ARC Centre of Excellence in Plant Energy Biology, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, Australia.
Plant J. 2012 Mar;69(5):743-54. doi: 10.1111/j.1365-313X.2011.04833.x. Epub 2011 Dec 2.
The process of chloroplast biogenesis requires a multitude of pathways and processes to establish chloroplast function. In cotyledons of seedlings, chloroplasts develop either directly from proplastids (also named eoplasts) or, if germinated in the dark, via etioplasts, whereas in leaves chloroplasts derive from proplastids in the apical meristem and are then multiplied by division. The snowy cotyledon 2, sco2, mutations specifically disrupt chloroplast biogenesis in cotyledons. SCO2 encodes a chloroplast-localized protein disulphide isomerase, hypothesized to be involved in protein folding. Analysis of co-expressed genes with SCO2 revealed that genes with similar expression patterns encode chloroplast proteins involved in protein translation and in chlorophyll biosynthesis. Indeed, sco2-1 accumulates increased levels of the chlorophyll precursor, protochlorophyllide, in both dark grown cotyledons and leaves. Yeast two-hybrid analyses demonstrated that SCO2 directly interacts with the chlorophyll-binding LHCB1 proteins, being confirmed in planta using bimolecular fluorescence complementation (BIFC). Furthermore, ultrastructural analysis of sco2-1 chloroplasts revealed that formation and movement of transport vesicles from the inner envelope to the thylakoids is perturbed. SCO2 does not interact with the signal recognition particle proteins SRP54 and FtsY, which were shown to be involved in targeting of LHCB1 to the thylakoids. We hypothesize that SCO2 provides an alternative targeting pathway for light-harvesting chlorophyll binding (LHCB) proteins to the thylakoids via transport vesicles predominantly in cotyledons, with the signal recognition particle (SRP) pathway predominant in rosette leaves. Therefore, we propose that SCO2 is involved in the integration of LHCB1 proteins into the thylakoids that feeds back on the regulation of the tetrapyrrole biosynthetic pathway and nuclear gene expression.
叶绿体生物发生的过程需要多种途径和过程来建立叶绿体功能。在幼苗的子叶中，叶绿体要么直接从原质体（也称为 eoplasts）发育而来（如果在黑暗中发芽），要么通过质体发育而来，而在叶子中，叶绿体则来自顶端分生组织的原质体，然后通过分裂增殖。雪绒花子叶 2（snowy cotyledon 2，SCO2）突变特异性地破坏子叶中的叶绿体生物发生。SCO2 编码一种定位于叶绿体的蛋白质二硫键异构酶，推测该酶参与蛋白质折叠。与 SCO2 共表达的基因分析表明，具有相似表达模式的基因编码参与蛋白质翻译和叶绿素生物合成的叶绿体蛋白。事实上，在黑暗生长的子叶和叶片中， sco2-1 积累了增加水平的叶绿素前体原叶绿素。酵母双杂交分析表明，SCO2 直接与叶绿素结合 LHCB1 蛋白相互作用，在植物体内使用双分子荧光互补（BIFC）得到证实。此外， sco2-1 叶绿体的超微结构分析表明，从内囊泡到类囊体的转运小泡的形成和运动受到干扰。SCO2 不与信号识别颗粒蛋白 SRP54 和 FtsY 相互作用，这两种蛋白被证明参与了 LHCB1 向类囊体的靶向。我们假设 SCO2 通过转运小泡为 LHCB 蛋白提供了一种替代的靶向途径，主要在子叶中，而信号识别颗粒（SRP）途径在罗勒叶中占主导地位。因此，我们提出 SCO2 参与了 LHCB1 蛋白整合到类囊体中，这反过来又反馈调节四吡咯生物合成途径和核基因表达。
