In particular, under warm temperature conditions, plants exhibit distinct morphological changes, such as elongation of hypocotyls and leaf petioles, formation of small, thin leaves, and leaf hyponasty that describes an upward bending of leaf petioles. These thermoresponsive morphological adjustments are termed thermomorphogenesis. Under warm temperature conditions, the PHYTOCHROME INTERACTING FACTOR 4 (PIF4) transcription factor is thermoactivated and stimulates the transcription of the YUCCA8 gene encoding an auxin biosynthetic enzyme, promoting hypocotyl elongation. Notably, these thermomorphogenic growth is influenced by daylength or photoperiod, displaying relatively high and low thermomorphogenic hypocotyl growth during the nighttime under short days and long days, respectively. We have recently reported that the photoperiod signaling regulator GIGANTEA (GI) thermostabilizes the REPRESSOR OF ga1-3 transcription factor, which is known to attenuate the PIF4-mediated thermomorphogenesis. We also found that the N-terminal domain of GI interacts with PIF4, possibly destabilizing the PIF4 proteins. We propose that the GI-mediated shaping of photoperiodic rhythms of hypocotyl thermomorphogenesis helps plant adapt to fluctuations in daylength and temperature environments occurring during seasonal transitions.