Barley telomeres shorten during differentiation but grow in callus culture

RAD50 function is essential for telomere maintenance in Arabidopsis

We have identified and characterized an Arabidopsis thaliana rad50 mutant plant containing a T-DNA insertion in the AtRAD50 gene and showing both meiotic and DNA repair defects. We report here that rad50/rad50 mutant cells show a progressive shortening of telomeric DNA relative to heterozygous rad50/RAD50 controls and that the mutant cell population rapidly enters a crisis, with the majority of the cells dying. Surviving rad50 mutant cells have longer telomeres than wild-type cells, indicating the existence in plants of an alternative RAD50-independent mechanism for telomere maintenance. These results report the role of a protein essential for double-strand break repair in telomere maintenance in higher eukaryotes.

Analysis of the G-overhang structures on plant telomeres: evidence for two distinct telomere architectures

Telomeres are highly conserved structures essential for maintaining the integrity of eukaryotic genomes. In yeast, ciliates and mammals, the G-rich strand of the telomere forms a 3' overhang on the chromosome terminus. Here we investigate the architecture of telomeres in the dicot plants Silene latifolia and Arabidopsis thaliana using the PENT (primer extension/nick translation) assay. We show that both Arabidopsis and Silene telomeres carry G-overhangs longer than 20-30 nucleotides. However, in contrast to yeast and ciliate telomeres, only half of the telomeres in Silene seedlings possess detectable G-overhangs. PENT reactions using a variety of primers and reaction conditions revealed that the remaining fraction of Silene telomeres carries either no overhangs or overhangs less than 12 nucleotides in length. G-overhangs were observed in Silene seeds and leaves, tissues that lack telomerase activity. These findings suggest that incomplete DNA replication of the lagging strand, rather than synthesis by telomerase, is the primary mechanism for G-overhang synthesis in plants. Unexpectedly, we found that the fraction of telomeres with detectable G-overhangs decreased from 50% in seedlings to 35% in leaves. The difference may reflect increased susceptibility of the G-overhangs to nuclease attack in adult leaves, an event that could act as a precursor for the catabolic processes accompanying leaf senescence

Characterization and developmental expression of single-stranded telomeric DNA-binding proteins from mung bean (Vigna radiata)

We have identified and characterized protein factors from mung bean (Vigna radiata) nuclear extracts that specifically bind the single-stranded G-rich telomeric DNA repeats. Nuclear extracts were prepared from three different types of plant tissue, radicle, hypocotyl, and root, in order to examine changes in the expression patterns of telomere-binding proteins during the development of mung bean. At least three types of specific complexes (A, B, and C) were detected by gel retardation assays with synthetic telomere and nuclear extract from radicle tissue, whereas the two major faster-migrating complexes (A and B) were formed with nuclear extracts from hypocotyl and root tissues. Gel retardation assays also revealed differences in relative amount of each complex forming activity in radicle, hypocotyl, and root nuclear extracts. These data suggest that the expression of telomere-binding proteins is developmentally regulated in plants, and that the factor involved in the formation of complex C may be required during the early stages of development. The binding factors have properties of proteins and are hence designated as mung bean G-rich telomere-binding proteins (MGBP). MGBPs bind DNA substrates with three or more single-stranded TTTAGGG repeats, while none of them show binding affinity to either double-stranded or single-stranded C-rich telomeric DNA. These proteins have a lower affinity to human telomeric sequences than to plant telomeric sequences and do not exhibit a significant binding activity to Tetrahymena telomeric sequence or mutated plant telomeric sequences, indicating that their binding activities are specific to plant telomere. Furthermore, RNase treatment of the nuclear extracts did not affect the complex formation activities. This result indicates that the single-stranded telomere-binding activities may be attributed to a simple protein but not a ribonucleoprotein. The ability of MGBPs to bind specifically the single-stranded TTTAGGG repeats may suggest their in vivo functions in the chromosome ends of plants.

