Evolution and development of a novel water conducting tissue in Marchantia polymorpha

Supervisors: Justin Goodrich, Gwyneth Ingram

Project Description:

The evolution of land plants from their aquatic ancestors required adaptations to a much drier environment:  for example land plants have a waterproof covering, the cuticle, which prevents desiccation and most have water transport systems to get water from soil to aerial tissues.  We have identified a transcription factor, ZHOUPI (ZOU) which is important for cuticle formation in Arabidopsis embryos and also for the breakdown of the endosperm tissue that surrounds the embryo in seed, allowing embryo growth.  The ZOU gene is conserved in all extant land plants and therefore was likely present in the common ancestor of land plants.  To understand how the ZOU gene regulatory network may have evolved, we are using the liverwort Marchantia polymorpha as a model organism representing one of the earliest diverging groups in land plant evolution.  We used CRISPR cas9 genome editing to inactivate the two Marchantia ZOU genes and strikingly we find that one is required for formation of pegged rhizoids, an external water conducting tissue.  Pegged rhizoids are a specialised type of rhizoid that undergo programmed cell death during differentiation, to create a hollow tube, and have secondary cell wall thickening  (the pegs) to prevent the tube collapsing.  There are similarities to the role in flowering plant endosperm, where ZOU promotes cell wall remodelling and programmed cell death.  To test whether these similarities reflect a common underlying pathway, we are using transcriptional profiling to compare zou mutant and wild type Marchantia plants and thus identify ZOU target genes.  The project will involve characterizing the role of the target genes in Marchantia pegged rhizoid development, using genome editing and over-expression constructs to make loss of function and gain of function mutants and analysing their effects on plant morphology.  It will provide a thorough training in plant molecular and developmental biology analysis.  

Fourquin C, Beauzamy L, Chamot S, Creff A, Goodrich J, Boudaoud A, Ingram G. 2016. Mechanical stress mediated by both endosperm softening and embryo growth underlies endosperm elimination in Arabidopsis seeds. Development 143: 3300-3305.
Shimamura M. 2016. Marchantia polymorpha: Taxonomy, Phylogeny and Morphology of a Model System. Plant Cell Physiol 57: 230-256.
Yang S, Johnston N, Talideh E, Mitchell S, Jeffree C, Goodrich J, Ingram G. 2008. The endosperm-specific ZHOUPI gene of Arabidopsis thaliana regulates endosperm breakdown and embryonic epidermal development. Development 135: 3501-3509.

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