Genetic control of chlorophyll biosynthesis: Effect of increased δ-aminolevulinic acid synthesis on the phenotype of the y-1 mutant of Chlamydomonas

Redox regulation of phototactic migration in the green alga Chlamydomonas reinhardtii and its possible application

Most flagellated green algae swim toward or away from the light source, i.e., display positive or negative phototaxis. Usually, the sign of phototaxis, positive or negative, is variable depending on various factors, but the mechanism that controls it has been unclear. Using Chlamydomonas reinhardtii, an excellent model organism for studying phototaxis, we have recently shown that cellular redox poise plays a key role; cells show positive phototaxis when treated with reactive oxygen species (ROS), whereas they show negative phototaxis when treated with ROS quenching reagents. Here we discuss the possible mechanism of the redox regulation of phototactic sign, questions to be clarified in the future and its possible application.

Reduction-oxidation poise regulates the sign of phototaxis in Chlamydomonas reinhardtii

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.

Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX

Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen ((1)O(2))-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F(v)/F(m) (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH(2)OH failed to restore the F(v)/F(m) ratio suggesting that the reduction of F(v)/F(m) reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q(A)(-) with S(2)) and B (due to recombination of Q(B)(-) with S(2)/S(3)) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F(695) band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by (1)O(2) produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings.

Three new yellow loci in Chlamydomonas reinhardtii

Three phenotypically 'yellow', mendelian mutants of Chlamydomonas reinhardtii have been isolated and tested for allelism with the yellow mutant y-1 a1 and with each other. The three mutants represent three new yellow loci, two of which are located on linkage group I. Like y-1a, the mutants accumulate protochlorophyllide when grown under dim light, but have a wildtype phenotype when grown in the light. We conclude that the control of light-independent protochlorophyllide reduction is more complex than has been thought previously.