Graduate School of Experimental Plant Sciences, Wageningen University, Wageningen, The Netherlands.
Plant Cell. 2009 Sep;21(9):2811-28. doi: 10.1105/tpc.108.064410. Epub 2009 Sep 4.
Rhizobium bacteria form N(2)-fixing organelles, called symbiosomes, inside the cells of legume root nodules. The bacteria are generally thought to enter the cells via an endocytosis-like process. To examine this, we studied the identity of symbiosomes in relation to the endocytic pathway. We show that in Medicago truncatula, the small GTPases Rab5 and Rab7 are endosomal membrane identity markers, marking different (partly overlapping) endosome populations. Although symbiosome formation is considered to be an endocytosis-like process, symbiosomes do not acquire Rab5 at any stage during their development, nor do they accept the trans-Golgi network identity marker SYP4, presumed to mark early endosomes in plants. By contrast, the endosomal marker Rab7 does occur on symbiosomes from an early stage of development when they have stopped dividing up to the senescence stage. However, the symbiosomes do not acquire vacuolar SNAREs (SYP22 and VTI11) until the onset of their senescence. By contrast, symbiosomes acquire the plasma membrane SNARE SYP132 from the start of symbiosome formation throughout their development. Therefore, symbiosomes appear to be locked in a unique SYP132- and Rab7-positive endosome stage and the delay in acquiring (lytic) vacuolar identity (e.g., vacuolar SNAREs) most likely ensures their survival and maintenance as individual units.
根瘤菌在豆科植物根瘤的细胞内形成固氮细胞器，称为共生体。这些细菌通常被认为是通过类似于内吞作用的过程进入细胞的。为了研究这一点，我们研究了共生体与内吞途径的关系。我们表明，在蒺藜苜蓿中，小 GTP 酶 Rab5 和 Rab7 是内体膜的身份标记，标记不同的（部分重叠的）内体群体。尽管共生体的形成被认为是一种类似于内吞作用的过程，但在它们的发育过程中，共生体不会在任何阶段获得 Rab5，也不会接受假定标记植物早期内体的跨高尔基网络身份标记 SYP4。相比之下，内体标记 Rab7 确实存在于共生体中，从它们停止分裂到衰老阶段的早期。然而，直到衰老开始，共生体才获得液泡 SNARE（SYP22 和 VTI11）。相比之下，从共生体形成开始，共生体就获得了质膜 SNARE SYP132，并贯穿其整个发育过程。因此，共生体似乎被锁定在一个独特的 SYP132 和 Rab7 阳性内体阶段，而获得（裂解）液泡身份（例如，液泡 SNARE）的延迟很可能确保了它们作为单个单元的存活和维持。
