Embryogenesis in Angiosperms: Development of the Suspensor.

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.