Downregulation of multiple CDK inhibitor ICK/KRP genes upregulates the E2F pathway and increases cell proliferation, and organ and seed sizes in Arabidopsis

Plant J. 2013 Aug;75(4):642-55. doi: 10.1111/tpj.12228. Epub 2013 May 30.


The ICK/KRP cyclin-dependent kinase (CDK) inhibitors are important plant cell cycle factors sharing only limited similarity with the metazoan CIP/KIP family of CDK inhibitors. Little is known about the specific functions of different ICK/KRP genes in planta. In this study, we created double and multiple mutants from five single Arabidopsis ICK/KRP T-DNA mutants, and used a set of 20 lines for the functional investigation of the important gene family. There were gradual increases in CDK activity from single to multiple mutants, indicating that ICK/KRPs act as CDK inhibitors under normal physiological conditions in plants. Whereas lower-order mutants showed no morphological phenotypes, the ick1 ick2 ick6 ick7 and ick1 ick2 ick5 ick6 ick7 mutants had a slightly altered leaf shape. The quintuple mutant had larger cotyledons, leaves, petals and seeds than the wild-type control. At the cellular level, the ICK/KRP mutants had more but smaller cells in all the organs examined. These phenotypic effects became more apparent as more ICK/KRPs were downregulated, suggesting that to a large extent ICK/KRPs function in plants redundantly in a dosage-dependent manner. Analyses also revealed increased expression of E2F-dependent genes, and elevated RBR1 as well as an increased level of phospho-RBB1 protein in the quintuple mutant. Thus, downregulation of multiple ICK/KRP genes increases CDK activity, upregulates the E2F pathway and stimulates cell proliferation, resulting in increased cell numbers, and larger organs and seeds.

Keywords: RBR1 and E2F; cell cycle; cell size; cyclin-dependent kinase inhibitor; plant growth; seed size.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis / cytology
  • Arabidopsis / genetics*
  • Arabidopsis / growth & development
  • Arabidopsis / metabolism
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism
  • Cell Count
  • Cell Proliferation
  • Cell Size
  • Cotyledon / cytology
  • Cotyledon / genetics
  • Cotyledon / growth & development
  • Cotyledon / metabolism
  • Cyclin-Dependent Kinase Inhibitor Proteins / genetics*
  • Cyclin-Dependent Kinase Inhibitor Proteins / metabolism
  • Cyclin-Dependent Kinases / genetics
  • Cyclin-Dependent Kinases / metabolism*
  • Down-Regulation
  • Flowers / cytology
  • Flowers / genetics
  • Flowers / growth & development
  • Flowers / metabolism
  • Gene Expression Regulation, Plant*
  • Models, Biological
  • Mutagenesis, Insertional
  • Phenotype
  • Plant Leaves / cytology
  • Plant Leaves / genetics
  • Plant Leaves / growth & development
  • Plant Leaves / metabolism
  • Plants, Genetically Modified
  • Seeds / cytology
  • Seeds / genetics
  • Seeds / growth & development
  • Seeds / metabolism


  • Arabidopsis Proteins
  • Cyclin-Dependent Kinase Inhibitor Proteins
  • RBR1 protein, Arabidopsis
  • Cyclin-Dependent Kinases