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First published online December 10, 2004; 10.1104/pp.104.048256

Plant Physiology 137:492-499 (2005)
© 2005 American Society of Plant Biologists

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State Transitions in Chlamydomonas reinhardtii. The Role of the Mehler Reaction in State 2-to-State 1 Transition1

Giorgio Forti* and Giovanni Caldiroli

Istituto di Biofisica del Consiglio Nazionale delle Ricerche, Sezione di Milano Dipartimento di Biologia, Milan 20133, Italy

The light intensity-dependent transition to state 1 of dark-adapted anaerobic state 2 Chlamydomonas reinhardtii cells is stimulated by oxygen and by other electron acceptors for photosystem I, such as oxaloacetate and methylviologen. This suggests that the transition to state 1 requires the oxidation of the intersystem chain by photosystem I photochemistry. On the other hand, the mere oxidation in the dark of the chain—by addition of O2—leads only to a slow and incomplete transition. The light-driven stimulation by O2 of the state 1 transition is saturated at an O2 concentration of 15 to 20 µM, definitely higher than that of respiration. We suggest that this may represent the affinity for oxygen of the Mehler reaction, a conclusion that is confirmed by the observations that mitochondrial respiration is apparently not involved in modulating state 2-to-state 1 transition. The catalysis of the state 2-to-state 1 transition upon illumination of anaerobically adapted algae might represent, therefore, a relevant physiological role of this process in C. reinhardtii.

1 This work was supported by a contribution from the Italian Ministry for Universities and Research under the project FIRB RBAU01E3CX.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.048256.

* Corresponding author; e-mail giorgio.forti{at}unimi.it; fax 39–02–5031–4815.

Received June 14, 2004; returned for revision September 3, 2004; accepted September 8, 2004.




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