• UEDA Minako
• SUZUKI Hidemasa

• Reproductive Development

• Imaging

In most angiosperms, the male and female gametes fuse (fertilize) deep within seeds, and zygotes undergo asymmetric cell division along the apical?basal axis. Since this first cell division determines the initial body axis in multicellular organisms, zygote polarization is an important step in plant ontogeny. In many plant species, the subsequent cell division patterns are non-uniform, hindering cell lineage tracing after several cell divisions. Conversely, Brassicaceae embryos exhibit stereotypic cell division patterns, simplifying cell lineage tracking. In Arabidopsis thaliana (Arabidopsis), apical cells vertically divide to produce daughter cells on the left and right sides, and the basal cells transversely divides to generate filamentous suspensors. During subsequent embryogenesis, the apical cell lineage gives rise to cotyledons and shoot apical meristems, and the basal cell lineage produces root tips. We are investigating the axis formation mechanism using Arabidopsis.

Schematic model of embryogenesis

Time-lapse image of the Arabidopsis zygote expressing microtubule and nucleus markers

To visualize the real-time dynamics of embryogenesis, an in vitro cultivation system of young seeds (ovules) was developed. Combining this with a two-photon excitation microscope that allows deep tissue observation enabled tracking of the zygote polarization details. This system revealed that the alignments of microtubules (MTs) and actin filaments (F-actins) are both reoriented after fertilization. The longitudinal MT arrays in the egg cell disappear upon fertilization, and then MTs form apical transverse rings in elongating zygotes. The mesh-like pattern of F-actin in the egg cell is also temporally disorganized after fertilization, aligning longitudinally in zygotes. In addition, the vacuole occupies the basal portion of the egg cell, and swiftly reduces the size upon fertilization. Then the vacuole forms tubular strands around the nucleus, moving gradually toward basal along F-actin. Since nuclei and mitochondria also migrate along F-actin, this cytoskeleton is thought to be the basis for the polar organelle positioning in zygotes. In elongating zygote, the mitochondria are connected to form a long filamentous structure along F-actin array, and in mature zygotes they accumulate polarly in the apical region. Mitochondria then temporarily separate during zygotic division, resulting in the apical cell inheriting a higher concentration of mitochondria than the basal cell. The ability to identify such temporal events is one of the major advantages of high-resolution live-cell imaging of the zygote.