Ryunosuke Irie and Koji Mikami

ABSTRACT

The conchocelis, the filamentous sporophyte generation of Bangiales, proliferates through tip growth in which only the tips of branch initial cells elongate and subsequently divide perpendicularly to the apical–basal axis to form filamentous structures. As was found in terrestrial plants, the tip growth of conchocelis in the marine red alga Pyropia yezoensis is regulated by auxin, actin polymerization, and phosphatidylinositol signaling. Although extracellular Ca2+ influx is critical for tip growth in terrestrial plants, little is known about its involvement in the tip growth of algae. We therefore investigated whether extracellular Ca2+ influx is required for tip growth in P. yezoensis conchocelis. Treatment of isolated single-celled conchocelis with a Ca2+ chelator, ethylene glycol tetraacetic acid, inhibited both the formation of branch initials and the tip growth of branches in a dose-dependent manner. These findings indicate that extracellular Ca2+ influx is indeed involved in the tip growth of conchocelis, suggesting that the basic regulatory mechanisms governing tip growth may be conserved between marine red algae and terrestrial plants. Further confirmation of this possibility will require the characterization of the spatiotemporal patterns of Ca2+ oscillations and F-actin accumulation, as well as the subcellular localization of phosphoinositides and their catabolic enzymes in the P. yezoensis conchocelis.