Functions of CDK/cyclin Complexes in Endoreplication Regulation by the CDK Inhibitor SIAMESE

Abstract

Arabidopsis trichome (leaf hair) is a specialized single cell extended from epidermal cell on the leaves, which is a typical endoreplication and is also known as endoreduplication. Several D-type cyclins were tested to check the cell division in trichome, and the trichome expressing either CYCB1;2 or CDKB2;2 cannot trigger cell division, even if simultaneous expression of CYCB1;2 and CDKB2;2 failed to produce mitosis in trichome. Only CYCD3;1 specifically promotes multicellular trichome. cdkb1;1cdbk1;2 double mutants and sim cdkb1;1cdkb1;2 triple mutants exhibit the phenotype similar to the wild type and very limited cell division respectively. Overexpression of a CDKB1;1 dominant-negative construct that has relatively low kinase activity suppresses cell division in sim mutant trichomes, suggesting that CDKB1;1 is required for cell division. However, the trichome overexpressing CYCD3;1 can induce cell division without the need of CDKB1;1, which suggested SIM acts as the inhibitor to suppress CDKB1;1/Cyclin activity, promoting mitosis by independence of CYCD3;1 way. Split luciferase complementation assays identified the interaction of SIM and CYCA2:3 rather than CYCD3;1. Yeast two-hybrid assay was used to verify this interaction and showed that motif A in SIM is required for this interaction especially conserved Thr-35 residue, but motif C in SIM is dispensable. Taken together, SIM might inhibit CYCD3;1 and CDKB1;1 in a parallel way in vivo which shields on the light on the endoreplication establishment in Arabidopsis trichomes.