Photoactivated phytochrome B (PHYB) binds to antagonistically acting PHYTOCHROME-INTERACTING transcription FACTORs (PIFs) to regulate hundreds of light responsive genes in Arabidopsis by promoting PIF degradation. However, whether PHYB directly controls the transactivation activity of PIFs remains ambiguous. Here we show that the prototypic PIF, PIF3, possesses a p53-like transcription activation domain (AD) consisting of a hydrophobic activator motif flanked by acidic residues. A PIF3mAD mutant, in which the activator motif is replaced with alanines, fails to activate PIF3 target genes in Arabidopsis, validating the functions of the PIF3 AD in vivo. Intriguingly, the N-terminal photosensory module of PHYB binds immediately adjacent to the PIF3 AD to repress PIF3’s transactivation activity, demonstrating a novel PHYB signaling mechanism through direct interference of the transactivation activity of PIF3. Our findings indicate that PHYB, likely also PHYA, controls the stability and activity of PIFs via structurally separable dual signaling mechanisms.

光活化光敏色素 B (PHYB) 与具有拮抗作用的光敏色素相互作用转录因子 (PIF) 结合，通过促进 PIF 降解来调节拟南芥中的数百个光响应基因。然而，PHYB 是否直接控制 PIFs 的反式激活活性仍然不明确。在这里我们展示了原型 PIF，PIF3，拥有一个类似 p53 的转录激活域 (AD)，它由一个侧接酸性残基的疏水性激活剂基序组成。PIF3mAD 突变体，其中激活剂基序被丙氨酸取代，未能激活拟南芥中的PIF3 靶基因，在体内验证 PIF3 AD 的功能。有趣的是，PHYB 的 N 端光敏模块紧邻 PIF3 AD 结合以抑制 PIF3 的反式激活活性，通过直接干扰 PIF3 的反式激活活性证明了一种新的 PHYB 信号传导机制。我们的研究结果表明，PHYB，也可能是 PHYA，通过结构上可分离的双重信号机制控制 PIF 的稳定性和活性。