Reproductive Induction is a Far-Red High Irradiance Response that is Mediated by Phytochrome and PHYTOCHROME INTERACTING FACTOR in Marchantia polymorpha.

Land plants have evolved a series of photoreceptors to precisely perceive environmental information. Among these, phytochromes are the sole photoreceptors for red light (R) and far-red light (FR), and play pivotal roles in modulating various developmental processes. Most extant land plants possess multiple phytochromes that probably evolved from a single phytochrome in the common ancestor of land plants. However, the ancestral phytochrome signaling mechanism remains unknown due to a paucity of knowledge regarding phytochrome functions in basal land plants. It has recently been reported that Mpphy, a single phytochrome in the liverwort Marchantia polymorpha, regulates typical photoreversible responses collectively classified as low fluence response (LFR). Here, we show that Mpphy also regulates the gametangiophore formation analogous to the mode of action of the far-red high irradiance response (FR-HIR) in angiosperms. Our phenotypic analyses using mutant plants obtained by CRISPR/Cas9-based genome editing revealed that MpFHY1, an ortholog of FAR-RED ELONGATED HYPOCOTYL1, as well as Mpphy is critical for the FR-HIR signaling in M. polymorpha. In addition, knockout of MpPIF, a single PHYTOCHROME INTERACTING FACTOR gene in M. polymorpha, completely abolished the FR-HIR-dependent gametangiophore formation, while overexpression of MpPIF accelerated the response. FR-HIR-dependent transcriptional regulation was also disrupted in the Mppif mutant. Our findings suggest that plants had already acquired the FR-HIR signaling mediated by phytochrome and PIF at a very early stage during the course of land plant evolution, and that a single phytochrome in the common ancestor of land plants could mediate both LFR and FR-HIR.