Abstract
Higher plants use the sedimentation of amyloplasts in statocytes as statolith to sense the direction of gravity during gravitropism. In Arabidopsis thaliana inflorescence stem statocyte, amyloplasts are in complex movement; some show jumping-like saltatory movement and some tend to sediment toward the gravity direction. Here, we report that a RING-type E3 ligase SHOOT GRAVITROPISM9 (SGR9) localized to amyloplasts modulates amyloplast dynamics. In the sgr9 mutant, which exhibits reduced gravitropism, amyloplasts did not sediment but exhibited increased saltatory movement. Amyloplasts sometimes formed a cluster that is abnormally entangled with actin filaments (AFs) in sgr9. By contrast, in the fiz1 mutant, an ACT8 semidominant mutant that induces fragmentation of AFs, amyloplasts, lost saltatory movement and sedimented with nearly statically. Both treatment with Latrunculin B, an inhibitor of AF polymerization, and the fiz1 mutation rescued the gravitropic defect of sgr9. In addition, fiz1 decreased saltatory movement and induced amyloplast sedimentation even in sgr9. Our results suggest that amyloplasts are in equilibrium between sedimentation and saltatory movement in wild-type endodermal cells. Furthermore, this equilibrium is the result of the interaction between amyloplasts and AFs modulated by the SGR9. SGR9 may promote detachment of amyloplasts from AFs, allowing the amyloplasts to sediment in the AFs-dependent equilibrium of amyloplast dynamics.
Highlights
Tropism in plants refers to the directional growth of organs in response to directional environmental stimuli, such as gravity, light, moisture, or touch, and is a consequence of the plants recognizing the directional information of the stimuli
The sgr9 inflorescence stem showed a greater lightoriented growth response than the wild type, similar to reports for other gravitropic mutants (Fukaki et al, 1996b; Yamauchi et al, 1997). These results demonstrate that sgr9 does not affect the phototropic response in inflorescence stems
We isolated a gene encoding a novel RING finger protein and showed that this protein is responsible for the reduced shoot gravitropism of the Arabidopsis sgr9 mutant (Figure 1G)
Summary
Tropism in plants refers to the directional growth of organs in response to directional environmental stimuli, such as gravity, light, moisture, or touch, and is a consequence of the plants recognizing the directional information of the stimuli. Since gravity acts upon mass, a number of organisms use relatively heavy cellular components, called statoliths or otoliths, to sense the direction of gravity. Higher plants have unique plastids, termed amyloplasts, that can function as plant statoliths for gravity sensing. Amyloplasts are present within specific cells (statocytes); these plastids accumulate dense starch granules and sink in the direction of gravity (Morita and Tasaka, 2004). Based on this observed property of these plastids, the starch statolith hypothesis proposed that
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