The RopGEF KARAPPO is Essential for the Initiation of Vegetative Reproduction in Marchantia

Many plants can reproduce vegetatively, producing clonal progeny from vegetative cells; however, little is known about the molecular mechanisms underlying this process. Liverwort ( Marchantia polymorpha ), a basal land plant, propagates asexually via gemmae, which are clonal plantlets formed in gemma cups on the dorsal side of the vegetative thallus. The initial stage of gemma development involves elongation and asymmetric divisions of a specific type of epidermal cell, called a gemma initial, which forms on the floor of the gemma cup. To investigate the regulatory mechanism underlying gemma development, we focused on two allelic mutants in which no gemma initial formed; these mutants were named karappo , meaning 9empty9. We used whole-genome sequencing of both mutants, and molecular genetic analyses to identify the causal gene, KARAPPO ( KAR ), which encodes a Rop guanine nucleotide exchange factor (RopGEF) carrying a PRONE catalytic domain. In vitro GEF assays showed that the full-length KAR protein and the PRONE domain have significant GEF activity toward MpRop, the only Rop GTPase in M. polymorpha . Moreover, genetic complementation experiments showed a significant role for the N- and C-terminal variable regions in gemma development. Our investigation demonstrated an essential role for KAR/RopGEF in the initiation of plantlet development from a differentiated cell, which may involve cell polarity formation and subsequent asymmetric cell division via activation of Rop signaling, implying a similar developmental mechanism in vegetative reproduction of various land plants.