Explants from ventralized embryos, which lack tissues expressing CE but close their Blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the Blastopore. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. Indirect evidence suggests that Blastopore closure during gastrulation of anamniotes, including amphibians such as Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. Taken together, our data indicate that transcriptional regulation of plekhg5 expression at the Blastopore lip determines bottle cell morphology via local polarized activation of Rho by Plekhg5, which stimulates apical actomyosin activity to induce apical constriction. Apical accumulation of actomyosin, apical constriction and bottle cell formation fail to occur in these embryos. Knockdown of plekhg5 inhibits activin-induced bottle cell formation and endogenous Blastopore lip formation in gastrulating frog embryos. Plekhg5 induces apical actomyosin accumulation and cell elongation. Plekhg5 protein is localized in the apical cell cortex via its pleckstrin homology domain, and the GEF activity enhances its apical recruitment. This activity is not shared by arhgef3, which encodes another organizer-specific RhoGEF. plekhg5 is expressed in the Blastopore lip and its expression is sufficient to induce ectopic bottle cells in epithelia of different germ layers in a Rho-dependent manner. Here, we identify a Rho guanine nucleotide exchange factor (GEF) gene plekhg5 as an essential regulator of apical constriction of bottle cells during Xenopus gastrulation. ABSTRACT Apical constriction regulates epithelial morphogenesis during embryonic development, but how this process is controlled is not understood completely.

Xenopus blastopore