ER-Localized PIN Carriers: Regulators of Intracellular Auxin Homeostasis

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.

Genome-Wide Identification and Characterization of PIN-FORMED (PIN) Gene Family Reveals Role in Developmental and Various Stress Conditions in Triticum aestivum L.

“…Most of the TaPIN proteins were predicted to be located on the plasma membrane ( Table 1 ). The plasma membrane-localized long PINs are involved in polar auxin transport, while ER-localized short PINs regulate intracellular auxin homeostasis [ 2 , 22 , 35 , 38 ]. Therefore, these results also suggest that PIN genes in wheat could have a similar function.…”

“…PIN proteins have been well characterized in Arabidopsis, which includes eight members that differ in the length of a middle region [ 34 , 36 , 37 ]. Five of the Arabidopsis PINs (AtPIN1-4 and AtPIN7) have a long hydrophilic loop situated at the plasma membrane, implicated in the directional and cell-to-cell auxin transport [ 2 , 22 , 38 ]. Further, these long AtPINs do not localize statically in the plasma membrane; however, they constitutively cycle between the plasma membrane and endosomal compartments, and their relocation is triggered by internal and external stimuli [ 39 ], while three AtPINs, namely, AtPIN5, AtPIN6 and AtPIN8, contain a smaller central hydrophilic domain, and both AtPIN5 and AtPIN8 are located in the endoplasmic reticulum, indicating a potential role in modulating intracellular auxin homeostasis [ 40 , 41 ].…”

Cytokinin-Controlled Gradient Distribution of Auxin in Arabidopsis Root Tip

“…In Arabidopsis thaliana , according to the length of the hydrophilic loop in the middle of the polypeptide chain, the PINs family is divided into two subfamilies: as auxin efflux carriers, PIN1, PIN2, PIN3, PIN4 and PIN7 contain a long hydrophilic loop and are located in the PM (plasma membrane) [ 117 , 119 , 120 ], while PIN5, PIN6 and PIN8 with a short hydrophilic loop are mainly located in the ER (endoplasmic reticulum), which are involved in intracellular auxin transport [ 117 , 119 , 120 , 134 ]. All of the PIN efflux carriers are expressed and active in the root tip and perform their respective functions [ 116 , 117 , 119 , 120 , 129 , 135 ].…”

“…Compared with the auxin efflux PINs located on the plasma membrane, which are involved in the intercellular transport of auxin, ER-localized PINs and PILSs mediate the intracellular transport of auxin [ 120 , 134 , 150 , 151 , 152 , 153 , 154 ]. ER-localized PINs are speculated to mediate auxin flow into (PIN5) or out (PIN8) of the ER lumen [ 120 , 152 , 154 ], or hypothetically from the ER lumen into the nucleus (PIN6 and PIN8) to open the auxin downstream genes’ transcription [ 150 , 152 ].…”

“…The roots of PIN5, PILS2, or PILS5 gain-of-function mutants become shorter; on the contrary, the roots of loss-of-function mutants become longer [ 21 , 146 , 148 ]. This is because ER-localization auxin transport carriers, PIN5 and PILSs negatively regulate PAT and auxin signaling [ 21 , 134 , 146 , 147 , 154 , 155 , 156 ]. Interestingly, unlike PINs involved in intercellular transport, transcription of PIN5 and PILS5 is induced by cytokinin ( Figure 1 ) [ 15 , 21 , 158 , 159 ], suggesting that PIN5 and PILS5 are other targets of cytokinin–auxin antagonism besides auxin conjugation enzymes GH3.5, GH3.6 and GH3.17.…”