The structure and prophylactic role of the angiosperm embryo sac and its associated tissues:Zea mays as a model

Abstract

Summar3: Various developmental phases can be distinguished inthe definition of the archesporium and the early life of the embryo, taking Zea mays (maize) as a model within the family Gramineae, and other families where pertinent: (1) the isolation of the megasporocyte and the functional spore derived from it; (2) the maturation of the specialized walls of the embryo sac, and their reinforcement by ensheathments derived from the contiguous nucellar cells during a sequence of phased genetic ablation; (3) the differentiation of the synergids, the associated flange, and the filiform apparatuses; (4) the blocking of the pollen tube pathway by secondary secretions in the micropylar region and the coagulation of the pollen tube cytoplasm within the filiform apparatuses during the process of fertilization; and finally (5) the development of a compound cutinized envelope of four fused layers (six where the outer integument is also involved) after fertilization. For the nascent haploid generation, the period of maximum vulnerability in respect to both pathogen invasion and the transition from diplophase control occurs during these phases. It is concluded that many of the protective features form a prophylactic shield and are key components of the angiosperms in general, which may have contributed to their evolutionary success as a group. Other physiological or biochemical adaptations or barriers may also supplement the mainly structural features described here.

Relationship between double fertilization and the cell cycle in male and female gametes of tobacco

“…Degeneration of one of the synergids, which is present in successfully fertilized embryo sacs, is viewed increasingly as the result of programmed cell death (PCD), with characteristic changes in morphological (An and You 2004) and molecular expression (Christensen et al 2002). The role of the degenerated synergid in fertilization has been attributed to providing a preferential entry point and a prophylactic portal that provides unique access to egg and central cell membranes (Heslop-Harrison et al 1999); however, in this study, it may also carry the role of a cell cycle trigger and locus of sperm maturation.…”

“…High concentrations of calcium are among the first indicators of synergid degeneration (Tian and Russell 1997a), and these are rapidly followed by cellular changes, breakdown of the vacuole and limited cellular degeneration. The degenerated synergid has also been viewed as a site with very low concentrations of oxygen (Stanley and Linskens 1967), which could alter respiratory metabolism and potentially represents a high stress environment (Heslop-Harrison et al 1999). Presumably, any of these characters could contribute to the environmental and developmental signals that trigger checkpoint release at the G 1 /S transition (Shen 2001).…”

Experimental Analysis of the Fertilization Process

“…Higashiyama et al (1997) estimated that they are present only briefly, whereas Wallwork and Sedgley (2000) report that the sperm cells remain in the synergid for more than 1 h. For only a short period after gamete delivery are four nuclei detectable in the receptive synergid, representing the nuclei of the synergid, the vegetative cell, and the two sperm cells (Higashiyama et al, 1997). Presumably, soon after pollen tube discharge, the DNA in the exposed synergid and vegetative nuclei breaks down in the nucleases of the embryo sac (Heslop-Harrison et al, 1999).…”

Ribosomal RNA of Hyacinthus orientalis L. female gametophyte cells before and after fertilization

“…Morphological and structural investigations of the female gametophyte and the fertilization process have been performed in several species of plants, including maize (Diboll 1968; Heslop-Harrison et al 1999), barley (Mogensen 1982), Plumbago (Russell 1985), Populus (Russell et al 1990), and Amaranthus (Coimbra and Salema 1999), and also in vitro in Zea (Faure et al 1994; Mól et al 1994; Kranz et al 1995) and wheat (Kumlehn et al 1999). Based on histochemical and ultrastructural investigations, the Angiospermae egg cell is considered to be relatively inactive.…”