Summary Abscisic acid (ABA) is the major hormone that controls plant's ability to survive in a harsh, changing environment. The ABA signaling pathway is conserved across all plants, including mosses, and is considered a very early adaptation to the terrestrial environment. ABA participates in the control of root growth, seed desiccation and dormancy, guard cell responses and cellular osmoprotection. The study of ABA is extremely vigorous, particularly since a key family of cytoplasmic receptors was identified and termed “one of the top breakthroughs of 2009”.