Figure 1. Origin of endosperm. The female gametophyte (embryo sac) is fertilized by the two sperm cells delivered by the pollen tube, which enters the micropyle (mp). One sperm cell fertilizes the egg cell leading to embryogenesis. The other sperm cell reactivates division in the central cell leading to endosperm development. After the double-fertilization process, the seed contains the embryo surrounded by the endosperm itself surrounded by the seed teguments of maternal origin (dark yellow). The maternal nutrients are delivered at the chalazal end (cz).

Figure 2. Endosperm development. After fertilization, the central cell nucleus undergoes a series of divisions not followed by cytokinesis, hence producing a syncytium (a–d). In the syncytium, the anterior (A) and posterior (P) poles are the sites of specific mitotic domains (m.d.) at each end of the peripheral endosperm. At the posterior pole, nuclei undergo endoreduplication and become larger (b). The posterior pole attracts nuclei from the peripheral endosperm (red arrow) into a multinucleated cyst visible in (d, e). The embryo (emb) occupies the anterior pole. The syncytial endosperm (syn end) undergoes a rapid cytokinetic event termed cellularization in the anterior and peripheral domain (e) leading to cellular endosperm (cel end). Scale bar in (d) represents 80 m. (d, e) Confocal sections of Arabidopsis seeds at 4 and 5 days after fertilization.

Figure 3. Epigenetic control of endosperm development. (a,b) Arabidopsis seed size is influenced by parental genome dosage. Crosses between tetraploid ovules and diploid pollen cause a reduction of seed size (b) in comparison to crosses between diploids (a). (c,d) Similar seed size differences are also caused by pollination with met1 plants. Differential interference contrast micrographs of Arabidopsis seeds at 6 days after fertilization with wild-type (WT) pollen (c) or with met1 pollen (d) show reduction of endosperm size but not of embryo size teguments (e) prototypic cycle of imprinting. During vegetative development both paternal (p) and maternal (m) alleles of imprinted genes are silenced by DNA methyltransferase 1 (MET1). During gametogenesis, silencing is maintained in the pollen while the DNA glycosylase DEMETER (DME) releases silencing in the central cell. After fertilization only the maternal allele is expressed in endosperm while MET1 maintains the paternal allele silenced.