Abbreviations: AGPase, ADPglucose pyrophosphorylase; BE, starch branching enzyme; BETL, Basal endosperm transfer cells; BG, beta glucan; Cr4, Crinkly4; CZE, chalazal endosperm; DBE, debranching enzyme; Dek1, Defective kernel1; ESR, embryo surrounding region; GBSS, granule bound starch synthase; LSU, large subunit; MCE, micropylar endosperm; PEN, peripheral endosperm; PPB, pre-prophase band; Sal1, Supernumerary aleurone layers1; SS, starch synthase; SSU, small subunit 47 Endosperm Development and Regulation of Starch Biosynthesis Stein Erik Lid * • Heidi Rudi Department of Plant and environmental Sciences, Norwegian University of Life Sciences, PO Box 5003, N-1432 Ås, Norway Corresponding author: * stein.lid@umb.no Keywords: AGPase, aleurone cells, cr4, dek1, sal1 ABSTRACT In angiosperms, double fertilization results in two fertilization products; the embryo and the endosperm. The endosperm represents a terminal organ limited to the seed stage during plant development, where its primary role is to nourish the growing embryo or germinating seedling. The endosperm of angiosperms has attracted much research attention primarily for two reasons. First, the endosperm represents our most important renewable source for food, feed and industrial raw material. Secondly, the relatively simple organization of the endosperm, with its limited number of cell types, also makes the endosperm an excellent system for basic studies of plant developmental biology. Therefore, progress in this field is not only expected to have an impact for developing plant varieties with improved characteristics for food and feed production, but for plant production in general. Due to the economic and agronomic importance of the monocots, much effort has been directed at understanding endosperm biology in cereal species. However, the use of Arabidopsis thaliana as a model species has in recent years contributed to the elucidation of several aspects of endosperm development. In this paper we will discuss both endosperm developmental biology as well as regulation of starch metabolism in the endosperm. 1. INTRODUCTION The life cycle of plants alternates between two generations, the spore producing diploid sporophyte that produces reproductive organs and the haploid gametophyte that produces gametes. In angiosperms, meiosis in the ovule produces the haploid megaspore, which goes through three rounds of mitoses to produce the female gametophyte, consisting of eight nuclei organized in seven cells (Fig. 1A); three antipodal cells located in the chalazal region, the egg cell at the micropylar end accompanied by two synergids, and two polar nuclei that migrate towards the central cytoplasm and fuse to form the diploid secondary endosperm nucleus in the central cell. At the time of fertilization, one generative haploid sperm cell fuses with the egg cell to form the zygote, the other with the diploid secondary endosperm nucleus to produce the triploid primary endosperm nucleus. The endosperm of the vast majority of flowering plants is therefore a biparental triploid, receiving two maternal and one paternal genome equivalents upon fertilization. In contrast to the embryo that subsequently develops into a mature plant with reproductive organs and thus gives rise to the next generation, the endosperm is a terminal organ, limited to the seed stage of the plant where its primary function is to nourish the growing embryo or seedling. The endosperm can be classified as persistent as it is in cereals and other monocots or non-persistent as it is in Arabidopsis and other eudicots. The persistent endosperm is maintained in the mature seed and the reserves utilized to support growth of the seedling after germination, whereas the majority of the non-persistent endosperm is degraded, and the reserves spent, during embryogenesis. In the latter case, the cotyledons subsequently support the germinating seedling. Different aspects of endosperm development have been reviewed in several recent papers (Becraft et al. 2001, Olsen 2001 2004a, Berger 2003, Costa et al. 2004). In this paper, our main emphasis will be discussing recent progress in the understanding of endosperm cell fate specification and regulatory properties of starch metabolism during endosperm differentiation. 2. PARENTAL INFLUENCE ON ENDOSPERM DEVELOPMENT Two contrasting hypotheses have been proposed for the evolutionary origin of the angiosperm endosperm, and which is correct remains unsolved. One states that the endosperm originated as an altruistic twin embryo that evolved into a nourishing organ for the surviving embryo, the other that the endosperm is a continuation of megagametophyte development triggered by fertilization (Friedman 1998). Despite the fact that both fertilization products contain the same genetic material, the endosperm and embryo embark upon highly divergent developmental pathways. How the fates of the two organs are initially separated at the molecular level is not known, however differential distribution or expression of maternal factors may play a role, as may dosage dependent gene expression (triploid endosperm contains two maternal and one paternal genome), as well as parent-of-origin specific genomic imprinting.