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JXB Advance Access published online on March 14, 2005
Journal of Experimental Botany, doi:10.1093/jxb/eri130
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
Received September 29, 2004
Accepted February 2, 2005

RESEARCH PAPER

The sources of carbon and reducing power for fatty acid synthesis in the heterotrophic plastids of developing sunflower (Helianthus annuus L.) embryos

Rafael Pleite 1, Marilyn J. Pike 2, Rafael Garcés 1, Enrique Martínez-Force 1, and Stephen Rawsthorne 2*

1 Instituto de la Grasa, CSIC, Avd. Padre García Tejero, 4. 41012, Seville, Spain
2 Department of Metabolic Biology, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK

* To whom correspondence should be addressed.
Stephen Rawsthorne, E-mail: steve.rawsthorne{at}bbsrc.ac.uk


  Abstract

The provision of carbon substrates and reducing power for fatty acid synthesis in the heterotrophic plastids of developing embryos of sunflower (Helianthus annuus L.) has been investigated. Profiles of oil and storage protein accumulation were determined and embryos at 17 and 24 days after anthesis (DAA) were selected to represent early and late periods of oil accumulation. Plastids isolated from either 17 or 24 DAA embryos did not incorporate label from [1-14C]glucose 6-phosphate (Glc6P) into fatty acids. Malate, when supplied alone, supported the highest rates of fatty acid synthesis by the isolated plastids at both stages. Pyruvate supported rates of fatty acid synthesis at 17 DAA that were comparable to those supported by malate, but only when incubations also included Glc6P. The stimulatory effect of Glc6P on pyruvate utilization at 17 DAA was related to the rapid utilization of Glc6P through the oxidative pentose phosphate pathway (OPPP) at this stage. Addition of pyruvate to incubations containing [1-14C]Glc6P increased OPPP activity (measured as 14CO2 release), while the addition of malate suppressed it. Observations of the interactions between the rate of metabolite utilization for fatty acid synthesis and the rate of the OPPP are consistent with regulation of the OPPP by redox control of the plastidial glucose 6-phosphate dehydrogenase activity through the demand for NADPH. During pyruvate utilization for fatty acid synthesis, flux through the OPPP increases as NADPH is consumed, whereas during malate utilization, in which NADPH is produced by NADP-malic enzyme, flux through the OPPP is decreased.

Keywords: Embryo; fatty acid synthesis; Helianthus annuus; oxidative pentose phosphate pathway; plastid; starch.
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