| Summary: | <p>microRNAs (miRNAs) are important regulators of gene expression and plant development. Some miRNAs, together with their target genes, have conserved functions among the major land plant lineages. An important miRNA-regulated developmental pathway is the ageing pathway that controls the juvenile to adult transition and the vegetative to reproductive transition in flowering plants. This thesis presents evidence that the function of HASTY (HST), a regulator of the ageing pathway through miRNA processing in the flowering plant Arabidopsis thaliana, is conserved in the bryophyte Marchantia polymorpha. MpHST negatively regulates the timing of the reproductive transition in male M. polymorpha, which is consistent with AtHST inhibiting phase transitions in A. thaliana. In contrary, MpHST positively regulates the reproductive transition in female M. polymorpha plants. This suggests a sex-specific function of MpHST in the dioecious liverwort and presents a new aspect of phase change regulation. This thesis also reports that another core regulator of miRNA biogenesis, DICER-LIKE 1 (DCL1), controls growth, morphogenesis and organ differentiation through miRNA processing in M. polymorpha, as it does in angiosperms. Moreover, a novel role was discovered for MpDCL1 – MpDCL1 represses the reproductive phase transition in the absence of the normally required far-red light inductive signal. This occurs in a sex-dependent manner – similar to the requirement of MpHST to delay phase transition in male plants, MpDCL1 represses phase change under non-inductive conditions in male plants. I propose that miRNAs, including sex-specific miRNAs, are involved in controlling phase change in the early divergent embryophyte M. polymorpha. This suggests that HST-regulated phase change operated in the common ancestor of liverworts and angiosperms, one of the first land plants.</p>
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