Graduate School of Biological Sciences, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara, Japan.
Plant Cell. 2011 May;23(5):1830-48. doi: 10.1105/tpc.110.079442. Epub 2011 May 20.
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.
高等植物利用沉降的淀粉粒在平衡细胞中作为平衡石来感知重力方向在向重性过程中。在拟南芥花序茎平衡细胞中，淀粉粒处于复杂的运动中；有些表现出跳跃式的盐沼运动，有些则倾向于向重力方向沉降。在这里，我们报告说，一个 RING 型 E3 连接酶 SHOOT GRAVITROPISM9（SGR9）定位于淀粉粒，调节淀粉粒动力学。在表现出向重性降低的 sgr9 突变体中，淀粉粒没有沉降，而是表现出增加的跳跃式运动。淀粉粒有时会形成一个异常缠绕在肌动蛋白丝（AFs）上的聚集体在 sgr9 中。相比之下，在 fis1 突变体中，一种半显性突变体 ACT8 诱导肌动蛋白丝的片段化，淀粉粒失去跳跃式运动并几乎静态沉降。Latrunculin B 的处理，一种肌动蛋白丝聚合的抑制剂，和 fiz1 突变体拯救了 sgr9 的向重性缺陷。此外，即使在 sgr9 中 fiz1 也会减少跳跃式运动并诱导淀粉粒沉降。我们的结果表明，在野生型内胚层细胞中，淀粉粒在沉降和跳跃式运动之间处于平衡状态。此外，这种平衡是淀粉粒和肌动蛋白丝相互作用的结果，由 SGR9 调节。SGR9 可能促进淀粉粒从肌动蛋白丝上脱离，从而使淀粉粒在依赖肌动蛋白丝的淀粉粒动力学平衡中沉降。
