Import of nuclear‐encoded proteins into carotenoid‐deficient young etioplasts

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Abstract
Young etioplasts with different carotenoid contents were assayed for their ability to import in vitro synthesized nuclear‐encoded proteins. The plastids were isolated from the basal 1. 5cm of dark‐grown wheat seedlings developed from seeds imbibed with 4 different concentrations of Norflurazon. an inhibitor of the carotenoid biosynthesis. Plastids isolated from plants treated with the two highest concentrations. 2. 8 and 28 mg l−1, of Norflurazon contained approximately 10 and 5% of the carotenoid contents, respectively, compared to the control. The total amounts of proteins in these plastids were approximately 68 and 60% compared to control plastids. Translocation assays employing the precursors of the small subunit of ribulose 1. 5‐bisphosphate carboxylase/oxygenase (pSS), and the non‐Photosynthetic heat‐shock protein 21 (pHSP21), showed that the rate of protein import was considerably lower in plastids with low carotenoid contents. The amounts of imported, processed SS were 11 and 10% after 2. 8 and 28 mg 1−1, respectively, compared to the control, whereas the amounts of HSP21 at these herbicide concentrations were 20 and 18%, respectively. The low apparent import in plastids of Norflurazon‐treated leaves was not an effect of intraorganellar degradation of imported proteins, nor were there any differences in the amounts of processed, protease‐protected protein when Norflurazon was added to the import reaction using control plastids. The low import capabilities are therefore discussed in relation to the possible role of the carotenoids in the translocation of cytosolically synthesized proteins into the plastidic compartment.