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 ...

BackgroundIn Arabidopsis thaliana, the family of cyclic nucleotide-gated channels (CNGCs) is composed of 20 members. Previous studies indicate that plant CNGCs are involved in the control of growth processes and responses to abiotic and biotic stresses. According to their proposed function as cation entry pathways these channels contribute to cellular cation homeostasis, including calcium and sodium, as well as to stress-related signal transduction. Here, we studied the expression patterns and regulation of CNGC19 and CNGC20, which constitute one of the five CNGC subfamilies.ResultsGUS, GFP and luciferase reporter assays were used to study the expression of CNGC19 and CNGC20 genes from Arabidopsis thaliana in response to developmental cues and salt stress. CNGC19 and CNGC20 were differentially expressed in roots and shoots. The CNGC19 gene was predominantly active in roots already at early growth stages. Major expression was observed in the phloem. CNGC20 showed highest promoter activity in mesophyll cells surrounding the veins. Its expression increased during development and was maximal in mature and senescent leaves. Both genes were upregulated in the shoot in response to elevated NaCl but not mannitol concentrations. While in the root, CNGC19 did not respond to changes in the salt concentration, in the shoot it was strongly upregulated in the observed time frame (6-72 hours). Salt-induction of CNGC20 was also observed in the shoot, starting already one hour after stress treatment. It occurred with similar kinetics, irrespective of whether NaCl was applied to roots of intact plants or to the petiole of detached leaves. No differences in K and Na contents of the shoots were measured in homozygous T-DNA insertion lines for CNGC19 and CNGC20, respectively, which developed a growth phenotype in the presence of up to 75 mM NaCl similar to that of the wild type.ConclusionTogether, the results strongly suggest that both channels are involved in the salinity response of different cell types in the shoot. Upon salinity both genes are upregulated within hours. CNGC19 and CNGC20 could assist the plant to cope with toxic effects caused by salt stress, probably by contributing to a re-allocation of sodium within the plant.