Calreticulin expression is upregulated during sexual reproduction of Hyacinthus orientalis, and the protein is localized both in the cytoplasm and a highly specialized cell wall within the female gametophyte. Several evidences indicate calreticulin (CRT) as an important calcium (Ca(2+))-binding protein that is involved in the generative reproduction of higher plants, including both pre-fertilization and post-fertilization events. Because CRT is able to bind and sequester exchangeable Ca(2+), it can serve as a mobile intracellular store of easily releasable Ca(2+) and control its local cytosolic concentrations in the embryo sac. This phenomenon seems to be essential during the late progamic phase, gamete fusion, and early embryogenesis. In this report, we demonstrate the differential expression of CRT within Hyacinthus female gametophyte cells before and during anthesis, during the late progamic phase when the pollen tube enters the embryo sac, and at the moment of fertilization and zygote/early endosperm activation. CRT mRNA and the protein localize mainly to the endoplasmic reticulum (ER) and Golgi compartments of the cells, which are involved in sexual reproduction events, such as those in sister synergids, the egg cell, the central cell, zygote and the developing endosperm. Additionally, immunogold research demonstrates selective CRT distribution in the filiform apparatus (FA), a highly specific component of the synergid cell wall. In the light of our previous data showing the total transcriptional activity of the Hyacinthus female gametophyte and the results presented here, we discuss the possible functions of CRT with respect to the critical role of Ca(2+) homeostasis during key events of sexual plant reproduction. Moreover, we propose that the elevated expression of CRT within the female gametophyte is a universal phenomenon in the cells involved in double fertilization in higher plants.
Keywords: Ca2+ homeostasis; Calreticulin; Egg apparatus; Embryo sac; Fertilization; Filiform apparatus.