Positional cues regulate dorsal organ formation in the liverwort Marchantia polymorpha

Bryophytes and vascular plants represent the broadest evolutionary divergence in the land plant lineage, and comparative analyses of development spanning this divergence therefore offer opportunities to identify truisms of plant development in general. In vascular plants, organs are formed repetitively around meristems at the growing tips in response to positional cues. In contrast, leaf formation in mosses and leafy liverworts occurs from clonal groups of cells derived from a daughter cell of the apical stem cell known as merophytes, and cell lineage is a crucial factor in repetitive organ formation. However, it remains unclear whether merophyte lineages are a general feature of repetitive organ formation in bryophytes as patterns of organogenesis in thalloid liverworts are unclear. To address this question, we developed a clonal analysis method for use in the thalloid liverwort Marchantia polymorpha, involving random low-frequency induction of a constitutively expressed nuclear-targeted fluorescent protein by dual heat-shock and dexamethasone treatment. M. polymorpha thalli ultimately derive from stem cells in the apical notch, and the lobes predominantly develop from merophytes cleft to the left and right of the apical cell(s). Sector induction in gemmae and subsequent culture occasionally generated fluorescent sectors that bisected thalli along the midrib and were maintained through several bifurcation events, likely reflecting the border between lateral merophytes. Such thallus-bisecting sectors traversed dorsal air pores and gemma cups, suggesting that these organs arise independently of merophyte cell lineages in response to local positional cues.