......- !l~ -g 'an 1 *It -!aiA Most morphogenetic events in flowering Plant Embryogenesis: plants occur in the postembryonic sporo- phyte after seed germination (Fig. 1) (2). Zygote to Seed Vegetative organ systems differentiate con- tinuously from root and shoot meristematic regions that are formed initially during em- Robert B. Goldberg,* Genaro de Paiva, Ramin Yadegari bryogenesis. The reproductive organs of the flower are differentiated from a repro- Most differentiation events in higher plants occur continuously in the postembryonic adult grammed shoot meristem after the seedling phase of the life cycle. Embryogenesis in plants, therefore, is concerned primarily with has become a mature plant (Fig. 1) (25). establishing the basic shoot-root body pattern of the plant and accumulating food re- Thus, a germline analogous to that found in serves that will be used by the germinating seedling after a period of embryonic dormancy animals (1) is not set aside during plant within the seed. Recent genetics studies in Arabidopsis have identified genes that provide embryogenesis. new insight into how embryos form during plant development. These studies, and others A mature flowering plant embryo con- using molecular approaches, are beginning to reveal the underlying processes that control tains two primary organ systems-the axis plant embryogenesis. and cotyledon (Fig. 1) (2). These organs have distinct developmental fates and are both composed of three basic, or primordial, tissue layers-protoderm, procambium, and A major problem in plant development is lecular approaches suggest that a plant em- ground meristem-which will become the to unravel the mechanisms operating dur- bryo has a modular structure and consists of epidermal, vascular, and parenchyma tissues ing embryogenesis that enable a plant to several regions that form autonomously dur- of the young seedling, respectively (2). The specify its body plan and tissue differentia- ing embryogenesis. axis, or hypocotyl-radicle region of the em- tion patterns. Although progress with a va- bryo, contains the shoot and root meristems riety of animal systems has been spectacular Embryos Begin the Diploid Phase and will give rise to the mature plant after in this regard (1), a detailed understanding of the Higher Plant Life Cycle seed germination (Fig. 1). By contrast, the of the events that govern plant embryo cotyledon is a terminally differentiated or- formation has yet to be realized. One obsta- The flowering plant life cycle is divided gan that accumulates food reserves that are cle in achieving this goal is the location of into haploid and diploid generations that utilized by the seedling for growth and de- on March 26, 2012 embryos within the plant and their relative are dependent on each other (Fig. 1) (2, velopment before it becomes photosynthet- inaccessibility to experimental manipula- 16-18). The haploid, or gametophytic, ically active (Fig. 1). The cotyledon func- tion, particularly at the early stages of em- generation begins after meiosis with spores tions primarily during seed germination and bryogenesis. In flowering plants, reproduc- that undergo mitosis and differentiate into senesces shortly after the seedling emerges tive processes occur within floral organs either a pollen grain (male gametophyte) or from the soil. Embryogenesis in higher (Fig. 1) (2). The egg cell is present in the an embryo sac (female gametophyte) (3, plants, therefore, serves (i) to specify meri- ovule, a multicellular structure that is buried 19-20). The pollen grain contains two stems and the shoot-root plant body pat- beneath several cell layers of the pistil, the sperm cells, whereas the embryo sac con- tern, (ii) to differentiate the primary plant www.sciencemag.org female reproductive organ (2-4). Because tains a single egg (Fig. 1). Other accessory tissue types, (iii) to generate a specialized egg cell formation, fertilization, and embry- cells within the haploid male and female storage organ essential for seed germination ogenesis occur within the pistil, it has been gametophytes help facilitate the pollination and seedling development, and (iv) to en- difficult to dissect the major events that take and fertilization processes (3, 19, 20). The able the sporophyte to lie dormant until place during the early stages of higher plant male and female gametophytes are derived conditions are favorable for postembryonic development. from specialized spore-forming cells within development. Recently, it has become feasible to iso- the reproductive organs of the flower (3, 4, late plant eggs and fertilize them in vitro in 21). By contrast, the diploid, or sporo- The Shoot-Root Body Plan Is Downloaded from order to investigate the initial events of phytic, generation begins after fertilization Generated During Early plant embryogenesis (5). In addition, genet- with the zygote and forms the mature plant Embryogenesis ic approaches have been used to identify with vegetative organs (leaf, stem, root) genes required for various embryogenic pro- and flowers that contain the