Trafficking to the Outer Polar Domain Defines the Root-Soil Interface

In animals, the interface between organism and environment is constituted by the epithelium [1]. In plants, the exchange of nutrients and signals between root and soil is crucial for their survival, but the cellular mechanisms underlying the epithelium-like function and specific localization of proteins to the root surface have not been identified [2]. Here we analyze the mechanism of polar delivery to the root-soil interface of the proteins BOR4, ABCG37, and PEN3, which transport nutrients [2], transport plant hormones, and are required for pathogen defense [3], respectively. The simultaneous visualization of these proteins and the apical and basal cargos in a single cell demonstrates that the outermost cell side represents an additional polar domain. Delivery to this outer polar domain depends on ARF GEF [4] and actin [5-8] function but does not require known molecular components of the apical or basal targeting. The outer polar delivery is, in contrast to known basal and apical cargos [9, 10], mediated by the polar secretion. Our findings show that the outermost cell membranes of roots define an additional polar domain in plant cells along with a specific, previously uncharacterized, polar targeting mechanism that is important for defining the functional, epithelium-like root-soil interface.