ABSTRACT The anaphase-promoting complex/cyclosome (APC/C) marks key cell cycle proteins for proteasomal breakdown, thereby ensuring unidirectional progression through the cell cycle. Its target recognition is temporally regulated by activating subunits, one of which is called CELL CYCLE SWITCH 52 A2 (CCS52A2). We sought to expand the knowledge of identified APC/C targets by using the severe growth phenotypes of CCS52A2 -deficient Arabidopsis thaliana plants as a readout in a suppressor mutagenesis screen, resulting in the identification of the previously undescribed gene called PIKMIN1 ( PKN1 ). PKN1 deficiency rescues the disorganized root stem cell phenotype of the ccs52a2-1 mutant, whereas an excess of PKN1 inhibits growth of ccs52a2-1 plants, indicating the importance of PKN1 abundance for proper development. Accordingly, the lack of PKN1 in a wild-type background negatively impacts cell division, while its ectopic expression promotes proliferation. PKN1 shows a cell cycle phase-dependent accumulation pattern, localizing to microtubular structures, including the preprophase band, the mitotic spindle, and phragmoplast. PKN1 is conserved throughout the plant kingdom, with its function in cell division being evolutionary conserved in the liverwort Marchantia polymorpha . Our data thus demonstrate that PKN1 represents a novel, plant-specific gene with a rate-limiting role in cell division, which is proteolytically controlled by the CCS52A2-activated APC/C. One-Sentence Summary PKN1 is a conserved plant-specific protein that is rate-limiting for cell division, likely through its interaction with microtubuli, and is proteolytically controlled by APC/C CCS52A2 .

The APC/CCCS52A2 E3-Ligase Complex targeted PKN1 Protein is a Plant Lineage-Specific Driver of Cell Division