Axis formation in plant embryogenesis: Cues and clues

Imagine a textbook entitled Developmental Biology that focuses entirely on plants, mentioning animals only for their peculiar way of making germ cells by setting aside a group of precursor cells early in the embryo. The converse has been, and still is, common practice. It is true that the regenerative potential of plants, which is indeed impressive, sets them apart from the more familiar animal models: individual cells can give rise to embryos in culture; localized groups of stem cells called meristems make the adult plant in a seemingly autonomous fashion not only during normal development, but also by regeneration from lumps of undifferentiated cells in culture. These special features notwithstanding, plants do develop from a fertilized egg cell, the zygote, during the normal course of their life cycle and, like animals, have to establish thecharacteristic body organization of the multicellular adult form. Are the underlying mechanisms the same or different? In the past few years, the genetic and molecular analysis of flower development in two plant model species, Arabidopsis and Antirrhinum (snapdragon), has established a network of MADS domain transcription factors and others that regulate this best-characterized process in plant development (Weigel and Meyerowitz, 1994). However, flower development is like putting the finishing touches on the adult plant and may thus not give clues to mechanisms that underlie earlier processes such as axis formation and the generation of the overall body organization. By drawing largely on recent genetic studies in Arabidopsis, I will briefly discuss postembryonic development that eventually culminates in the formation of flowers, but mainly focus on pattern formation in the embryo that establishes the basic body organization of flowering plants. The Shoot Me&tern: Linking Up the Embryo with the Flower Pattern formation in animals is largely confined to embryogenesis such that the future adult form is represented in the body organization of the mature embryo. By contrast, plant embryogenesis produces a juvenile form, the seedling, that lacks most structures of the adult plant. Embryogenesis in essence organizes two groups of stem cells at the opposite ends of the body axis, the primary meristems of the shoot and the root. These meristems then add new structures to the seedling, thus generating the species-specific adult form during postembryonic development (Steeves and Sussex, 1989). Regardless of the appearance of the adult plant, the shoot meristem is organized essentially the same way in different plant species. Two functional units can be distinguished within the meristem: a central zone, which is required for self-renewal and integrity of the meristem, and a peripheral zone, which makes primordia of lateral organs and their associated secondary shoot meristems, such as leaves and flowers (Steeves and Sussex, 1989). Whether leaves or flowers are produced depends on the physiological state of the meristem. In embryonic flower (emf) mutant seedlings, for example, the primary shoot meristem skips the vegetative phase of making leaves altogether, producing flowers directly (Sung et al., 1992). Shoot meristem identity seems to be conferred by the continuous expression of genes like the maize homeobox gene Knotted7 (Knl), whose ectopic expression can cause the formation of shoot meristems on leaves (Sinha et al., 1993; Jackson et al., 1994). A putative Arabidopsis homolog of Knl, the SHOOT MEW STEM-LESS (STM) gene, is required for shoot meristem formation both in the embryo and during regeneration from tissue culture (Barton and Poethig, 1993). How the activity of such shoot meristem-specific genes is established and