Telomerase is a highly regulated ribonucleoprotein complex that stabilizes eukaryotic genomes by replenishing telomeric repeats on chromosome ends. Defects in telomerase RNP components involving the catalytic subunit TERT or the RNA template TER lead to stem cell-related diseases such as dyskeratosis congenita and idiopathic pulmonary fibrosis, while inappropriate telomerase expression is a rate-limiting step in carcinogenesis. In this study we report the discovery of a novel negative regulatory mechanism for telomerase that stems from duplication and diversification of key components of the telomerase RNP in the flowering plant Arabidopsis thaliana. We show that Arabidopsis encodes three distinct TERs: TER1, TER2 and a processed form of TER2 termed TER2S. Although all three RNAs can serve as templates for telomerase in vitro, in vivo they have different expression patterns, assemble into distinct RNPs with different protein binding partners, and play opposing roles in telomere maintenance. The TER1 RNP is analogous to the telomerase enzyme previously described in other eukaryotes, but the TER2 RNP is a negative regulator of telomerase activity and telomere maintenance in vivo. Furthermore, we demonstrate that the Protection Of Telomeres (POT1) paralogs in Arabidopsis (POT1a, POT1b and POT1c) are novel TER binding proteins. This finding is striking because in yeast and vertebrates, POT1 is an essential component of the telomere capping complex and functions to distinguish the chromosome terminus from a double-strand break. Thus, our data argue that Arabidopsis POT1 proteins have migrated off of the chromosome terminus and onto the telomerase RNP, indicating that duplication and diversification of Arabidopsis telomerase may be the end result of the co-evolution of the TER and POT1 RNP components. Additionally, given the dire consequences of misregulating telomerase in human cells, our discovery of a novel negative regulatory mechanism for telomerase in plants strongly suggests that additional modes of telomerase control remain to be elucidated in vertebrates.

The adverse effect of Cu on growth is apparent from the reduction in root length of the Cu-treated Arabidopsis roots. Arabidopsis seems to be more resistant to Cu in comparison with that of soybean in terms of root growth. The increase of the levels of H2O2 was observed in Cu-treated Arabidopsis roots. The lignin biosynthesis related enzymes, PODs and laccases were enhanced during the Cu treatments. The lignin contents slightly increased in Cu-treated Arabidopsis roots. To our surprise, the Arabidopsis can be tolerant to high concentration of Cu (200 µM), because only part of high levels of H2O2 accumulated in Cu-treated tissues are used by PODs to synthesize the lignin.

The adverse effect of Cd on growth is apparent from the reduction in root length of the Cd-treated Arabidopsis thaliana roots. The increase of the levels of H2O2 was observed in Cd-treated A. thaliana roots. The lignin biosynthesis related enzymes, POXs and laccases were enhanced during the Cd treatments. The lignin contents slightly increased in Cd-treated A. thaliana roots¡]48 h¡^. The A. thaliana can be tolerant to high concentration of Cd (500 µM), and only part of high levels of H2O2 accumulated in Cd-treated tissues are used by POXs to synthesize the lignin.

The adverse effect of Zn on growth is apparent from the reduction in root length of the Zn-treated Arabidopsis roots. The levels of H2O2 were increased rapidly in Zn-treated Arabidopsis roots. The lignin biosynthesis realated enzymes, peroxidases and laccases were enhanced during the Zn treatments. The lignin contents increased in Zn-treated Arabidopsis roots. Arabidopsis can be tolerant to high concentration of Zn (4 mM), because part of high levels of H2O2 accumulated in Zn-treated tissues are utilized by peroxidases to synthesize the lignin.

Telomerase is the key enzyme synthesizing telomeric DNA in most eukaryotic organisms. In mammals, telomerase expression is abundant in the germline cells but is undetectable in most other differentiated organs. Intensive studies of telomerase have focused on human cancerous cells, where over 90% of all cancerous tissues examined have telomerase activity. In wild-type Arabidopsis, telomerase expression is abundant in reproductive organs and dedifferentiated tissues such as flowers, siliques and calli but barely detectable in vegetative tissues (both rosette and cauline leaves). In this study, a biochemical screen strategy was developed for isolation of telomerase activating mutants in Arabidopsis thaliana. Through screening of Arabidopsis activation-tagged lines by a PCR-based TRAP assay, two tac (for telomerase activator) mutants were isolated. RT-PCR analysis of AtTERT expression revealed that different mechanisms are involved in alternating telomerase activity in tac1 and tac2. We cloned and characterized the TAC1 gene. TAC1 encodes a single zinc finger protein and acts synergistically with auxin to induce telomerase expression without altering cell cycles. Telomere length was unperturbed in the mutant, but other phenotypes, such as altered root development and the ability of cells to grow in culture without exogenous auxin, indicated that TAC1 not only is part of the previously reported link between auxin and telomerase expression, but also potentiates other classic responses to this phytohormone. DNA microarrays were used to analyze the expression profile of the tac1 mutant and revealed that several drought-induced genes were up-regulated 3 to 10 fold in the tac1-1D mutant. RT-PCR analysis further confirmed this up-regulation for five of these genes. Investigation of root growth also indicated that tac1-1D roots were ~20% longer relative to wild-type. Further experiments demonstrated that over-expression of TAC1 does confer drought tolerance, but not salt tolerance. In addition, our preliminary result showed that treatment with a low concentration of IAA could induce drought tolerance in wild-type Arabidopsis. Although plants with constitutive expression of telomerase have no practical utility, the ability of TAC1 to confer drought tolerance could have significant agricultural applications.

Los glucosinolatos son compuestos cianogénicos naturales que se degradan por la acción de tioglucosidasas endógenas (mirosinasas) para dar lugar a isotiocianatos, tiocianatos y nitrilos; muchos de ellos han captado la atención por sus diversas propiedades biológicas, entre otras, como agentes preventores del cáncer, biopesticidas, antiafrodisiacos. Luego del secuenciamiento del genoma de Arabidopsis, se ha elucidado mejor la ruta de biosíntesis de los glucosinolatos, identificandose los primeros reguladores de la ruta, estudios de su función, así como relaciones evolutivas entre rutas parecidas. / Glucosinolates are natural cyanogenic compounds that are broken down by the action of endogenous tioglucosidases (mirosinases) to produce isothiocyanates, thiocyanate, and nitriles; many of them call the attention to researchers by diverse biological properties, such as cancer preventing, biopesticides, anti-aphrodisiacs. After the sequencing of Arabidopsis genome was completed, the glucosinolates biosynthetic process has been better elucidated, identifying the first regulators of the pathway, function research, and also the evolution relationship between similar pathways.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 231-256). Also available on the Internet.

Thesis (Ph. D.)--West Virginia University, 2009. / Title from document title page. Document formatted into pages; contains ix, 89 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 82-88).

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 151-164). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Missouri--Columbia, 2003. / "August 2003." Typescript. Vita. Includes bibliographical references (leaves 158-174). Also issued on the Internet.