Running title:

The mitosis-to-endocycle transition requires the controlled inactivation of M-phaseassociated cyclin-dependent kinase (CDK) activity. Previously, the B-type CDKB1;1 has been identified as an important negative regulator of endocycle onset. Here, we demonstrate that CDKB1;1 co-purifies and associates with the A2-type cyclin CYCA2;3. Co-expression of CYCA2;3 with CDKB1;1 triggered ectopic cell divisions and inhibited endoreduplication. Moreover, the enhanced endoreduplication phenotype observed after overexpression of a dominant-negative allele of CDKB1;1 could be partially complemented by CYCA2;3 co-overexpression, illustrating that both subunits unite in vivo to form a functional complex. CYCA2;3 protein stability was found to be controlled by CCS52A1, an activator of the anaphase-promoting complex. We conclude that CCS52A1 participates in the endocycle onset by down-regulating CDKB1;1 activity through the destruction of CYCA2;3. www.plantphysiol.org on July 19, 2017 Published by Downloaded from Copyright © 2009 American Society of Plant Biologists. All rights reserved.