Press Releases
Jun. 21, 2011

How green alga cells switch between positive and negative phototaxis.

Presenters
  • Ken-ichi Wakabayashi (Department of Biological Sciences, University of Tokyo)
  • Yuka Misawa (Department of Biological Sciences, University of Tokyo)
  • Shota Mochiji (Department of Biological Sciences, University of Tokyo)
  • Ritsu Kamiya (Department of Biological Sciences, University of Tokyo)

Abstract

In many phototrophic microorganisms and plants, chloroplasts change their positions relative to the incident light to achieve optimal photosynthesis. In the case of motile green algae, cells change their swimming direction by switching between positive and negative phototaxis, i.e., swimming toward or away from the light source, depending on environmental and internal conditions. However, little is known about the molecular signals that determine the phototactic direction. Using the green alga Chlamydomonas reinhardtii , we found that cellular reduction-oxidation (redox) poise plays a key role: Cells always exhibited positive phototaxis after treatment with reactive oxygen species (ROS) and always displayed negative phototaxis after treatment with ROS quenchers. The redox-dependent switching of the sign of phototaxis may contribute in turn to the maintenance of cellular redox homeostasis.

Paper information

Ken-ichi Wakabayashi, Yuka Misawa, Shota Mochiji, and Ritsu Kamiya
(Department of Biological Sciences, Graduate School of Science, University of Tokyo)

Title
Reduction-oxidation poise regulates the sign of phototaxis in Chlamydomonas reinhardtii
Journal
Proceedings of the National Academy of Sciences of the United States of America (PNAS)
Figure 1

Fig.1
(Left) A micrograph of Chlamydomonas cell. Bar: 10µm. (Right) A schematic diagram of Chlamydomonas cell. It swims with two flagella like human's breast stroke. It perceives light at the eyespot.

Figure 2

Fig.2
Phototaxis assay of Chlamydomonas cells. Cell suspensions were photographed after illumination from the right side for 15 minutes (green arrows). Under control conditions, wild-type and agg1 (a mutant that shows strong negative phototaxis) cells showed positive and negative phototaxis, respectively. Both strains exhibited positive phototaxis after treatment with hydrogen peroxide, a kind of reactive oxygen species (ROS), whereas they both showed negative phototaxis after treatment with TEMPOL, a ROS quenching reagent.