Specific association of a small protein with the telomeric DNA-protein complex during the onset of leaf senescence in Arabidopsis thaliana

Telomeres and their changes in length throughout the life span of cells have been intensively investigated in different organisms. Telomere length is assumed to control replicative senescence in mammalian cells. However, only very few data are available on the developmental dynamics of plant telomeres. Here, changes of telomere length and DNA-protein structure of Arabidopsis thaliana telomeres were analysed in different stages of development, with the main focus resting on the transition from pre-senescent to senescent leaves. The lengths of the telomeres, ranging from ca. 2.0 to 6.5 kb, do not significantly change during plant development indicating that telomere length is not involved in differentiation and replicative senescence nor in post-mitotic senescence of A. thaliana. In dedifferentiated cultured cells a slight increase in length can be determined. The nucleoprotein structure of the telomeric DNA was investigated by gel mobility shift assays, with synthetic oligonucleotides and nuclear protein extracts derived from four defined stages of post-mitotic leaf senescence. In all four stages, a highly salt-resistant DNA-protein complex was formed with the double-stranded as well as with the single-stranded G-rich telomeric DNA. An additional DNA-protein complex was identified in nuclear protein extracts isolated from plants in the transition stage from pre-senescence to senescence. The protein components of the DNA-protein complexes were analysed on native PAGE and SDS-PAGE gels. A protein of 67 kDa (ATBP1) bound to the telomeric DNA in all developmental stages. An additional protein of merely 22 kDa (ATBP2) was associated via protein-protein interaction with ATBP to form a higher-order complex exclusively during the onset of senescence. DNA interaction of this higher-order protein complex seems to be restricted to double-stranded telomeric DNA. The defined period of ATBP1/ATBP2 complex formation with the telomeric DNA probably indicates that ATBP2 is involved in the onset of post-mitotic leaf senescence by either disturbing an established or establishing an additional function exhibited by the telomeres in the interphase nuclei.

Disruption of the telomerase catalytic subunit gene from Arabidopsis inactivates telomerase and leads to a slow loss of telomeric DNA

Telomerase is an essential enzyme that maintains telomeres on eukaryotic chromosomes. In mammals, telomerase is required for the lifelong proliferative capacity of normal regenerative and reproductive tissues and for sustained growth in a dedifferentiated state. Although the importance of telomeres was first elucidated in plants 60 years ago, little is known about the role of telomeres and telomerase in plant growth and development. Here we report the cloning and characterization of the Arabidopsis telomerase reverse transcriptase (TERT) gene, AtTERT. AtTERT is predicted to encode a highly basic protein of 131 kDa that harbors the reverse transcriptase and telomerase-specific motifs common to all known TERT proteins. AtTERT mRNA is 10-20 times more abundant in callus, which has high levels of telomerase activity, versus leaves, which contain no detectable telomerase. Plants homozygous for a transfer DNA insertion into the AtTERT gene lack telomerase activity, confirming the identity and function of this gene. Because telomeres in wild-type Arabidopsis are short, the discovery that telomerase-null plants are viable for at least two generations was unexpected. In the absence of telomerase, telomeres decline by approximately 500 bp per generation, a rate 10 times slower than seen in telomerase-deficient mice. This gradual loss of telomeric DNA may reflect a reduced rate of nucleotide depletion per round of DNA replication, or the requirement for fewer cell divisions per organismal generation. Nevertheless, progressive telomere shortening in the mutants, however slow, ultimately should be lethal.

Auxin induction of cell cycle regulated activity of tobacco telomerase

Telomerase activity was measured at each phase of the cell cycle in synchronized tobacco (Nicotiana tabacum) BY-2 cells in suspension culture with the use of the telomeric repeat amplification protocol assay. The activity was low or undetectable at most phases of the cell cycle but showed a marked increase at early S phase. The induction of telomerase activity was not affected by the S phase blockers aphidicolin (which inhibits DNA polymerase alpha) or hydroxyurea (which inhibits ribonucleotide reductase), but it was prevented by olomoucine, an inhibitor of Cdc2/Cdk2 kinases that blocks G(1)-S cell cycle transition. These results suggest that the induction of telomerase activity is not directly coupled to DNA replication by conventional DNA polymerases, but rather is triggered by the entry of cells into S phase. Various analogs of the plant hormone auxin, including indole-3-acetic acid, alpha-naphthaleneacetic acid, and 2,4-dichlorophenoxyacetic acid, potentiated the increase in telomerase activity at early S phase; the growth-inactive analog 2,3-dichlorophenoxyacetic acid, however, had no such effect. Potentiation by indole-3-acetic acid of the induction of telomerase activity was dose dependent. Together, these data indicate that telomerase activity in tobacco cells is regulated in a cell cycle-dependent manner, and that the increase in activity at S phase is specifically inducible by auxin.

