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Chromosome Missegregation Causes Somatic Polyploidization in Plants

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Chromosome Missegregation Causes Somatic Polyploidization in Plants bioRxiv preprint doi: https://doi.org/10.1101/438648; this version posted October 9, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC 4.0 International license. Chromosome missegregation causes somatic polyploidization in plants Elena Kozgunova1*, Momoko Nishina2, Gohta Goshima2* 1International Collaborative Programme in Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan 2Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan *Correspondence should be addressed to [email protected]; [email protected] u.ac.jp Polyploidy, or genome multiplication, is mitotic arrest, apoptosis, and/or aneuploidy- common in plants and greatly impacts inducing chromosome missegregation (1). evolution and speciation. It is generally Most of our knowledge on kinetochore considered that genome duplication in function and impact on the genome stability plants mainly arises through sexual is derived from animal and yeast studies (2). reproduction via unreduced gamete Another major group of eukaryotes, plants, fusion. On the contrary, polyploidization also possesses conserved kinetochore in somatic cells occurs in the differentiated proteins (3–5). Although the localization and tissues; thus, it is not inherited. In this loss-of-function phenotype of some plant study, we show that chromosome kinetochore proteins have been reported missegregation during somatic cell before (6–15), the data are mostly obtained division frequently induces from fixed cells of specific tissues. No polyploidization in the moss comprehensive picture of plant kinetochore Physcomitrella patens. We carried out a protein dynamics and functions can be drawn comprehensive characterization of as of yet. For example, plants appear to have kinetochore components in moss and lost 12 out of 16 components that form found that kinetochore malfunction CCAN (constitutive centromere associated induces lagging chromosomes, which network) in animal and yeast cells (2, 5), yet inhibit the assembly of cytokinetic how residual CCAN subunits act in plants is machinery. The resultant cells were unknown. polyploid, proceeded to the next cell cycle, The moss Physcomitrella patens is an and eventually developed into polyploid emerging model system for plant cell plants. We also showed that bi-nucleated biology. The majority of its tissues are in a Arabidopsis embryos that were generated haploid state, and, owing to an extremely by artificial inhibition of cytokinesis were high rate of homologous recombination, gene able to execute the next round of cell disruption and fluorescent protein tagging of division, suggesting that mitotic errors endogenous genes are easy to obtain in the could potentially lead to inheritable first generation (16). Another remarkable polyploidization in diverse plant species. feature is its regeneration ability; for Introduction example, differentiated gametophore leaf cells, when excised, are efficiently The kinetochore is a macromolecular reprogrammed to become stem cells (17, 18). complex that connects chromosomes to Thus, genome alteration (mutation, spindle microtubules and plays a central role polyploidy), even in a somatic cell, can in chromosome segregation. Kinetochore potentially spread through the population. malfunction causes checkpoint-dependent 1 bioRxiv preprint doi: https://doi.org/10.1101/438648; this version posted October 9, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC 4.0 International license. In this study, we aimed to comprehensively RNAi system, knockdown of target genes characterize conserved kinetochore proteins was induced by the addition of β-estradiol to in a single plant cell type, the P. patens the culture medium 4–6 days prior to live- caulonemal apical cell. Unexpectedly, we imaging (19). We first targeted CENP-A, the observed that most of the kinetochore only constitutive centromeric protein proteins displayed localization patterns identified in P. patens. Since RNAi distinct from their animal counterparts. sometimes exhibits an off-target effect, we Furthermore, kinetochore malfunction prepared two independent RNAi constructs. resulted in chromosome missegregation and As expected, we observed a significant polyploidy via perturbing the function of the mitotic delay and chromosome cytokinetic machinery. alignment/segregation defects in the CENP- A RNAi lines (Fig. S8; Movie S4). These Results phenotypes can be