Megasporogenesis in Arabidopsis thaliana L.: an ultrastructural study

In this study, megasporogenesis of the plant model Arabidopsis thaliana was investigated by electron microscopy for the first time. The data described here could constitute a reference for future investigations of Arabidopsis mutants. During the beginning of meiosis the megaspore mother cell shows a polarity created by unequal distribution of organelles in the cytoplasm. P lastids accumulate in the chalazal region and long parallel saccules of end oplasmic reticulum, small vacuoles and some dictyosomes are found in the mi cropylar region. Plasmodesmata are abundant in the chalazal cell wall. The nucleus is almost centrally localized and contains a prominent excentric nu cleolus and numerous typical synaptonemal complexes. After the second divis ion of meiosis the four megaspores are separated by thin cell walls crossed by numerous plasmodesmata and do not show significant cellular organizatio n. The young functional megaspore is characterized by a large nucleus and a large granular nucleolus. The cytoplasm is very electron dense due to the abundance of free ribosomes and contains the following randomly distributed organelles: mitochondria, a few short saccules of endoplasmic reticulum, d ictyosomes and undifferentiated plastids. However, there is no apparent pol arity, except for the distribution of some small vacuoles which are more ab undant in the micropylar region of the cell. The degenerating megaspores ar e extremely electron dense and do not show any substructure.