Gravity-regulated differential auxin transport from columella to lateral root cap cells

Abstract

Gravity-induced root curvature has long been considered to be regulated by differential distribution of the plant hormone auxin. However, the cells establishing these gradients, and the transport mechanisms involved, remain to be identified. Here, we describe a GFP-based auxin biosensor to monitor auxin during Arabidopsis root gravitropism at cellular resolution. We identify elevated auxin levels at the root apex in columella cells, the site of gravity perception, and an asymmetric auxin flux from these cells to the lateral root cap (LRC) and toward the elongation zone after gravistimulation. We differentiate between an efflux-dependent lateral auxin transport from columella to LRC cells, and an effluxand influx-dependent basipetal transport from the LRC to the elongation zone. We further demonstrate that endogenous gravitropic auxin gradients develop even in the presence of an exogenous source of auxin. Live-cell auxin imaging provides unprecedented insights into gravity-regulated auxin flux at cellular resolution, and strongly suggests that this flux is a prerequisite for root gravitropism.gravitropic root curvature ͉ polar auxin transport ͉ auxin carrier proteins G ravity plays a major role in plant morphogenesis by determining the directional growth of plant organs (gravitropism). Roots orient at a preferred angle with respect to gravity [their gravitropic set-point angle (GSA); ref. 1], allowing efficient exploration of the soil (root gravitropism). Main roots of Arabidopsis seedlings, for instance, have a GSA of 0°and grow parallel to the gravity vector. Changes in gravity vector orientation (gravistimulation) induce root curvature, resulting in realignment of the root tip to the GSA. Root curvature is a consequence of gravity signal perception, involving amyloplast sedimentation in the columella cells of the root cap (2), and differential growth induced on opposite flanks in the elongation zone (EZ). In the 1920s, the Cholodny-Went hypothesis and various interpretations of it ever since have proposed that this differential growth within the EZ is mediated by an asymmetric distribution of the plant hormone auxin (3). Supportive evidence for an auxin asymmetry in the EZ after gravistimulation has come from the analyses of radio-labeled auxin distribution, or differential induction of auxin-response promoters (4). It has been questioned, however, whether auxin gradients are necessary or sufficient to cause root gravitropism (3, 5). Furthermore, it is not clear as to how the gravisensing events in the columella cells can give rise to changes in auxin concentration in the EZ. Recently, the gravity-dependent relocation of an auxin efflux carrier protein in columella cells suggested gravity-regulated changes of auxin transport right at the site of gravity perception in the root cap (6). However, differential auxin fluxes through the cap cells and their contribution to gravitropic root curvature remain to be demonstrated. In the work presented here, we applied a GFP-based auxin biosensor to study gravity-induced ...

COP1 mediates the coordination of root and shoot growth by light through modulation of PIN1- and PIN2-dependent auxin transport in Arabidopsis

“…5J), resembling that observed during etiolation (Fig. 3M) or in light-grown pin2 mutant (Ottenschläger et al, 2003). Our results support a scenario in which dark-induced reduction in PIN1 activity in the hypocotyl leads to lower auxin levels in the root, which in turn promotes a reduction in PIN1 and PIN2 levels at the PM, thus impairing root growth.…”

An auxin-responsive 1-aminocyclopropane-1-carboxylate synthase is responsible for differential ethylene production in gravistimulated Antirrhinum majus L. flower stems

“…Even now, asymmetric distribution of auxin, as already described by the Cholodny-Went theory (Went and Thimann 1937), is still considered to be the main causative factor for differential growth although the observed changes in free auxin levels are often rather small, non-existent or transient (Mertens and Weiler 1983;Clifford et al 1985;Schwark and Bopp 1993;Philosoph-Hadas et al 2001). Recent findings regarding the lateral relocation of the auxin efflux regulator PIN3 upon gravistimulation and the observed differential expression of a synthetic DR5::GUS auxin reporter element, however, strongly support the auxinredistribution theory (Friml et al 2002;Ottenschla¨ger et al 2003). Rapid cycling of PIN3 proteins between the plasma membrane and other compartments of the cell in gravisensing tissues (starch sheath layer, columella) provides a mechanism to rapidly respond to changes in orientation by redirecting auxin efflux and differential growth (Friml et al 2002).…”

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

“…1). In the root, auxin regulates the gravitropic response (Marchant et al 1999;Ottenschlager et al 2003), maintains the root apical meristem (Jiang and Feldman 2005) and promotes the initiation of lateral roots (Benková et al 2003;Casimiro et al 2003Casimiro et al , 2001De Smet et al 2007). Therefore, understanding the distribution and transport of auxin within root tissue could help explain key processes in root development.…”

“…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).…”