Abstract
The most motile phototrophic organisms exhibit photo-induced behavioral responses (photobehavior) to inhabit better light conditions for photosynthesis. The unicellular green alga Chlamydomonas reinhardtii is an excellent model organism to study photobehavior. Several years ago, we found that C. reinhardtii cells reverse their phototactic signs (i.e., positive and negative phototaxis) depending on the amount of reactive oxygen species (ROS) accumulated in the cell. However, its molecular mechanism is unclear. In this study, we isolated seven mutants showing positive phototaxis, even after the induction of negative phototaxis (ap1~7: always positive) to understand the ROS-dependent regulatory mechanism for the phototactic sign. We found no common feature in the mutants regarding their growth, high-light tolerance, and photosynthetic phenotypes. Interestingly, five of them grew faster than the wild type. These data suggest that the ROS-dependent regulation of the phototactic sign is not a single pathway and is affected by various cellular factors. Additionally, the isolation and analyses of mutants with defects in phototactic-sign regulation may provide clues for their application to the efficient cultivation of algae.
Highlights
For motile phototrophic organisms, photo-induced behavioral responses are important to maintain optimal light exposure for their photosynthetic activities
This study introduced random mutations to wild-type C. reinhardtii to screen for mutants showing positive phototaxis, even after treatment with reactive oxygen species (ROS) scavengers that induce negative phototaxis
The mutant library was subjected to phototaxis screening (Figure 1A)
Summary
The unicellular green alga Chlamydomonas reinhardtii is an ideal model organism for studying photo responses because it exhibits striking photobehavior immediately after photoreception [1,2]. The typical photobehavior in C. reinhardtii includes phototaxis and photoshock response. Phototaxis is a behavior in which cells swim toward or away from incident light (called positive or negative phototaxis, respectively). Photoshock response is a behavior in which cells stop swimming or swim backward for a short period after a sudden change in light intensity. Both behaviors are regulated by the following two organelles: the eyespot and cilia
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