Promotes cell expansion-dependent organ growth, probably via a brassinosteroids signaling pathway. Acts downstream of BRI1. Expressed in cotyledons, roots, flowers, siliques and leaves. Present throughout the cotyledons as well as in the mature regions of roots in young seedlings. Expressed in expanding leaves, but not in leaf primordia and juvenile leaves. In flowers, accumulates in sepals and stamen filaments, as well as at the apices and bases of siliques. By brassinosteroids. The OSR domain is sufficient to promote organ growth. Smaller lateral organs including cotyledons and leaves. Belongs to the plant organ size related (OSR) protein family. Truncated N-terminus.