# Phosphoinositide Signaling and Actin Polymerization Are Critical for Tip Growth in the Marine Red Alga Pyropia yezoensis

## TL;DR

This study shows that phosphoinositide signaling and actin polymerization are essential for tip growth in the marine red alga Pyropia yezoensis.

## Contribution

The study reveals distinct roles of phosphoinositide turnover and actin polymerization in branch initiation and tip growth in Pyropia yezoensis.

## Key findings

- Pharmacological inhibitors of phosphoinositide signaling reduced side-branch formation and inhibited branch elongation.
- Actin inhibitors cytochalasin B and latrunculin B affected tip growth differently, indicating varied roles in branch initiation and elongation.
- Phosphoinositide turnover and actin polymerization play critical and diverse roles in regulating tip growth in Pyropia yezoensis.

## Abstract

In the marine red alga Pyropia yezoensis, filamentous phases of the life cycle, e.g., the conchocelis (sporophyte) and conchosporangium (conchosporophyte), proliferate by tip growth. In this study, we investigated the possible involvement of phosphoinositide turnover and actin polymerization in the spontaneous initiation and tip growth of new branches in isolated single-celled conchocelis cells using pharmacological treatments. Treatment with LY294002 and U73122, specific inhibitors of phosphoinositide-phosphate 3-kinase and phospholipase C, respectively, reduced side-branch formation and inhibited the elongation of branches. In addition, two inhibitors of the actin cytoskeleton, cytochalasin B (CCB) and latrunculin B (LAT-B), had similar effects on tip growth. However, CCB did not alter the branching rate of single-celled conchocelis, whereas LAT-B did. As CCB and LAT-B affect actin polymerization through different mechanisms, this result suggests differences in the contributions of actin polymerization to branch initiation versus tip growth. These findings demonstrate the critical and diverse functional roles played by phosphoinositide turnover and actin polymerization in the regulation of the initiation and maintenance of tip growth in the conchocelis phase of P. yezoensis.