Molecular Mechanisms of Root Gravitropism

Abstract

Plant shoots typically grow against the gravity vector to access light, whereas roots grow downward into the soil to take up water and nutrients. These gravitropic responses can be altered by developmental and environmental cues. In this review, we discuss the molecular mechanisms that govern the gravitropism of angiosperm roots, where a physical separation between sites for gravity sensing and curvature response has facilitated discovery. Gravity sensing takes place in the columella cells of the root cap, where sedimentation of starch-filled plastids (amyloplasts) triggers a pathway that results in a relocalization to the lower side of the cell of PIN proteins, which facilitate efflux of the plant hormone auxin efflux. Consequently, auxin accumulates in the lower half of the root, triggering bending of the root tip at the elongation zone. We review our understanding of the molecular mechanisms that control this process in primary roots, and discuss recent insights into the regulation of oblique growth in lateral roots and its impact on root-system architecture and soil exploration.

Control of Adventitious Root Architecture in Rice by Darkness, Light, and Gravity

“…Phototropic and gravitropic responses are mediated by auxin that can be transported polarly by auxin efflux carriers. Directed auxin flow allows for an asymmetric auxin distribution and, hence, for differential growth rates on the upper and lower sides of a root or on the shoot side facing the light versus the shaded side and, hence, to organ bending (Fankhauser and Christie, 2015;Su et al, 2017). In rice, AR growth direction is determined by gravity to only a minor degree.…”

A new wrinkle in our understanding of the role played by auxin in root gravitropism

“…2 from Su et al . (), published with permission.) The auxin‐maximum center is illustrated by red shading of the cells, whereas auxin flows are represented by red arrows whose widths reflect flux levels.…”

“…The roots of flowering plants constitute a good system to investigate gravitropism. Indeed, a physical separation between the primary site of gravity sensing (the columella cells of the root cap) and the location of curvature response (the elongation zone) allows careful assignment of contributing processes to specific steps in the pathway (reviewed in Su et al ., ). The primary mechanism of gravity sensing in flowering plants involves the sedimentation of starch‐filled plastids (amyloplasts) in specialized cells named statocytes (Kiss et al ., ).…”

“…Amyloplast sedimentation within the statocytes, or their repositioning, appears to trigger a signal transduction pathway that leads to a relocalization of auxin efflux facilitators of the PIN family to the bottom membrane of the statocytes. This triggers the formation of a lateral auxin gradient across the cap, which is transmitted toward the elongation zone where it triggers a downward curvature (Chauvet et al ., ; reviewed in Su et al ., ).…”

“…This gravitropic response requires machinery that senses the gravity vector and transduces the corresponding physical information into a physiological signal, which is transmitted to the elongation zone where it promotes a gravitropic curvature. The plant hormone auxin constitutes a signal that is transmitted from the site of gravity sensing to the site of curvature response (reviewed in Su et al ., ). A ground‐breaking report by Zhang et al . in this issue of New Phytologist (pp.…”