The apical bias of Rab11 vesicle transport depends on the activation of apical actomyosin contractility. (A) Diagram depicting optogenetic inhibition of Myosin II activation by Opto-Rho1DN. (B) Bottom: upon continuous blue light stimulation, Rho1DN is immediately recruited to the plasma membrane (yellow arrows). Apical myosin II (red boxes) disappears ∼30 s after stimulation. Top: A control embryo at similar stages of furrow formation. Scale bar, 10 μm. (C–F) The apical bias of the transport is abolished in <1 min after Opto-Rho1DN stimulation (C, n = 3 embryos). This change is caused by a significant increase in the number of basally directed transport events (D), which is not observed in wildtype embryos (E, n = 3 embryos). There is no significant difference on average trajectory velocity, length and duration of apical and basal bound transports between 0–23 s and 35–58 s post stimulation windows (F, mean ± SD). One-sample, two-tailed t test against 50% was used for comparison to hypothetical unbiased transport. Error bars stand for SD. Two-sample, two-tailed unpaired t test was used for comparison between conditions. ***, P < 0.001; **, P < 0.01; *, P < 0.05; n.s., P > 0.05. (G and H) Microtubules do not exhibit obvious changes in intensity or overall organization in the first 2 min after Opto-Rho1DN stimulation (0 min). (G) En face maximum intensity projection of the apical surface (5 μm thick) showing microtubules (GFP-Jupiter) in an embryo stimulated at the stage comparable to that in B. Scale bar, 5 μm. (H) Cross-section view showing an embryo stimulated at a stage slightly earlier than that in G. N = 6 embryos. Scale bar, 10 μm.