Together with CDKB1-1, promotes both the last division in the stomatal cell lineage as well as the number of stomata (PubMed:20675570). In collaboration with MYB124 and MYB88, restrict the G1/S transition and chloroplast and nuclear number during stomatal formation, and normally maintain fate and developmental progression throughout the stomatal cell lineage (PubMed:24123248). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] Interacts with CKS1. Expressed in flowers. No apparent stomatal abnormalities. The double mutant cdkb1;1 cdkb1;2 has a reduced number of abnormal stomata consisting in single guard cells (GC) (PubMed:20675570). The quadruple mutant flp-1 myb88 cdkb1;1 cdkb1;2 has a reduced number of large single guard cells blocked at mitosis, with strongly altered shape and size and characterized by enlarged nucleus due to endomitosis and endocycling, as well as extensive chloroplast replication (PubMed:24123248). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily. Truncated C-terminus.