The experimental taxonony of flowering plants is now firmly established, and major advances in our knowledge of the Pteridophytes has come from similar studies. The present study is attempt to apply the methods and approach of experimental taxonomy to a group of mosses which have not yet been investigated in this way.

No description available.

The moss genus Phascum is common in the Northern Hemisphere. Its species may be frequently encountered by botanists studying the moss flora of bare earth in fallow fields and gardens. Species within this genus have been recognised by taxonomists since at least 1741. The present form of the generic name dates from 1753. Yet Phascum still appears to be poorly understood. Even with available modern treatments, in floras such as Nyholm (1960) and Lawton (1971), bryologists find it difficult to recognise taxa and seem hesitant to name species. Why are these taxa difficult to identify? Is it because they are a complex of "good" but difficult to recognise species and varieties or a loose assemblage of intergrading forms and ecotypes? The present work attempts to answer the following questions: - 1. Is Phascum divisible into recognisable entities in the Northern Hemisphere? And, if it is divisible, what should the subunits be correctly called? 2. If the species Phascum cuspidatum Schreb. ex Hedw. is proved to be a valid taxon, is it possible to subdivide it into infraspecific units? And, if it is divisible, at what level should the subunits be recognised and what should they be correctly called? The problem was tackled in three phases. First, in order to cover the nomenclatural questions, using the Index Muscorum edited by Wijk (1967) as a starting point, all the relevant publications which contained citations of putative taxa or new combinations within the genus were assembled. From the information provided in these works a chronological survey of the taxonomy of the genus from 1770 to the present day was compiled (Chapter I). Hopefully this synopsis, of what turned out to be an extremely complex literature, will provide a reference point not only for this study but for any further researches in the genus in Europe and North America. Alterations and amendments to the information provided in the Index Yuscorun, together with other notes and comments, are incorporated in the survey where appropriate. Secondly, from these taxonomic publications, a list of the definitive characters used in the descriptions of the many putative taxa was compiled. This information was used to provide direction in a study of the taxonomic usefulness of these characters and thus the validity of the proposed taxa. This work, which included some growth experiments, was carried out within the eight fields of study suggested by Turrill (1942). Attention was paid to both traditional and more modern taxonomic methods, including computational techniques. An attempt was also made to fill the gaps in areas not covered by this work by reference to the specialised published literature within some of these eight fields (Chapter II). Thirdly, the genus Phascum is discussed in terms of both historical treatment and current practice concerning generic and specific concepts. And, finally, from the results of the studies carried out and with due reference to the literature and specimens seen the genus Phascum is redefined in terms of its component, taxa. These are thereafter typified and named, as far as possible at the present state of knowledge, in accordance with the International Code of Botanical nomenclature (I.C.B.N.) of 1972. An indication is provided of work which still remains to be carried out (Chapter III).

No description available.

The evolution of specialised rooting structures was one of the key morphological innovations that allowed the first multicellular plants to colonise land more than 470 million years ago. The first land plants likely resembled modern bryophytes with a rooting structure that consists of filamentous rooting cells called rhizoids, which are morphologically similar to the root hairs of vascular plants. This thesis describes identification of genes required for rhizoid development in a model bryophyte, the liverwort Marchantia polymorpha. The aim was to identify the conserved and novel aspects of the gene regulatory networks for filamentous rooting cell development in land plants. Agrobacterium-mediated T-DNA transformation of haploid M. polymorpha spores was used to generate 150,000 M. polymorpha T-DNA insertion lines, which were screened for defects in rhizoid development. 204 rhizoid defective mutants were isolated which allowed identification of 25 genes required for M. polymorpha rhizoid development. The role of the class I RSL (ROOT HAIR DEFECTIVE SIX-LIKE) genes as positive regulators of root hair and rhizoid differentiation was found to be conserved between liverworts and vascular plants. This suggests that the class I RSL gene mechanism is ancient and originated before the two lineages diverged more than 440 million years ago. Furthermore, the M. polymorpha class I RSL gene MpRSL1 is required for development of gemmae and mucilage papillae, which similarly to rhizoids form from single epidermal cells that elongate to form an outgrowth. Moreover, MpRSL1 is under negative feedback regulation of the FEW RHIZOIDS1 (FRH1) microRNA. Many genes required for rhizoid elongation were similar with those required for root hair elongation, suggesting that these genes are likely to have been part of the genetic network downstream of class I RSL genes in the common ancestor of liverworts and vascular plants.