Developmental control of telomere lengths and telomerase activity in plants

Telomere lengths and telomerase activity were studied during the development of a model dioecious plant, Melandrium album (syn Silene latifolia). Telomeric DNA consisted of Arabidopsis-type TTTAGGG tandem repeats. The terminal positions of these repeats were confirmed by both Bal31 exonuclease degradation and in situ hybridization. Analysis of terminal restriction fragments in different tissues and ontogenetic stages showed that telomere lengths are stabilized precisely and do not change during plant growth and development. Telomerase activity tested by using a semiquantitative telomerase repeat amplification protocol correlated with cell proliferation in the tissues analyzed. Highest activity was found in germinating seedlings and root tips, whereas we observed a 100-fold decrease in telomerase activity in leaves and no activity in quiescent seeds. Telomerase also was found in mature pollen grains. Telomerase activity in tissues containing dividing cells and telomere length stability during development suggest their precise control during plant ontogenesis; however, the telomere length regulation mechanism could be unbalanced during in vitro dedifferentiation.

Developmental control of telomere lengths and telomerase activity in plants

Telomere lengths and telomerase activity were studied during the development of a model dioecious plant, Melandrium album (syn Silene latifolia). Telomeric DNA consisted of Arabidopsis-type TTTAGGG tandem repeats. The terminal positions of these repeats were confirmed by both Bal31 exonuclease degradation and in situ hybridization. Analysis of terminal restriction fragments in different tissues and ontogenetic stages showed that telomere lengths are stabilized precisely and do not change during plant growth and development. Telomerase activity tested by using a semiquantitative telomerase repeat amplification protocol correlated with cell proliferation in the tissues analyzed. Highest activity was found in germinating seedlings and root tips, whereas we observed a 100-fold decrease in telomerase activity in leaves and no activity in quiescent seeds. Telomerase also was found in mature pollen grains. Telomerase activity in tissues containing dividing cells and telomere length stability during development suggest their precise control during plant ontogenesis; however, the telomere length regulation mechanism could be unbalanced during in vitro dedifferentiation.

Development patterns of telomerase activity in barley and maize

Eukaryotic chromosomes terminate with specialized structures called telomeres. Maintenance of chromosomal ends in most eukaryotes studied to date requires a specialized enzyme, telomerase. Telomerase has been shown to be developmentally regulated in man and a few other multicellular organisms, while it is constitutively expressed in unicellular eukaryotes. Recently, we demonstrated telomerase activity in plant extracts using the PCR-based TRAP (Telomeric Repeat Amplification Protocol) assay developed for human cells. Here we report telomerase activities in two grass species, barley and maize, using a modified, semi-quantitative TRAP assay. Telomerase was highly active in very young immature embryos and gradually declined during embryo development. The endosperm telomerase activity was detectable, but significantly lower than in the embryo and declined during kernel development with no detectable activity in later stages. Telomerase activity in dissected maize embryo axis was several orders of magnitude higher than in the scutellum. Telomerase activity was not detected in a range of differentiated tissues including those with active meristems such as root tips as well as the internode and leaf base. The role of telomerase repression during differentiation and the relationship between chromosome healing and telomerase activity is discussed.

Telomeres in the haemopoietic system

The limited life span of most blood cells requires the continuous production of cells, which in adults exceeds 10(12) cells/day. This impressive production of cells (approximately 4 x 10(16) cells over a lifetime) is achieved by the proliferation and differentiation of committed progenitor cells, which themselves are derived from a population of pluripotent stem cells with self-renewal potential. Paradoxically, the large majority of stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 55 divisions can yield 4 x 10(16) cells. First, it was shown that 'candidate' stem cells purified from fetal and adult tissue showed dramatic functional differences in turn-over time and the ability to produce cells with stem cell properties, Second, these functional differences were found to correlate with a measurable loss of telomere repeats despite the presence of low but readily detectable levels of telomerase in all purified cell fractions. In order to address questions about the role of telomeres in normal and malignant haemopoiesis, we developed a quantitative fluorescence in situ hybridization technique. Here we review the characteristics of this novel tool to assess the number of telomere repeats at the end of individual chromosomes and provide an overview of recent observations.

