Bgs4 and its β(1,3)glucan are required for correct straight CAR constriction and for coupling septum synthesis to CAR contraction and PM extension. (A–D) Bgs4 absence causes misdirected (arrows) CAR (A and C; Rlc1 and Cdc15) and septa (B and C; Bgs1, Bgs3, Psy1, and CW staining). Cells were grown as in Fig. 2 A. (D) Percentage of misdirected CAR and septa. Error bars indicate SD. (E) The defect in Bgs4 function (shown in the cwg1-1 mutant) produces similar misdirected CAR contraction and septum progression (arrows). During misdirected ingression, the CAR (Rlc1) stays attached to the PM (Psy1). CW staining shows the details of wavy PS (bottom right, wavy arrow), indicative of relaxed CAR with multiple changes in septum synthesis direction. Cells were grown as in Fig. 2 H and observed by time-lapse. (F) TEM details of rigid and straight WT septum formation with simultaneous PS and SS synthesis from the start (left). Formation in Bgs4 absence of weak twisted and misdirected (arrows) septa with no SS and defects in the last stages of PS synthesis (right). Cells were grown as in A. (G and H) Bgs4 is essential for coupling PS growth to CAR contraction and PM extension. (G) Magnification of septum formation as in F. (H) Model of advanced CAR and PM ingression uncoupled from delayed PS synthesis. A relaxed CAR devoid of tensile force causes misdirected septa. Arrows, CAR and PM edge; arrowheads, PS edge. AR, annular rudiment; Cw, cell wall. Bars: (cells) 5 µm; (septum details) 1 µm.