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.

Cytokinin signaling is involved in root hair elongation in response to phosphate starvation.

Root hair, single-celled tubular structures originating from the epidermis, plays a vital role in the uptake of nutrients from the soil by increasing the root surface area. Therefore, optimizing root hair growth is crucial for plants to survive in fluctuating environments. Root hair length is determined by the action of various plant hormones, among which the roles of auxin and ethylene have been extensively studied. However, evidence for the involvement of cytokinins has remained elusive. We recently reported that the cytokinin-activated B-type response regulators, ARABIDOPSIS RESPONSE REGULATOR 1 (ARR1) and ARR12 directly upregulate the expression of ROOT HAIR DEFECTIVE 6-LIKE 4 (RSL4), which encodes a key transcription factor that controls root hair elongation. However, depending on the nutrient availability, it is unknown whether the ARR1/12-RSL4 pathway controls root hair elongation. This study shows that phosphate deficiency induced the expression of RSL4 and increased the root hair length through ARR1/12, though the transcript and protein levels of ARR1/12 did not change. These results indicate that cytokinins, together with other hormones, regulate root hair growth under phosphate starvation conditions.