Nuclear fusions contribute to polyploidization of the gigantic nuclei in the chalazal endosperm of Arabidopsis
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Somatic polyploidization is recognized as a means to increase gene expression levels in highly active metabolic cells. The most common mechanisms are endoreplication, endomitosis and cell fusion. In animals and plants the nuclei of multinucleate cells are usually prevented from fusing. Here, we report that the nuclei from the syncytial cyst of the chalazal endosperm of Arabidopsis thaliana (L.) Heynh. are polyploid with some intermediate ploidy levels that cannot be attributed to endoreplication, suggesting nuclear fusion. Analysis of isolated nuclei, together with fluorescent in situ hybridization (FISH), revealed that nuclei from the chalazal endosperm are two or three times bigger than the nuclei from the peripheral endosperm and have a corresponding increase in ploidy. Together with the consistent observation of adjoined nuclei, we propose that nuclear fusion contributes, at least in part, to the process of polyploidization in the chalazal endosperm. Confocal analysis of intact seeds further suggested that free nuclei from the peripheral endosperm get incorporated into the chalazal cyst and likely participate in nuclear fusions.
KeywordsArabidopsis Chalazal endosperm FISH Nuclear fusion Polyploidization
Bacterial artificial chromosome
Fluorescent in situ hybridization
Nucleolar organizing region
Nuclear cytoplasmic domain
We gratefully acknowledge Philippa Barrell for the protocol of improved Feulgen staining and for comments on the manuscript, and two anonymous reviewers for helpful suggestions. C.B. was supported by a fellowship from the Roche Research Foundation; the project was also supported by the University of Zürich and a Searle Scholarship to U.G.
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