Functional analyses of growth and development in the liverwort Marchantia polymorpha
Abstract: Land plants developed from a freshwater charophycean algae about 500 million years ago. Today, they consist of two main clades, the vascular plants and the non-vascular bryophytes including hornworts, liverworts and mosses. To adapt to challenges within a terrestrial habitat, the first land plants evolved a diversity of hormonal and genetic pathways regulating growth and development. Analyses regarding these networks are mainly based on the angiosperm Arabidopsis thaliana. Genes of other land plant lineages that are inexistent in Arabidopsis are often not considered in functional studies, resulting in an incomplete picture of land plant evolution. The remarkable phylogenetic position of bryophytes makes them interesting for studies of gene function as they might carry different characteristics compared to e.g. angiosperms. In difference to vascular plants, the liverwort Marchantia polymorpha harbors a small, low genetic redundant genome containing most gene families present in Arabidopsis. Thus making it an advantageous model organism to determining specific gene function. This thesis focuses on describing how dormancy and the circadian clock regulate growth in Marchantia. To avoid growth during unfavorable environmental, plants apply dormancy programs. Marchantia applies dormancy in gemmae, small asexual propagules produced by the shoot in a cup. Gemmae are dormant in the cup until they are dispersed by rain and subsequently germinating. I show that high levels of absisic acid (ABA), inhibits gemmae germination within the cup. Gemmae with a manipulated MpCYP707A, a gene involved in catabolism of ABA and seed dormancy regulation in Arabidopsis, showed altering dormancy suggesting that ABA homeostasis is fundamental for regulation of gemmae dormancy. Because dormant gemmae are not physically attached to the cup it has been speculated that the signal maintaining dormancy of gemmae is a gas. I found that gemmae mutated in positive and negative regulators of the ethylene signaling pathway showed decreased and increased dormancy respectively, suggesting that ethylene regulates dormancy through ABA.I also found that the circadian clock in Marchantia regulates growth of the thallus, possibly by affecting auxin levels. The circadian clock in land plants appears in structural differences between species. I showed that the gene MpDET1 has a conserved structure but harbors a different function compared to Arabidopsis. In Arabidopsis, growth is regulated by the clock through PIF genes, but in Marchantia, this pathway appears independent of PIF. Although the clock mechanism appear well conserved in land plants, its structure and function has evolved, creating diversity between land plant groups.
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