Involved in the control of the cell cycle. Essential for both G1/S and G2/M (mitosis) phase transitions. Functions in cell morphogenesis as well as cell proliferation. Required for cell division (entry into mitosis) of the generative cell in male gametogenesis. Required to trigger guard mother cells (GMC) symmetric divisions at the late stage of stomatal development, probably via the regulation of G1 to S transition in the cell cycle. Required for the function of SPCH in entering the stomatal lineage (PubMed:25680231). Promotes divisions in the guard cells (GCs) after the guard mother cells (GMC) symmetric division when in the presence of CYCD3-2 via the phosphorylation of SPCH (PubMed:24687979, PubMed:25680231). 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] CDK kinase activated by CDKF-1. CDK kinase activity inhibited by KRP1/ICK1, KRP2/ICK2, KRP3/ICK6, KRP4/ICK7, KRP5/ICK3, KRP6/ICK4 and KRP7/ICK5. Down-regulated by phosphorylation by WEE1. Interacts with CDT1A, CYCA2-3, CYCD2-1, CYCD3-1, CYCD4-1, CYCD4-2, CYCH1-1, CYCU1-1, CYCU2-1, CYCU2-2, CYCU3-1, CYCU4-1, CYCU4-2, CYCU4-3, CKS1, KRP2/ICK2, KRP3/ICK6, KRP4/ICK7, KRP6/ICK4, KRP7/ICK5, and C-terminal domain of KRP1/ICK1. Interacts with WEE1 and TIF4A-1/EIF4A-1. Interacts with PAS2; when phosphorylated at Tyr-15. Interacts with SMR3, SMR4, SMR5, SMR6, SMR8 and At4g14310. Binds to CYCD3-2 (PubMed:24687979). Component of a DREAM-like complex which modulates a variety of developmentally regulated genes and of the mitotic genes in proliferating and differentiated cells. Interacts with MYB3R3 at later and with MYB3R4 at earlier stages of leaf development (PubMed:26069325). May interact with SPCH (PubMed:25680231). Mainly cytoplasmic. Nuclear distribution increases after binding to ICK1/KRP1. Expressed in roots, stems, flowers and siliques. Expressed throughout the cell cycle. 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. By nematode infection in roots. Down-regulated by salt stress in root meristem and replication blocking agents (hydroxyurea and aphidicolin). Phosphorylated at Tyr-15 by WEE1. Phosphorylation at Thr-161 is important for the kinase activity and substrate binding. Binding to the anti-phosphatase PAS2 prevents dephosphorylation. Plants display lethal male gametophyte (PubMed:16460514, PubMed:25680231). Impaired stomata formation with arrested guard mother cells (GMC) divisions (PubMed:25680231). Impaired last mitotic division in the male gametophyte, leading to 50 percent of pollen with two gametes (PubMed:25680231). The double mutant flp-1 myb88 displays an enhanced stomatal phenotype with more and larger stomatal clusters. Triple mutants cdka;1 flp-1 myb88 don't have guard cells stacks but accumulates sGCs (PubMed:24687979). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.