Isolation and functional analysis of the 5'-flanking region of carrot top1beta gene coding for the beta isoform of DNA topoisomerase I

Differential subcellular localization of DNA topoisomerase-1 isoforms and their roles during Caenorhabditis elegans development

DNA topoisomerase-1 (TOP-1) resolves the topological problems associated with DNA replication, transcription and recombination by introducing temporary single-strand breaks in the DNA. Caenorhabditis elegans TOP-1 has two isoforms, TOP-1α and TOP-1β. TOP-1β is broadly localized to the nuclei of many cells at all developmental stages and concentrated in nucleoli in embryo gut and oogenic cells. However, TOP-1α is specifically localized to centrosomes, neuronal cells, excretory cells and chromosomes of germ cells in embryonic and larval stages. Reporter gene analysis also shows that top-1 transcription is highly activated in several sensory neurons, speculating the possible role of TOP-1α in neuronal development. From RNA interference (RNAi) experiments, we demonstrated that C. elegans TOP-1 is required for chromosomal segregation, germline proliferation and gonadal migration, which are all correlated with the expression and activity of TOP-1. Therefore, our findings may provide an insight into a new role of TOP-1 in development of multicellular organisms.

Response to UV-C radiation in topo I-deficient carrot cells with low ascorbate levels

In animal cells, recent studies have emphasized the role played by DNA topoisomerase I (topo I) both as a cofactor of DNA repair complexes and/or as a damage sensor. All these functions are still unexplored in plant cells, where information concerning the relationships between DNA damage, PCD induction, and topo I are also limited. The main goal of this study was to investigate the possible responses activated in topo I-depleted plant cells under oxidative stress conditions which induce DNA damage. The carrot (Daucus carota L.) AT1-beta/22 cell line analysed in this study (characterized by an antisense-mediated reduction of top1beta gene expression of approximately 46% in association with a low ascorbate content) was more sensitive to UV-C radiation than the control line, showing consistent cell death and high levels of 8-oxo-dG accumulation. The topo I-depleted cells were also highly susceptible to the cross-linking agent mitomycin C. The death response was associated with a lack of oxidative burst and there were no changes in ascorbate metabolism in response to UV-C treatment. Electron and fluorescence microscopy suggested the presence of three forms of cell death in the UV-C-treated AT1-beta/22 population: necrosis, apoptotic-like PCD, and autophagy. Taken together, the data reported here support a reduced DNA repair capability in carrot topo I-deficient cells while the putative relationship between topo I-depletion and ascorbate impairment is also discussed.

Methyl jasmonate disrupts shoot formation in tobacco thin cell layers by over-inducing mitotic activity and cell expansion

The aim of the present study was to determine early cyto-histological events associated with the reduced number of shoots formed at the end of culture in tobacco (Nicotiana tabacum L.) thin cell layers treated with methyl jasmonate (MJ) [S. Biondi et al. (2001) J Exp Bot 52:1-12]. The results show that 0.1-10 microM MJ strongly stimulated mitotic activity early in culture relative to untreated controls. Treatment with MJ also induced anomalous mitoses. Enhanced proliferative growth was confirmed by northern analysis and in situ hybridisation using cDNA probes of the G1/S phase-specific genes ubiquitin carboxyl-extension protein (ubi-CEP), topoisomerase 1 (top1) and ribonucleotide reductase (RNR). The formation of meristematic cell clusters on day 5 was also enhanced by 1 muM MJ, but subsequent development of these cell clusters into meristemoids and shoot primordia was reduced by all MJ concentrations in a dose-dependent manner. Cell expansion was stimulated by MJ concentrations ranging from 0.001 to 10 microM; expanded cells frequently occurred around and within meristemoids and shoot primordia, and displayed thickened and suberised cell walls; cell wall thickness increased with increasing MJ concentration. These cytological events caused alterations in the tunica and stem differentiation of the shoot dome. The apparently paradoxical role of MJ, which deregulates shoot formation through a stimulation of growth events, i.e., mitotic activity and cell expansion, is discussed.