Cytokinin–CLAVATA cross-talk is an ancient mechanism regulating shoot meristem homeostasis in land plants (PNAS)

The shoot apical meristem (SAM) maintains a small pool of pluripotent stem cells, which are the source of all above-ground plant tissues. In Arabidopsis thaliana, CLAVATA3 (CLV3), CLAVATA1(CLV1) and RECEPTOR-LIKE PROTEIN KINASE2 (RPK2) repress WUSCHEL (WUS) expression. WUS induces CLV3, forming a local feedback loop to maintain the stem cell population. Cytokinin is also involved in the WUS-CLV regulation, as well as SAM formation, maintenance, and growth. In contrast to flowering plants, the moss SAM is self-generated from a single stem cell and there is no stem cell maintenance through WUS function. Since the functions of CLV1 and RPK2 are conserved in the moss Physcomitrium patens, Cammarata et al. investigated whether CLV and cytokinin cross-talk is conserved in a similar way despite lacking the WUS interaction. Exogenous cytokinin treatment, CRISPR mutants, and mathematical modeling revealed that CLV1 and RPK2 act additively in parallel pathways to regulate stem cell specification. Mutants of CLV and RPK form ectopic stem cells in response to exogenous cytokinin, supporting the hypothesis that PpCLV1 and PpRPK2 act upstream of cytokinin-stem cell induction. This work shows that CLV and cytokinin cross-talk is conserved to maintain SAM homeostasis in P. patens, demonstrates how the pathways between different plant lineages are conserved, and is a valuable contribution to our understanding of the underlying genetic interactions.  (Summary by Andrea Gómez Felipe @andreagomezfe) PNAS 10.1073/pnas.2116860119