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 ...

No abstract

Abstract. We modified a video digitizer system to allow short-term high-resolution measurements of root elongation in intact seedlings of Arabidopsis thaliana (L.) Heynh.. We used the system to measure the kinetics of promotion and inhibition of root elongation by applied auxin and to determine the dose-response relationship for auxin action on elongation in roots of wild-type seedlings and seedlings of mutants (axrl, auxl, and axr2) with altered auxin responsiveness. Roots of the mutants showed less inhibition in the presence of inhibitory concentrations of auxin than did roots of the wild type. The latent period preceding the change in elongation rate after auxin application was the same for axrl and axr2 as for the wild type whereas the latent period for auxl was about twice as long as for the wild type. Low concentrations (ca. 10 11 M) of auxin induced substantial promotion of root elongation in the wild type and in axr2.

between the meristem and the point at which rapid elongation is initiated as a special zone in which growth is nearly isodiametric. They referred to this region as the postmitotic "isodiametric" growth zone, The cells that show rapid graviinduced enhancement of elongation rate (Ishikawa et 1991) include cells in the postmitotic isodiametric growth zone (compare Baluska et al., 1990, andIshikawa et al., 1991).There is evidence that cells between the meristem and the CEZ also play a special role in the ability of roots to adapt to inhibitory levels of auxin. Burstrom (1957) reported that the elongation of roots of wheat seedlings was initially inhibited by IAA but SOOn recovered even in the presente of the hormone. The resumption of elongation resulted from initiation of rapid elongation in cells near the meristem including the apical region of the elongation zone. Cells that were in the main 'One at the time Of auxin treatment remained inhibited. Similar data were obtained by Hejnowicz (1961) using wheat roots and bY Goodwin (1972) using roots Of PhkUm.In this study, we examined the auxin response of cells throughout the apical region of the root. Because IAA redistribution is thought to mediate gravitropic responses, we compared the elongation rate distribution pattern in auxintreated roots with that of gravistimulated roots. In view of the suggestion that adaptation to localized changes in auxin leve1 contributes to the kinetics of gravitropism (Evans, 1991), we also examined the gravitropic response of roots exposed to IAA before stimulation. We discuss the surprising finding that roots whose elongation rate is reduced to zero by auxin treatment still exhibit a rapid and strong gravitropic response upon gravistimulation. Preliminary results of these studies were reported by Evans et al. (1990) and Ishikawa and Evans (1992).We used a video digitizer system to (a) measure changes i n the pattern of longitudinal surface extension in primary roots of maize (Zea mays 1.) upon appkcation and withdrawal of auxin and (b) compare these patterns during gravitropism in control roots and roots pretreated with auxin. Special attention was paid to the distal elongation zone (DEZ), arbitrarily defined as the region between the meristem and the point within the elongation zone at which the rate of elongation reaches 0.3 of the peak rate. For roots i n aqueous solution, the basal limit of the DEZ is about 2.5 mm behind the tip of the root cap. Auxin suppressed elongation throughout the elongation zone, but, after 1 to 3 h, elongation resumed, primarily as a result of induction of rapid elongation i n the DEZ.Withdrawal of auxin during the period of strong inhibition resulted i n exceptionally rapid elongation attributable to the initiation of rapid elongation in the DEZ plus recovery i n the main elongation In a study of surface extension patterns in maize roots (Ishikawa et al., 1991), we reported that the pattern of differential elongation responsible for gravitropic curvature is complex. During rapid downward curvature, ...