The evolution of RHO of plant (ROP) proteins and their morphogenetic functions

The ability of a cell to polarise, and direct cell growth or orient cell division, for example, is fundamental for the morphogenesis of multicellular organisms. A key molecular system for signalling cell polarity in diverse eukaryotes involves the RHO family of small GTPases. Since its origin in early eukaryotes, the RHO family has evolved independently in different lineages, and the plant-specific subfamily of RHO - RHO of plants (ROP) - was established in the streptophyte algal ancestors of land plants. Insights from both bryophytes and vascular plants have revealed conserved roles for ROP signalling in land plant morphogenesis. Synthesising our current understanding of how ROP signalling evolved and how it regulates morphogenesis in extant land plants, we propose that this polarity signalling system was co-opted to spatially coordinate morphogenetic mechanisms that evolved in the algal ancestors of land plants.