Heterochromatin in the model plant Arabidopsis thaliana is confined to small pericentromeric regions of all five chromosomes and to the nucleolus organizing regions. This clear differentiation makes it possible to study spatial arrangement and functional properties of individual chromatin domains in interphase nuclei. Here, we present the organization of Arabidopsis chromosomes in young parenchyma cells. Heterochromatin segments are organized as condensed chromocenters (CCs), which contain heavily methylated, mostly repetitive DNA sequences. In contrast, euchromatin contains less methylated DNA and emanates from CCs as loops spanning 0.2-2 Mbp. These loops are rich in acetylated histones, whereas CCs contain less acetylated histones. We identified individual CCs and loops by fluorescence in situ hybridization by using rDNA clones and 131 bacterial artificial chromosome DNA clones from chromosome 4. CC and loops together form a chromosome territory. Homologous CCs and territories were associated frequently. Moreover, a considerable number of nuclei displayed perfect alignment of homologous subregions, suggesting physical transinteractions between the homologs. The arrangement of interphase chromosomes in Arabidopsis provides a well defined system to investigate chromatin organization and its role in epigenetic processes.

The correlation between environmental stress and DNA methylation has been studied by following the methylation status of cytosine residues in the DNA of pea root tips exposed to water deficit. DNA methylation was evaluated by two complementary approaches: (i) immunolabelling by means of a monoclonal antibody against 5‐methylcytosine; (ii) MSAP (Methylation‐Sensitive Amplified Polymorphism) to verify if methylation and de‐methylation in response to water deficit may be related to specific DNA sequences. Immunolabelling showed that water stress induces cytosine hypermethylation in the pea genome. Regarding the CCGG target sequence, an increase in methylation specifically in the second cytosine (about 40 % of total site investigated) was revealed by MSAP analyses. In addition, MSAP band profile detected in three independent repetitions was highly reproducible suggesting that, at least for the CCGG target sequence, methylation was addressed to specific DNA sequences.

This study aimed to provide new information about phyto-toxicology of nano-TiO2 on plant systems. To contribute to the evaluation of the potential harmful effects of the nanoparticles on monocots and dicots we considered their effects on seed germination and root elongation applying a concentration range from 0.2 to 4.0% in the plants Zea mays L. and Vicia narbonensis L. Moreover, we achieved a genotoxicity study at cytological level in root meristems by means of traditional cytogenetic approach, to evidence possible alterations in mitotic activity, chromosomal aberrations, and micronuclei release. From these analyses it comes out that nano- TiO2 particles, after short-term exposure and under our experimental conditions, delayed germination progression for the first 24 h in both materials. Root elongation was affected only after treatment with the higher nano-TiO2 concentration. Further significant effects were detected showing mitotic index reduction and concentration-dependent increase in the aberration emergence that evidenced a nano-TiO2- induced genotoxic effect for both species

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