Molecular Plant Advance Access originally published online on August 11, 2008
Molecular Plant 2009 2(2):236-248; doi:10.1093/mp/ssn041
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Mutants, Overexpressors, and Interactors of Arabidopsis Plastocyanin Isoforms: Revised Roles of Plastocyanin in Photosynthetic Electron Flow and Thylakoid Redox State
a Dipartimento di Produzione Vegetale, Università degli studi di Milano c/o Parco Tecnologico Padano Via Einstein, Loc. Cascina Codazza, I-26900 Lodi, Italy
b Lehrstuhl für Botanik, Department Biologie I, Ludwig-Maximilians-Universität München, Großhaderner Str. 2, D-82152 Planegg-Martinsried, Germany
c Abteilung für Pflanzenzüchtung und Genetik, Max-Planck-Institut für Züchtungsforschung, Carl-von-Linné-Weg 10, D-50829 Köln, Germany
d Institute of Chemistry and Umeå Plant Science Centre (UPSC), Umeå University, SE-901 87 Umeå, Sweden
e Institut de Biologie Physico-Chimique, UMR 7141 CNRS-Université P. et M. Curie, 13, rue Pierre et Marie Curie, F-75005 Paris, France
f Dipartimento di Biologia, Università degli studi di Milano, Via Celoria 26, I-20133 Milano, Italy
g Dipartimento Scientifico e Tecnologico, Università degli studi di Verona, Strade le Grazie 15, I-37134 Verona, Italy
h Institut für Biochemie der Pflanzen, Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany
1 To whom correspondence should be addressed. E-mail leister{at}lrz.uni-muenchen.de, fax +49-89-2180-74599, tel. +49-89-2180-74550.
Two homologous plastocyanin isoforms are encoded by the genes PETE1 and PETE2 in the nuclear genome of Arabidopsis thaliana. The PETE2 transcript is expressed at considerably higher levels and the PETE2 protein is the more abundant isoform. Null mutations in the PETE genes resulted in plants, designated pete1 and pete2, with decreased plastocyanin contents. However, despite reducing plastocyanin levels by over 
90%, a pete2 null mutation on its own affects rates of photosynthesis and growth only slightly, whereas pete1 knockout plants, with about 60–80% of the wild-type plastocyanin level, did not show any alteration. Hence, plastocyanin concentration is not limiting for photosynthetic electron flow under optimal growth conditions, perhaps implying other possible physiological roles for the protein. Indeed, plastocyanin has been proposed previously to cooperate with cytochrome c6A (Cyt c6A) in thylakoid redox reactions, but we find no evidence for a physical interaction between the two proteins, using interaction assays in yeast. We observed homodimerization of Cyt c6A in yeast interaction assays, but also Cyt c6A homodimers failed to interact with plastocyanin. Moreover, phenotypic analysis of atc6-1 pete1 and atc6-1 pete2 double mutants, each lacking Cyt c6A and one of the two plastocyanin-encoding genes, failed to reveal any genetic interaction. Overexpression of either PETE1 or PETE2 in the pete1 pete2 double knockout mutant background results in essentially wild-type photosynthetic performance, excluding the possibility that the two plastocyanin isoforms could have distinct functions in thylakoid electron flow.