Abstract
Auxin plays crucial roles in multiple developmental processes, such as embryogenesis, organogenesis, cell determination and division, as well as tropic responses. These processes are finely coordinated by the auxin, which requires the polar distribution of auxin within tissues and cells. The intercellular directionality of auxin flow is closely related to the asymmetric subcellular location of PIN-FORMED (PIN) auxin efflux transporters. All PIN proteins have a conserved structure with a central hydrophilic loop domain, which harbors several phosphosites targeted by a set of protein kinases. The activities of PIN proteins are finely regulated by diverse endogenous and exogenous stimuli at multiple layers—including transcriptional and epigenetic levels, post-transcriptional modifications, subcellular trafficking, as well as PINs’ recycling and turnover—to facilitate the developmental processes in an auxin gradient-dependent manner. Here, the recent advances in the structure, evolution, regulation and functions of PIN proteins in plants will be discussed. The information provided by this review will shed new light on the asymmetric auxin-distribution-dependent development processes mediated by PIN transporters in plants.
Highlights
Auxin is a universal hormone in plants, which participates in many aspects of plant developmental and growth processes [1]
With the help of these auxin carriers, the auxin can be transported in a polar manner within cells, where specific maxima and minima levels of auxin can be sensed by the downstream regulation modules, such as the TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX (TIR1/AFB)-Auxin/INDOLE-3-ACETIC ACID (Aux/IAA)-AUXIN RESPONSE FACTOR (ARF) signal pathway, to transform endogenous and exogenous stimuli into gene reprogramming events [5,18,19]
Much progress has been made in understanding the roles of PIN proteins on developmental and growth processes
Summary
Auxin is a universal hormone in plants, which participates in many aspects of plant developmental and growth processes [1]. To facilitate polar auxin transport (PAT), several auxin transporters were identified, such as AUXIN1/LIKE-AUX1 (AUX/LAX), PIN-FORMED (PIN), ATP-binding cassette (ABC) transporters, nitrate transporter 1.1 (NRT1.1), PIN-Like transporters (PILS) and WALLS ARE THIN 1 (WAT1) [7,9,10,11,12,13,14,15,16,17]. These auxin carriers can be generally divided into two groups based on the subcellular locations and functions. The information provided by this review will shed new light on the molecular mechanisms of the auxin maxima mediated by the activities of PIN efflux carriers, as well as the roles of PIN proteins in regulating plant development and growth
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