reproductive How the embryo acquires its three-dimen- cesses, including pattern formation (6, 7). organs (anther and pistil) (Fig. 1). sional shape with specialized organs and Genetic manipulation of Arabidopsis thali- Two fertilization events occur in flower- tissues, and what gene networks orchestrate ana, by both chemical mutagenesis (8-12) ing plants (2, 22). One sperm unites with embryonic development remain major un- and insertional mutagenesis (13-15), has the egg cell to produce a zygote and initiate resolved problems. From a descriptive point identified a large number of mutants that embryogenesis. The other unites with a spe- of view, plant embryogenesis can be divided are blocked at different stages of embryo- cialized cell within the embryo sac (central into three general phases in which distinct genesis. In this review we outline the major cell) to initiate the differentiation of the developmental and physiological events oc- insights that have been derived from studies endosperm, a triploid tissue that is neither cur: (i) postfertilization-proembryo, (ii) ofArabidopsis embryo mutants, and we sum- gametophytic nor sporophytic in origin (Fig. globular-heart transition, and (iii) organ ex- marize gene transcription experiments in 1) (23). The endosperm is present during pansion and maturation (26-28) (Fig. 2 other plants that provide new information seed development and provides nutrients for and Table 1). Although there is consider- about the processes regulating higher plant either the developing embryo, the germinat- able variation in how embryos in different embryogenesis. Both the genetic and mo- ing seedling, or both (23). Fertilization also plant taxa form (29), the overall trends are causes the ovule, containing the embryo and remarkably similar (29). We summarize the Authors are in the Department of Biology, University of endosperm, to develop into a seed and the Capsella and Arabidopsis pattern of embryo California, Los Angeles, CA 90024-1606, USA. ovary to differentiate into a fruit, which development (29-33) because (i) it is one *To whom correspondence should be addressed. facilitates seed dispersal (Fig. 1) (24). of the most well-studied forms of plant em- SCIENCE * VOL. 266 * 28 OCTOBER 1994 605 liiiiiiiiniiinii!iii -- ....- Fig. 1. The life cycle of a Pollen flowering plant with em- Egg cell formation, grains ?"' phasis on egg cell forma- pollination, and fertilization Stigma Sperm cells tion and seed develop- FTube ment. [Adapted from (16, Pistil Megaspore Surviving Embryo Female Style nucleus 26).] mother cell megaspore sac gametophyte (n) (2n) (n) Male us I \ Anti.iFipdal gametophyte Nucellu ' - b Central Aceells Meiosis Mitosis cell s l -_ ___ Ovary-fruitvary-ft(n) Ovule 'Synergid Flower cells Funiculus /Integuments Micropyle yW Germinating Dormant Developing Fertilized endosperm seed seed seed nucleus (3n) Endosperm 0* Fertilized meristem Root Axis lvEmbryo eeloping egg (2n) Cotyledons j embryo Seed development and germination bryogenesis, dating back to the classical the suspensor (Fig. 2). In Arabidopsis, the to be passed from the maternal sporophyte studies of Hanstein, Schaffner, and Soueges suspensor contains only 7 to 10 cells (Fig. into the developing proembryo (Fig. 2) with Capsella (30-32), (ii) it has an invari- 2). The suspensor anchors the embryo prop- (35). The suspensor senesces after the heart on March 26, 2012 ant division pattern during the early stages, er to the surrounding embryo sac and ovule stage and is not a functional part of the which allows cell lineages to be traced his- tissue and serves as a conduit for nutrients embryo in the mature seed. Derivatives of tologically (33), and (iii) recent studies mutants have provided with Arabidopsis Postfertilization Globular-heart transition new insights into the processes that control I a 5 A embryo development (6, 9). l II1 A Proembryo C Asymmetric cleavage of the zygote results in -cPd ti the formation of an embryo with a suspensor A and embryo proper that have distinct develop- www.sciencemag.org mental fates. The zygote in Arabidopsis and 0 Capsella has an asymmetric distribution of cellular components-the nucleus and most Zygote 2-cell of the cytoplasm are present in the upper 2/4-cell Heart EP 8-cell portion of the cell, whereas a large vacuole EP 16-cell embryo dominates the middle to lower portion (Fig. EP Globular embryo 2). This spatial asymmetry is derived from Downloaded from the egg cell (30). The zygote divides asym- metrically into two distinct-sized daughter Organ expansion and maturation cells a small, upper terminal cell and a Torpedo embryo Walking-stick embryo Mature embryo large, lower basal cell-which establish a CE polarized longitudinal axis within the em- SM bryo (Fig. 2) (2, 30-33). Histological stud- ies over the course of the past 125 years BeEn - SC have indicated that the terminal and basal -Pd A- cells give rise to different regions of the - SM mature embryo (29-33). The small termi- EGm nal cell gives rise to the embryo proper that -Pc will form most of the mature embryo (Fig.