Ionic requirements for establishment of an embryonic axis in Pelvetia zygotes

Abstract

Previous reports have shown that fertilized Pelvetia fastigiata (J. Ag.) DeToni eggs generate a transcellular ionic current which correlates temporally and spatially with establishment of a rhizoid/thallus axis. In order to understand more fully the ionic controls of development, we have determined the ionic requirements for axis formation induced by unilateral light. Formation of the developmental axis was independent of the presence of individual ions in artificial seawater. This finding indicates that transcellular circulation of a particular ion is not obligatory for polarization, and it confirms earlier work showing that calcium circulation is not fundamental to axis establishment in Fucus zygotes. Polarization was not, however, completely independent of ionic conditions; zygotes were unable to form an axis in pure sucrose solutions. Single salts were added to sucrose to determine which ions were sufficient to permit polarization. Salts of impermeant monovalent cations and salts of divalent cations supported polarization weakly or not at all. By contrast, zygotes photopolarized well in KCl, and other alkali metals substituted for K(+) with varying effectiveness (Rb(+)>Na(+)> Cs(+)≫ Li(+)). The anion was unimportant; a variety of different potassium salts all supported polarization equally well. The mechanism by which KCl promotes polarization is not yet understood, but may involve transport through K(+) channels.

Ca2+ and Calmodulin Dynamics during Photopolarization in Fucus serratus Zygotes

“…Removal of free Ca 2+ from the culture medium and the addition of high concentrations of EGTA had no significant effect on photopolarization. This confirms previous data (Hurst and Kropf, 1991), but is different from a recent study on the photopolarization of P.fastigiata zygotes (Robinson, 1996a). Using a protocol similar to ours, that study simultaneously exposed zygotes to 4 mM EGTA in Ca …”

“…First, zygotes respond normally to polarizing stimuli in media containing 10 to 100 PM Ca2+, 109 times less than the [Ca2+] of sea water, suggesting that Ca2+ circulation is not essential for polar axis formation (Hurst and Kropf, 1991), although these findings have themselves been questioned by recent results (Robinson, 1996a). Second, chlorotetracycline fluorescence imaging of membrane-bound Ca2+ displayed no asymmetric distribution during polar induction (Kropf and Quatrano, 1987).…”

“…Indeed, plant cells possess a rich and poorly understood reserve of extracellular Ca 2+ bound to the cell wall, and several hours of exposure to EGTA is required to remove this Ca 2+ pool (Saunders and Hepler, 1983). Such a procedure was not applicable to the experiments described here, nor was it used in the experiments of Hurst and Kropf (1991). Instead, this eventuality was investigated by direct inhibition of plasma membrane Ca 2+ channels.…”

Radial endoplasmic reticulum arrays co-localize with radial F-actin in polarizing cells of brown algae

“…In fucoid zygotes a cGMP dependent redistribution of the peripheral Ca 2þ channels (Robinson et al, 1999) is mainly responsible for the Ca 2þ gradient. However, when the zygotes are placed in a Ca 2þ -free medium, the formation of the Ca 2þ gradient and the emergence of the rhizoid proceeds normally (Hurst & Kropf, 1991;Love et al, 1997), while the blockage of intracellular Ca 2þ release by various inhibitors (e.g. nifedipine) significantly reduces the photopolarization of Fucus zygotes (Love et al, 1997).…”

Fucus Embryogenesis: A Model to Study the Establishment of Polarity.

“…Axis formation is not dependent on any one specific ion, but it has been shown that K+ salts alone (10 mM KCI, with sucrose for an osmoticum) can support axis formation (Hurst and Kropf, 1991). It is not known whether this requirement for K+ is directly related to axis formation or is indirectly related through some essential housekeeping function such as maintenance of turgor pressure.…”