Development of the male reproductive cells and fertilization in Nicotiana and Cymbidium: Three-dimensional ultrastructure and quantitative cytology

The generative cell (GC) and sperm cell of Nicotiana tabacum and Cymbidium goeringii were studied during development and fertilization using TEM, 3-D reconstruction and quantitative analysis.;A male germ unit is present in the pollen tube (PT) of tobacco, characterized by the structural connection of the two sperm cells and a physical association between the vegetative nucleus and GC or leading sperm cell (S;Incomplete exclusion of plastids during the first haploid mitosis results in an average of 2.81 plastids in each newly-formed GC in tobacco. These plastids are incompletely eliminated by autophagy during GC maturation, resulting in an average of 0.48 plastids per GC in the pollen grain at anthesis. A low frequency of plastids is observed in the GC and sperm cells during the whole period of PT growth. In contrast, an average of 80 mitochondria per GC or sperm pair is retained throughout the development of the reproductive cells. During double fertilization, male cytoplasm is transmitted into both the egg cell and central cell.;The mitosis of the GC within the PT of tobacco is characterized by: (1) absence of a metaphase plate, (2) oblique metaphase spindle, (3) typical anaphase and (4) cytokinesis with centrifugal cell plate formation.;Serial sections revealed an average of 0.44 mitochondria in the sperm nucleus in PTs of tobacco. These are presumably trapped during the reformation of the nuclear envelope of the GC mitosis due to spatial constraints in the PT. These nuclear mitochondria appear to be transmitted into female nuclei during karyogamy.