Ultrastructure and Histochemistry of the Ovule, Fertilization, and Formation of the Zygote in a Tropical Acacia Hybrid (Acacia mangium Willd. × Acacia auriculiformis A. Cunn. ex Benth.)

Abstract

The ultrastructure of the ovule of the Acacia hybrid (Acacia mangium Willd. × Acacia auriculiformis A. Cunn. ex Benth.) was studied before and after pollen penetration to examine fertilization and formation of the zygote using electron microscopy and histochemistry. At anthesis, the ovarian chamber is filled with a secretion that stains positively with PAS and Amido black for insoluble carbohydrate and proteins, respectively. We suggest that this may be secreted from abundant ovarian trichomes rather than the ovules and play an important role in supporting pollen tube growth and guidance. The mature ovule lacks a distinct micropyle. The embryo sac contains cell types similar to those of many angiosperms. About 12 h after pollination, a pollen tube is observed to penetrate the micropylar nucellus and enters the embryo sac. One of the two synergids is the site of pollen tube penetration and its degeneration is triggered by the pollen tube penetration of the nucellus. After pollen tube discharge, no cytoplasmic bodies of the sperm cells are observed in the degenerate synergid. The sperm nuclei are observed in the egg and central cells ca. 3 d after open pollination. Following alignment of the sperm between the polar nuclei, polar nuclear fusion starts with the linkage of their nuclear envelopes, followed by fusion of nucleoplasm through the nuclear bridges. ER is likely involved in this process. Triple fusion then occurs. The fusion of the sperm with the egg nucleus occurs by alignment and contact of both nuclei and is less associated with the ER. No sperm cytoplasm is observed in the egg cell. After fertilization, the zygote shows ultrastructural changes when compared with the egg cell and appears to be metabolically inactive.