Comparative analysis of the nucleosomal structure of rye, wheat and their relatives

Analysis of the structure of chromatin in cereal species using micrococcal nuclease (MNase) cleavage showed nucleosomal organization and a ladder with typical nucleosomal spacing of 175-185 bp. Probing with a set of DNA probes localized in the authentic telomeres, subtelomeric regions and bulk chromatin revealed that these chromosomal regions have nucleosomal organization but differ in size of nucleosomes and rate of cleavage between both species and regions. Chromatin from Secale and Dasypyrum cleaved more quickly than that from wheat and barley, perhaps because of their higher content of repetitive sequences with hairpin structures accessible to MNase cleavage. In all species, the telomeric chromatin showed more rapid cleavage kinetics and a shorter nucleosome length (160 bp spacing) than bulk chromatin. Rye telomeric repeat arrays were shortest, ranging from 8 kb to 50 kb while those of wheat ranged from 15 kb up to 175 kb. A gradient of sensitivity to MNase was detected along rye chromosomes. The rye-specific subtelomeric sequences pSc200 and pSc250 have nucleosomes of two lengths, those of the telomeric and of bulk nucleosomes, indicating that the telomeric structure may extended into the chromosomes. More proximal sequences common to rye and wheat, the short tandem-repeat pSc119.2 and rDNA sequence pTa71, showed longer nucleosomal sizes characteristic of bulk chromatin in both species. A strictly defined spacing arrangement (phasing) of nucleosomes was demonstrated along arrays of tandem repeats with different monomer lengths (118, 350 and 550 bp) by combining MNase and restriction enzyme digestion.

The molecular genetic analysis of leaf senescence

The cloning of genes induced during leaf senescence and the study of their modes of regulation conducted in the past two years have revealed some of the molecular mechanisms underlying leaf senescence. The identification of genetic mutants that control leaf senescence in Arabidopsis thaliana opened up new possibilities for genetically analyzing leaf senescence in a model system. Encouraging experimental data with transgenic plants show that manipulation of leaf senescence may greatly contribute to the improvement of important agronomic traits such as crop yield and the storage life of the harvested tissues.

Characterization and developmental patterns of telomerase expression in plants

Telomerase activity is developmentally regulated in mammals. Here we examine telomerase activity in plants, whose development differs in fundamental ways from that of animals. Using a modified version of the telomere repeat amplification protocol (TRAP) assay, we detected an activity in extracts from carrots, cauliflower, soybean, Arabidopsis, and rice with all the characteristics expected for a telomerase synthesizing the plant telomere repeat sequence TTTAGGG. The activity was dependent on RNA and protein components, required dGTP, dATP, and dTTP, but not dCTP, and generated products with a seven nucleotide periodicity. Telomerase activity was abundant in cauliflower meristematic tissue and undifferentiated cells from Arabidopsis, soybean, and carrot suspension cultures, but was low or not detectable in a sampling of differentiated tissues from mature plants. Telomerase from cauliflower meristematic tissues exhibited relaxed DNA sequence requirements, which might reflect the capacity to form telomeres on broken chromosomes in vivo. The dramatic differences in telomerase expression and their correlation with cellular proliferation capacity mirror changes in human telomerase levels during differentiation and immortalization. Hence, telomerase activation appears to be a conserved mechanism involved in conferring long-term proliferation capacity.

Telomerase activity in plant extracts

Telomeres of most eucaryotes terminate in long stretches of short, guanine-rich repeats. Telomerase, a specialized enzyme with reverse transcriptase-like activity, has been shown to synthesize these repeats in many lower eucaryotes and several animal species. Although a sequence (TTTAGGG)n that matches the eucaryotic consensus sequence Tx(A)Gy is present in several plant species, the activity and expression patterns of plant telomerase have not been reported. Here we document the presence of telomerase activity in plant tissues using a modification of the human Telomeric Repeat Amplification Protocol (TRAP) assay. Telomerase activity was detected in barley embryo, anther and carpel tissues and in immature seeds of Arabidopsis thaliana, but not in barley leaf tissue.

Telomerase activity in plant cells

Telomerase is a ribonucleoprotein enzyme which elongates the G-rich strand of telomeric DNA to compensate for the progressive reduction in its length due to incomplete replication of chromosome ends, which in human somatic cells leads to cell cycle arrest upon shortening of telomeres to a critical length. To examine the possible involvement of telomerase in metabolism of plant genetic material, we used cells of Nicotiana tabacum strain TBY-2, a stable long-term culture which has kept a constant pattern of restriction fragments from chromosome termini during its 6 month period of cultivation in our laboratory. In a direct assay for telomerase, a 5' end-labeled plant telomeric oligonucleotide 5' (TTTAGGG)(3')6 was elongated in a TBY-2 cell extract, showing a pausing pattern which is a characteristic feature of telomerases from other organisms. The elongation was inhibited by RNase A pretreatment of the extract. We conclude that plant cells possess telomerase which is used for maintenance of their telomeres.