Mechanisms of the Early Phases of Plant Gravitropism

Abstract

Gravitropism is directed growth of a plant or plant organ in response to gravity and can be divided into the following temporal sequence: perception, transduction, and response. This article is a review of the research on the early events of gravitropism (i.e., phenomena associated with the perception and transduction phases). The two major hypotheses for graviperception are the protoplast-pressure and starch-statolith models. While most researchers support the concept of statoliths, there are suggestions that plants have multiple mechanisms of perception. Evidence supports the hypothesis that the actin cytoskeleton is involved in graviperception/transduction, but the details of these mechanisms remain elusive. A number of recent developments, such as increased use of the molecular genetic approach, magnetophoresis, and laser ablation, have facilitated research in graviperception and have allowed for refinement of the current models. In addition, the entire continuum of acceleration forces from hypo- to hyper-gravity have been useful in studying perception mechanisms. Future interdisciplinary molecular approaches and the availability of sophisticated laboratories on the International Space Station should help to develop new insights into mechanisms of gravitropism in plants.

A gene essential for hydrotropism in roots

“…thaliana roots display phototropism as well as gravitropism and hydrotropism (4)(5)(6). The roots display a negative phototropism in response to unilateral illumination of white or blue light.…”

“…Roots control their growth orientation by displaying tropisms in response to environmental cues such as gravity, light, physical contact, and moisture gradients (1)(2)(3)(4)(5)(6)(7). These responses are important features for the survival of sessile terrestrial plants in stressful environments such as drought conditions.…”

“…These responses are important features for the survival of sessile terrestrial plants in stressful environments such as drought conditions. Although there have been many studies conducted on the gravitropic response (4,8,9), other tropisms in roots largely have been unexplored. Although some studies have identified the existence of hydrotropism (the directed growth of roots in a moisture gradient), it has proven difficult to differentiate between this response and gravitropism because the latter always interferes with the former here on Earth (3).…”

Shoot circumnutation and winding movements require gravisensing cells

“…The graviresponse also participates in the regulation of apical dominance (5). Intense studies of these forms of gravimorphogenesis led to many discoveries of the molecular mechanisms of gravitropism (6,7). However, the mechanisms for other types of gravimorphogenesis remain unclear, despite their importance.…”

Where's the water? Hydrotropism in plants

“…These authors also suggested that the earlyperception phase of hydrotropism occurs in the root cap, because roots grow normally but do not respond to moisture gradients after removal of the cap. Later, Takahashi et al (13) used a starchless mutant of Arabidopsis, which has a greatly reduced sensitivity to gravity compared with WT (14,15), to show it is enhanced in root hydrotropism.…”