COP1 conveys warm temperature information to hypocotyl thermomorphogenesis

Summary Plants adjust their architecture to optimize growth and reproductive success under changing climates. Hypocotyl elongation is a pivotal morphogenic trait that is profoundly influenced by light and temperature conditions. While hypocotyl photomorphogenesis has been well characterized at the molecular level, molecular mechanisms underlying hypocotyl thermomorphogenesis remains elusive. Here, we demonstrate that the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 ( COP 1) conveys warm temperature signals to hypocotyl thermomorphogenesis. To investigate the roles of COP 1 and its target ELONGATED HYPOCOTYL 5 ( HY 5) during hypocotyl thermomorphogenesis, we employed Arabidopsis mutants that are defective in their genes. Transgenic plants overexpressing the genes were also produced. We examined hypocotyl growth and thermoresponsive turnover rate of HY 5 protein at warm temperatures under both light and dark conditions. Elevated temperatures trigger the nuclear import of COP 1, thereby alleviating the suppression of hypocotyl growth by HY 5. While the thermal induction of hypocotyl growth is circadian‐gated, the degradation of HY 5 by COP 1 is uncoupled from light responses and timing information. We propose that thermal activation of COP 1 enables coincidence between warm temperature signaling and circadian rhythms, which allows plants to gate hypocotyl thermomorphogenesis at the most profitable time at warm temperatures.