The Cytoskeleton and Gravitropism in Higher Plants

Abstract

The cellular and molecular mechanisms underlying the gravitropic response of plants have continued to elude plant biologists despite more than a century of research. Lately there has been increased attention on the role of the cytoskeleton in plant gravitropism, but several controversies and major gaps in our understanding of cytoskeletal involvement in gravitropism remain. A major question in the study of plant gravitropism is how the cytoskeleton mediates early sensing and signal transduction events in plants. Much has been made of the actin cytoskeleton as the cellular structure that sedimenting amyloplasts impinge upon to trigger the downstream signaling events leading to the bending response. There is also strong molecular and biochemical evidence that the transport of auxin, an important player in gravitropism, is regulated by actin. Organizational changes in microtubules during the growth response phase of gravitropism have also been well documented, but the significance of such reorientations in controlling differential cellular growth is unclear. Studies employing pharmacological approaches to dissect cytoskeletal involvement in gravitropism have led to conflicting results and therefore need to be interpreted with caution. Despite the current controversies, the revolutionary advances in molecular, biochemical, and cell biological techniques have opened up several possibilities for further research into this difficult area. The myriad proteins associated with the plant cytoskeleton that are being rapidly characterized provide a rich assortment of candidate regulators that could be targets of the gravity signal transduction chain. Cytoskeletal and ion imaging in real time combined with mutant analysis promises to provide a fresh start into this controversial area of research.

The Vegetative Vacuole Proteome ofArabidopsis thalianaReveals Predicted and Unexpected Proteins[W]

“…The direct involvement of the cytoskeleton and the vacuole in the gravitropic response has also been recently documented (Blancaflor, 2002;Kato et al, 2002b). A small number of proteins potentially involved in anchoring the vacuole to the cytoskeleton were identified.…”

Hypocotyl Directional Growth in Arabidopsis: A Complex Trait

“…Changes in auxin, ethylene, gravity signaling, or alteration in cell wall properties alter hypocotyl growth direction (De Grauwe et al, 2005;Vandenbussche et al, 2011). The cytoskeleton also plays a crucial role in optimal hypocotyl direction as evident by the hypocotyl phenotypes of seedlings harboring mutations in genes encoding various microtubule-interacting proteins (Blancaflor, 2002;Bisgrove, 2008). Exogenous BR application or enhanced endogenous BR signaling compromised the ability of dark-grown seedlings to penetrate a hard medium.…”

Amyloplasts and Vacuolar Membrane Dynamics in the Living Graviperceptive Cell of the Arabidopsis Inflorescence Stem

“…Findings from the actin disruption experiment suggest that the saltatory movement of amyloplasts through transvacuolar strands is dependent upon the actin cytoskeleton. The actin cytoskeleton has been proposed as a major player in plant gravitropism (Yoder et al, 2001;Blancaflor, 2002;Hou et al, 2003). Recent studies, however, showed that an intact actin cytoskeleton is not required for gravity perception in inflorescence stems and roots (Yamamoto and Kiss, 2002;Hou et al, 2003) and likely acts to downregulate gravitropism by continuously resetting the gravitropic-signaling system (Hou et al, 2004).…”