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Meristem regulation in the early divergent land plant Marchantia polymorpha


Type

Thesis

Change log

Authors

Rebmann, Marius 

Abstract

Meristems are key features of land plant development, enabling post-embryonic organogenesis through precise spatial patterning of cell division and differentiation. Extensive work on flowering plant meristems has revealed gene circuits driving meristem patterning, which are increasingly targeted to engineer crop development. However, genetic redundancy and morphological complexity present powerful obstacles to explore engineering of meristem regulation. Marchantia polymorpha is an early divergent land plant which has received re-surging interest as a model plant, following the publication of its genome which revealed extraordinarily low levels of genetic redundancy. Marchantia’s genetic simplicity is mirrored by a simple body plan. Asexual propagules called gemma are a particularly attractive model to study meristem regulation owing to their small size, disc shaped morphology and open development. This permits facile live imaging of early development of whole plants at cellular resolution. Despite these benefits, our understanding of the Marchantia meristem remains rudimentary compared to other plant models. This dissertation describes the use of single cell RNA-sequencing and novel marker lines to define the cell composition of the Marchantia meristem and the use of genetic and experimental perturbation to interrogate the gene networks and phytohormone patterning systems governing meristem maintenance and initiation. I describe the use of a proximal promoter library comprising a near complete collection of promoter elements for all Marchantia transcription factors, to identify novel meristem markers. Fluorescent reporters for approximately 20% of all Marchantia transcription factors were screened in gemma, identifying novel cell type markers. I present the analysis of a single cell RNA-sequencing dataset comprising approximately 7’000 cells from developing gemmalings. I characterise Marchantia cell types based on their transcriptomic profiles and identify corresponding cell identities in vivo using marker genes. The data captured broad developmental gradients of proximal-distal and dorsal-ventral patterning as well as resolving specific cell lineages such as rhizoid cells at unprecedented resolution. I define central stem cells as tissue organisers of the Marchantia meristem. I show that central stem cells are auxin sources and develop novel markers to study auxin response in gemma. I identify a ERF/AP2 transcription factor Mp ERF20 as a positive regulator of central stem cell fate. I interrogate the regulation of the division zone by cytokinins and identify Mp CYCD1 as a key regulator of cell division rates in the Marchantia meristem. I show that Mp CYCD1 overexpression can be leveraged as a tool to induce ectopic cell divisions. I characterise meristem regeneration in Marchantia explants using marker lines and precise surgical manipulation. I observe the activation of proliferation markers and establish auxin transport reorganisation as a critical driver of meristem regeneration. Using this data I propose a model for meristem maintenance and regeneration in Marchantia which will form an important framework for future attempts to engineer growth in this simple morphogenic system.

Description

Date

2021-12-01

Advisors

Haseloff, Jim

Keywords

Marchantia Polymorpha, Meristem, Plant genomics, scRNA-seq, MpCYCD1, MpERF20, Plant stem cells, Cytokinin, Auxin, Plant tissue regeneration

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
BBSRC (1943399)
Biotechnology and Biological Sciences Research Council (1943399)