Characterization of Blue Light Signal Transduction Chains That Control Development and Maintenance of Sexual Competence in Chlamydomonas reinhardtii

Abstract

Blue light induces the differentiation of Chlamydomonas reinhardtii pregametes to gametes. The light-induced conversion of pregametes to gametes is protein synthesis dependent and proceeds only after a lag phase. Upon incubation in the dark, gametes lost their mating ability, resulting in dark-inactivated gametes. Reillumination rapidly restored mating competence and this was shown to be independent of protein synthesis. Apparently, differentiation and maintenance of gametic competence are both regulated by light. Whether one or two light-activated signal pathways are involved was investigated using pharmacological compounds that affect signal transduction. Compounds that affected pregamete-togamete conversion affected the expression of a gamete-specific gene in a similar fashion. Other drugs affected only dark-inactivated gametes, suggesting that reactivating gametes requires a separate signaling pathway. Combined treatments provided evidence for the consecutive action of a phosphatase and a protein kinase C-like kinase in the light-induced reactivation process.

Phototropin is the blue-light receptor that controls multiple steps in the sexual life cycle of the green alga Chlamydomonas reinhardtii

“…Studies at the biochemical level have identified a protein kinase C-like activity and a tyrosine kinase (20,46). Does light signaling during gametogenesis and zygote germination use the same or different pathways?…”

“…Reillumination of these dark-inactivated gametes rapidly restores their mating competence. For this step, 450-nmwavelength light was shown to be most effective (20). To test a possible function of phototropin in this process, gametes of the RNAi20 strain were generated from pregametes and then incubated in the dark for 60 min.…”

“…After a period of maturation, the zygotes are ready for meiosis and germination (18). In this cycle, light has been shown to be required at three steps: gamete formation, the maintenance of mating competence in gametes, and zygote germination (19)(20)(21)(22). Gametogenesis requires the consecutive action of two extrinsic signals.…”

“…These dark-inactivated gametes, by irradiation with blue light, regain their mating ability. However, although the light-induced conversion of pregametes to gametes is a slow process that requires cytoplasmic protein synthesis, the restoration of mating by light occurs rapidly and in the absence of cytoplasmic protein synthesis (20,24). Separate signaling pathways seem to control these photoresponses.…”

Mating-Induced Shedding of Cell Walls, Removal of Walls from Vegetative Cells, and Osmotic Stress Induce Presumed Cell Wall Genes in Chlamydomonas

“…This signal becomes effective in the late phase of sexual differentiation Weissig and Beck, 1991;Beck and Acker, 1992). Irradiation with blue light also is required to maintain the gametes' mating competence (Pan et al, 1997).…”