Outer, inner and planar polarity in the Arabidopsis root

- AIP1-2-dependent and TWD1-dependent actin function or organization mediate planar polarity.
- A lipid microdomain underlies polar D6PK and ROP recruitment during hair initiation.
- Distinct ACT7 functions in TGN trafficking and in EXO84b outer domain polarity.
- Phosphorylation-dependent and AP2-dependent endocytosis maintain NIP5:1 outer domain polarity.
- SGN1 outer membrane polarity in CIF1/2-SGN3 signalling of Casparian strip placement.

Plant roots control uptake of water and nutrients and cope with environmental challenges. The root epidermis provides the first selective interface for nutrient absorption, while the endodermis produces the main apoplastic diffusion barrier in the form of a structure called the Casparian strip. The positioning of root hairs on epidermal cells, and of the Casparian strip around endodermal cells, requires asymmetries along cellular axes (cell polarity). Cell polarity is termed planar polarity, when coordinated within the plane of a given tissue layer. Here, we review recent molecular advances towards understanding both the polar positioning of the proteo-lipid membrane domain instructing root hair initiation, and the cytoskeletal, trafficking and polar tethering requirements of proteins at outer or inner plasma membrane domains. Finally, we highlight progress towards understanding mechanisms of Casparian strip formation and underlying endodermal cell polarity.