Liam Dolan

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.

A fundamental question in developmental biology is how highly complex, yet reproducible multicellular body plans form from a single cell. The multicellular haploid body of the land plant Marchantia polymorpha develops from a single isolated cell - the spore - that divides asymmetrically. This produces a small terminally differentiating basal cell, and a large proliferative apical cell that gives rise to the multicellular sporeling body. The genetic basis for the establishment of asymmetry, first within the spore, and later within the sporeling body has remained virtually unknown. Here, we show that the plant specific RHO-type GTPase - RHO OF PLANTS (ROP) - polarises to the spore basal pole to ensure spore division is highly and reproducibly asymmetric. We demonstrate this highly asymmetric division is necessary to specify the terminal differentiation of the basal cell. Furthermore, we show that ROP-mediated polarised outgrowth is required to establish asymmetry within the sporeling body derived from the apical cell. Our discovery highlights how ROP function confers developmental robustness and contributes to the establishment of asymmetry during plant development from a single isolated cell.

Summary: MicroRNAs (miRNAs) are small non‐coding RNA molecules essential for growth and development in eukaryotes. In plants, the master gene DICER‐LIKE 1 ( DCL1 ) catalyzes the biogenesis of miRNAs by processing double‐stranded precursors that give rise to mature miRNAs. We sought to understand the function and evolution of microRNAs using Marchantia polymorpha , a model bryophyte that allows comparative approaches to infer characteristics of the ancestral land plant. We functionally characterized loss‐of‐function mutants of Mp DCL1 a generated by means of CRISPR‐Cas9‐mediated genomic edition and the miR166/CLASS III HD‐ZIP regulatory circuit in Marchantia polymorpha . We report that Mp DCL1a is required for the biogenesis of miRNAs and uncovered a central role for miR166/Homeodomain Zipper Class III‐regulated auxin synthesis in the specification of cell identity, patterning, meristem function, laminar expansion, and the development of the body in the last common ancestor of the bryophytes and vascular plants. Our findings indicate that DCL1 , Class III HD‐ZIP, miR166, and auxin functioned in the development of the body of the last common ancestor of extant land plants and provide a novel working framework to interrogate the basic principles of cell specification and patterning in plants.

Founder cells comprise the earliest state of commitment to form one organ or branch, and no other [1]. Comparative lineage analyses concur that cell fate is established later in flowering plants than in animals, and is determined by position rather than lineage, but both systems harbor large numbers of organ progenitors in lieu of a few stem cells [2]. An exciting new serial-editing strategy, applied here for the first time in plants, holds great promise for determining the cell lineage history of every structure in the plant body [3]. Unfortunately, the authors' conclusion that every shoot branch and lateral organ of the Arabidopsis plant derives from exactly three founder cells is unsupported by both the classical plant literature and more recent studies of plant cell lineage, calling for more cautious interpretation of the data and for better integration with complementary experimental strategies.

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.