Cyclin-dependent protein kinase (CDK) inhibitor that functions as a repressor of mitosis in the endoreduplication cell cycle (PubMed:10952891, PubMed:11882294, PubMed:17098811, PubMed:17764505, PubMed:19717615, PubMed:20194967). Inhibits the kinase activity of CYCD3-1/CDKA-1, CYCD2-1/CDKA-1 and CYCB1-1/CDKB1-1 complexes in a dose dependent manner (PubMed:26546445). Cooperates with SMR1 and SMR2 to promote endoreplication during leaf development (PubMed:26546445). Required for normal trichome endoreplicating cell cycles (PubMed:10952891, PubMed:11882294, PubMed:17098811, PubMed:17764505, PubMed:19717615, PubMed:20194967). Positive regulator of effector-triggered immunity (ETI) (PubMed:25455564). Interacts with CDKA-1 (PubMed:17098811, PubMed:26546445). Interacts with CYCD2-1, CYCD3-2 and CYCD4-1 (PubMed:17098811). Interacts with CDKB1-1 (PubMed:20706207, PubMed:26546445). Interacts with CPR5 (PubMed:25455564). Expressed in the shoot apical meristem, leaf primordia and the elongation zone of the root. Down-regulated by gibberellin (PubMed:19576768). Up-regulated by zeocin treatment (PubMed:21613568). Multicellular trichomes with nuclei showing reduced levels of endoreduplication. Plants over-expressing SIM are dwarf with serrated leaves containing enlarged cells with increased levels of nuclear DNA.