Gravity signal transduction in primary roots

Abstract

Aims: The molecular mechanisms that correlate with gravity perception and signal transduction in the tip of angiosperm primary roots are discussed.

Scope: Gravity provides a cue for downward orientation of plant roots, allowing anchorage of the plant and uptake of the water and nutrients needed for growth and development. Root gravitropism involves a succession of physiological steps: gravity perception and signal transduction (mainly mediated by the columella cells of the root cap); signal transmission to the elongation zone; and curvature response. Interesting new insights into gravity perception and signal transduction within the root tip have accumulated recently by use of a wide range of experimental approaches in physiology, biochemistry, genetics, genomics, proteomics and cell biology. The data suggest a network of signal transduction pathways leading to a lateral redistribution of auxin across the root cap and a possible involvement of cytokinin in initial phases of gravicurvature.

Conclusion: These new discoveries illustrate the complexity of a highly redundant gravity-signalling process in roots, and help to elucidate the global mechanisms that govern auxin transport and morphogenetic regulation in roots.

Fig.1.

Gravitropism directs downward root growth in Arabidopsis thaliana. (A) A 4-d-old arabidopsis seedling grown on vertical agar-based medium in the light was positioned horizontally (gravistimulated) and allowed to respond. After 3.5 h, the root has initiated a downward gravitropic curvature (arrowhead), whereas the hypocotyl is curving upward. (B) The fountain model of auxin transport in a 5-d-old arabidopsis root tip, stained with propidium iodine to mark the cells. Arrows indicate the direction of auxin transport in different tissues. The arrows' width indicates the relative intensity of transport, and their colour represents the combination of PIN proteins expressed in these tissues (Blilou et al., 2005): green = PIN1, 3, 7; yellow = PIN1, 2, 4; blue = PIN1, 3, 4, 7; purple = PIN1, 2. The red mark delineating the central columella cells of the cap represents the distribution of PIN3 at the plasma membrane of these cells. PIN3 is symmetrically distributed in vertical roots (left panel), but accumulates quickly in the lower membrane upon gravistimulation (right panel). (C) The columella cells of the root cap perceive gravity through the sedimentation of amyloplasts. A columella cell is schematized before (left) and after (right) gravistimulation. Grey circles represent amyloplasts. In all panels the gravity vector is directed downward (white arrows).