Plant eyes in the dark: How a blue-light photoreceptor senses and functions without light

Light is both a source of energy for photosynthesis and a key environmental signal that regulates plant growth. Seedlings grown in darkness exhibit elongated hypocotyls and shorter roots, while light promotes shorter hypocotyls and longer roots. Cryptochromes (CRYs), as blue-light receptors, mediate many blue-light-dependent processes, but their roles under darkness or non-blue-light conditions remain unclear. Zeng et al. discovered that Arabidopsis CRY2 suppresses root elongation in darkness by inhibiting cell division in the root apical meristem. Under blue light, CRY2 forms oligomers, which disrupt this suppression, thereby promoting root cell division and elongation. This study reveals that CRY2 retains biological activity in its non-light-activated state, with blue light effectively “switching off” this activity to regulate root development. In darkness, CRY2 interacts and inhibits the activities of FORKED-LIKE (FL) proteins that promote cell division by enhancing the transcription of division-related genes. In contrast, blue-light-induced CRY2 oligomerization prevents this interaction, releasing FL activity to stimulate root growth. By integrating light and dark cues through this dual-regulation mechanism, plants achieve a balance between aboveground and underground development. This study answers the long-standing question of CRY2’s function in darkness, revealing its “darkness-dependent” activity alongside its known “blue-light-dependent” role. It expands our understanding of photoreceptor biology and offers new strategies to regulate crop growth and architecture through photoreceptor manipulation. (Summary by Hao Chen) Science  doi.org/10.1016/j.cell.2024.10.031