Abstract

Plants are permanently situated in a fixed location and thus are well adapted to sense and respond to environmental stimuli and developmental cues. At the cellular level, several of these responses require delicate adjustments that affect the activity and steady-state levels of plasma membrane proteins. These adjustments involve both vesicular transport to the plasma membrane and protein internalization via endocytic sorting. A substantial part of our current knowledge of plant plasma membrane protein sorting is based on studies of PIN-FORMED (PIN) auxin transport proteins, which are found at distinct plasma membrane domains and have been implicated in directional efflux of the plant hormone auxin. Here, we discuss the mechanisms involved in establishing such polar protein distributions, focusing on PINs and other key plant plasma membrane proteins, and we highlight the pathways that allow for dynamic adjustments in protein distribution and turnover, which together constitute a versatile framework that underlies the remarkable capabilities of plants to adjust growth and development in their ever-changing environment.

Introduction

Plasma membrane proteins play key roles in mediating responses to the endogenous and environmental cues that regulate various developmental and metabolic events. Intrinsic membrane proteins have been implicated in various transport processes across membranes as well as in perception and further transmission of regulatory signals. Crosstalk at the plasma membrane interface is all the more important for sessile organisms such as higher plants, which are required to maintain a diverse spectrum of plasma membrane receptor and transport proteins in order to be able to properly sense and respond to fluctuations in their environment (Geldner and Robatzek, 2008; Otegui and Spitzer, 2008; Richter et al., 2009; Robinson et al., 2012; Korbei and Luschnig, 2013). In plants, as in any other eukaryote, this is achieved by a complex system of internal membranes that serves to transport proteins to their site of action and degradation. Exchange of material between these membranes is achieved by vesicles that bud off from one membrane, mature and fuse with another in a highly controlled fashion.