During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP+ by way of photosystem I (PSI). This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b6f complex mediates electron transport between the two photosystems and generates the proton gradient (DeltapH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b6f complex. Such cyclic flow generates DeltapH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP+ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.

Expression of miR398 is induced in response to copper deficiency and is involved in the degradation of mRNAs encoding copper/zinc superoxide dismutase in Arabidopsis thaliana. We found that SPL7 (for SQUAMOSA promoter binding protein–like7) is essential for this response of miR398. SPL7 is homologous to Copper response regulator1, the transcription factor that is required for switching between plastocyanin and cytochrome c  6 in response to copper deficiency in Chlamydomonas reinhardtii. SPL7 bound directly to GTAC motifs in the miR398 promoter in vitro, and these motifs were essential and sufficient for the response to copper deficiency in vivo. SPL7 is also required for the expression of multiple microRNAs, miR397, miR408, and miR857, involved in copper homeostasis and of genes encoding several copper transporters and a copper chaperone, indicating its central role in response to copper deficiency. Consistent with this idea, the growth of spl7 plants was severely impaired under low-copper conditions.

RNA editing is a process of RNA maturation involved in the insertion, deletion or modification of nucleotides. In organellar transcripts of higher plants, specific cytidine residues are converted into uridine residues. In many cases, editing results in the restoration of conserved amino acid residues, a process that is essential for protein function in plastids. Despite the technical breakthrough in establishing systems in vivo and in vitro for analysing RNA editing, its machinery still remains to be identified in higher plants. Here we introduce a genetic approach and report the discovery of a gene responsible for the specific RNA editing event in the chloroplast.