Exposure of Arabidopsis plants to high temperature (28 degrees C) results in a dramatic change in plant development. Responses to high temperature include rapid extension of plant axes, leaf hyponasty, and early flowering. These phenotypes parallel plant responses to the threat of vegetational shade and have been shown to involve the hormone auxin. In this work, we demonstrate that high temperature-induced architectural adaptations are mediated through the bHLH transcriptional regulator PHYTOCHROME INTERACTING FACTOR 4 (PIF4). Roles for PIF4 have previously been established in both light and gibberellin (GA) signaling, through interactions with phytochromes and DELLA proteins, respectively. Mutants deficient in PIF4 do not display elongation responses or leaf hyponasty upon transfer to high temperature. High temperature-mediated induction of the auxin-responsive gene IAA29 is also abolished in these plants. An early flowering response to high temperature is maintained in pif4 mutants, suggesting that architectural and flowering responses operate via separate signaling pathways. The role of PIF4 in temperature signaling does not, however, appear to operate through interaction with either phytochrome or DELLA proteins, suggesting the existence of a novel regulatory mechanism. We conclude that PIF4 is an important component of plant high temperature signaling and integrates multiple environmental cues during plant development.

The DELLA proteins are a family of putative transcriptional regulators that inhibit the cell proliferation and expansion that drives the growth of plant organs. Plant growth is stimulated via destruction of DELLA proteins in the 26S proteasome. Recent advances have shown that the 'classical' phytohormones gibber-ellin (GA), auxin and ethylene control plant growth by influencing DELLA-destruction. This paper outlines the discovery, molecular characterization and biology of the DELLA proteins, and summarizes the evidence that they act as integrators of response to multiple growth-regulatory signals.

DELLAs are a family of nuclear proteins that act as growth repressors throughout the life cycle of higher plants. Derepression is mediated through the gibberellic acid (GA)‐dependent degradation of DELLAs and the key components of the GA‐DELLA signalling pathway (the GA receptor and the F‐box protein involved in DELLA destruction) have recently been identified. It is becoming increasingly clear that DELLAs promote a plant's survival by integrating its growth responses to a wide range of endogenous and environmental signals.