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The SCO2 protein disulphide isomerase is required for thylakoid biogenesis and interacts with LCHB1 chlorophyll a/b binding proteins which affects chlorophyll biosynthesis in Arabidopsis seedlings

Authors

  • Sandra K. Tanz,

    1. ARC Centre of Excellence in Plant Energy Biology, University of Western Australia, 35 Stirling Highway, Crawley 6009, Western Australia, Australia
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    • These authors contributed equally.

  • Joachim Kilian,

    1. Zentrum für Molekularbiologie der Pflanzen, Plant Physiology, University of Tübingen, 72076 Tübingen, Germany
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    • These authors contributed equally.

  • Christoffer Johnsson,

    1. ARC Centre of Excellence in Plant Energy Biology, Research School of Biology, Australian National University Canberra, Canberra, Acton ACT 0200, Australia
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  • Klaus Apel,

    1. Institute of Plant Sciences, ETH Zurich, 8092 Zurich, Switzerland
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    • Present address: Boyce Thompson Institute for Plant Research, Ithaca, NY 144853-1801, USA.

  • Ian Small,

    1. ARC Centre of Excellence in Plant Energy Biology, University of Western Australia, 35 Stirling Highway, Crawley 6009, Western Australia, Australia
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  • Klaus Harter,

    1. Zentrum für Molekularbiologie der Pflanzen, Plant Physiology, University of Tübingen, 72076 Tübingen, Germany
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  • Dierk Wanke,

    1. Zentrum für Molekularbiologie der Pflanzen, Plant Physiology, University of Tübingen, 72076 Tübingen, Germany
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  • Barry Pogson,

    1. ARC Centre of Excellence in Plant Energy Biology, Research School of Biology, Australian National University Canberra, Canberra, Acton ACT 0200, Australia
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  • Verónica Albrecht

    Corresponding author
    1. ARC Centre of Excellence in Plant Energy Biology, Research School of Biology, Australian National University Canberra, Canberra, Acton ACT 0200, Australia
      (fax +612 6125 0313; e-mail veronica.albrecht@anu.edu.au).
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(fax +612 6125 0313; e-mail veronica.albrecht@anu.edu.au).

Summary

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.

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