Mutants, Overexpressors, and Interactors ofArabidopsis Plastocyanin Isoforms: Revised Rolesof Plastocyanin in Photosynthetic Electron Flowand Thylakoid Redox State

ABSTRACT Two homologous plastocyanin isoforms are encoded by the genes PETE1 and PETE2 in the nuclear genome ofArabidopsis thaliana. The PETE2 transcript is expressed at considerably higher levels and the PETE2 protein is the moreabundant isoform. Null mutations in the PETE genes resulted in plants, designated pete1 and pete2, with decreased plastocyanincontents. However, despite reducing plastocyanin levels by over ;90%, a pete2 null mutation on its own affectsrates of photosynthesis and growth only slightly, whereas pete1 knockout plants, with about 60–80% of the wild-typeplastocyanin level, did not show any alteration. Hence, plastocyanin concentration is not limiting for photosynthetic electronflow 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 wefind no evidence for a physical interaction between the two proteins, using interaction assays in yeast. We observedhomodimerization 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 twoplastocyanin-encoding genes, failed to reveal any genetic interaction. Overexpression of either PETE1 or PETE2 in the pete1pete2 double knockout mutant background results in essentially wild-type photosynthetic performance, excludingthe possibility that the two plastocyanin isoforms could have distinct functions in thylakoid electron flow.