A cis-acting bidirectional transcription switch controls sexual dimorphism in the liverwort.

Plant life cycles alternate between haploid gametophytes and diploid sporophytes. While regulatory factors determining male and female sexual morphologies have been identified for sporophytic reproductive organs, such as stamens and pistils of angiosperms, those regulating sex-specific traits in the haploid gametophytes that produce male and female gametes and hence are central to plant sexual reproduction are poorly understood. Here, we identified a MYB-type transcription factor, MpFGMYB, as a key regulator of female sexual differentiation in the haploid-dominant dioicous liverwort, Marchantia polymorpha MpFGMYB is specifically expressed in females and its loss resulted in female-to-male sex conversion. Strikingly, MpFGMYB expression is suppressed in males by a cis-acting antisense gene SUF at the same locus, and loss-of-function suf mutations resulted in male-to-female sex conversion. Thus, the bidirectional transcription module at the MpFGMYB/SUF locus acts as a toggle between female and male sexual differentiation in M. polymorpha gametophytes. Arabidopsis thaliana MpFGMYB orthologs are known to be expressed in embryo sacs and promote their development. Thus, phylogenetically related MYB transcription factors regulate female gametophyte development across land plants.

Generative Cell Specification Requires Transcription Factors Evolutionarily Conserved in Land Plants.

Land plants differentiate germ cells in the haploid gametophyte. In flowering plants, a generative cell is specified as a precursor that subsequently divides into two sperm cells in the developing male gametophyte, pollen. Generative cell specification requires cell-cycle control and microtubule-dependent nuclear relocation (reviewed in [1-3]). However, the generative cell fate determinant and its evolutionary origin are still unknown. In bryophytes, gametophytes produce eggs and sperm in multicellular reproductive organs called archegonia and antheridia, respectively, or collectively called gametangia. Given the monophyletic origin of land plants [4-6], evolutionarily conserved mechanisms may play key roles in these diverse reproductive processes. Here, we showed that a single member of the subfamily VIIIa of basic helix-loop-helix (bHLH) transcription factors in the liverwort Marchantia polymorpha primarily accumulated in the initial cells and controlled their development into gametangia. We then demonstrated that an Arabidopsis thaliana VIIIa bHLH transiently accumulated in the smaller daughter cell after an asymmetric division of the meiosis-derived microspore and was required for generative cell specification redundantly with its paralog. Furthermore, these A. thaliana VIIIa bHLHs were functionally replaceable by the M. polymorpha VIIIa bHLH. These findings suggest the VIIIa bHLH proteins as core regulators for reproductive development, including germ cell differentiation, since an early stage of land plant evolution.