Land plants evolved from a group of aquatic algae known as charophytes and molecular evidence suggests that they were pre-adaptated to life on land. Early land plants necessarily required mechanisms to survive dehydration and the plant hormone abscisic acid (ABA) is known to play a vital role in this conferring desiccation tolerance in all land plants. The basal non-vascular land plants, made up of the liverworts, hornworts and mosses, rely heavily on ABA-mediated vegetative dehydration/desiccation tolerance (D/DT) as they lack anatomical adaptations to retain water and this trait remains a developmentally regulated feature of the angiosperm seed. ABA non-responsive (anr) mutants were identified in the model bryophyte Physcomitrella patens and genotyping of segregating populations enabled the mapping of the PpANR locus. This locus encodes a trimodular MAP3 kinase comprising an N-terminal PAS domain, a central EDR domain and a C-terminal MAPKKK-like domain (“PEK” structure). Mutants of PpANR showed dehydration hypersensitivity and an inability to respond to exogenous ABA demonstrating the vital role of PpANR in the ABA-dependent osmotic stress responses. RNA-seq analysis of wild-type and anr mutant plants also revealed potential components of the wild-type ABA-dependent osmotic stress response not yet characterised in bryophytes. Phylogenetic analysis reveals PpANR to be part of a basal plant-specific subfamily of MAP3Ks closely related and possibly ancestral to the “EK” structured negative ethylene regulator CTR1 and the “PK” structured positive ABA regulators Raf10/11. The establishment of these subfamilies in the charophytes suggests them as potential vital components of ancestral water stress responses. The PAS domain likely originated from a domain swap from histidine kinases in the green algae and the solving of the crystal structure of this domain reveals it to form a homodimer with each domain taking the canonical PAS fold structure. A model is suggested for a key role of PpANR in an ancestral ABA-dependent osmotic stress signalling pathway.