An Evolutionarily Conserved Abscisic Acid Signaling Pathway Regulates Dormancy in the Liverwort Marchantia polymorpha.

Dormancy is a key process allowing land plants to adapt to changing conditions in the terrestrial habitat, allowing the cessation of growth in response to environmental or physiological cues, entrance into a temporary quiescent state, and subsequent reactivation of growth in more favorable environmental conditions [1-3]. Dormancy may be induced seasonally, sporadically (e.g., in response to drought), or developmentally (e.g., seeds and apical dominance). Asexual propagules, known as gemmae, derived via clonal reproduction in bryophytes, are often dormant until displaced from the parent plant. In the liverwort Marchantia polymorpha, gemmae are produced within specialized receptacles, gemma cups, located on the dorsal side of the vegetative thallus [4]. Mature gemmae are detached from the parent plant but may remain in the cup, with gemma growth suppressed as long as the gemmae remain in the gemma cup and the parental plant is alive [5]. Following dispersal of gemmae from gemma cups by rain, the gemmae germinate in the presence of light and moisture, producing clonal offspring [6]. In land plants, the plant hormone abscisic acid (ABA) regulates many aspects of dormancy and water balance [7]. Here, we demonstrate that ABA plays a central role in the control of gemma dormancy as transgenic M. polymorpha gemmae with reduced sensitivity to ABA fail to establish and/or maintain dormancy. Thus, the common ancestor of land plants used the ABA signaling module to regulate germination of progeny in response to environmental cues, with both gemmae and seeds being derived structures co-opting an ancestral response system.