Anna Maria Innocenti

Amplification of ribosomal cistrons during the maturation of metaxylem in the root ofAllium cepa

SummaryA mixture of 18 S and 25 S3H-rRNA fractions was used for cytological hybridization with DNA of immature metaxylem cells ofAllium cepa in different stages of development. Labelling by3H-rRNA was detected over nucleolus associated DNA of almost all investigated stages and in correspondence of DNA bodies of one stage. Utilizingin vitro DNA-rRNA hybridization technique it was shown that DNA, extracted from root portions where extra synthesis of DNA occurs in metaxylem cells, contains approximately 6-fold rDNA than DNA extracted from meristems. Since the frequency of metaxylem cells among root cells is very low, the detected difference demonstrates a conspicuos amplification process. To our knowledge this is the first report on the quantitative estimation of amplification of ribosomal cistrons in plants.

Some Cytological Aspects of the Differentiation of Metaxylem in the Root of Allium Cepa

SUMMARYA cytological study was accomplished on the immature metaxylem of Allium cepa roots in the histological region between 500 and 3000 μ from the tip. Labelling experiments with 3H-thymidine, 3H-uridine and 3H-lysine were also carried out.The results show that in metaxylem maturation four morphologically detectable stages can be distinguished: 1) Cells close to the region of the apical initials undergo a process of chromosome endoreduplication.2) Cells of the histological region between 800 and 1200 μ from the root tip show amplification of the genes of the nucleolar organizing region.3) Cells of the histological region between 1200 and 2000 μ from the root tip show extrusion of the nucleolus from the nucleus (about 30% of cells show the phenomenon). Nucleoli, either extruded or not extruded, incorporate 3H-thymidine in their peripheral area in concomitance with chromatin incorporation. On the contrary, the inner part of the nucleolus is active in DNA synthesis when chromatin is almost inactive.4) Aft...

Differentiation of metaxylem cell line in the root of Allium cepa. I. DNA heterogeneity and ribosomal cistrons of two different stages of differentiation.

The differentiation processes of the metaxylem cell line in the root ofAllium cepa are characterized by amplification phenomena of repetitive DNA sequences mainly localized in heterochromatic regions of metaphase chromosomes. Moreover, these sequences are heavily methylated. This paper presents additional results on variation in endogenous DNA methylation in different developing root segments. The results show that methylation is higher in apical meristematic cells than the differentiating segments; contrastingly, total RNA synthesis seems to be correlated with undermethylation. Addition of labelled methyl groups to DNA by eukaryotic methylase, DNA digestions with different restriction enzymes specific for methylated sites and HPLC analysis confirmed the above results. Moreover, variation in methylation levels during differentiation occur not only at the internal cytosine of the-CCG-sites, but also at external cytosine. Furthermore, methylation affects other sites containing the trinucleotides-CXG-. In conclusion, root differentiation inAllium cepa seems to be correlated with gene activation modulated by the methylation/demethylation of particular DNA sequences.SummaryDNA was extracted from three root segments ofAllium cepa: i) an apical portion 500 μm long from the tip (meristem); ii) a second portion 4 mm long (I root segment containing metaxylem cells in the initial stages of differentiation); iii) a third portion 6 mm long (II root segment containing metaxylem cells in further stages of differentiation). A mixture of homologous 18 S and 25 S3H-rRNA was used for “invitro” DNA-rRNA hybridization. The following percent saturation values were detected in the three samples: 0.08 in meristem DNA (samplea), 0.129 in I root segment DNA (sampleb), and 0.105 in II root segment DNA (samplec).Thermal denaturation of DNA and the derivative curves of the melting profiles evidenced five DNA families which were differently represented in the three DNA samples. DNA elution by thermal chromatography on hydroxyapatite followed by hybridization with3H-rRNA, revealed that ribosomal cistrons melt between 90 and 91 °C, corresponding to a G-C content of 50.7%. Moreover, the amount of the DNA family containing ribosomal cistrons was greater in sampleb andc, in sampleb to a greater extent, as compared with samplea. On the other hand, one DNA family melting at a higher temperature (92–93 °C) was drastically increased in samplec.Buoyant density profiles of unsonicated DNA showed no peaks in the three DNA samples. Upon somcation, a heavy shoulder was observed in the profile of sampleb. As the density of ribosomal cistrons and that of shoulder were very similar, it seems possible that the two fractions contain many DNA sequences in common.The present studies demonstrate that the proportion of ribosomal cistrons and other DNA families does not keep constant during the development of the metaxylem cell line.

Cytophotometric Determination of Histone Content in Cell Nuclei of Proliferating and Non-Proliferating Root Meristem Cells of Triticum Durum

SUMMARYCytophotometric analysis of nuclear DNA and histone content were made in dividing and nondividing root meristems of Triticum durum cv. CAPPELLI. It was demonstrated that the nuclei in the pre-DNA synthesis phase of the nuclear cycle (G1), all having a 2C DNA content, show very different amounts of fast-green stainable histone. The 2C nuclei in nonproliferating meristems of caryopses at 15 days before harvest have a histone content nearly twice that of 2C nuclei in proliferating meristems of germinating caryopses.The possible significance of the quantitative variations in histone contents is discussed.

Cyclic Changes of Histones/DNA Ratio in Differentiating Nuclei of Metaxylem Cell Line in Allium Cepa Root Tip

SUMMARYCytophotometric analysis of the nuclear DNA and histones content were carried out in procambial cells with develop into metaxylem in Allium cepa root tip. The collected data indicate that histones/DNA ratio is not constant. It is in fact highest during the endoreduplication process (stages 1 and 4), decreases in extrasynthesis of nucleolus associated DNA (stages 2 and 5) and reaches its lowest values during extra DNA release (stages 3 and 6).The results concerning stages 1, 2 and 3 are discussed in connection with the nuclear ultrastructure of the same stages.

NUCLEAR METABOLIC ASPECTS AND ROOT CAP RENEWAL IN ALLIUM CEPA

SUMMARYThe cap cells of Allium cepa, labeled with 3HTdR, are displaced from the meristem and sloughed from the cap; after 72h the whole cap appears renewed. During the cap cell differentiation processes, some nuclear cytopbotometric and ultrastructural aspects were analyzed. Though all the nuclei of the root cap differentiate with the same Dna content per nucleus, histones/Dna ratio shows differences both between and within columella and peripheral cell nuclei. Besides, histones/Dna ratio shows a wider heterogeneity in mature than in undifferentiated cells. The ultrastructural observations are in agreement with histones/Dna data, showing changes in chromatin structure.

Different nuclear damage pattern during the lifespan of Secale cereale polymorphic seeds

SUMMARYSecale cereale produces, in the same ear, brown and black caryopses, which differ widely in size and morphology. These phenotypic differences are correlated to different variations in both germination and viability during seeds lifespan. Spontaneous mutagenesis analysis demonstrates that the two kinds of seeds display a different nuclear damage during storage. The brown caryopses are less damaged during the dormancy period, but more damaged during aging, mainly in the last months of life. This different pattern of dormancy and aging in seeds produced within the same ear might be of significance for specie propagation and safeguard.