ABSTRACT
The shape of modular organisms depends on the branching architecture, which in plants is determined by the fates of generative centers called meristems. The branches of the liverwort Marchantia polymorpha are derived from two adjacent meristems that develop at thallus apices. These meristems may be active and develop branches or may be dormant and do not form branches. The relative number and position of active and dormant meristems define the overall shape and form of the thallus. We show that the clade III SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor MpSPL1 is required for meristem dormancy. The activity of MpSPL1 is regulated by the liverwort-specific Mpo-MR13 miRNA, which, in turn, is regulated by PIF-mediated signaling. An unrelated PIF-regulated miRNA, MIR156, represses a different SPL gene (belonging to clade IV) that inhibits branching during the shade avoidance response in Arabidopsis thaliana. This suggests that a conserved light signaling mechanism modulates branching architecture in liverworts and angiosperms and therefore is likely operated in the last common ancestor. However, PIF-mediated signaling represses the expression of different miRNA genes with different SPL targets during dichotomous, apical branching in liverworts and during lateral, subapical branching in angiosperms. We speculate that the mechanism that acts downstream of light and regulates meristem dormancy evolved independently in liverworts and angiosperms.
