COP1 integrates light signals to ROP2 for cell cycle activation.

The exquisite regulation of cell division at the shoot apex according to the external environmental cues enables plants to adapt the ever-changing environment. We have recently shown that light direct signaling and carbohydrate (sugar) energy signaling are both essential for the activation of cell division at the shoot apex. Light is converted to auxin signal to activate small GTPase 2 (ROP2). Subsequently, the activated ROP2 directly interacts with Target of Rapamycin (TOR) protein kinase, a pivotal regulator of cell division, to promote its kinase activity. However, neither light nor auxin alone can activate TOR kinase without the presence of sugar. In this addendum, we showed that Constitutive Photomorphogenesis 1 (COP1) acts as an upstream factor of ROP2. COP1 regulates ROP2 and TOR activity in an auxin dependent manner. The development of true leaves in the cop1-6 mutant under darkness is compromised by auxin biosynthesis inhibitor yucasin and TOR chemical inhibitor torin2. Together, our results suggested that COP1 regulates auxin-ROP2-TOR signaling in response to light.