The first land plants faced a harsh terrestrial environment when they emerged from the water over 470 million years ago, and one of the key adaptations that allowed them radiate across the land was the development of a rooting system. To investigate the genetic mechanism that controlled the differentiation of rooting cells in ancient land plants, I carried out a mutant screen to identify genes that regulate rhizoid development in the liverwort Marchantia polymorpha, a member of the earliest-diverging lineage of land plants. I used insertional mutagenesis to generate a population of 105,000 lines from which I selected 61 mutants with defects in rhizoid development, and identified 10 genes that are part of the network of genes that influence the differentiation and growth of rhizoids. Eight of these are late-acting genes that are required for the elongation of rhizoids by tip growth, while two are transcription factors that direct early events in the adoption of rhizoid fate. I identified the bHLH transcription factor MpROOT HAIR DEFECTIVE 6-LIKE1 (MpRSL1) as a key regulator of rhizoid differentiation, as gain-of-function mutations in MpRSL1 cause rhizoids to develop in ectopic locations. The homologues of MpRSL1 in the angiosperm Arabidopsis and the moss Physcomitrella control the differentiation of their root hairs and rhizoids, respectively, which suggests that a gene regulatory network that included RSL genes controlled the development of filamentous rooting cells in the last common ancestor of all land plants. I also identified MpWIP, which encodes a member of a plant-specific family of zinc finger proteins, as a putative regulator of the development of both rhizoids and the cells of the air pore complex, a second specialized epidermal cell type. WIP genes have not been implicated in the control of rooting cell development in other species, so this role in Marchantia may be either inherited from the earliest land plants or a derived character. This work demonstrates the suitability of M. polymorpha as a subject for large-scale mutageneses and screens for gene discovery. The genes I have found to be involved in rhizoid development indicate that the last common ancestor of all land plants already possessed a gene regulatory network that controlled the development of rooting cells, and that at least some of its components, such as RSL genes, have been conserved in its descendents since the divergence of the liverworts and other land plants.

No description available.

No description available.

No description available.

Land plants evolved from a group of aquatic algae known as charophytes and molecular evidence suggests that they were pre-adaptated to life on land. Early land plants necessarily required mechanisms to survive dehydration and the plant hormone abscisic acid (ABA) is known to play a vital role in this conferring desiccation tolerance in all land plants. The basal non-vascular land plants, made up of the liverworts, hornworts and mosses, rely heavily on ABA-mediated vegetative dehydration/desiccation tolerance (D/DT) as they lack anatomical adaptations to retain water and this trait remains a developmentally regulated feature of the angiosperm seed. ABA non-responsive (anr) mutants were identified in the model bryophyte Physcomitrella patens and genotyping of segregating populations enabled the mapping of the PpANR locus. This locus encodes a trimodular MAP3 kinase comprising an N-terminal PAS domain, a central EDR domain and a C-terminal MAPKKK-like domain (“PEK” structure). Mutants of PpANR showed dehydration hypersensitivity and an inability to respond to exogenous ABA demonstrating the vital role of PpANR in the ABA-dependent osmotic stress responses. RNA-seq analysis of wild-type and anr mutant plants also revealed potential components of the wild-type ABA-dependent osmotic stress response not yet characterised in bryophytes. Phylogenetic analysis reveals PpANR to be part of a basal plant-specific subfamily of MAP3Ks closely related and possibly ancestral to the “EK” structured negative ethylene regulator CTR1 and the “PK” structured positive ABA regulators Raf10/11. The establishment of these subfamilies in the charophytes suggests them as potential vital components of ancestral water stress responses. The PAS domain likely originated from a domain swap from histidine kinases in the green algae and the solving of the crystal structure of this domain reveals it to form a homodimer with each domain taking the canonical PAS fold structure. A model is suggested for a key role of PpANR in an ancestral ABA-dependent osmotic stress signalling pathway.