ROP GTPase regulates polarised cell growth and cell division orientation during tissue development and organogenesis inMarchantia polymorpha

SUMMARY Cell polarity – broadly defined as the asymmetric distribution of cellular activities and or subcellular components within a cell – determines the geometry of cell growth and division during development. RHO GTPase proteins regulate the establishment of cell polarity and are conserved among eukaryotes. RHO Of Plant (ROP) proteins are a subgroup of RHO GTPases that are required for cellular morphogenesis in plants. However, how ROP proteins modulate the geometry of cell growth and division during the morphogenesis of plant tissues and organs is not well understood. To investigate how ROP proteins function during tissue development and organogenesis, we characterised the function of the single copy ROP gene of the liverwort Marchantia polymorpha . M. polymorpha develops morphologically complex three-dimensional tissues and organs exemplified by air chambers and gemmae, respectively. Mp rop loss of function mutants form defective air chambers and gemmae, indicating ROP function is required for tissue development and organogenesis. During air chamber and gemma development in wild-type, the MpROP protein is enriched to sites of polarised growth at the cell surface and accumulates at the expanding cell plate of dividing cells. Consistent with these observations, polarised cell growth is lost, and cell divisions are misoriented in Mp rop mutants. We propose that ROP regulates both polarised cell growth and cell division orientation in a coordinated manner to orchestrate tissue development and organogenesis in land plants.