Figure 3.

Fe-responsive proteins with relevance for photosynthesis in Cyanobacteria, Chlamydomonas, and land plants. Proteins that are discussed in this review for each species are presented (a) Fe-responsive proteins in Cyanobacteria, relatives to the ancestors of chloroplasts. On the outer membrane, Fe³⁺ is chelated by a siderophore and taken up through a TonB Dependent Transporter (TBDT). TonB spans the periplasmic space and the inner membrane to facilitate uptake through the TBDT. On the inner membrane, Fe is taken up as Fe³⁺ via the Fe Uptake Transporter, FutABC, system. Fe can also be taken up as Fe²⁺ after reduction, through Ferrous Iron Transporter B (FeoB). Fe is required for the photosynthetic electron transport chain. During Fe deficiency, Iron Stress Induced protein A (IsiA) can protect Photosystem I (PSI) and Ferredoxin (Fd) can be replaced by the non-Fe-requiring Flavodoxin (Fld) in several species. Two operons exist for Fe–S cluster assembly: Iron Sulfur Cluster (isc) and Sulfur Utilization Factor (suf). Fe is sequestered by ferritin molecules: bacterioferritin and MrgA. (b) Fe-responsive proteins in Chlamydomonas reinhardtii. Chlamydomonas takes up Fe³⁺ by a ferric iron permease yeast homologue (FTR1) after Fe²⁺ is oxidized by Ferric Oxidase 1 (FOX1). Fe is sequestered in the vacuole via Vacuolar Iron Transporter 1 (VIT1) and can be exported from vacuoles via Natural Resistance Associated Macrophage-like Protein 3 (NRAMP3). Chloroplast Fe is sequestered by Ferritin (FER). The ROS-scavenging SuperOxide Dismutases (SOD), FeSOD and MnSOD, are regulated in response to Fe deficiency. (c) Fe-responsive proteins in the leaf mesophyll cell with a focus on chloroplast proteins. From the xylem, Fe can be loaded into the phloem by Oligo Peptide Transporter 3 (OPT3) or to the mesophyll cell. For import into the mesophyll cell, Fe is exported from the xylem by Yellow-Stripe Like 1/3 (YSL1/3) presumably in a Fe³⁺-nicocianamine (NA) complex. Fe³⁺ is reduced at the leaf plasma membrane by Ferric Reductase Oxidase 6 (FRO6) and Fe²⁺ is taken up into the cell. Fe is reduced at the chloroplast envelope by FRO7 and taken up into the stroma by Permease in Chloroplasts 1 (PIC1). ATP Binding Cassette (ABC) proteins, ABCI11, ABCI10, and ABCI12 may also take up Fe into the chloroplast. YSL4/6 is proposed to be a chloroplast Fe exporter. Multiple Antibiotic Resistance1 (MAR1) may transport NA or citrate (CA) into the chloroplast to sequester free Fe. FER is also required to sequester Fe. Fe–S clusters are formed by the SUF pathway in the chloroplast and transfer molecules insert these Fe–S clusters into photosynthetic proteins. Heme and chlorophyll are produced in the tetrapyrrole pathway. Many enzymes for chlorophyll and heme production are Fe-responsive, including Genome Uncoupled 5 (GUN5), Glutamyl-tRNA reductase 1 (HEMA1), and Copper Response Defect 1 (CRD1). Within the chloroplast, during Fe deficiency, ROS-scavenging molecules, Stromal Ascorbate Peroxidase (sAPX) and FeSOD are downregulated in Fe deficiency. Catalase (CAT) is maintained or slightly downregulated during Fe deficiency. Fe is sequestered in the vacuole, where it is imported by VIT1 and exported by NRAMP3/4. YSL4/6 may also be a vacuolar Fe exporter. (d) Fe requirement of photosynthetic electron transport chain proteins. Symbols are explained in the legend.