Phototropin-mediated blue light signaling orients the asymmetry of Marchantia polymorpha spores

Multicellular organisms produced by sexual reproduction develop from single cells, and the asymmetry of these cells can define the orientation of the earliest developmental axes. The haploid multicellular stage of the plant, Marchantia polymorpha, develops from a single cell—the spore—that divides asymmetrically, producing an apical germ cell that generates the plant body and a smaller basal cell that differentiates as an anchoring germ rhizoid cell. We show that the orientation of this asymmetric cell division is controlled by an external environmental cue—blue light—that is perceived by the photoreceptor phototropin. This process requires the NCH1 (NRL PROTEIN FOR CHLOROPLAST MOVEMENT 1) protein for downstream signaling. Together, these findings define core elements of the mechanism by which a directional environmental signal orients cell division, which, in turn, orients the first axis of symmetry.