The root response to gravity: from the macro to the nanoscale

Plants are sessile organisms which adapt to their everchanging environment. The root is buried is the soil and continuously explores its surroundings. Indeed, while growing downwards to anchor the plants in the ground, it has to avoid obstacles and seek for nutrients and water. This seeking mechanism depends on the root perception of gravity. Through differential growth, the root is able to align according to the gravity vector. The growth is regulated at the cellular level by an increase of the plant hormone auxin, which activates the small Rho Guanine triphosphatase (Rho GTPase) of plant 6 (ROP6) at the plasma membrane to inhibit endocytosis and trigger cytoskeleton reorganization. Through a collaborative work, four French laboratories addressed the question of ROP6 membrane dynamics upon gravistimulation. Based on cellular biology, biochemistry and super resolution imaging approaches, they discovered that ROP6 is organized into nanoclusters at the plasma membrane of plant cells in response to auxin. The stabilization of ROP6 in these nanoclusters is required for signaling and thus the regulation of gravitropic bending. The formation of these nanoclusters is dependent upon the membrane lipid phosphatidylserine, which directly interact with ROP6. Using a genetic toolkit, the authors uncovered that phosphatidylserine is rate limiting for the ROP6-dependent nanocluster formation, which in turn tunes the cellular read outs. This work, not only explain the fine mechanism of the root response to gravity from the developmental level to the nanoscale but also provide a valuable insight towards the understanding of small GTPase signaling in eukaryotic system.