Male gametophyte development inPlumbago zeylanica: cytoplasm localization and cell determination in the early generative cell

Summary

The behavior of the generative cell during male gametophyte development inPlumbago zeylanica was examined by epifluorescence microscopy and electron microscopy with organelle nucleoid as a cytoplasm marker. When the thin sections stained with 4′,6-diamidino-2-phenylindoIe (DAPI) were observed under an epifluorescence microscope, two types of fluorescence spots were detected in the cytoplasm of the pollen cells before the second mitosis. The spots emitting stronger fluorescence were confirmed as plastid nucleoids and those emitting dimmer fluorescence were mitochondrial nucleoids. Before the first mitosis, both plastid and mitochondrial nucleoids distributed randomly in the cytoplasm of the microspore. A small lenticular generative cell formed with attachment to the interior of the intine after the mitosis. Small vacuoles were found in the lenticular cell. In the cytoplasm of the lenticular cell, both plastid nucleoids and the small vacuoles were distributed randomly at the very beginning but began to migrate in opposite directions immediately. Plastid nucleoids aggregated to the side of the cell that faces the pollen center and the small vacuoles aggregated to the side of the cell that attaches to the inline. As the result, the lenticular generative cell appeared highly polarized in cytoplasm location soon after the first mitosis. In accordance with the definition of the cytoplasm polarization, the primary wall between the generative and the vegetative cells began to flex and the lenticular generative cell started to protrude towards the pollen center. When the generative cell peeled away from the inline, it was spherical in shape with the pole that aggregated plastids towards the vegetative nucleus. But the cell direction appeared to be transformed immediately. The pole that aggregated small vacuoles turned to the position towards the vegetative nucleus and the pole that aggregated plastid nucleoids turned to the position countering to the vegetative nucleus. A cellular protuberance formed at the edge of the pole that aggregated small vacuoles and elongated into a tapered end that got into contact with the vegetative nucleus. The polarization of the cytoplasm kept constant throughout the second mitosis. The small vacuoles that apportioned to the sperm cell which attached the vegetative nucleus (the leading sperm cell) disappeared during sperm cell maturation. Plastid nucleoids were apportioned to the other sperm cell (the trailing sperm cell) completely. Mitochondrial nucleoids became undetectable after the second mitosis.