observation-of-photobehavior-in-chlamydomonas-reinhardtii

Observation of Photobehavior in <em>Chlamydomonas reinhardtii</em>

For the survival of the motile phototrophic microorganisms, being under proper light conditions is crucial. Consequently, they show photo-induced ...

reactivation-of-demembranated-cell-models-in-chlamydomonas-reinhardtii

Reactivation of Demembranated Cell Models in <em>Chlamydomonas reinhardtii</em>

Since the historical experiment on the contraction of glycerinated muscle by adding ATP, which Szent-Gy&#246;rgyi demonstrated in the mid-20th century, in ...

Hosei University

The biflagellate green alga Chlamydomonas reinhardtii exhibits both positive and negative phototaxis to inhabit areas with proper light conditions. 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. 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. This mutant has a single amino acid substitution in phytoene synthase, an enzyme that functions in the carotenoid-biosynthesis pathway. The eyespot contains large amounts of carotenoids and is crucial for phototaxis. Most lts1-211 cells have no detectable eyespot and reduced carotenoid levels. Interestingly, the reversed phototactic-sign phenotype of lts1-211 is shared by other eyespot-less mutants. In addition, we directly showed that the cell body acts as a convex lens. 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. These results suggest that light-shielding property of the eyespot is essential for determination of phototactic sign.

Eyespot-dependent determination of the phototactic sign in Chlamydomonas reinhardtii.

The unicellular green alga  is a model organism for various studies in biology. CC-124 is a laboratory strain widely used as a wild type. However, this strain is known to carry  mutation, which causes cells to swim away from the light source (negative phototaxis), in contrast to the cells of other wild-type strains, which swim toward the light source (positive phototaxis). Here we identified the causative gene of  () using AFLP-based gene mapping and whole genome next-generation sequencing. This gene encodes a 36-kDa protein containing a Fibronectin type III domain and a CHORD-Sgt1 (CS) domain. The gene product is localized to the cell body and not to flagella or basal body.

Identification of the  mutation responsible for negative phototaxis in a "wild-type" strain of .

Channelrhodopsins (ChRs) are the light-gated ion channels that have opened the research field of optogenetics. They were originally identified in the green alga Chlamydomonas reinhardtii, a biciliated unicellular alga that inhabits in freshwater, swims with the cilia, and undergoes photosynthesis. It has various advantages as an experimental organism and is used in a wide range of research fields including photosynthesis, cilia, and sexual reproduction. ChRs function as the primary photoreceptor for the cell's photo-behavioral responses, seen as changes in the manner of swimming after photoreception. In this chapter, we will introduce C. reinhardtii as an experimental organism and explain our current understanding of how the cell senses light and shows photo-behavioral responses.

Channelrhodopsin-Dependent Photo-Behavioral Responses in the Unicellular Green Alga Chlamydomonas reinhardtii.

Photo-induced behavioral responses (photobehaviors) are crucial to the survival of motile phototrophic organisms in changing light conditions. Volvocine green algae are excellent model organisms for studying the regulatory mechanisms of photobehavior. We recently reported that unicellular Chlamydomonas reinhardtii and multicellular Volvox rousseletii exhibit similar photobehaviors, such as phototactic and photoshock responses, via different ciliary regulations. To clarify how the regulatory systems have changed during the evolution of multicellularity, we investigated the photobehaviors of four-celled Tetrabaena socialis. Surprisingly, unlike C. reinhardtii and V. rousseletii, T. socialis did not exhibit immediate photobehaviors after light illumination. Electrophysiological analysis revealed that the T. socialis eyespot does not function as a photoreceptor. Instead, T. socialis exhibited slow accumulation toward the light source in a photosynthesis-dependent manner. Our assessment of photosynthetic activities showed that T. socialis chloroplasts possess higher photoprotection abilities against strong light than C. reinhardtii. These data suggest that C. reinhardtii and T. socialis employ different strategies to avoid high-light stress (moving away rapidly and gaining photoprotection, respectively) despite their close phylogenetic relationship.

Noriko Ueki

Science Research Center

Hosei University

Observation of Photobehavior in Chlamydomonas reinhardtii

Reactivation of Demembranated Cell Models in Chlamydomonas reinhardtii