3D imaging reveals robustness and plasticity of cell division in rice early embryogenesis.

Embryogenesis is an essential process involving a series of formative cell divisions that contribute to establishing the plant's body axis. In many dicotyledons, the asymmetric cell division of the zygote gives rise to two daughter cells, which develop into two distinct cell lineages. In contrast, the fate of the two daughter cells and their contribution to the body axis formation remain poorly understood in the monocots. To address this question, we developed a method for three-dimensional imaging of early rice embryos. Our observations demonstrated that both an egg cell and two synergids are polarized prior to fertilization and are anchored to the micropylar end of the ovule via a cell wall-like structure stained with SR2200. Upon fertilization, the zygote undergoes an asymmetric cell division with a ventrally tilted division plane. The following cell divisions are not strictly synchronized between the apical and basal lineages, exhibiting non-stereotypic patterns up to the globular stage of embryogenesis. Furthermore, we examined the role of auxin signaling in rice embryogenesis using the auxin response sensor DR5rev::NLS-3xVENUS. The reporter activity was first detected at the center of the globular embryos and subsequently extended along the apical-basal axis as embryogenesis progressed. Our results highlight the importance of the progressive establishment of the body axes within cell populations during early embryogenesis.