Microrheological Insights into the Dynamics of Amyloplasts in Root Gravity-Sensing Cells

Molecular mechanisms of gravity perception and signal transduction in plants

“…Amyloplasts are enmeshed in this actin network, with their envelopes being in contact with actin filaments (Driss-Ecole et al 2003). Amyloplasts are located in actin hollows, which may form the cages confining these organelles (Zheng et al 2015). This arrangement suggests that the actin network could be easily destroyed during amyloplast sedimentation.…”

“…This arrangement suggests that the actin network could be easily destroyed during amyloplast sedimentation. Artificial disruption of actin filaments results in a more rapid and free diffusive sedimentation of amyloplasts after an orientation change and increased gravisensitivity in roots, hypocotyls, and inflorescence stems (Hou et al 2003(Hou et al , 2004Nakamura et al 2011;Saito et al 2005;Yamamoto and Kiss 2002;Yoder et al 2001;Zheng et al 2015). Similarly, the act2-5 mutation in the ACTIN2 gene induces hypersensitivity to gravitropic stimuli and mutation in ACT8-facilitated amyloplast sedimentation (Lanza et al 2012;Nakamura et al 2011).…”

“…Ubiquitin ligases like WAV3 may target actin-regulatory proteins for degradation, thus modulating amyloplast movement and gravitropism. Plants mutated in components of the ARP2/3 actin-related protein nucleating complex, a complex which binds to existing filaments to initiate their branching, exhibit defects in gravitropism (Reboulet et al 2010;Zheng et al 2015), and inhibited cooperative cage escape movement of root amyloplasts. This indicates their stronger actindependent cage confinement in the mutant (Zheng et al 2015).…”

Mechano-biological Coupling of Cellular Responses to Microgravity

“…ARP2/3-dependent actin network dominates cage-effect on amyloplast movements and the cooperative cage-escaping motion of amyloplasts during sedimentation. This finding suggests a potential gravity-sensing mechanism dictating a linear frustration effect of actin cytoskeleton upon the conversion of mechanical stimulation of amyloplasts into gravitropic signals (Zheng et al 2015).…”

“…Finally, a linear correlation is observed among the initial mechanical stimulation strength in the columella cells estimated by the inverse local apparent viscosity, the subsequent intercellular signal transduction of the asymmetric accumulation of auxin ratio in root cap, and the final gravity response of the root curvature angle. A possible gravity sensing mechanism is suggested giving rise to a linear frustration of the actin cytoskeleton on the conversion of mechanical stimulation into gravitropic signals (Zheng et al 2015).…”