Sistemas de defesa antioxidativo contra a toxicidade induzida por arsênio em aguapé (Eichhornia crassipes (Mart.) Solms)

• Ano de defesa: 2011
• Autor(a) principal: Reis, Iulla Naiff Rabelo de Souza
• Orientador(a): Oliveira, Juraci Alves de
• Banca de defesa: Cambraia, José , Mello, Jaime Wilson Vargas de , Ribeiro, Sylvia Therese Meyer
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Viçosa
• Programa de Pós-Graduação: Doutorado em Fisiologia Vegetal
• Departamento: Controle da maturação e senescência em órgãos perecíveis; Fisiologia molecular de plantas superiores
• País: BR
• Palavras-chave em Português: Fitorremediação; Macrófita aquática; Enzimas antioxidantes; Glutationa; Anatomia vegetal
• Palavras-chave em Inglês: Phytoremediation; Aquatic macrophyte; Antioxidant enzymes; Glutathione; Plant anatomy
• Área do conhecimento CNPq: CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::FITOTECNIA::FISIOLOGIA DE PLANTAS CULTIVADAS
• Link de acesso: http://locus.ufv.br/handle/123456789/984

Water hyacinth (Eichhornia crassipes (Mart.) Solms) was subjected to toxic concentrations of arsenic (As) to analyze the absorption capacity of this metalloid by this macrophyte and its toxic effects on growth and structural changes in leaves and roots, as well as studies of biochemical mechanisms related to metabolism and glutathione antioxidant involved in tolerance to As. In the first experiment, plants were grown in nutrient solution containing 0; 3; 7; 10 e 13 μM As, in the form of sodium arsenate for three days. In this period, there were symptoms of toxicity that changed according to the concentration of this metalloid in the nutrient solution. The As contents in roots increased with increasing concentration of As in the nutrient solution up to 9 μM, while in leaves the levels of this element increased linearly with As concentration. When treated with 3 μM As, the plants were able removing 97% of this element from the solution and, although there was reduction in removal efficiency at higher concentrations, it appears that even under 13 μM the plants were able to remove over 50% of this element, accumulating large amounts of As. The values of bioconcentration factor (BCF) were lower than 500, while the values of transfer factor (TF) in plants exposed to levels greater than 7 μM As, were greater than 1. This indicates that, as it increases the absorption of As by E. crassipes, there is increased translocation of this element to the shoot. The lower levels of chlorophyll and carotenoids found in E. crassipes probably compromised its photosynthetic capacity, justifying the reduction in relative growth rate of plants exposed to higher concentrations of As. Moreover, there was increase of anthocyanin content with increasing concentrations of As. In plants exposed to moderate concentrations of As, there was reduction in the levels of hydrogen peroxide (H2O2), indicating the efficient action of the enzymes involved in combating oxidative stress. The treatment with As increased the activity of superoxide dismutase (SOD) in leaves, up to 8 μM, which may represent part of a defense mechanism to oxidative stress caused by this element. The increase in activity of catalase (CAT) and a reduction in the levels of H2O2, in the roots of plants exposed to 6 μM of As, suggest that this enzyme operates effectively in the removal of this compound in this organ. As exposure inhibited the activity of peroxidase (POXs) in roots, however, leaves showed greater stimulation of enzyme activity. Thus, probably the reduction in levels of H2O2, observed in roots was due to the action of CAT, while POXs seem to have been responsible for the efficient removal of H2O2 in leaves. The activity of ascorbate peroxidase (APX) was stimulated in the presence of moderate concentrations of As in leaves, indicating that this enzyme should efficiently contribute to the removal of H2O2 produced in this organ. In the second experiment, plants were grown in nutrient solution containing or not 7 μM As. As exposure increased the activity of ATP sulfurylase (ATPS) in the roots, but did not alter the enzyme activity in leaves. On the other hand, As reduced activity of glutathione peroxidase (GSH-Px) and increased the activity of glutathione reductase (GR) both in leaves and roots. The glutathione sulfotransferase (GST) operates more effectively in the roots, where there was an increase in activity of this enzyme, indicating GSH conjugation to metalloid in these organs. The As treatment did not affect the activity of γ-glutamylcysteine synthetase (γ-ECS) in the roots, but stimulated the action of this enzyme in leaves. Furthermore, there was a significant reduction in total glutathione contents and increase of total thiols and non-protein thiols contents, which suggests a higher phytochelatin synthesis. The anatomical analysis revealed significant damage only in the apical region of the leaves, where initiate the toxic symptoms to As. Thus, the increase in activity of enzymes involved in glutathione metabolism is an important mechanism for tolerance and accumulation of As in E. crassipes. It appears that E. crassipes has a high absorption efficiency of As, and in concentrations up to 7 μM As, this plant has an increased activity of antioxidant enzymes contributing to the removal of excess reactive oxygen species arising from the toxicity of this element. Furthermore, the increased activity of enzymes involved in glutathione metabolism may represent an important mechanism for tolerance and accumulation of As in E. crassipes.