Figure 1

Flowering plant fertilization and embryo development. Micrographs of (a) pollen (detail of nuclei only) and ovule before fertilization, (b) ovule after fertilization, showing endosperm development (red dots are individual nuclei in the syncytial endosperm); and (c) a developing seed; all images are false-colored to indicate endosperm (yellow) and the embryo (green). All images are from Arabidopsis. In the pollen grain, the two sperm nuclei (fluorescent dots) are supported by the larger haploid vegetative nucleus; in the ovule, the haploid egg cell is supported by the diploid central cell. Angiosperms have a 'double fertilization', in which one sperm nucleus combines with the egg nucleus to generate the diploid zygote, and the other sperm nucleus combines with the larger central cell nucleus to generate a triploid endosperm. White arrows indicate the potential flow of small RNAs from non-inherited supporting cells into the gametes or the embryo.