ChRs localize to the plasma membrane over the carotenoid layers, which reflect and amplify the light signal coming from the outside of the cell (the “front side” of ChRs) while blocking the light from the inside of the cell (the “rear side” of ChRs) ( Fig. During step 1, the eyespot plays a crucial role in directional photoreception. Recent studies suggested that the Chlamydomonas phototactic pathway primarily consists of four steps: ( i) photoreception by ChRs ( ii) excitation of the cellular membrane ( iii) increase in intraflagellar and ( iv) a change in the beating balance between the two flagella, i.e., the cis-flagellum (the one closest to the eyespot) and the trans-flagellum (the one farthest from the eyespot) ( Fig. These results suggest that light-shielding property of the eyespot is essential for determination of phototactic sign. The lens effect of the cell body condenses the light coming from the rear onto the photoreceptor in the absence of carotenoid layers, which can account for the reversed-phototactic-sign phenotype of the mutants. In addition, we directly showed that the cell body acts as a convex lens. Interestingly, the reversed phototactic-sign phenotype of lts1-211 is shared by other eyespot-less mutants. Most lts1-211 cells have no detectable eyespot and reduced carotenoid levels. The eyespot contains large amounts of carotenoids and is crucial for phototaxis. This mutant has a single amino acid substitution in phytoene synthase, an enzyme that functions in the carotenoid-biosynthesis pathway. Taking advantage of this property, we isolated a mutant, lts1-211, which displays a reduction-oxidation (redox) dependent phototactic sign opposite to that of the wild type. It has been shown that treatment of cells with reactive oxygen species (ROS) reagents biases the phototactic sign to positive, whereas that with ROS scavengers biases it to negative. The biflagellate green alga Chlamydomonas reinhardtii exhibits both positive and negative phototaxis to inhabit areas with proper light conditions.
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