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

Polar auxin transport controls multiple aspects of plant development including differential growth, embryo and root patterning and vascular tissue differentiation. Identification of proteins involved in this process and availability of new tools enabling 'visualization' of auxin and auxin routes in planta largely contributed to the significant progress that has recently been made. New data support classical concepts, but several recent findings are likely to challenge our view on the mechanism of auxin transport. The aim of this review is to provide a comprehensive overview of the polar auxin transport field. It starts with classical models resulting from physiological studies, describes the genetic contributions and discusses the molecular basis of auxin influx and efflux. Finally, selected questions are presented in the context of developmental biology, integrating available data from different fields.

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Polar auxin transport - old questions and new concepts?

The Physiology of in Vitro Asexual Embryogenesis

Publisher Summary This chapter focuses on megagametophytes and examines the female germ unit from four new perspectives: (1) the ultrastructure and cytochemistry of megagametophyte development and cellularization, (2) the organization and function of the female germ unit in reproduction, (3) the isolation and characterization of embryo sacs and their component cells, and (4) the involvement of the cytoskeleton in megagametophyte development and function. The development of the megagametophyte is divided into two connected developmental phases: (1) megasporogenesis, which entails the formation and maturation of the initial products of meiosis, followed by (2) megagametogenesis, which begins with the mitotic division of the meiotic products and continues through the cellularization and maturation of the megagametophyte. The female germ unit is comprised of the egg, two synergids, and the central cell. As such, it constitutes the minimum number of cells required to (1) receive the pollen tube, (2) cause the discharge of the sperm into the receptive portion of the female gametophyte, and (3) undergo double fertilization.

Female Germ Unit: Organization, Isolation, and Function

The female gametophyte develops from the megaspore formed in the nucellar tissue of the ovule. The megaspore enlarges and after some mitotic divisions a small coenocyte differentiates. In general, a megagametophyte consists, after cytokinesis, of an egg apparatus, a central cell, and antipodal cells.

The Female Gametophyte

Massive wall ingiowths occur in the micropylar end of the embryo sac of sunflower Helianthus annuus L These projections arise after the deposition of the common wall separating the two synergids, apparently as a result of Golgi activity After fertilization, endosperm cell wall is laid down over the embryo sac wall projections suggesting an altered pathway for solutes into the embryo sac The possible nutritional significance of these projections is discussed

Helianthus annuus Embryogenesis: Embryo Sac Wall Projections Before and After Fertilization

Four types of wall formation occur in the endosperm of chickweed Stellaria media (L.) Cyrill. Freely growing walls form at the edges of the micropylar region of the embryo sac during the early heart stage of embryogenesis; Golgi and dilated endoplasmic reticulum may be sources of precursors for these anastomosing walls which form independently of a mitotic spindle Other walls form by means of cell plates and in a manner similar to wall regeneration reported in protoplasts. Also, wall ingrowths of the transfer cell type form on the embryo sac wall in the micropylar region and protrude into the endosperm.

The Fine Structure of the Change from the Free-Nuclear to Cellular Condition in the Endosperm of Chickweed Stellaria media

Adventive embryogenesis in wild carrot is enhanced by the lowering of the effective auxin concentration either by removal from the medium or by the addition of 2,4,6-T (2,4,6-trichlorophenoxyacetic acid) or both. Growth is similar in undifferentiated and differentiated tissue until the embryos begin to elongate; the elongation is slower in the presence of 2,4,6-T and a greater homogeneity of developmental stages occurs. Also, root hairs do not form in the presence of 2,4,6-T. Aerobic respiration is stimulated by 2,4,6-T and a synergistic effect with DNP also occurs. In addition, DNP exerts a greater effect on older tissue than on younger tissue.

William Newcomb

Papers

Helianthus annuus Embryogenesis: Embryo Sac Wall Projections Before and After Fertilization

The Fine Structure of the Change from the Free-Nuclear to Cellular Condition in the Endosperm of Chickweed Stellaria media

The Effects of 2,4,6-Trichlorophenoxyacetic Acid on Embryogenesis in Wild Carrot Tissue Cultures