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Review
. 2020 Nov 10;9(11):1527.
doi: 10.3390/plants9111527.

ER-Localized PIN Carriers: Regulators of Intracellular Auxin Homeostasis

Nayyer Abdollahi Sisi  1   2 , Kamil Růžička  1
Affiliations
Review

ER-Localized PIN Carriers: Regulators of Intracellular Auxin Homeostasis

Nayyer Abdollahi Sisi et al. Plants (Basel). .

Abstract

The proper distribution of the hormone auxin is essential for plant development. It is channeled by auxin efflux carriers of the PIN family, typically asymmetrically located on the plasma membrane (PM). Several studies demonstrated that some PIN transporters are also located at the endoplasmic reticulum (ER). From the PM-PINs, they differ in a shorter internal hydrophilic loop, which carries the most important structural features required for their subcellular localization, but their biological role is otherwise relatively poorly known. We discuss how ER-PINs take part in maintaining intracellular auxin homeostasis, possibly by modulating the internal levels of IAA; it seems that the exact identity of the metabolites downstream of ER-PINs is not entirely clear as well. We further review the current knowledge about their predicted structure, evolution and localization. Finally, we also summarize their role in plant development.

Keywords: ER-PINs; PIN proteins; PIN5; PIN8; auxin metabolism; auxin transport.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A ClustalW-based maximum likelihood [17] phylogenetic tree of the PIN proteins in A. thaliana, rooted with KfPIN [18]. PM: PINs present on PM, ER: PINs showing ER or dual PM and ER localization.
Figure 2
Figure 2
(a) The membrane topology, structural properties, and proposed direction of PIN-mediated auxin transport on PM and ER. The composition of the main structural units is illustrated on PM-PINs. (b) A scheme of the ER-PIN-mediated auxin transport, including the downstream IAA metabolism. Conversion of IAA to oxIAA or its conjugation to amino acid occurs most probably in the cytoplasm, while IAA de-conjugation was seen in the ER lumen. The subcellular localization of IAA conjugation with sugar moieties is unknown [23].
Figure 3
Figure 3
A simplified phylogenetic tree of the major PIN lineages in plants as proposed by Bennett et al. [22], including algal PIN from Klebsormidium flaccidum [18]. The taxonomical groups where the subcellular localization of the representative PIN transporters was experimentally confirmed are marked with a solid line, and the predicted localization is denoted by the dashed line.
See this image and copyright information in PMC

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