Endoreplication, a modified cell cycle in which DNA is replicated without subsequent cell division, plays an important but poorly understood role in plant growth and in plant responses to biotic and abiotic stress. The Arabidopsis () () gene encodes the first identified member of the SIAMESE-RELATED (SMR) family of cyclin-dependent kinase inhibitors. controls endoreplication during trichome development, and mutant trichomes divide several times instead of endoreplicating their DNA. The SMR family is defined by several short linear amino acid sequence motifs of largely unknown function, and family members have little sequence similarity to any known protein functional domains. Here, we investigated the roles of the conserved motifs in site-directed Arabidopsis mutants using several functional assays. We identified a potential cyclin-dependent kinase (CDK)-binding site, which bears no resemblance to other known CDK interaction motifs. We also identified a potential site of phosphorylation and two redundant nuclear localization sequences. Surprisingly, the only motif with similarity to the other family of plant CDK inhibitors, the INHIBITOR/INTERACTOR OF CDC2 KINASE/KIP-RELATED PROTEIN proteins, is not required for function in vivo. Because even highly divergent members of the family are able to replace function in Arabidopsis trichomes, it is likely that the results obtained here for will apply to other members of this plant-specific family of CDK inhibitors.

Endoreplication, also known as endoreduplication, is a modified cell cycle in which DNA is replicated without subsequent cell division. Endoreplication plays important roles in both normal plant development and in stress responses. The SIAMESE (SIM) gene of Arabidopsis (Arabidopsis thaliana) encodes a cyclin-dependent kinase (CDK) inhibitor that plays a central role in establishing endoreplication, and is the founding member of the SIAMESE-RELATED (SMR) family of plant-specific CDK inhibitor genes. However, there has been conflicting evidence regarding which specific cyclin/CDK complexes are inhibited by SIM in vivo. In this work, we use genetic evidence to show that SIM likely inhibits both CDKA;1-and CDKB1-containing CDK complexes in vivo, thus promoting endoreplication in developing Arabidopsis trichomes. We also show that SIM interacts with CYCA2;3, a binding partner of CDKB1;1, via SIM motif A, which we previously identified as a CDK-binding motif. By contrast, SIM motif C, which has been indicated as a cyclin binding motif in other contexts, appears to be relatively unimportant for interaction between SIM and CYCA2;3. Together with earlier results, our work suggests that SIM and other SMRs likely have a multivalent interaction with CYC/CDK complexes.

One sentence summary:The cyclin-dependent kinase inhibitor SIAMESE (SIM) 9 targets both CDKA;1 and CDKB1 complexes to establish endoreplication, and that SIM 10 interacts with the cyclin CYCA2;3 via SIM Motif A. Abstract 25 Endoreplication, also known as endoreduplication, is a modified cell cycle in 26 which DNA is replicated without subsequent cell division. Endoreplication plays 27 important roles in both normal plant development and in stress responses. The 28 SIAMESE (SIM) gene of Arabidopsis (Arabidopsis thaliana) encodes a cyclin-dependent 29 kinase inhibitor that plays an central role in establishing endoreplication, and is the 30 founding member of the SIAMESE-RELATED (SMR) family of plant-specific cyclin-31 dependent kinase inhibitors genes. However, there has been conflicting evidence 32 regarding which specific cyclin/CDK complexes are inhibited by SIM in vivo. In this 33 work, we use genetic evidence to show that SIM likely inhibits both CDKA;1-and 34 CDKB1-containing CDK complexes in vivo to promote endoreplication in developing 35Arabidopsis trichomes. We also show that SIM interacts with CYCA2;3, a binding 36 partner of CDKB1;1, via SIM Motif A, which we previously identified as a CDK-binding 37 motif. In contrast, SIM Motif C, which has been indicated as a cyclin binding motif in 38 other contexts, appears to be relatively unimportant for interaction between SIM and 39 CYCA2;3. Together with earlier results, our work suggests that SIM and other SMRs 40 likely have a multivalent interaction with CYC/CDK complexes. 41 42 43Classically, the eukaryotic cell cycle is divided into four phases: G1, S, G2 and 44 M, which are followed by cytokinesis. Cell cycle regulation depends in large part on 45 specific cyclin and cyclin-dependent kinase (CDK) complexes that regulate both the 46 G1→S and the G2→M transitions, which are the two major cell cycle checkpoints 47