Ultrastructural observations of the isthmus of dividing chloroplasts

采用连续切片透明电子显微镜对一种绿藻（Closterium ehrenbergii）收缩叶绿体峡部的超微结构进行了三维观察。峡部外侧（细胞质侧）被大量同心定向的电子不透明纤维结构包围。三维重建显示，这些结构作为一个整体，在地峡周围旋转了几次。这种分布在分裂叶绿体峡部周围的电子不透明纤维结构与其他植物的分布有很大的不同。与高等植物不同，内膜基质表面未检测到电子不透明的环状结构。微管抑制剂氨丙磷甲基（APM）被观察到不能抑制塑质分裂（Hashimoto, 1992），但它并没有破坏电子不透明的纤维结构，这表明这些结构与微管无关。电子不透明纤维结构的环状分布显然与观察到的分裂叶绿体峡部被罗丹明结合的phalloidin染色相一致。内质网（ER）几乎覆盖了“间期”叶绿体的所有表面。当叶绿体进入分裂期时，内质网在分裂叶绿体的峡部分离。当叶绿体收缩。子核进入叶绿体的沟槽。这种核迁移被认为是由一个杆状的微管中心驱动的。这些观察结果表明，在梭菌中，叶绿体分裂和其他细胞器的循环可能是协同调节的。

Ultrastructure of the isthmus of constricting chloroplasts of a green alga Closterium ehrenbergii was three-dimensionally observed by transparent electron microscopy of serial sections. The isthmus was encircled outside (cytoplasmic side) by a large number of concentrically orientated electron-opaque fibrous structures. Three-dimensional reconstruction showed that these structures as a whole form belts turning around the isthmus several times. This type of the distribution of electron-opaque fibrous structures around the isthmus of the dividing chloroplasts is quite different from those described for the other plants. No electron-opaque ring-like structures were detected on the stromal surface of the inner envelope membrane as observed in higher plants. Microtubule inhibitor amiprophos methyl (APM) which was observed not to inhibit plastokinesis (Hashimoto, 1992) did not disrupt the electron-opaque fibrous structures, indicating the structures be not relevant with microtubules. The ring-like distribution of the electron-opaque fibrous structure apparently coincide with the observation that the isthmus region of the dividing chloroplast was stained with rhodamine-conjugated phalloidin.Endoplasmic reticulum (ER) covers almost all over the surface of 'interphase' chloroplasts. When the chloroplast entered division phase, ER was separated at the isthmus region of the dividing chloroplast. As chloroplasts constricted. daughter nuclei moved into the groove the chloroplasts. Such nuclear migration was suggested to be driven by a rodshaped microtubule center. These observations suggest that in Closterium the cycle of chloroplast division and other organelles may be cooperatively regulated.