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Summary: Carotenoids are the most universal and most widespread pigments in nature. They have played pivotal roles in the evolution of photosensing mechanisms in microbes and of vision in animals. Several groups of phytoflagellates developed a photoreceptive organelle called the eyespot apparatus (EA) consisting of two separable components: the eyespot, a cluster of carotenoid‐rich globules that acts as a reflector device, and actual photoreceptors for photobehaviors. Unlike other algal eyespots, the eyespot of Euglenophyta lacks reflective properties and is generally considered to act as a shading device for the photoreceptor (paraflagellar body, PFB) for major photomovements. However, the function of the eyespot of Euglenophyta has not yet been fully proven. Here, we report that the blocking carotenoid biosynthesis in Euglena gracilis by suppressing the phytoene synthase gene (crtB) caused a defect in eyespot function resulting in a loss of phototaxis. Raman spectroscopy and transmission electron microscopy suggested that EgcrtB‐suppressed cells formed eyespot globules but had a defect in the accumulation of carotenoids in those packets. Motion analysis revealed the loss of phototaxis in EgcrtB‐suppressed cells: a defect in the initiation of turning movements immediately after a change in light direction, rather than a defect in the termination of cell turning at the appropriate position due to a loss of the shading effect on the PFB. This study revealed that carotenoids are essential for light perception by the EA for the initiation of phototactic movement by E. gracilis, suggesting one possible photosensory role of carotenoids in the EA for the phototaxis. Significance Statement: The eyespot apparatus (EA) of Euglena gracilis consists of a cluster of cytosolic carotenoid‐rich globules, called the eyespot, and the actual photoreceptive organelle, the paraflagellar body (PFB). The eyespot of this alga has been speculated to be just a shading device for the PFB. This study revealed by single‐cell tracking techniques that the eyespot is necessary for phototaxis of this alga and that carotenoids participate in light perception by the EA to initiate phototactic movement. [ABSTRACT FROM AUTHOR]

Ferredoxin-NADP+-oxidoreductase (FNR) mediates electron transfer between ferredoxin (Fd) and NADP+; therefore, it is a key enzyme that provides the reducing power used in the Calvin cycle. Other than FNR, nitrite reductase, sulfite reductase, glutamate synthase, and Fd-thioredoxin reductase also accept electrons from Fd, an electron carrier protein in the stroma. Therefore, the regulation of electron partitioning in the chloroplast is important for photosynthesis and other metabolic pathways. The regulatory mechanism of electron partitioning, however, remains to be elucidated. We found, by taking advantage of a gain-of-function approach, that expression of two rice (Oryza sativa) full-length cDNAs of leaf-type FNRs (OsLFNR1 and OsLFNR2) led to altered chlorophyll fluorescence and growth in Arabidopsis (Arabidopsis thaliana) and rice. We revealed that overexpression of the OsLFNR1 and OsLFNR2 full-length cDNAs resulted in distinct phenotypes despite the high sequence similarity between them. Expression of OsLFNR1 affected the nitrogen assimilation pathway without inhibition of photosynthesis under normal conditions. On the other hand, OsLFNR2 expression led to the impairment of photosynthetic linear electron transport as well as Fd-dependent cyclic electron flow around photosystem I. The endogenous protein level of OsLFNR was found to be suppressed in both OsLFNR1- and OsLFNR2-overexpressing rice plants, leading to changes in the stoichiometry of the two LFNR isoforms within the thylakoid and soluble fractions. Thus, we propose that the stoichiometry of two LFNR isoforms plays an important role in electron partitioning between carbon fixation and nitrogen assimilation. [ABSTRACT FROM AUTHOR]

Mutant resources are indispensable for the characterization of the functions of genes. There are two types of mutants, loss-of-function and gain-of-function mutants. Recently, we have developed a novel system in plants that uses a gain-of-function approach and is named as the FOX (full-length cDNA overexpressor gene) hunting system. In this system, Arabidopsis full-length cDNAs (fl-cDNAs) are randomly over-expressed under the control of the cauliflower mosaic virus (CaMV) 35S promoter in Arabidopsis plants. These transgenic plants, or Arabidopsis FOX lines, possess ectopically expressed fl-cDNAs in their genome. Chemical genomics is a newly emerging field that connects chemical biology with genomes. Since each FOX line expresses an excess amount of the protein from the transgene it can be resistant or hypersensitive to bioactive chemicals when the protein is the target for the chemical. In this protocol, we describe the procedure for identification of the fl-cDNAs responsible for the target of the chemical or for the signal transduction pathway involving the chemical.