Structural analysis of embryogenesis and endosperm formation in celery-leafed buttercup (Ranunculus sceleratus L.)

Embryo and endosperm development were studied in celery-leafed buttercup (Ranunculus sceleratus L.) by light and electron microscopy. The first pollen tubes entered embryo sacs within 8 h after pollination. The two first divisions of the zygote were transversal and the three-celled pro-embryo was linear. Its basal cell gave rise to a multicellular suspensor with the hypophysis, and its middle and upper cell formed the embryo proper. Because of this pattern of cell division, the embryogenic pattern of R. sceleratus differs from the Onagrad Type to which Ranunculus supposedly belongs. The suspensor showed limited growth and similarly the embryo proper remained in the early torpedo stage in the mature seed. Endosperm was nuclear initially. Cellularization was preceded by alveolation and coincided with the accumulation of storage products in lipid droplets and amyloplasts. Starch grains gradually disappeared whereas protein bodies and lipid droplets accumulated during further development. At maturity, the endosperm occupied the greater part of the seed and its cytological features varied around the embryo. The endosperm cells surrounding the embryo suspensor persisted whereas those surrounding the embryo proper degenerated. The site-specific degeneration occurred after the establishment of the protoderm and points to the interaction between embryo proper and endosperm. ‘Multivesicular structures’ were observed in the endosperm at the alveolation stage. They may be involved in the transport of metabolites between the apoplast and symplast.