The endosperm of flowering plants is a tissue that acquires resources from the maternal sporophyte and is in turn digested by the developing embryo in its own seed. In this respect, the endosperm takes the role filled by the female gametophyte in gymnosperms. Endosperm is a tissue unique to angiosperms. At fertilization, a male gametophyte (pollen tube) releases two sperm into the female gametophyte. One sperm fuses with the egg to form a diploid zygote, and the other sperm fuses with two female-gametophyte nuclei (known as polar nuclei) to form the triploid primary-endosperm nucleus. Therefore, the endosperm, which develops from this triploid nucleus, has an unusual genetic composition. Each nucleus contains one copy of the paternal contribution to the associated embryo and two copies of the maternal contribution. A minority of flowering plant species have endosperms of different genetic composition, but almost all share the characteristic that more of the endosperm genome is derived from the mother than from the father. Double fertilization and the formation of endosperm are often considered the major features that distinguish flowering plants from other seed plants. Because of this importance, many authors have suggested adaptive explanations for the unusual genetic constitution of endosperm. Heterozygote vigor (Brink and Cooper 1940) can explain the presence of a paternal contribution but not why the maternal contribution should be doubled. Polyploid vigor (Stebbins 1974) can explain the advantage of triploid over diploid endosperm but not the weighting toward the maternal contribution. In other words, the polyploid-vigor explanation does not explain why genome doubling should occur in the female gametophyte before fertilization, particularly given that endosperm cells often increase their ploidy by other means after fertilization (D'Amato 1984). More-recent hypotheses have treated the endosperm as a participant in conflicts of interest among mother, father, and offspring. Conflict arises because sibling offspring of one mother tend to compete with each other. Each offspring's prospects of survival and eventual reproduction are sensitive to the quantity of resources it obtains from the mother, and when some offspring obtain more resources, others tend to obtain less. Because the mother is equally related to (has an equal genetic investment in) each of her offspring, her inclusive fitness is greatest when all nonabortive offspring are provisioned equally (Smith and Fretwell 1974; Trivers 1974). However, each offspring is more closely related to itself than to its siblings. For this reason, individual offspring would benefit from some redistribution of the mother's resources toward themselves and away from their

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Parent-Specific Gene-Expression and the Triploid Endosperm

Endosperm has been studied from a variety of vantage points: evolution, role in seed development and germination, genetics, physiology, and biochemistry. This tissue represents a renewable, biodegradable source of materials; much effort has been directed to improve its use in feed and food making as well as its refinement to secondary products such as oils and plastics. Although there is a vast literature dealing with each of these topics, we still have a remarkably superficial understanding of most of them. There has been revitalized interest in understanding the endosperm in relation to seed-specific developmental processes. lnformation from these studies could provide a basis for developing more efficient approaches for plant improvement and use. Recent advances in molecular biology have created the possibility for detailed study of many of the genetic and molecular mechanisms involved in endosperm development. This research could conceivably lead to answers to many basic questions in developmental biology as well as to new tools that enhance practical uses of endosperm. In this review, it is not our intention to present a comprehensive overview of what is known about endosperm. Rather, we have chosen to summarize some of the research that has been done, making note of comprehensive reviews and pointing out important questions that remain open to scientific inquiry.

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Endosperm Origin, Development, and Function

Endoreduplication is a form of nuclear polyploidization that results in multiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it generally occurs in cells that are terminally differentiated. In plants, endoreduplication is well documented in the endosperm and cotyledons of developing seeds, but it also occurs in many tissues throughout the plant. It is thought that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly investigated. Numerous observations have been made of endoreduplication, or at least extra cycles of S-phase, as a consequence of mutations in genes controlling several aspects of cell cycle regulation. However, until recently there were few studies directed at the molecular mechanisms responsible for this specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activity and oscillations in the activity of S-phase cyclin-dependent kinase.

