## 1 Introduction  
Carotenoids are pigments with important structural, light-harvesting, and photoprotective roles in photosynthetic pigment-protein complexes and membranes [1, 2]. Carotenoid biosynthesis in phototrophs (for reviews see [3, 4]) begins with the condensation of the products of the 2-C-methyl-D-erythnitol 4-phosphate (MEP) pathway: isopentyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). This generates geranyl pyrophosphate (GPP), which is converted to geranylgeranyl pyrophosphate (GGPP) (via farnesyl-pyrophosphate) by condensation with two further IPP molecules, prior to the production of 15-_cis_-phytoene (phytoene) by the condensation of two molecules of GGPP. How phytone is converted to carotenoids with longer conjugation length (\(N\)) polyene chains differs between phototrophic organisms. In phototrophic bacteria, phytoene desaturase (Crttl) catalyzes sequential desaturations of phytoene (colorless with three conjugated carbon-carbon double bonds; \(N=3\)) to produce neurosporene (yellow, three desaturations of phytoene; \(N=9\)) or lycopene (orange/red; four desaturations of phytoene; \(N=11\)) [3; 4]. Neurosoprene and/or lycopene are subsequently modified by additional enzymes to produce the carotenoids that are utilized for light harvesting and photoprotection. Conversely, oxygenic phototrophs (cyanobacteria, algae, and plants), green sulfur bacteria (GSB), and Chloroacidobacteria use a multi-enzyme pathway to synthesize lycopene from phytoene (Figure 1); the four enzymes are phytoene desaturase (PDS/CrtP; catalyzes two desaturations of lycopene), zeta(\(\zeta\))-carotene isomerase (Z-ISO), \(\zeta\)-carotene desaturase (ZDS/CrtQ; catalyzes two desaturations of \(\zeta\)-carotene), and the prolycopene isomerase (CRT-ISO/CrtH) [3; 4]. Figure 1: Overview of carotenoid biosynthesis in oxygenic phototrophs focusing on the steps in the conversion of phytoene to lycopene. The products of the MEP pathway, IPP and DMAPP, are condensed to form GGPP by the enzyme GGPP synthase (GGPPS). Two molecules of GGPP are subsequently condensed by the enzyme phytoene synthase (PSY), producing 15-_cis_-phytoene. Phytoene desaturase (PDS) and \(\zeta\)-carotene desaturase (ZDS) introduce four double bounds by desaturation of the carotenoid backbone (blue highlight) to produce all-_trans_-lycopene, with _cis_-to-_trans_ isomerization of bonds in the intermediate species (red highlight) carried out by Z-ISO and CRT-ISO; these _cis_-bonds are photo-labile and can be isomerized non-enzymatically by light (indicated by orange arrows). Subsequent modification of all-_trans_-lycopene (enzymatic steps not shown) yields the caroten and xanthophyll species used for light harvesting, photoproduction, and structural stabilization of proteins and lipid membranes. Z-ISO is the most recently identified early carotenoid biosynthesis enzyme; the _Zea mays_ (maize) enzyme has been demonstrated to be a heme B-containing integral membrane protein that catalyzes the redox-regulated _cis_-to-_trans_ conversion of the 15-_cis_ double bond in 9,15,9\({}^{\prime}\)-tri-_cis_-\(\zeta\)-carotene to form 9,9\({}^{\prime}\)-di-_cis_-\(\zeta\)-carotene [5; 6]. Z-ISO homologs are present in plants, algae, diatoms, and cyanobacteria; the enzymes from maize and _Arabidopsis thaliana_ (hereafter _Arabidopsis_) are the best studied [5; 7; 8] but Z-ISO has also been experimentally identified in _Oryza sativa_ (rice) [9; 10], the filamentous cyanobacterium _Arthospira_[11], the eukaryotic microalgae Euglena [7; 11], and the fruits _Citrus sinensis_[12] and tomato [13]. Interestingly, GSB and species of the _Chloracidobacterium_ genus appear to lack Z-ISO [8; 11]. Maize and _Arabidopsis_ mutants accumulate 9,15,9\({}^{\prime}\)-tri-_cis_-\(\zeta\)-carotene and lack carotenoids in the dark, as well as having delayed greening and a lower carotenoid content when exposed to light [8]. In rice, Z-ISO links carotenoid and plant hormone biosynthesis to regulate/modulate photosynthesis and tillering [9; 10].