Spatiotemporal asymmetric auxin distribution: a means to coordinate plant development

Abstract

The plant hormone auxin plays crucial roles in regulating plant growth development, including embryo and root patterning, organ formation, vascular tissue differentiation and growth responses to environmental stimuli. Asymmetric auxin distribution patterns have been observed within tissues, and these so-called auxin gradients change dynamically during different developmental processes. Most auxin is synthesized in the shoot and distributed directionally throughout the plant. This polar auxin transport is mediated by auxin influx and efflux facilitators, whose subcellular polar localizations guide the direction of auxin flow. The polar localization of PIN auxin efflux carriers changes in response to developmental and external cues in order to channel auxin flow in a regulated manner for organized growth. Auxin itself modulates the expression and subcellular localization of PIN proteins, contributing to a complex pattern of feedback regulation. Here we review the available information mainly from studies of a model plant, Arabidopsis thaliana, on the generation of auxin gradients, the regulation of polar auxin transport and further downstream cellular events.

Context, Specificity, and Self-Organization in Auxin Response

“…Although spatial differences in the activity of these homeostatic mechanisms contribute significantly to patterns of auxin accumulation it is active and directed cell-to-cell movement of auxin that is the primary determinant of the gradients and highly asymmetric localizations of auxin that drive so much of development (Benkova et al 2003;Tanaka et al 2006). Active intercellular auxin transport is mediated by a system of membrane proteins prominent among which are the AUX1/LAX family of amino acid permeases for auxin influx and the PIN and ABCB (or PGP) families of transmembrane proteins for auxin efflux from the cell (Bennett et al 1996;Tanaka et al 2006;Bandyopadhyay et al 2007).…”

“…From the specification of apical-basal axis in the embryo to the control of anisotropic growth in response to gravity and light, this simple molecule is able to regulate both pattern and growth throughout the life cycle of the plant (Leyser 2005). The principal reason for this capacity for control is that auxin, unlike other known plant hormones, can be actively transported in a coordinated, directional manner such that gradients may be formed that, in various guises, direct numerous distinct developmental phenomena (Tanaka et al 2006). Although the phenomenology of auxin movement and distribution in the growing plant has been largely established (Bennett et al 1996;Benkova et al 2003;Friml et al 2003;Blilou et al 2005;Bandyopadhyay et al 2007;Grieneisen et al 2007;Petersson et al 2009), the link between differences in auxin concentration and outputs in terms of changes in growth and development is not well understood.…”

Fine tuning of auxin signaling by miRNAs

“…Auxins constitute the oldest and one of the most well studied class of phytohormones. An expanse of literature is available on the various aspects of auxin action (for recent reviews see Fleming 2006;Hartig and Beck 2006;Quint and Gray, 2006;Teale et al, 2006;Tanaka et al, 2006;Able, 2007;Spaepen et al, 2007;Vieten et al, 2007). Recent years have witnessed a tremendous leap in our understanding of the role of auxin and mechanism of its regulation (Dharmasiri et al, 2005;Woodward et al, 2005).…”

Multiscale modelling of auxin transport in the plant-root elongation zone

“…In this paper, we use asymptotic methods to gain understanding of the transport of the hormone auxin through plant tissue. Auxin plays an important role in plant growth and development (Benjamins and Scheres 2008;Friml 2003;Tanaka et al 2006;Vieten et al 2007), and an asymptotic analysis is particularly applicable since plant tissue has a regular structure and we can distinguish the roles of the several cell-scale components of the auxin flux. Thus, the analysis presented provides insight into the underlying dynamics and complements the many computational cell-scale auxin-transport models that are currently in development [see Berleth et al (2007); Jönsson and Krupinski (2010); Kramer (2008); Kramer et al (2008); Krupinski and Jönsson (2010) and Smith and Bayer (2009) for recent reviews].…”

“…in the rootward direction) in the centre of the root and in an upwards (shootward) direction though the root's outer layers. The flux through the root's outer layers is important in producing a gravitropic response (Rashotte et al 2000;Swarup et al 2005): if the root is not orientated downwards (in the direction of gravity) the columella cells, located at the root tip, create a lateral auxin gradient (Ottenschlager et al 2003); due to the upward flux though the root's outer layers, the elongation-zone cells on the underside of the root receive more auxin than those on the upper side, which causes them to reduce their elongation rate; within minutes, this process causes the root to bend and reorientate in the direction of gravity (Friml 2003;Swarup et al 2005;Tanaka et al 2006).…”