ERECTA family signaling controls cell fate specification during ovule initiation in Arabidopsis

The number of seeds produced by a plant depends on the number of ovules initiated within the carpels by a meristematic tissue, the placenta. Previous studies indicated that ERECTA-LIKE1 (ERL1) and ERL2 receptors and their extracellular ligand EPIDERMAL PATTERNING FACTOR-LIKE2 (EPFL2) promote seed density while ERECTA inhibits it. Here, we show that all three receptors and two ligands, EPFL1 and EPFL2, promote ovule initiation. After initiation, EPFL1 and EPFL2 redundantly regulate the elongation of nascent ovules and promote the growth of the funiculus. The synergy of EPFL1 and EPFL2 with CUP-SHAPED COTYLEDON boundary genes in the regulation of ovule number and shape suggests cooperation in their function. Ovule initiation starts with the specification of ovule founder cells that are separated by boundary cells. During ovule bulging, a new cell type, characterized by a high proliferative rate, forms between ovule founder and boundary cells. In the epfl1 epfl2 mutant, marker genes for ovule founder and boundary cells are expressed much more broadly. Our data indicate that ERECTA family signaling is necessary for efficient cell fate separation at the earliest stages of ovule initiation. Later, it promotes the formation of a zone of high proliferation at the base of the ovule.