Conserved CKI1-mediated signaling is required for female germline specification in Marchantia polymorpha

In land plants, gametes derive from a small number of dedicated haploid cells.1 In angiosperms, one central cell and one egg cell are differentiated in the embryo sac as female gametes for double fertilization, while in non-flowering plants, only one egg cell is generated in the female sexual organ, called the archegonium.2,3 The central cell specification of Arabidopsis thaliana is controlled by the histidine kinase CYTOKININ-INDEPENDENT 1 (CKI1), which is a two-component signaling (TCS) activator sharing downstream regulatory components with the cytokinin signaling pathway.4,5,6,7 Our phylogenetic analysis suggested that CKI1 orthologs broadly exist in land plants. However, the role of CKI1 in non-flowering plants remains unclear. Here, we found that the sole CKI1 ortholog in the liverwort Marchantia polymorpha, MpCKI1, which functions through conserved downstream TCS components, regulates the female germline specification for egg cell development in the archegonium. In M. polymorpha, the archegonium develops three-dimensionally from a single cell accumulating MpBONOBO (MpBNB), a master regulator for germline initiation and differentiation.8 We visualized female germline specification by capturing the distribution pattern of MpBNB in discrete stages of early archegonium development, and found that MpBNB accumulation is restricted to female germline cells. MpCKI1 is required for the proper MpBNB accumulation in the female germline, and is critical for the asymmetric cell divisions that specify the female germline cells. These results suggest that CKI1-mediated TCS originated during early land plant evolution and participates in female germ cell specification in deeply diverged plant lineages.