Sensing and responding to the environment is an adaptation which is essential to life. In biology, environmental stimuli and cues are sensed by receptors, which in turn activate signal transduction and downstream responses. Downstream responses are often mediated by transcriptional regulators, which can coordinate responses to environmental stimuli.

GRAS-domain transcriptional regulators are a family of plant proteins, which are highly conserved across land plants and have diverse functionality. Many of these roles involve responding to environmental stimuli, and the signalling processes involving most GRAS proteins are fairly well understood in flowering plants. This thesis investigates the GRAS family in liverworts. MpGRAS7, a GRAS gene in the liverwort Marchantia polymorpha (common name: Marchantia) was found to be transcriptionally responsive to stress, and it was hypothesised that it had a role in controlling biological responses to the environment.

My phylogenetic analyses found that this gene belongs to a previously uncharacterized sub-clade of GRAS transcriptional regulators (SCARECROW-LIKE A), which is conserved across land plants but lost in the model plant Arabidopsis thaliana. In Marchantia, I found the gene to be transcriptionally responsive to abiotic and biotic stimuli, and also enriched in vegetative and generative reproductive organs. I have produced CRISPR knockout mutants in Marchantia polymorpha and its AM-compatible sister species, Marchantia paleacea, in order to investigate the functions of SCLA in liverworts. Phenotyping revealed MpGRAS7 to be involved in the vegetative to generative reproductive transition of Marchantia polymorpha. I also found SCLA to be a positive regulator of oomycete pathogen colonisation in both species. This thesis explores upstream regulation of MpGRAS7 by far-red light and drought stresses, as well as potential downstream targets. This investigation is the first characterisation of the functionality of this sub-clade of genes in any plant species, and thus contributes significant new knowledge to the fields of plant environmental signalling and development.