The sporophyte-to-gametophyte transition: The haploid generation comes of age

Flowering plants alternate between two multicellular generations: the diploid sporophyte and haploid gametophyte. Despite its small size, the gametophyte has significant impacts on plant genetics, evolution, and breeding. Each male pollen grain and female embryo sac is a multicellular organism with independent gene expression, a functioning metabolism, and specialized cell types. In this review, we describe recent progress in understanding the process in which the haploid genome takes over expression from its diploid parent - the sporophyte-to-gametophyte transition. The focus is on pollen, but similar concepts may also apply to the female gametophyte. Technological advances in single-cell genomics offer the opportunity to characterize haploid gene expression in unprecedented detail, positioning the field to make rapid progress.