FIGURE 1.

Schizogenous intercellular space (ICS) formation in Marchantia polymorpha. (A) Schematic representation of ICS formation during air chamber development. After a periclinal cell division to generate the protodermal and sub-protodermal cell layers, ICSs first appear as an initial aperture, between the anticlinal walls of protodermal cells. The base of the initial aperture broadens. The primary air chamber is formed via anticlinal cell divisions and protodermal and subprotodermal cell growth surrounding the ICS. (B) Hypothetical model of NOP1-mediated signaling involved in schizogenous ICS formation. Extracellular signals such as peptide ligands that are perceived by a receptor, Y, promote ICS formation via modulation of regulatory factor X that otherwise suppresses activation of cell-wall-remodeling enzymes. In the modulation of regulatory factor X, receptor Y signaling activates the E3 ligase NOP1 via phosphorylation of the ARM-repeat domain. NOP1 catalyzes the ubiquitination and subsequent degradation of regulatory factor X, which promotes cell separation through transcriptional activation of cell-wall-remodeling genes. In nop1, cell separation is constitutively suppressed by the action of regulatory factor X.