Polarity and morphogenesis of flat plant structures
 

Developing a complex multicellular body of a particular form requires several tightly regulated processes, such as setting up body axes, establishing cell- and tissue polarity and specifying cell fates followed by differentiation into distinct cell types that together form functional organs. Our research focuses on the morphogenesis of flat photosynthetic plant structures that develop big surfaces to efficiently capture light and atmospheric CO₂ – an essential ability of plants that fundamentally shapes the life and climate on our planet. We aim to discover the core mechanisms and developmental programs that enable flat plant structures to develop de novo.

Our primary model plant is Marchantia polymorpha, which forms a flat thallus as photosynthetically active plant body from a single-celled spore. Spores germinate in isolation from the parental plant, bare and exposed to the environment, which makes tracking of sporeling development easy. The prothallus is the first flat body structure of the sporeling that develops by spatiotemporally oriented cell divisions in 5 consecutive stages. At the last stage, an apical stem cell is specified at the prothallus margin that produces daughter cells to generate the multilayered thallus body with distinct dorsal and ventral sides. Sporeling development thus presents a so far underexplored, novel and powerful system that enables us to answer fundamental questions about plant morphogenesis, body axes and pattern formation in response to distinct environmental cues that are difficult to approach with the genetically and morphologically more complex vascular plant models.

Our tools: The genome of Marchantia polymorpha is fully sequenced, genetic redundancy is low, the thallus is haploid and transformation is easy, allowing us to utilize the full state-of-the-art repertoire of microscopy, omics, forward and reverse genetics, biochemical and modelling approaches.

Research Projects

1. Epidermal patterning and cell division orientation during prothallus development

Prothallus development follows a reproducible sequence of symmetric and asymmetric cell divisions that generate the plate, the disc and eventually the flabellum (Wallner et al. 2024). Cell polarity is required to orient asymmetric cell divisions. This means that proteins, other signalling molecules, cytoskeletal structures, cell wall components or external mechanical forces are asymmetrically distributed prior to mitosis. Which intra- or extracellular components orient the divisions in the prothallus, is so far unknown.

In this project we aim to discover:

- Which genes regulate the divisions of the distinct prothallus stages?
- Which subcellular components polarize prior to the asymmetric cell divisions?
- What is enforcing epidermal flattening?

2. Formation of the dorsoventral axis in response to external and internal signals

Body axes are set up de novo during sporeling development in response to light (Wallner et al. 2025). The prothallus is initially comprised of a one-cell-thick layer that transitions to multiple cell layers as the flabellum develops. Around the same time, the transcription factor MpC3HDZ polarizes to cells at the light-facing (future dorsal) prothallus side to specify the dorsoventral body axis (Wallner and Dolan 2024, Spencer et al. 2024). How the light signal is translated into an internal dorsoventrality signal is unknown. We want to understand:

- What regulates the 2D to 3D transition of the prothallus?
- How are external (light) signals integrated to orient (pro)thallus polarity?
- What is the regulatory network of MpC3HDZ in setting up dorsoventrality?

3. Outlook: discovering the core regulators of flat plant body structures

Tracheophytes (vascular plants) and bryophytes (non-vascular plants) share a common land plant ancestor but evolved their flat photosynthetic structures independently. How similar or diverged are the developmental programs that direct the morphogenesis of leaves and thalli? What are the minimal regulatory networks required to form a flat plant organ?

By extrapolating our findings on Marchantia polymorpha (pro)thallus development to the genetically more complex vascular leaf of Arabidopsis thaliana, we aim to identify the minimal core regulators that allow formation of a flat photosynthetic structure. This knowledge could find future applications in combating rising CO₂ levels.

 

Selected publications:

Wallner ES, Edelbacher N, Dolan L; De novo meristem development in Marchantia requires light and an apical auxin signaling minimum. Current Biology (2025), 10.1016/j.cub.2025.11.016

Specer V, Wallner ES, Jandrasits K, Edelbacher N, Mosiolek M, Dolan L; The three-dimensional anatomy and dorsoventral asymmetry of the mature Marchantia polymorpha meristem develops from a symmetrical gemma meristem. Development (2024), 10.1242/dev.204349

Wallner ES, Dolan L; Reproducibly oriented cell divisions pattern the prothallus to set up dorsoventrality and de novo meristem formation in Marchantia polymorpha. Current Biology (2024), 10.1016/j.cub.2024.07.099

Wallner ES, Mair A, Handler D, McWhite C, Xu SL, Dolan L, Bergmann DC; Spatially resolved proteomics of the Arabidopsis stomatal lineage identifies polarity complexes for cell divisions and stomatal pores. Developmental Cell (2024), 10.1016/j.devcel.2024.03.001

Wallner ES, Dolan L, Bergmann DC; Arabidopsis stomatal lineage cells establish bipolarity and segregate differential signaling capacity to regulate stem cell potential. Developmental Cell (2023), 10.1016/j.devcel.2023.07.024

Wallner ES; The value of asymmetry: how polarity proteins determine plant growth and morphology. Journal of Experimental Botany (2020), 10.1093/jxb/eraa329