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

Thickness of cotton fiber, referred to as fiber maturity, is a key determinant of fiber quality, lint yield, and textile performance. The cotton immature fiber (im) mutant has been used to study fiber maturity since its fiber is thinner than the wild type near isogeneic line (NIL), Texas Marker-1 (TM-1). The im phenotype is caused by a single recessive mutation of a pentatricopeptide repeat (PPR) gene that reduces the activity of mitochondrial complex I and up-regulates stress responsive genes. However, the mechanisms altering the stress responses in im mutant are not well understood. Thus, we characterized growth and gas exchange in im and TM-1 under no stress and also investigated their stress responses by comparing gas exchange and transcriptomic profiles under high temperature. Phenotypic differences were detected between the NILs in non-fiber tissues although less pronounced than the variation in fibers. At near optimum temperature (28±3°C), im maintained the same photosynthetic performance as TM-1 by means of greater stomatal conductance. In contrast, under high temperature stress (>34°C), im leaves reduced photosynthesis by decreasing the stomatal conductance disproportionately more than TM-1. Transcriptomic analyses showed that the genes involved in heat stress responses were differentially expressed between the NIL leaves. These results indicate that the im mutant previously reported to have low activity of mitochondrial complex I displays increased thermosensitivity by impacting stomatal conductance. They also support a notion that mitochondrial complex I activity is required for maintenance of optimal photosynthetic performance and acclimation of plants to high temperature stress. These findings may be useful in the future efforts to understand how physiological mechanisms play a role in determining cotton fiber maturity and may influence stress responses in other crops.

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The abscisic acid (ABA) signaling pathway is the key defense mechanism against drought stress in plants, yet the connectivity of cellular molecules related to gene expression in response to ABA is little understood. A dynamic model of the core components of the ABA signaling pathway was built using ordinary differential equations to understand the connectivity. Parameter values of protein-protein interactions and enzymatic reactions in the model were implemented from the data obtained by previously conducted experiments. On the other hand, parameter values of gene expression and translation were determined by comparing the kinetics of gene expression in the model to those of ABA-induced RD29A (response to desiccation 29A) in actual plants. Based on the analyses of the optimized model, we hypothesized that the translation rate of PP2C (protein phosphatase type 2C) is downregulated by ABA to increase the ABRE (ABA-responsive element) promoter activity. The hypotheses were preliminarily supported by newly conducted experiments using transgenic Arabidopsis plants that carry a luciferase expression cassette driven by the RD29A promoter (RD29A::LUC). The model suggests that identifying a mechanism that alters PP2C translation rate would be one of the next research frontiers in the ABA signaling pathway.