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Unidirectional transport of orthophosphate across the envelope of isolated cauliflower-bud amyloplasts

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Abstract

Isolated amyloplasts from cauliflower (Brassica oleracea L. var botrytis) buds are able to export orthophosphate unidirectionally into the incubation medium. This orthophosphate transport appears to be protein-mediated, as indicated by the following observations: (i) low temperature and the presence of inhibitors of protein-mediated transport reduced the rate of orthophosphate export, and (ii) the rate of orthophosphate export became saturated with rising internal substrate concentrations. Micromolar concentrations of 4,4′-diisothiocyano-2,2′-stilbene disulphonic acid inhibited the rate of unidirectional orthophosphate export, thus indicating the involvement of the amyloplastic glucose-6-phosphate (Glc6P)translocator in the unidirectional export of orthophosphate. The effect of rising concentrations of orthophosphate upon the activity of ADP glucose pyrophosphorylase in desalted extracts was determined. Orthophosphate given in concentrations similar to those measured in the amyloplastic stroma under conditions of steady-state rates of Glc6P-dependent starch synthesis inhibited the activity of ADP-glucose pyrophosphorylase significantly. However, even under strong limiting substrate conditions the residual activity was sufficient to catalyze the flux of carbon into starch. The maximal rates of orthophosphate transport (in the counter-exchange mode) by isolated spinach (Spinacia oleracea L.) chloroplasts and by isolated cauliflower-bud amyloplasts were also determined. These rates were compared with the maximal rates of undirectional orthophosphate export by these plastids. From these measurements we can conclude that, compared with spinach chloroplasts, isolated amyloplasts of cauliflower exhibit a fivefold greater ratio of unidirectional orthophosphate transport to maximal rate of orthophosphate transport in the counter-exchange mode compared to spinach chloroplasts. The determined rate of maximal unidirectional orthophosphate export is sufficient to catalyze the release of additional inorganic phosphate liberated in the amyloplastic stroma during the process of Glc6P-dependent starch synthesis.

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Abbreviations

ADPGlcPPase:

ADP-glucose pyrophosphorylase

DIDS:

4,4′-diisothiocyano-2,2′-stilbene disulphonic acid

Glc1P:

glucose-1-phosphate

Glc6P:

glucose-6-phosphate

PGA:

3-phosphoglyceric acid

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Authors and Affiliations

  1. Pflanzenphysiologie, Fachbereich Biologie/Chemie, Universität Osnabrück, Barbarastraße 11, D-49069, Osnabrück, Germany

    H. -Ekkehard Neuhaus & Uwe Maaß

Authors
  1. H. -Ekkehard Neuhaus
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  2. Uwe Maaß
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Correspondence to H. -Ekkehard Neuhaus.

Additional information

The authors gratefully acknowledge stimulating discussions on and support for this work by Prof. Dr. Renate Scheibe (Pflanzenphysiologie, Universität Osnabrück, Osnabrück, Germany). We thank Gudrun Henrichs for excellent technical assitance and Dr. Antje von Schaewen for help during preparation of the manuscript. The work was funded by the Deutsche Forschungsgemeinschaft (SFB 171, C16).

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Neuhaus, H.E., Maaß, U. Unidirectional transport of orthophosphate across the envelope of isolated cauliflower-bud amyloplasts. Planta 198, 542–548 (1996). https://doi.org/10.1007/BF00262640

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