Flowering plants form various organs, such as the flower, leaves, root and stem, which develops along its body axis. One of the most fundamental axes is the apical-basal (shoot-root) axis, i.e. the apical (top part) develops into shoots, containing flowers, stems and leaves, and the basal (bottom part) grows into roots. The fertilized egg cell (zygote), which is the origin for plants, establishes the plant’s body axis from its first cell division. In Arabidopsis thaliana, the zygote possesses the nucleus at the apical cell tip and large vacuoles at the basal end. Despite the obvious asymmetry, the real-time dynamics of the zygote polarization steps was poorly understood, because the zygote develops deep in the flower. Therefore we established a live-cell imaging system to visualize the intracellular dynamics of the zygote through the seed. By combining this system with various fluorescent reporters, specific inhibitors and mutants, we assessed the dynamics and roles of intracellular structures in zygote polarization. For example, we found that the vacuoles form thin tubular strands along longitudinal array of actin filaments, and polarly position to the basal cell end, supporting apical-directed nuclear migration and thus asymmetric division of the zygote and subsequent embryo pattern formation.

In addition of the visualization of intracellular dynamics, we are screening the key regulators of polarization events by using transcriptome approach. By combining these approaches with further techniques, such as chemical biology and image quantification, we aim to understand the initiation steps of plant ontogeny from the single-celled zygote.