Abstract
Micronuclei are biomarkers of genotoxic effects and chromosomal instability. They are formed when chromosome fragments or whole chromosomes fail to disjoin into daughter nuclei. We present qualitative and quantitative analyses of the involvement of specific chromosome regions of chromosomes Bd4 and Bd5 in the formation of micronuclei of Brachypodium distachyon root tip cells following maleic hydrazide (MH) treatment and X-radiation. This is visualised by cytomolecular approaches using bacterial artificial chromosome (BAC)-based multicolour fluorescence in situ hybridisation (mcFISH) in combination with 5S and 25S rDNA probes. The results showed that the long arm of submetacentric chromosome Bd4 forms micronuclei at twice the frequency of its short arm, suggesting that the former is more prone to double-strand breaks (DSBs). In contrast, no difference was observed in the frequency of micronuclei derived from the long and short arms of submetacentric chromosome Bd5. Interestingly, the proximal region of the short arm of Bd5 is more prone to DSBs than its distal part. This demonstrates that 5S rDNA and 35S rDNA loci are not “hot spots” for DNA breaks after the application of these mutagens.
Highlights
The integrity and stability of DNA in a nucleus are essential for the correct execution of cellular metabolism at interphase and the accurate distribution of genetic information during cell division
We show the composition and putative origin of micronuclei from the long and short arms of these two chromosomes in Brachypodium root-tip meristematic cells after maleic hydrazide (MH) treatment and X-radiation, and use these observations to infer the distribution of double-strand breaks (DSBs)
DSBs from MH Treatment and X-Radiation Are Non-Randomly Distributed along the Arms of Chromosome Bd4
Summary
The integrity and stability of DNA in a nucleus are essential for the correct execution of cellular metabolism at interphase and the accurate distribution of genetic information during cell division. Despite the fact that recent studies have provided some insights into the effects of various mutagens in plant cells [1,2,3,4], our knowledge of potential double-strand break (DSB) “hot spots” in the plant genome is still limited. Hsu [5] proposed a bodyguard hypothesis, which states that euchromatin centrally localised in the nucleus may be protected from chemical agents and X-radiation by surrounding constitutive heterochromatin at the nuclear periphery. This hypothesis gains support from studies of ethyl methanesulfonate (EMS)-treated maize cells, it does not appear to apply to Crepis capillaris cells subjected to maleic hydrazide (MH) treatment [6]. Other factors could be responsible for the non-random distributions of chromosome aberrations, such as the spatial organisation of the nucleus at interphase, the diverse transcriptional activity of specific chromosome regions, and chromosome size [12]
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