Johannes Roetzer

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.

Ultraviolet-B radiation (UV-B) poses a major challenge to all forms of plant life. The liverwort Marchantia (Marchantia polymorpha) serves as a key model organism for studying signaling pathways and inferring their evolution throughout the green lineage. Marchantia expresses key components of UV-B signaling, including the photoreceptor UV RESISTANCE LOCUS 8 (MpUVR8), the WD40-repeat protein REPRESSOR OF UV-B PHOTOMORPHOGENESIS (MpRUP), the E3 ubiquitin ligase complex CONSTITUTIVELY PHOTOMORPHOGENIC 1/SUPPRESSOR OF phyA-105 (MpCOP1/MpSPA), and the transcriptional regulator ELONGATED HYPOCOTYL 5 (MpHY5). Here, we show that whereas the core UVR8 photocycle is well conserved, regulatory roles of SPA and HY5 diverged during plant evolution. Unlike the case in Arabidopsis (Arabidopsis thaliana) and in contrast to the strong Mpcop1 mutant phenotype, Mpspa mutants developed only a very weak constitutive photomorphogenesis phenotype, indicating that COP1 function is much more independent of SPA in Marchantia than in Arabidopsis. Moreover, in contrast to Arabidopsis SPAs, Mpspa showed enhanced UV-B acclimation and UV stress tolerance, indicating that MpSPA is a negative regulator of MpUVR8 signaling. Similar to Arabidopsis HY5/HYH, MpHY5 functioned antagonistically to MpCOP1, but its role in UV-B-mediated gene expression changes was more limited. Our findings demonstrate that although core components of UV-B signaling existed in the last common ancestor of extant land plants, regulatory interactions have diversified in different lineages since their divergence more than 400 million years ago.

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 and signals in an NCH1-dependent manner. This defines core elements of the mechanism by which a directional environmental signal orients cell division, which in turn orients the first axis of symmetry.

The growth orientation of the Marchantia polymorpha thallus – a system of dorsiventralized, indeterminate axes – is modulated by light. We show that red and blue light act antagonistically to control thallus growth tropisms, with red light signalling promoting epinasty and blue light signalling promoting hyponasty. We found that loss-of-function mutations in the blue light receptor Mp PHOT led to epinasty, while loss-of-function mutations in the red light receptor Mp PHY resulted in hyponasty. We hypothesize that these antagonistic activities of blue and red light signalling are balanced in white light, resulting in the development of flat thalli. Using time-resolved transcriptomics, we identified genes that were rapidly induced upon light exposure. Among these genes were all six members of the M. polymorpha BBX gene family. Mutants harbouring loss-of-function mutations in two of the six MpBBX transcription factors developed defective thallus tropisms. Mp bbx1 loss-of-function mutants formed hyponastic thalli, while Mp bbx5 loss-of-function mutants developed epinastic thalli. Double mutants Mpbbx1 Mp bbx5 grew flat, supporting the hypothesis that they function antagonistically. Together, these data indicate that phototropin-mediated blue light and phytochrome-mediated red light signalling antagonistically modulate thallus tropism, and that BBX transcription factors also act antagonistically to regulate thallus flatness.

Ultraviolet-B radiation (UV-B) poses a major challenge to all forms of plant life. The liverwort Marchantia polymorpha (Marchantia) serves as a key model organism to study signaling pathways and to infer their evolution throughout the green lineage. Marchantia expresses key components of UV-B signaling, including the photoreceptor UV RESISTANCE LOCUS 8 (MpUVR8), the WD40-repeat protein REPRESSOR OF UV-B PHOTOMORPHOGENESIS (MpRUP), the E3 ubiquitin ligase complex CONSTITUTIVELY PHOTOMORPHOGENIC 1 / SUPPRESSOR OF phyA-105 (MpCOP1/MpSPA), and the transcriptional regulator ELONGATED HYPOCOTYL 5 (MpHY5). Here, we show that MpUVR8 exists as a homodimer in its ground-state in vivo, then monomerizes and accumulates in the nucleus upon UV-B activation. Activated MpUVR8 interacts with MpCOP1, triggering growth inhibition, genome-wide gene expression changes, biosynthesis of UV-absorbing metabolites, and photoprotection, which overall contributes to UV-B stress tolerance. MpRUP facilitates redimerization of MpUVR8 and Mp rup null mutants show enhanced UV-B photomorphogenesis, demonstrating that MpRUP efficiently represses MpUVR8 signaling. Unlike the case in Arabidopsis and in contrast to the strong Mp cop1 mutant phenotype, Mp spa mutants develop only a very weak constitutive photomorphogenesis phenotype, indicating that COP1 function is much more independent of SPA in Marchantia than in Arabidopsis. Moreover, in contrast to Arabidopsis SPAs, Mp spa is linked with a hyper-responsive UV-B phenotype, suggesting that MpSPA is a negative regulator of MpUVR8 signaling. Similar to Arabidopsis HY5/HYH, MpHY5 functions antagonistically to MpCOP1, but its role in UV-B-mediated gene expression changes is more limited. Our findings demonstrate that although core components of UV-B signaling existed in the last common ancestor of extant land plants, regulatory interactions have diversified in different lineages since their divergence more than 400 million years ago.