Investigating the hows and whys of DNA endoreduplication

The endosperm of cereals functions as a storage tissue in which the majority of starch and seed storage proteins are synthesized. During its development, cereal endosperm initiates a cell death program that eventually affects the entire tissue with the exception of the outermost cells, which differentiate into the aleurone layer and remain living in the mature seed. To date, the cell death program has been described for maize and wheat endosperm, which exhibits common and unique elements for each species. The progression of endosperm programmed cell death (PCD) in both species is accompanied by an increase in nuclease activity and the internucleosomal degradation of nuclear DNA, hallmarks of apoptosis in animals. Moreover, ethylene and abscisic acid are key to mediating PCD in cereal endosperm. The progression of the cell death program in developing maize endosperm follows a highly organized pattern whereas in wheat endosperm, PCD initiates stochastically. Although the essential characteristics of cereal endosperm PCD are now known, the molecular mechanisms responsible for its execution remain to be identified.

Programmed cell death during endosperm development

The zygote in flowering plants usually divides transversely to form a terminal cell, which gives rise to the embryo proper, and a vacuolated basal cell, which often divides rapidly to form a structure known as the suspensor. Angiosperm suspensors vary widely in size and morphology from a single cell to a massive column of several hundred cells (Maheshwari, 1950; Wardlaw, 1955; Lersten, 1983). In most cases, the suspensor functions early in embryogenesis and then degenerates during later stages of development and is not present in the mature seed. Classically, the suspensor was thought to play a passive role in embryo development by holding the embryo proper in a fixed position within the seed (Maheshwari, 1950). It now appears from extensive structural, biochemical, and physiological studies with a variety of angiosperms that the suspensor plays an active role early in development by promoting continued growth of the embryo proper. In addition, growth of the suspensor during early stages of development may be inhibited by the embryo proper (Marsden and Meinke, 1985). Analysis of reproductive development in angiosperms must therefore include a consideration of developmental interactions that occur between the embryo proper and suspensor. Although the suspensor appears to play a critical role in zygotic embryogenesis, it usually fails to develop when somatic embryos are produced in culture. The suspensor should therefore be viewed as a specialized structure that functions primarily to facilitate continued development of the embryo proper within the seed. In this review, we present an overview of the structure and function of the angiosperm suspensor and discuss recent attempts to analyze the development of the suspensor through a combination of descriptive, experimental, and genetic approaches. The recent identification of a large collection of Arabidopsis mutants with abnormal suspensors provides a unique opportunity to examine the underlying genetic factors that influence suspensor development.

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Embryogenesis in Angiosperms: Development of the Suspensor.

In the older literature on angiosperm morphology many examples of very large cells with giant or “hypertrophied” nuclei within differentiated tissues, including reproductive tissues, have been reported, and the connection between nuclear size and trophic activity of the cell has been stressed (references in Schnarf 1929, Tischler 1944, Maheshwari 1950). Goldstein (1928) even attempted to establish a correlation between nuclear form and functional activities of normal and pathological cells. In the absence of adequate knowledge of the mechanisms responsible for the multiplication of the genome, the large size and the variable form (irregular, crenate, lobate, constricted, furrowed, etc.) of nuclei were generally assumed to result from fusion of nuclei and/or amitosis. The significance of restitutional mitosis as a mechanism of doubling the chromosome number was not realized, and was generally regarded as a pathological process. The situation began to change in the late 1930’s—early 1940’s following the discovery of endomitosis in Homoptera, e.g., the pondskaters of the genus Gerris (Geitler 1939), and the proposition that the tetraploid mitoses “with paired chromosomes” (now called “diplochromosomes”) in poly somatic root tips of Spinacia oleracea are due to a double chromosome reproduction at interphase (Berger 1941). After more than 30 years of research on the nuclear cytology of differentiated tissues, it is now clear that the “supernumerary chromonemal reproduction” at interphase (Lorz 1947), better called “chromosome endoreduplication” (Levan and Hauschka 1953), is the commonest and most widespread process of cell polyploidization in both plants and animals (Brodsky and Uryvaeva 1977, D’Amato 1977 a, Nagl 1978).

F. D’Amato

Papers

Role of Polyploidy in Reproductive Organs and Tissues