Calcium and the photopolarization of Pelvetia zygotes

The initially apolar zygotes of the brown algae, Fucus and Pelvetia, form their main axes during the hours following fertilization and each cell expresses its axis by germinating at one location. The germinating region is destined to become the rhizoid and the rest of the zygote gives rise to the thallus. In response to unilateral blue light, the zygotes organize their developmental axes so that the rhizoids emerge on the shaded side, away from the light source. In the research reported here, the signal-transduction elements involved in the photopolarization of Pelvetia fastigiata De Toni zygotes have been investigated. It was found that exposure of zygotes to 90- or 150-min pulses of unilateral light in the absence of extracellular Ca2+ completely eliminated photopolarization; that is, the cells formed their rhizoid-thallus axes randomly with respect to the light direction, while controls similarly exposed to light in normal (10 mM) Ca2+ were well polarized. When the cells were incubated in Ca2+-free sea water for an hour before being given the light pulse (while still in Ca2+-free sea water), they exhibited an unusual negative polarization: they formed their rhizoids on the hemisphere nearer the light source. Organic and inorganic calcium-channel blockers reduced or abolished photopolarization when present during light pulses. Reducing external Ca2+ to one-tenth of normal has the paradoxical effect of increasing calcium influx into Pelvetia zygotes. When zygotes were given light pulses in reduced extracellular calcium, the degree of photopolarization was increased substantially. These data are consistent with the idea that the formation of an intracellular gradient of [Ca2+] is an essential part of the polarization process. The fungus-derived calmodulin antagonist, ophiobolin A, blocked or greatly delayed germination when present continuously at a concentration of 100-300 nM. However, when present at 300 nM during a brief light pulse, it markedly increased the sensitivity of the cells to light. These results suggest that calmodulin may be the mediator of intracellular [Ca2+] gradients in the photopolarization process.