The evolution of abscisic acid (ABA) and ABA function in lower plants, fungi and lichen

Abscisic acid (ABA)-the universal stress hormone of cormophytes-was detected in very low concentrations in almost all organisms tested from a range of cyanobacteria, algae, bryophytes, fungi and lichens and higher plants (Fig. 1). There are a few reports only on stress-induced ABA biosynthesis in cyanobacteria and algae. This extra ABA is released to the external medium. Application of external ABA has been shown to produce weak and contradicting effects on development and metabolism of algae. In most studies, extremely high concentrations of external ABA have been applied, those being far beyond any physiological concentration range. It is, therefore, extremely difficult to discuss those data satisfactorily. When organisms start to colonise terrestrial habitats (e.g. aquatic liverworts, mosses), endogenous ABA is increased even under mild drought stress, then desiccation protecting mechanisms are stimulated and the formation of terrestrial organs is induced. The same can be observed in water ferns (Marsilea) and in a range of heterophyllous angiosperms. Sporophytes of hornwort and mosses that bear true stomata, have particularly high ABA levels and their stomata respond to ABA as is the case in cormophytes, although a significant regulatory function of these stomata does not exist. Fungi produce large amounts of ABA that are released into the external medium and do not seem to have a function for the fungus. Fungal ABA, however, may be significant in associations of fungi with cyanophytes and algae (lichens), in mycorrhizal associations and in the rhizosphere of higher plants.