Polar transport in plants mediated by membrane transporters: focus on mechanisms of polar auxin transport
Introduction
The movement of functional molecules in a single direction between cells, termed polar transport, regulates development and adaptation to surrounding environments. Many functional molecules possess limited membrane permeability, and their movement depends on transporters that localize to plasma membranes (PMs). PIN-FORMED proteins (PINs) are PM-localized auxin efflux carriers that regulate the direction of cellular auxin export via their polar localization. The coordinated polar localization of PINs within a tissue determines the direction of cell-to-cell transport of auxin [1, 2], thereby linking cellular and tissue polarity and providing the basis for multiple developmental processes, including embryogenesis, vascular formation, initiation of lateral organs, and tropic growth [3, 4] (Figure 1a). A large number of experiments support the importance of membrane dynamics in polar localization of PINs [5, 6, 7, 8, 9, 10]. Although PINs can be statically located at the polar domain of the PM, it has been proposed that PINs undergo continuous cycling between PMs and endosomal compartments [9, 10, 11, 12, 13]. Particularly, polar recycling has been proposed to regulate polar localization of PIN proteins. These cycling models are consistent with the dynamic PIN polarity changes seen during plant development and plant responses to environmental signals, and therefore, they have been widely accepted [14, 15, 16].
In this review, current knowledge regarding the mechanisms underlying polar auxin transport is provided by classifying into three regulatory systems for PINs: polar localization, control of abundance at PMs, and biochemical activity. Recently identified nutrient transporters that localize to PMs in a polar manner are also discussed as regulators of nutrient transport from soil to root vasculature [17, 18, 19, 20, 21, 22] (Figure 1b,c).
Section snippets
Control of polar localization
Polar localization of PINs is proposed to be established by the combination of three distinct membrane functions: (1) polar recycling from endosomal compartments, (2) reducing lateral diffusion within polar domains to retain polar localization, and (3) spatial restriction of endocytosis for the removal of laterally-mislocalized PIN proteins that counteract lateral diffusion of PINs [23]. In this section, our current knowledge about each of these steps is described.
PINs are proposed to undergo
Mechanisms of lateral nutrient transport
Soil nutrients can enter the central vasculature of plant systems, via concentric layers of epidermis, cortex, and endodermis, by three directional transport (hereafter referred to as lateral transport) pathways: apoplastic, symplastic, and trans-cellular route (for details, see review [49]). Recent research identified a battery of influx and efflux carriers for nutrients localized to the lateral (inner/central or outer/peripheral) side of the cell, indicating that mechanisms similar to auxin
Conclusions
Knowledge regarding the regulatory mechanisms governing polar transport has advanced substantially in recent decades. Substantial numbers of polar-localized transporters have been identified, and cell biological analysis has shown that polar localization and turnover of these transporters are tightly regulated by membrane functions to allow plants to adjust growth and development in response to changing environments. Although the general concepts of polar transport regulation are largely
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
I apologize to the authors whose works are not cited because of space constraints. I thank Peter Marhavy and Tomoko Dainobu for critical reading of the manuscript.
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