Bgs4 β(1,3)glucan confers the septum strength needed for a rigid straight septum. (A and B) Absence of Bgs4 generates curved (A) and defective septa with no CW-stained PS at the septum base (B). Details of PS (arrows) retracted from the cell wall (B, right). Cells were grown as in Fig. 1 B. Arrows, cell wall; arrowheads, CW-stained PS base. Error bars indicate SD. (C and D) Bgs4 absence generates flexible curved septa, oscillating according to the changes in internal pressure between sister cells. Cells were grown in MM+S+T for 9 h and visualized by time-lapse. (C) WT rigid straight septum formation and progressive cell separation (top). Flexible curved septa with two oscillating cycles (arrow), ending with cell lysis in one (middle) or both cells (bottom). (D) Kymograph of a flexible curved septum with four waving cycles (arrow). Bars: (cells) 5 µm; (septum details) 1 µm.