| Summary: | Since their origin from a unicellular ancestor 630–890 million years ago, the streptophytes, which comprise streptophyte algae and land plants, have evolved mechanisms to develop morphologically complex body plans. Much remains unknown about how such mechanisms evolved and how they regulate plant morphogenesis. To start, I hypothesise that cell polarity signalling, which can spatially regulate fundamental cellular processes like growth and division, contributed to morphological evolution in the streptophytes. In this context, the aim of this thesis is to investigate the evolution and function of RHO Of Plant (ROP) signalling, the plant specific form of the ancient eukaryotic RHO cell polarity signalling. First, I report phylogenetic and sequence analyses resulting in the discovery that ROP signalling genes became established early in the streptophyte lineage, during the period when multicellular filamentous body plans are believed to have evolved from unicellular/colonial forms. Second, by functionally characterising the single <i>ROP</i> gene in the liverwort <i>Marchantia polymorpha</i>, I demonstrate that ROP protein regulates the morphogenesis of complex plant tissues by controlling cell growth, division, and possibly adhesion. Finally, through a series of cross-species complementation experiments with <i>M. polymorpha rop</i> mutants, I show that ROP function has remained largely conserved since the time land plants last shared a common ancestor with the filamentous streptophyte algae, <i>Klebsormidium nitens</i>. Collectively, the findings from this thesis reveal that an ancient signalling mechanism which became established early in the streptophyte lineage is required for the morphogenesis of complex tissues in extant land plants. They also indicate a potential role of ROP signalling function in the morphological evolution of early streptophytes.
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