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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:713973 |
| Date | January 2015 |
| Creators | Jones, Victor Arnold Shivas |
| Contributors | Dolan, Liam ; Harberd, Nicolas |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:fd764164-55da-4b71-8397-f5a9e0f6ac4c |
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