First published online November 24, 2004; 10.1104/pp.104.052324
Plant Physiology 136:4265-4274 (2004)
© 2004 American Society of Plant Biologists
BIOENERGETICS AND PHOTOSYNTHESIS
The Role of Plastocyanin in the Adjustment of the Photosynthetic Electron Transport to the Carbon Metabolism in Tobacco1
Mark Aurel Schöttler*,
Helmut Kirchhoff and
Engelbert Weis
Institut für Botanik, Westfälische Wilhelms-Universität, 48 149 Munster, Germany
We investigated adaptive responses of the photosynthetic electron transport to a decline in the carbon assimilation capacity. Leaves of different ages from wild-type tobacco (Nicotiana tabacum) L. var Samsun NN and young mature leaves of tobacco transformants with impaired photoassimilate export were used. The assimilation rate decreased from 280 in young mature wild-type leaves to below 50 mmol electrons mol chlorophyll–1 s–1 in older wild-type leaves or in transformants. The electron transport capacity, measured in thylakoids isolated from the different leaves, closely matched the leaf assimilation rate. The numbers of cytochrome (cyt)-bf complexes and plastocyanin (PC) decreased with the electron transport and assimilation capacity, while the numbers of photosystem I (PSI), photosystem II, and plastoquinone remained constant. The PC to PSI ratio decreased from five in leaves with high assimilation rates, to values below one in leaves with low assimilation rates, and the PC versus flux correlation was strictly proportional. Redox kinetics of cyt-f, PC, and P700 suggest that in leaves with low electron fluxes, PC is out of the equilibrium with P700 and cyt-f and the cyt-f reoxidation rate is restricted. It is concluded that the electron flux is sensitive to variations in the number of PC, relative to PSI and cyt-bf, and PC, in concert with cyt-bf, is a key component that adjusts to control the electron transport rate. PC dependent flux control may serve to adjust the electron transport rate under conditions where the carbon assimilation is diminished and thereby protects PSI against over-reduction and reactive oxygen production.
1 This work was supported by the Studienstiftung des Deutschen Volkes (to M.A.S.) and by the Deutsche Forschungsgemeinschaft (to H.K.).
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.052324.
* Corresponding author; e-mail schoett{at}uni-muenster.de; fax 0049–251–8323823.
Received August 24, 2004;
returned for revision October 7, 2004;
accepted October 7, 2004.
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