Defining where cell growth occurs on the surface of a cell is a crucial step in the development of three-dimensional tissues and organs in all multicellular organisms from whales and pine trees to mice and mint. Eukaryotes have evolved a conserved family of proteins, the RHO GTPase signaling molecules, that define the sites of growth on the surface of cells during development. The plant version of these proteins – “RHO GTPase of plants” (ROP) – were known to control the growth of single cells that are extremely polar – like filamentous root hairs or pollen tubes. However, the role of ROP in the development of multicellular arrays like tissues has remained missing.
Now, GMI senior scientist Liam Dolan and Hugh Mulvey, a postdoctoral fellow in the Dolan group, have discovered the role of ROP in the development of tissues in the liverwort Marchantia polymorpha that has a single copy of the ROP gene. The researchers show that ROP protein accumulates at the growth sites on the cells' surface during tissue development. Using CRISPR/Cas9-mediated gene editing, the scientists generated plants with a defective ROP gene. The shape of the cells in these “gene-edited” plants was defective; instead of forming brick-shaped cells, they formed spherical cells, like footballs. These cell shape defects disrupt the formation of complex three-dimensional tissues and organs, including the air chambers through which these plants “breathe”. The disruption of tissue development in plants with defective ROP function demonstrates that the normal function of these proteins is to coordinate cell growth and division during morphogenesis – the formation of plant organs.
ROP proteins are required to shape the plants and flowers that grow around us, and the GMI scientists suggest that these proteins controlled the shape and form of the first plants that grew on land over 450 million years ago.