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

Matthieu Pierre Platre

doi:10.5802/crbiol.23

Abstract

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.

Highlights

Auxin as a master regulator of root growth during gravity responseAuxin is a key factor in regulating root growth upon gravistimulation
We looked at the downstream cellular gravity responses and observed, again, a direct correlation
Rop6 is a GTPase which is able to switch from an active form when bound to guanosine diphosphate (GTP), to the inactive form, when bound to guanosine diphosphate (GDP)

Summary

Introduction

Auxin is a key factor in regulating root growth upon gravistimulation. It is mainly produced in the leaves and transported downward to the root tip through the auxin carrier PIN FORMED 1 (PIN1; [1, 2]). The higher auxin concentration promotes the activation of the “non-genomic” auxin signaling pathway in the root epidermis which in turn triggers ROP6 activation [6, 7] This cellular signaling cascade inhibits the endocytosis of cargo proteins and reorganizes microtubules in order to restrain cellular growth (Figure 1, Panel 3; [8,9]). This pathway creates a differential growth, which on the upper part is not affected while in the lower is decreased. This local regulation of cellular growth promotes bending at the organ level in order to realign the root according to the gravity vector (Figure 1, Panel 2)

Phosphatidylserine levels tune the root response to gravity

Phosphatidylserine is a player in the “nongenomic” auxin signaling pathway

Conclusion

L’importance de la phosphatidylsérine pour la réponse à la gravité

La phosphatidylsérine est un acteur de la voie « non-génomique » de l’auxine