Fertilization in Nicotiana tabacum: ultrastructural organization of propane-jet-frozen embryo sacs in vivo

Abstract

Abstract. Ovules of Nicotiana tabacum L. were cryofixed with a propane-jet freezer and freeze-substituted in acetone to examine technique-dependent changes in preand post-fertilization embryo sacs using rapidly frozen material. Freezing quality was acceptable in 10% of the embryo sacs in the partially dissected ovules, with icecrystal damage frequently evident in vacuoles and nuclei. One of the two synergids begins to degenerate before pollen-tube arrival in cryofixed material, with breakdown of the plasma membrane and large chalazal vacuole delayed until the penetration of the pollen tube. Early synergid degeneration involved characteristic increases in cytoplasmic electron density and the generation of cytoplasmic bodies to the intercellular space through "pinching-off'. Upon pollen-tube arrival, the male gametes are released through a terminal aperture into the degenerate synergid. Sperm cells undergo morphological alteration before gametic fusion: their mitochondrial electron density increases, the endoplasmic reticulum dilates, cytoplasm becomes finely vacuolated and the surrounding pollen plasma membrane is lost, causing the sperm cells and vegetative nucleus to dissociate. Discharge of the pollen tube results in the formation of numerous enucleated cytoplasmic bodies which are either stripped or shed from sperm cells and pollen-tube cytoplasm. Two so-called X-bodies are found in the degenerate synergid after pollen-tube penetration: the presumed vegetative nucleus occurs at the chalazal end and the presumed synergid nucleus near the micropylar end.

Synergid Cell Death in Arabidopsis Is Triggered following Direct Interaction with the Pollen Tube

“…These differences are likely due to the harsher fixation methods required for TEM analysis. The early steps of synergid cell death have also been described using TEM in other species, including cotton (Jensen and Fisher, 1968), barley (Hordeum vulgare; Cass and Jensen, 1970), Quercus gambelii (Mogensen, 1972), Proboscidea louisianica (Mogensen, 1978), soybean (Glycine max; Dute et al, 1989), tobacco (Nicotiana tabacum; Mogensen and Suthar, 1979;Huang et al, 1993;Huang and Russell, 1994), Brassica campestris (van Went and Cresti, 1988;Sumner, 1992), and sunflower (Yan et al, 1991). Although these observations vary to some extent, the early stages in general are associated with an increase in the electron density of the cytoplasm and breakdown of the nucleus.…”

“…Synergid cell death occurs before arrival of the pollen tube at the female gametophyte in some species (Jensen and Fisher, 1968;Cass and Jensen, 1970;Maze and Lin, 1975;Mogensen, 1978;Mogensen and Suthar, 1979;Wilms, 1981;Dute et al, 1989;Kuroiwa, 1989;Russell et al, 1990;Yan et al, 1991;Huang et al, 1993;Huang and Russell, 1994) and after arrival in other species (van der Pluijm, 1964;Schulz and Jensen, 1968;van Went, 1970avan Went, , 1970bNewcomb, 1973). Similarly, observations of the cell death process are variable.…”

Regeneration of Fertile Barley Plants from Mechanically Isolated Protoplasts of the Fertilized Egg Cell.

“…In a recent study of tobacco in which propane-jet-frozen embryo sacs were analyzed at the ultrastructural leve1 (Huang et al, 1993), a distinct wall was observed around the zygote.…”

Pollination induces mRNA poly(A) tail‐shortening and cell deterioration in flower transmitting tissue

“…When pollen tubes penetrate the style, transmitting tissue cells deteriorate, some of them rupture, further enriching this secretory matrix with cellular materials for pollen tube growth (Herrero and Dickinson, 1979;Jensen and Fisher, 1969). In the ovary of some plants, pollination induces the disintegration of one of the two synergid cells inside the embryo sac (Cass and Jensen, 1970;Huang et al, 1993;Jensen and Fisher, 1968). Substances released by the ruptured synergid cell may play a role in attracting pollen tubes into the ovules.…”