Abstract Dormancy is a crucial trait that allows land plants to withstand harsh terrestrial environments by pausing growth until favorable conditions return. The liverwort Marchantia polymorpha produces dormant propagules, gemmae, for asexual reproduction. The phytohormone abscisic acid (ABA) is known to regulate dormancy in the seeds of flowering plants and the gemmae of M. polymorpha. Here, we identify a basic helix-loop-helix protein, MpHYPNOS (MpHYP), as a central regulator of gemma dormancy. MpHYP expression induces germination arrest in gemmae. Transcriptome analysis revealed that MpHYP represses a D-type cyclin gene MpCYCD;1 and induces genes involved in ABA biosynthesis. Repression of MpCYCD;1 expression and germination arrest are both maintained even in an ABA-receptor mutant. By contrast, Mphyp and ABA-related mutants lacked the desiccation tolerance observed in wild-type gemmae. We propose that MpHYP regulates gemma dormancy through two parallel pathways: an ABA-independent pathway that suppresses germination through cell cycle repression, and an ABA-dependent pathway that enhances desiccation tolerance via ABA biosynthesis. These findings highlight an integrated regulatory mechanism of dormancy via phytohormonal and cell cycle control in bryophytes.
Yoshimura et al. (Tue,) studied this question.