Polar Auxin Transport

Abstract

Auxins, a group of plant signalling compounds, ensure proper growth and development of the plant in relation to both external and internal stimuli. Within a plant, auxin is distributed asymmetrically, thus creating local auxin maxima and minima. Such asymmetric auxin distribution underlies many developmental and stress adaptation processes and facilitates their spatial and temporal coordination. The appropriate local auxin level is achieved by regulation of biosynthesis, metabolism and through active, cell‐to‐cell transport relying on auxin carriers that are embedded in the plasma membrane. Owing to the physical–chemical properties of the auxin molecule, the asymmetric (polar) localisation of auxin efflux carriers, analogous in neighbouring cells, determines the direction of intercellular auxin flow (polar auxin transport). With respect to the important function of polar auxin transport for plant development, the polar auxin transport machinery is subject to a tight control at multiple regulatory levels. Key Concepts Auxin flow through polar auxin transport pathways relies on concerted action of passive diffusion and auxin transporters. Complex machinery consisting of both auxin carriers and regulatory networks modulates polar auxin transport. Polar auxin transport machinery, together with auxin biosynthesis and its further metabolism, contributes to establishment and control of auxin homeostasis and therefore to the availability of auxin for signalling pathways. Detailed quantitative data are needed for further understanding of polar auxin transport regulation. Mathematical modelling based on detailed quantitative data represents a promising tool to understand contribution of individual processes to resulting polar auxin flow.

Arp2/3 Complex Is Required for Auxin-Driven Cell Expansion Through Regulation of Auxin Transporter Homeostasis

“…We also show here that Arp2/3 mutants have changed auxin metabolism in respect to increased pool of inactivated auxin, as well as increased auxin concentration in pavement cells. One of the most important factors that regulate auxin levels within and outside the cell is polar auxin transport (Lacek et al, 2017). We tested whether transporter localization was also an issue in pavement cell formation.…”

“…Auxin has been shown to be a major hormone involved in cell shape and patterning (Teale et al, 2006;Gallavotti, 2013;Saini et al, 2013). Auxin-driven cell morphogenesis relies in the correct localization of auxin carriers, which regulate this hormone's cell-to-cell transport in order to create different concentration gradients, and failure to efficiently transport auxin across cells results in reduced tissue differentiation (Lacek et al, 2017). Therefore, correct auxin transporter localization is of utmost importance for correct auxin distribution.…”

The TOR–Auxin Connection Upstream of Root Hair Growth

“…Cell fate establishment requires fine-tuned cellular events, including the activity of specific receptor-like protein kinases and transcriptional regulators, substantially depending on auxin availability and signaling [ 75 , 77 , 78 , 79 ], which are described in Section 4 . PIN2 facilitates auxin distribution from the very root tip shootwards, over the lateral root cap and epidermis cells, and back to the root tip over the cortex, resembling a reverse fountain [ 21 , 80 , 81 ]. In the two cell files of the epidermis, the trichoblasts and atrichoblasts, PIN2 undergoes differential intracellular trafficking, showing higher internalization rates, followed by lower protein abundance at the plasma membrane (PM) in trichoblasts [ 82 ].…”

“…Loss-of-function mutations in PIN1 exhibit a pointed inflorescence stem that fails to produce flowers, emphasizing not only the role of auxin, but also of auxin transport in this process [ 121 , 122 ]. In the root, the redistribution of auxin towards the elongation zone takes place in the root tip and involves the activity of PIN3, PIN4, and PIN7 that localize to the PM of root cap columella cells [ 81 , 124 , 125 ] Auxin flow continues through PIN2 over the lateral root cap cells and epidermis shootward towards the differentiation zone, and back over the cortex to the root meristem [ 21 , 80 , 81 ]. Auxin enhances the cell proliferation rate in the root meristem [ 119 , 123 ], and when the energy status of the whole plant drops, auxin distribution from the shoot to the root is reused to stop root growth by downregulating PIN expression over the SnRK1 signaling pathway [ 126 ].…”

Lessons Learned from the Studies of Roots Shaded from Direct Root Illumination

“…Finally, differential root development under direct illumination results in altered shoot growth, as shoots with shaded roots accumulate less mass and anthocyanins, demonstrating differential distribution of resources throughout the plant and fitness depending on the experimental growth conditions chosen [26]. The regulation of root growth (e.g., the timing of lateral root and root hair emergence, directional root growth) is highly dependent on the finely tuned distribution of numerous signaling molecules, including sugars, ROS, phytohormones, and other small molecules whose availability per cell is strongly modulated by internal and external conditions [4,16,19,29,33,[35][36][37]. Therefore, the direct interplay of phytohormones, light, and sugar signaling pathways in roots exposed to direct illumination compared to shaded roots has been the target of several recent studies [4,21,23,24,[26][27][28]32,[37][38][39][40].…”