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Centromere and telomere biology of plants

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Eukaryote growth and reproduction implies mitotic and meiotic cell divisions. Mitosis results in formation of two identical daughter cells from a single parental cell, meiosis gives rise of gametes (sex cells), each with half the number of chromosomes from the parental cell. Correct segregation of chromosomes ...

Eukaryote growth and reproduction implies mitotic and meiotic cell divisions. Mitosis results in formation of two identical daughter cells from a single parental cell, meiosis gives rise of gametes (sex cells), each with half the number of chromosomes from the parental cell. Correct segregation of chromosomes during mitosis and meiosis ensures genome stability, while defects in these processes result in aneuploidization or polyploidization, often leading to cell death or cancer. Centromeres and telomeres are important chromosomal domains required for the proper separation of genetic material during both mitosis and meiosis.
Centromeres are formed by centromeric DNA and a protein complex, the kinetochore, and are involved in sister chromatid cohesion, proper microtubule attachment, chromosome movement and cell cycle regulation. Centromeric DNA is composed of tandem repeats and/or transposable elements that have evolved fast and are therefore highly variable even among closely related species. Kinetochore assembly at centromere begins with the incorporation of centromeric histone H3 variant (cenH3) into centromeric nucleosomes. cenH3 incorporation is not determined by the sequences of centromeric repeats, but is regulated epigenetically, since cenH3 incorporation and formation of neocentromeres can occur at sequences without typical centromere repeats. Even in simple eukaryotes such as budding yeast more than 65 proteins form the kinetochore. Many of these kinetochore proteins were identified and functionally characterized in yeast, Drosophila or mammals, but only few in plants. The best characterized kinetochore protein in plants is cenH3. However, the multi-step process of centromere formation is still poorly understood.
Telomeres are chromatin domains present at the end of each chromosome arm. Their basic functions include solving of two basic problems inherent to linear chromosomes: the end-replication problem, i.e. compensation of replicative shortening of chromosome ends, and end-protection problem, i.e. masking the natural chromosome ends from being recognized as unrepaired chromosome breaks. In contrast to centromere, telomere repeats are highly conserved but numerous exceptions exist in which telomere DNA sequence does not correspond to the phylogenetic position of an organism. The most common molecular tool to replenish incompletely replicated telomeres is the ribonucleoprotein enzyme complex called telomerase, but telomerase-independent strategies of telomere elongation were also described as alternative or backup systems. The end-protection is mediated by telomere-binding proteins (including also DNA repair factors) and local DNA structures such as telomeric loops or G-quadruplexes. The knowledge of protective proteins in plants, analogous to vertebrate shelterin complex, is just fragmentary and some proteins involved in telomere protection and telomerase recruitment in plants were characterized only recently. Plant telomerase (in contrast to its mammalian counterpart) is regulated reversibly but the mechanism(s) of regulation are not known. On top of that, telomeres (if sufficiently long) form nucleosomal arrays with specific histone marks challenging the common concept of telomeres as heterochromatic structures. As an epigenetic feature observed exclusively in plants, cytosines in telomeric DNA are partially methylated and changes in methylation state were recently shown to have important regulatory implications.
We welcome all types of articles (original research, methods, hypotheses, opinions and reviews) that provide new insights into assembly and function of centromeres and telomeres in plants, including their regulation and their interactions, as well as potential utility in crop improvement.


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