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The cell cycle is a complex sequence of events by which cells grow and divide mitotically or meiotically. Mitosis results in the generation of two identical daughter cells, while meiosis generates gametes as a prerequisite for sexual reproduction. To study the localization and dynamics of proteins involved in the regulation and proceeding of the cell cycle, life cell imaging of proteins fused to fluorescent tags can be performed. However, in some cases this approach cannot be applied, e.g., due to low fluorescence intensity, fast bleaching, or degradation of recombinant proteins by the proteasome pathway. Instead, immunolabeling with protein-specific antibodies offers a useful approach for the analysis of intact cells. Alternatively, immunolabeling can also be applied to isolated and/or flow-sorted nuclei of particular cell cycle stages (G1, S, and G2) or of different endopolyploidy levels. The following chapter will detail indirect immunolabeling protocols to analyze the subcellular localization and distribution of cell cycle-specific proteins in Arabidopsis thaliana., (© 2022. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

KINETOCHORE NULL2 (KNL2) is involved in recognition of centromeres and in centromeric localization of the centromere-specific histone cenH3. Our study revealed a cenH3 nucleosome binding CENPC-k motif at the C terminus of Arabidopsis thaliana KNL2, which is conserved among a wide spectrum of eukaryotes. Centromeric localization of KNL2 is abolished by deletion of the CENPC-k motif and by mutating single conserved amino acids, but can be restored by insertion of the corresponding motif of Arabidopsis CENP-C. We showed by electrophoretic mobility shift assay that the C terminus of KNL2 binds DNA sequence-independently and interacts with the centromeric transcripts in vitro. Chromatin immunoprecipitation with anti-KNL2 antibodies indicated that in vivo KNL2 is preferentially associated with the centromeric repeat pAL1 Complete deletion of the CENPC-k motif did not influence its ability to interact with DNA in vitro. Therefore, we suggest that KNL2 recognizes centromeric nucleosomes, similar to CENP-C, via the CENPC-k motif and binds adjoining DNA., (© 2017 American Society of Plant Biologists. All rights reserved.)

The cell cycle is a complex sequence of events by which cells grow and divide mitotically or meiotically. Mitosis results in the generation of two identical daughter cells, while meiosis generates gametes as a prerequisite for sexual reproduction. To study the localization and dynamics of proteins involved in the regulation and proceeding of the cell cycle, life cell imaging of proteins fused to fluorescent tags can be performed. However, in some cases this approach cannot be applied, e.g., due to low fluorescence intensity, fast bleaching or degradation of recombinant proteins by the proteasome pathway. Instead, immunolabeling with protein-specific antibodies represents a useful approach for the analysis of intact cells. Alternatively, immunolabeling can also be applied to isolated and/or flow-sorted nuclei of particular cell cycle stages (G1, S, and G2) or of different endopolyploidy levels. This chapter details indirect immunolabeling protocols to analyze the subcellular localization and distribution of cell cycle-specific proteins in Arabidopsis thaliana.

The centromere-the primary constriction of monocentric chromosomes-is essential for correct segregation of chromosomes during mitosis and meiosis. Centromeric DNA varies between different organisms in sequence composition and extension. The main components of centromeric and pericentromeric DNA of Brassicaceae species are centromeric satellite repeats. Centromeric DNA initiates assembly of the kinetochore, the large protein complex where the spindle fibers attach during nuclear division to pull sister chromatids apart. Kinetochore assembly is initiated by incorporation of the centromeric histone H3 cenH3 into centromeric nucleosomes. The spindle assembly checkpoint acts during mitosis and meiosis at centromeres and maintains genome stability by preventing chromosome segregation before all kinetochores are correctly attached to microtubules. The function of the spindle assembly checkpoint in plants is still poorly understood. Here, we review recent advances of studies on structure and functional importance of centromeric DNA of Brassicaceae, assembly and function of cenH3 in Arabidopsis thaliana and characterization of core SAC proteins of A. thaliana in comparison with non-plant homologues.

The centromeric histone H3 variant cenH3 is an essential centromeric protein required for assembly, maintenance, and proper function of kinetochores during mitosis and meiosis. We identified a kinetochore null2 (KNL2) homolog in Arabidopsis thaliana and uncovered features of its role in cenH3 loading at centromeres. We show that Arabidopsis KNL2 colocalizes with cenH3 and is associated with centromeres during all stages of the mitotic cell cycle, except from metaphase to mid-anaphase. KNL2 is regulated by the proteasome degradation pathway. The KNL2 promoter is mainly active in meristematic tissues, similar to the cenH3 promoter. A knockout mutant for KNL2 shows a reduced level of cenH3 expression and reduced amount of cenH3 protein at chromocenters of meristematic nuclei, anaphase bridges during mitosis, micronuclei in pollen tetrads, and 30% seed abortion. Moreover, knl2 mutant plants display reduced expression of suppressor of variegation 3-9 homologs2, 4, and 9 and reduced DNA methylation, suggesting an impact of KNL2 on the epigenetic environment for centromere maintenance.

Potassium (K (+) ) is an important nutrient for plants. It serves as a cofactor of various enzymes and as the major inorganic solute maintaining plant cell turgor. In a recent study, an as yet unknown role of K (+) in plant homeostasis was shown. It was demonstrated that K (+) gradients in vascular tissues can serve as an energy source for phloem (re)loading processes and that the voltage-gated K (+) channels of the AKT2-type play a unique role in this process. The AKT2 channel can be converted by phosphorylation of specific serine residues (S210 and S329) into a non-rectifying channel that allows a rapid efflux of K (+) from the sieve element/companion cells (SE/CC) complex. The energy of this flux is used by other transporters for phloem (re)loading processes. Nonetheless, the results do indicate that post-translational modifications at S210 and S329 alone cannot explain AKT2 regulation. Here, we discuss the existence of multiple post-translational modification steps that work in concert to convert AKT2 from an inward-rectifying into a non-rectifying K (+) channel.

The essential mineral nutrient potassium (K(+)) is the most important inorganic cation for plants and is recognized as a limiting factor for crop yield and quality. Nonetheless, it is only partially understood how K(+) contributes to plant productivity. K(+) is used as a major active solute to maintain turgor and to drive irreversible and reversible changes in cell volume. K(+) also plays an important role in numerous metabolic processes, for example, by serving as an essential cofactor of enzymes. Here, we provide evidence for an additional, previously unrecognized role of K(+) in plant growth. By combining diverse experimental approaches with computational cell simulation, we show that K(+) circulating in the phloem serves as a decentralized energy storage that can be used to overcome local energy limitations. Posttranslational modification of the phloem-expressed Arabidopsis K(+) channel AKT2 taps this "potassium battery," which then efficiently assists the plasma membrane H(+)-ATPase in energizing the transmembrane phloem (re)loading processes.