explained by a deficiency Endogenous localization analysis of in proper kinetochore-microtubule conserved kinetochore proteins in P. attachment. Consequently, micronuclei were patens observed in the daughter cells, a hallmark of To observe the endogenous localization of aneuploidy. We concluded that CENP-A, like putative kinetochore components, we in many organisms, is essential for equal inserted a fluorescent tag in-frame at the C- chromosome segregation during mitosis in or N-terminus of eighteen selected proteins moss. (Fig. S1). Histone H3-like CENP-A (CenH3) Surprisingly, we also observed cells with two localization was determined by ectopic large nuclei in both RNAi lines (Fig. 2B, 1 h Citrine-CENP-A expression, as tagging 18 min), which is the typical outcome of likely perturbs its function. Consistent with cytokinesis failure in this cell type (20–22). their sequence homology, many of the To check if a similar phenotype is observed proteins were localized to the kinetochore at after the depletion of another kinetochore least transiently during the cell cycle. protein, we made conditional RNAi line for However, many proteins also showed SKA1 (the localization of SKA1-Citrine is unexpected localization (or disappearance) at shown on Fig. S6D, Movie S3), an outermost certain cell cycle stages (Fig. 1; Fig. S2–S7; kinetochore component that does not directly Movies S1-S3). Most surprising were CCAN interact with CENP-A. As expected, mitotic protein dynamics: CENP-X, CENP-O and delay and chromosome missegregation were CENP-S did not show kinetochore observed in the RNAi line (Fig. S8; Movie enrichment at any stages (Fig. 1; Fig. S3; S4). In addition, cytokinesis failure was also Movie S1), whereas CENP-C also detected (Fig. 2B; Movie S5). To verify that dissociated from the kinetochore transiently the observed phenotype of SKA1 was not due in the post-mitotic phase (Fig. S4; Movie S2). to an off-target effect, we ectopically Thus, we could not identify any expressed RNAi-insensitive SKA1-Cerulean “constitutive” kinetochore proteins other in the RNAi line and observed the rescue of than CENP-A. all the above phenotypes (Fig. S9). Kinetochore malfunction causes Although we could not detect any chromosome missegregation and kinetochore enrichment of the CCAN subunit cytokinesis failure CENP-X, we analyzed its RNAi lines. We failed to obtain knockout lines for all the Interestingly, we observed similar targeted kinetochore proteins, except for the phenotypes to CENP-A and SKA1, including spindle checkpoint protein Mad2, strongly cytokinesis failure (Fig. 2B; Fig. S8; Movie suggesting that they are essential for moss S4). CENP-X RNAi phenotypes were viability. We therefore selected conditional rescued by the ectopic expression of CENP- RNAi lines for functional analyses. In this X-Cerulean that was resistant to the RNAi 2 bioRxiv preprint doi: https://doi.org/10.1101/438648; this version posted October 9, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC 4.0 International license. construct (Fig. S9). Thus, CENP-X has lost Polyploid plants are regenerated from its kinetochore localization in moss, but is isolated multi-nucleated cells. still essential for chromosome segregation Lagging chromosomes are a major cause of and cell division. aneuploidy in daughter cells, which is During plant cytokinesis, a bipolar particularly deleterious for haploid cells. microtubule-based structure known as the However, the above observation supports a phragmoplast is assembled between different scenario, whereby cytokinesis segregating chromatids. The cell plate then failure induced by lagging chromosomes forms in the phragmoplast midzone (~4 min allows a cell to have a duplicated genome set after anaphase onset in P. patens caulonemal in two or more nuclei. On the other hand, cells) and gradually expands towards the cell whether animal somatic cells that have failed cortex, guided by the phragmoplast (20). We cytokinesis can re-enter the cell cycle or not observed that microtubules reorganized into remains an ongoing debate (23–25). To phragmoplast-like structures upon address whether moss cells can recover from chromosome segregation in every cell, severe cell division defects and continue their regardless of the severity of chromosome cell cycle, we first analyzed the DNA content missegregation (e.g. 32 min in Fig. 2B). of cells in the CENP-A exon-targeting RNAi However, by analyzing a total of 44 cells line, in which multi-nucleated cells were from SKA1 (9 cells), CENP-X (18 cells) and most prevalent. For comparison,
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