Redox chemistry-the transfer of electrons or hydrogen atoms--is central to energy conversion in respiration and photosynthesis. In photosynthesis in chloroplasts, two separate, light-driven reactions, termed photosystem I and photosystem II, are connected in series by a chain of electron carriers. The redox state of one connecting electron carrier, plastoquinone, governs the distribution of absorbed light energy between photosystems I and II by controlling the phosphorylation of a mobile, light-harvesting, pigment- protein complex. Here we show that the redox state of plastoquinone also controls the rate of transcription of genes encoding reaction-centre apoproteins of photosystem I and photosystem II. As a result of this control, the stoichiometry between the two photosystems changes in a way that counteracts the inefficiency produced when either photosystem limits the rate of the other. In eukaryotes, these reaction-centre proteins are encoded universally within the chloroplast. Photosynthetic control of chloroplast gene expression indicates an evolutionary explanation for this rule: the redox signal-transduction pathway can be short, the response rapid, and the control direct.