Evolutionary origin of abscisic acid beyond land plants

Abscisic acid (ABA) is a central regulator of plant stress responses and development, but its evolutionary history extends far beyond terrestrial plants (Embryophyta). Emerging phylogenomic evidence reveals that the plastidial ABA biosynthetic pathway originates from cyanobacteria-indicating that plastid-based ABA production predates terrestrialization. In contrast, the signaling framework evolved through stepwise modular innovations. The protein phosphate 2C-sucrose non-fermenting 1-related protein kinase2 core is ancient, whereas the canonical ABA-binding capacity of pyrabactin resistance/pyrabactin resistance 1-like/regulatory component of ABA receptors emerged during the evolution of land plants (Embryophyta) from ancestral Zygnematophyceae algae, with further refinement in angiosperms. By integrating noncanonical receptors, such as chloroplast Mg-chelatase H, we propose a continuous evolutionary trajectory of ABA from prokaryotes to embryophytes, highlighting how receptor innovation facilitated the transition to a terrestrial lifestyle.