Probable cyclin-dependent protein kinase (CDK) inhibitor that functions as a repressor of mitosis in the endoreduplication cell cycle (PubMed:17098811, PubMed:26546445). Cooperates with SIM and SMR2 to promote endoreplication during leaf development (PubMed:26546445). Specifically regulates endoreduplication in epidermal pavement cells to produce the cell size pattern (PubMed:20485493). Is necessary for giant cell formation (PubMed:20485493). Positive regulator of effector-triggered immunity (ETI) (PubMed:25455564). Interacts with CDKB1-1 (PubMed:20706207). Interacts with CPR5 (PubMed:25455564). Expressed in roots, leaves, stems, siliques and flowers (PubMed:17098811). Expressed in the root elongation zone (PubMed:24399300). Up-regulated by double-stranded DNA breaks-inducing treatments (PubMed:17227549). Up-regulated by zeocin treatment (PubMed:21613568). Increased number of smaller cells and loss of giant cells in sepals due to both a decrease in endoreduplication and an average decrease in the duration of the cell cycle. Over-expression of SMR1 produces additional giant cells on sepals.