Ectopic, but not endogenous, expression of the transcription factor gene Mp <i>SETA</i> disrupts epidermal tissue formation in <i>Marchantia polymorpha</i>

Basic helix-loop-helix (bHLH) transcription factors form dimers that regulate the expression of their downstream genes and drive morphogenesis in multicellular eukaryotes. We recently revealed that a bHLH heterodimer consisting of MpSETA (also known as MpSMF) and MpICE2 (also known as MpSCRM2) coordinately controls the development of the seta, a diploid sporophytic tissue, in the liverwort Marchantia polymorpha. Ectopic expression of MpSETA in the haploid thallus causes abnormalities in epidermal tissue formation, but whether native MpSETA has any role in haploid tissue formation was unclear. Here we investigated the effects of constitutive MpSETA expression in thalli using a steroid-inducible system. Ectopic MpSETA expression resulted in developmental defects in air pore and air chamber formation, ectopic pegged rhizoid differentiation, and thallus dwarfism. However, Mpseta knockout mutants did not exhibit obvious defects in thallus formation, and we did not detect expression of MpSETA in the wild-type thallus. Inducing MpSETA in the Mpice2 mutant background partially alleviated these phenotypes, suggesting that they are mediated in part by MpSETA-MpICE2 heterodimers. We conclude that although ectopic MpSETA expression affects thallus development, the native MpSETA gene mainly has a role in the diploid sporophyte development.