Sexual reproduction in animals and plants shares common elements, including sperm and egg production, but unlike animals, little is known about the regulatory pathways that determine the sex of plants. Here we use mutants and gene silencing in a fern species to identify a core regulatory mechanism in plant sexual differentiation. A key player in fern sex differentiation is the phytohormone abscisic acid (ABA), which regulates the sex ratio of male to hermaphrodite tissues during the reproductive cycle. Our analysis shows that in the fern Ceratopteris richardii , a gene homologous to core ABA transduction genes in flowering plants [SNF1-related kinase2s ( SnRK2 s)] is primarily responsible for the hormonal control of sex determination. Furthermore, we provide evidence that this ABASnRK2 signaling pathway has transitioned from determining the sex of ferns to controlling seed dormancy in the earliest seed plants before being co-opted to control transpiration and CO 2 exchange in derived seed plants. By tracing the evolutionary history of this ABA signaling pathway from plant reproduction through to its role in the global regulation of plantatmosphere gas exchange during the last 450 million years, we highlight the extraordinary effect of the ABASnRK2 signaling pathway in plant evolution and vegetation function.