Autonomous endosperm development in flowering plants: how to overcome the imprinting problem?

Abstract

In the vast majority of sexually reproducing flowering plants, a ratio of 2 maternally derived genomes to 1 paternally derived genome (2m:1p) is essential for normal endosperm development, and therefore ultimately for seed development. Even in many pseudogamous apomicts, where the embryo develops without a paternal contribution, fertilisation of the endosperm to obtain the correct 2m:1p parental ratio is still necessary. How do autonomous apomicts, where both embryo and endosperm develop autonomously, circumvent this requirement? The background for the 2m:1p requirement is that the parental genomes are epigenetically different; in either genome, a set of genes is silenced in a sex-specific way by genomic imprinting. Removal of the imprints from the maternally derived endosperm genome leads to expression of normally maternally silenced genes, and effectively supplies the missing paternal genome. In Arabidopsis, we propose that a combination of the fie mutation and hypomethylation of the genome creates such a situation in the endosperm genome. As a result, in a fie mutant, hypomethylated ovule complete autonomous endosperm development takes place in the absence of fertilisation.