The march of the PINs: developmental plasticity by dynamic polar targeting in plant cells

Abstract

Development of plants and their adaptive capacity towards ever-changing environmental conditions largely depend on the spatial distribution of the plant hormone auxin. At the cellular level, various internal and external signals are translated into specific changes in the polar, subcellular localization of auxin transporters from the PIN family thereby directing and redirecting the intercellular fluxes of auxin. The current model of polar targeting of PIN proteins towards different plasma membrane domains encompasses apolar secretion of newly synthesized PINs followed by endocytosis and recycling back to the plasma membrane in a polarized manner. In this review, we follow the subcellular march of the PINs and highlight the cellular and molecular mechanisms behind polar foraging and subcellular trafficking pathways. Also, the entry points for different signals and regulations including by auxin itself will be discussed within the context of morphological and developmental consequences of polar targeting and subcellular trafficking.

Figure 1

Auxin gradients during plant development. (A, B) DR5 promoter activity during embryogenesis, visualizes the auxin response first in all apical cells of octant embryos (A) and then only in the root stem cell niche of early heart (B) embryos. (C) During lateral root initiation DR5::GUS activity increases in the lateral root founder cells. Arrowheads indicate the asymmetric divided pericycle cells. (D) A restricted auxin gradient in the tip of root meristems maintains the stem cell niche. (E) DR5::GUS staining precedes the initiation of vascular strands in young leaves.

Figure 2

Subcellular trafficking mechanisms controlling PIN polarity and degradation. BFA, brefeldinA; EE, early endosomes; ER, endoplasmatic reticulum; MVB, multi-vesiculate bodies; PVC, pre-vacuolar compartments; RE, recycling endosomes; TGN, trans-golgi network; WM, wortmannin.

Figure 3

Schematic representation of transcytosis and phosphorylation-dependent polarity changes. EE, early endosomes; P indicates phosphorylated protein; PID, PINOID; RE, recycling endosomes; TGN, trans-golgi network.

Figure 4

PIN polarity changes during plant development. (A, B) Apolar PIN1 localization in apical cells of a 16-cell stage embryo (A), changes to basal PIN polar localization in early heart stage embryos (B). (C, D) PIN1 polarity changes from the anticlinal (C) to the periclinal cell sides (D) during lateral root initiation. (E, F) PIN3 polarization in response to gravity. Arrowheads indicate polar localization; arrows illustrate the gravity vector.