Abstract: Light microscopic immunohistochemical techniques with horse radish peroxidase(HRP)-conjugated second antibody and protein A-gold immunoelectron microscopic techniques were used to study the distribution of calmodulin (CaM) in unfertilized and fertilized embryo sacs in Nicotiana tabacum var. mocrophylla. Before fertilization, CaM was richer in the egg apparatus cells and antipodal cells than in the central cell. During the course from pollination to fertilization, the persistent synergid contained more CaM than the degenerated synergid. Meanwhile, two distinct bands rich in CaM were observed between the egg apparatus and the central cell, and gradually fused with each other appearing arc shape. When the two polar nuclei had fused, this CaM-rich band began to disappear. After fertilization, CaM level was still high in the zygote and the persistent synergid but low in the endosperm cells. Although there was no evidence about the polar distribution of CaM in the zygote, distinguishable difference, however, existed between the apical cell and the basal cell of a proembryo, being higher in the former than in the latter. The function of CaM during double fertilization and early embryogenesis as well as the temopral relationship between the CaM-rich band and the actin corona reported by other investigators are discussed.

Key words: Calmedulin, Embryo sac, Immunocytochemical localization, Nicotiana tabacum var. Macrophylla