May control G2/M (mitosis) phase progression. Plays a role in regulating seedling growth in darkness via regulation of hypocotyl cell elongation and cotyledon cell development. Plays a role in stomatal development. Required to suppress endoreduplication. Together with CDKB1-2, 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, PubMed:24687979). 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] Phosphorylation at Thr-14 or Tyr-15 inactivates the enzyme, while phosphorylation at Thr-176 activates it. Interacts with CKS1 (PubMed:9276444, PubMed:11319029). Interacts with CYCU3-1 (PubMed:15197472). Interacts with SIM, SMR1 and SMR2 (PubMed:20706207). Highly expressed in guard cells and stomatal precursor cells of cotyledons. Expressed in roots, stems, flowers and siliques. Preferentially expressed in the S and G2 phases. Expressed in actively dividing cells: root and shoot apical meristems, leaf primordia and emerging lateral root meristem. Expressed in light-grown seedlings from 1 up to 7 days after germination with a peak at 2 and 3 days. Observed in late guard mother cells (GMC), newly formed guard cells, and immature stomata, thus being expressed just before and after the symmetric division of stomatal differentiation (PubMed:20675570). Repressed by MYB88 and MYB124 during stomatal development. 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, PubMed:24123248, PubMed:24687979). CDKA-1 partially rescue abnormal stomata phenotype of cdkb1;1 cdkb1;2 (PubMed:24687979). 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.