Fig. 5.

Evolution of plastid electron transport systems. a Photosynthetic plastid such as Chlamydomonas reinhardtii. The photosynthetic electron transport system comprises linear electron transport, plastid terminal oxidase (PTOX)-mediated electron sink, cyclic electron flow, and FNR-mediated NADPH-Fd electron transfer [3,4,5,6]. Solid arrows indicate the direction of reactions. Dotted arrows indicate the electron flow. b The non-photosynthetic plastid lacking the photosynthetic pigment system but retaining the PTOX-mediated sink of electrons and reducing power (NADPH). NADPH-Fd electron transfer was also performed. Gray arrows indicate the postulated direction and flow from the model in a, not experimentally confirmed. Chlamydomonad sp. NrCl902 is highly likely to possess this type of plastid. c Distribution of the plastid-targeted proteins/plastid functions in eukaryotes bearing non-photosynthetic plastids. Dark gray boxes enclosed by solid lines indicate the presence of corresponding proteins or functions localized in plastids, whereas those without solid lines indicate the presence of homologous sequence with no plastid-targeting sequence. Highlighted in light gray are the genes of the electron sink machineries. Data of Polytomella spp. (Volvocales), Helicosporidium sp. (Trebouxiophycea), Monotropa hypopithys (Eudicot land plant), Nitzschia sp. (Bacillariophyceae), “Spumella” sp. (Chrysophyceae), Rhodelphis spp. (Rhodelphidia), and Plasmodium falciparum (Apicomplexa) are derived from previous studies [8, 11,12,13,14, 16,17,18]. Data of Polytomella, Monotropa, Nitzschia, and Spumella are of transcriptome, while the others are of genome. 1 Only this is the plant-type while the others are the divergent-type [34]. 2 Enolase and phosphoglycerate mutase are absent. 3 Flavodoxin (Fld) is used instead of ferredoxin (Fd). d Non-photosynthetic plastid with only NADPH-Fd electron transfer, such as a non-photosynthetic plastid of Helicosporidium sp. AAA: aromatic amino acids, Asp-Lys: aspartate-lysine conversion, BCAA: branched chain amino acids, FA: fatty acids, Fe-S: iron sulfur cluster, FNR: Ferredoxin-NADP⁺ reductase, HPD: 4-hydroxyphenylpyruvate dioxygenase, HST: homogentisate solanesyltransferase, IPP: isopentenyl pyrophosphate, MMT: MPBQ/MSBQ methyltransferase, NDH2: type 2 NADH dehydrogenase, PSI: photosystem I, PSII: photosystem II, Cyt b6/f: cytochrome b6/f, PC: plastocyanin, PQ: plastoquinone, Phy: phytoene, PPP: pentose phosphate pathway, ζ-Car: ζ-Carotene, Lyc: lycopene, and e⁻: electron