Cotton embryogenesis: The entrance and discharge of the pollen tube in the embryo sac

Abstract

An analysis of the entrance and discharge of the pollen tube into the embryo sac of Gossypium hirsutum was made with the light and electron microscopes. The following sequence of events is seen in cotton: 1. One of the two synergids begins to degenerate following pollination but before the pollen tube reaches the embryo sac. This degeneration is marked by the swelling and darkening of the organelle membranes, the collapse of the vacuoles, and the disappearance of the plasma membrane. Striking chemical changes accompany the structural degeneration. 2. The pollen tube grows into the degenerating synergid through the filiform apparatus. The tube ceases growth while the tip is still in the synergid. A pore develops on the chalazal side of the tube in a subterminal position. 3. The pollen tube cytoplasm and the sperm are discharged into the degenerating synergid through the pore in the tube. Following discharge a plug forms at the pore. None of the discharge leaves the synergid except the sperm nuclei which enter the egg or central cell directly from the synergid. The X-bodies present in the synergid are the remains of the sperm cytoplasm. The data stress the importance of the degenerating synergid in pollen tube discharge and the entrance of the sperm nuclei into the egg and central cell. A hypothesis is presented to explain the passage of the sperm nuclei into the egg and central cell. The data show clearly that the pollen tube does not destroy the synergid it enters, and that the degenerating synergid following pollen-tube discharge is remarkably stable.

The Egg Cell: Development and Role in Fertilization and Early Embryogenesis.

“…Up to 50% of the dry weight of the synergid vacuole is calcium, according to standardless semiquantitative energy-dispersive x-ray analysis (Chaubal and Reger, 1992a). The synergid is also believed to serve as the source of a chemical signal that directs the pollen tube to enter the micropyle, penetrate one synergid, and release its gametes (Jensen et al, 1985).…”

“…A number of mechanisms that would reduce the likelihood of male cytoplasmic transmission prior to sperm cell deposition have been proposed: (1) organellar diminution through the pinching off of cellular processes (see Mogensen, 1992) or production of enucleated cytoplasmic bodies ; (2) alteration of organelles (Vaughn et al, 1980); (3) modification of organellar DNA through molecular means (Day and Ellis, 1984), including specific nucleases (e.g., nuclease C; Nakamura et al, 1992); or (4) low ratios of paternal to maternal organelles (Russell, 1987), resulting in a high probability of paternal organelle extinction (Birky, 1983) or in levels of expression that are below the threshold for detection (Milligan, 1992). Just prior to fusion, the sperm cells may shed their cytoplasm outside the egg and central cells, as in cotton (Jensen and Fisher, 1968), or just outside of the egg cell, as in barley (Mogensen, 1988).…”

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.…”

Reddish-Brown Granules Observed within the Cytoplasm of Generative Cells of Pollen Grains in <i>Hymenocallis littoralis</i> (Amaryllidaceae)

“…The transmission of the cytoplasm of two sperm cells into female cells (egg cell and central cell) differs depending on species (Russell et al 1990, Russell 1992. The cytoplasm of both sperm cells is excluded during the union of sperm cells with female cells in Gossypium hirsutum (Jensen and Fisher 1968) and Spinacia oleracea (Wilms 1981); the sperm cell cytoplasm is transmitted into the central cell but not into the egg cell in Hordeum vulgare (Mogensen 1988) and probably also in Populus deltoides (Russell et al 1990); the transmission of the cytoplasm of both sperm cells, one into the egg cell and the other into the central cell, takes place in Oenothera erythrosepala (Meyer and Stubbe 1974), Plumbago zeylanica (Russell 1980(Russell , 1983, and Nicotiana tabacum (Yu et al 1994).…”