Independent evolution of lateral inhibition mechanisms in different lineages of land plants: MpFEW RHIZOIDS1 miRNA-mediated lateral inhibition controls rhizoid cell patterning in Marchantia polymorpha

Lateral inhibition patterns differentiated cells during development in bacteria, metazoans and land plants. Tip-growing rhizoid cells develop among flat epidermal cells in the epidermis of the early diverging land plant Marchantia polymorpha. We show that the majority of rhizoid cells develop individually but some develop in linear, one-dimensional clusters of between two and seven rhizoid cells in wild type plants. The distribution of rhizoid cells can be accounted for within a simple model of lateral inhibition. The model also predicted that, in the absence of lateral inhibition, rhizoid cell clusters would be two-dimensional with larger clusters than those formed with lateral inhibition. Rhizoid differentiation in Marchantia polymorpha is positively regulated by the ROOT HAIR DEFECTIVE SIX-LIKE1 (MpRSL1) basic Helix Loop Helix (bHLH) transcription factor which is directly repressed by the FEW RHIZOIDS1 (MpFRH1) miRNA. To test if MpFRH1 miRNA acts during lateral inhibition we generated loss-of-function mutants that did not produce the MpFRH1 miRNA. Two-dimensional clusters of rhizoids develop in Mpfrh1loss-of-function (lof) mutants as predicted by the model for plants that lack lateral inhibition. Furthermore, clusters of up to nine rhizoid cells developed in the Mpfrh1lof mutants compared to a maximum number of seven observed in wild type. The higher steady state levels of MpRSL1 mRNA in Mpfrh1lof mutants indicate that MpFRH1-mediated lateral inhibition involves the repression of MpRSL1 activity. Together the modelling and genetic data indicate that the pattern of cell differentiation in the M. polymorpha epidermis is consistent with a lateral inhibition process in which MpFRH1 miRNA represses MpRSL1. This discovery suggests that novel mechanisms of lateral inhibition may operate in different lineages of land plants, unlike metazoans where the conserved Delta-Notch signaling system controls lateral inhibition in diverse metazoan lineages.