The Role of Glutathione in the Uptake and Metabolism of Sulfur and Selenium. In: Grill D, Tausz M, De Kok LJ, editors. Significance of Glutathione to Plant Adaptation to the Environment. Dordrecht: Springer Netherlands; 2001. pp. 57-99. [DOI: 10.1007/0-306-47644-4_4]

The fascinating facets of plant selenium accumulation - biochemistry, physiology, evolution and ecology

Contents 1582 I. 1582 II. 1583 III. 1588 IV. 1590 V. 1592 1592 References 1592 SUMMARY: The importance of selenium (Se) for medicine, industry and the environment is increasingly apparent. Se is essential for many species, including humans, but toxic at elevated concentrations. Plant Se accumulation and volatilization may be applied in crop biofortification and phytoremediation. Topics covered here include beneficial and toxic effects of Se on plants, mechanisms of Se accumulation and tolerance in plants and algae, Se hyperaccumulation, and ecological and evolutionary aspects of these processes. Plant species differ in the concentration and forms of Se accumulated, Se partitioning at the whole-plant and tissue levels, and the capacity to distinguish Se from sulfur. Mechanisms of Se hyperaccumulation and its adaptive significance appear to involve constitutive up-regulation of sulfate/selenate uptake and assimilation, associated with elevated concentrations of defense-related hormones. Hyperaccumulation has evolved independently in at least three plant families, probably as an elemental defense mechanism and perhaps mediating elemental allelopathy. Elevated plant Se protects plants from generalist herbivores and pathogens, but also gives rise to the evolution of Se-resistant specialists. Plant Se accumulation affects ecological interactions with herbivores, pollinators, neighboring plants, and microbes. Hyperaccumulation tends to negatively affect Se-sensitive ecological partners while facilitating Se-resistant partners, potentially affecting species composition and Se cycling in seleniferous ecosystems.

Comparative effects of selenate and selenite on selenium accumulation, morphophysiology, and glutathione synthesis in Ulva australis

The capacity of Ulva australis Areschoug to tolerate and accumulate selenium (Se) supplied in the form of selenate or selenite was investigated. The macroalga was provided for 3 and 7 days with concentrations of selenate (Na2SeO4) or selenite (Na2SeO3) ranging from 0 to 400 μM. U. australis exhibited the highest ability to accumulate selenium when fed with 100 μM selenate and 200 μM selenite after 7 days, and accumulation values were respectively 25 and 36 ppm Se. At the same concentrations, stimulation of the synthesis of chlorophylls and carotenoids was observed. Elevated doses of selenate or selenite decreased Se accumulation inside algal cells, perhaps through repression of membrane transporters. This effect was more pronounced in thalli cultivated with selenate. There were no morphological and ultrastructural alterations in thalli exposed to Se. However, selenite induced the increase of the oxidized fraction of glutathione (GSSG), perhaps because of its capacity to bind the thiol group of reduced glutathione (GSH). In conclusion, this study highlights the capacity of U. australis to resist to very high concentrations of selenite and selenate, which are normally toxic to other organisms. Also, the lack of bioconcentration in U. australis indicates that this alga does not facilitate delivery of Se in the food chain and remains safe for consumption when it grows in water bodies contaminated with Se. Its potential for the removal of excess Se from water bodies appears limited.

Exogenous selenium pretreatment protects rapeseed seedlings from cadmium-induced oxidative stress by upregulating antioxidant defense and methylglyoxal detoxification systems

The protective effect of selenium (Se) on antioxidant defense and methylglyoxal (MG) detoxification systems was investigated in leaves of rapeseed (Brassica napus cv. BINA sharisha 3) seedlings under cadmium (Cd)-induced oxidative stress. Two sets of 11-day-old seedlings were pretreated with both 50 and 100 μM Se (Na(2)SeO(4), sodium selenate) for 24 h. Two concentrations of CdCl(2) (0.5 and 1.0 mM) were imposed separately or on the Se-pretreated seedlings, which were grown for another 48 h. Cadmium stress at any levels resulted in the substantial increase in malondialdehyde and H(2)O(2) levels. The ascorbate (AsA) content of the seedlings decreased significantly upon exposure to Cd stress. The amount of reduced glutathione (GSH) increased only at 0.5 mM CdCl(2), while glutathione disulfide (GSSG) increased at any level of Cd, with concomitant decrease in GSH/GSSG ratio. The activities of ascorbate peroxidase (APX) and glutathione S-transferase (GST) increased significantly with increased concentration of Cd (both at 0.5 and 1.0 mM CdCl(2)), while the activities of glutathione reductase (GR) and glutathione peroxidase (GPX) increased only at moderate stress (0.5 mM CdCl(2)) and then decreased at 1.0 mM severe stress (1.0 mM CdCl(2)). Monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II) activities decreased upon exposure to any levels of Cd. Selenium pretreatment had little effect on the nonenzymatic and enzymatic components of seedlings grown under normal conditions; i.e., they slightly increased the GSH content and the activities of APX, GR, GST, and GPX. On the other hand, Se pretreatment of seedlings under Cd-induced stress showed a synergistic effect; it increased the AsA and GSH contents, the GSH/GSSG ratio, and the activities of APX, MDHAR, DHAR, GR, GPX, CAT, Gly I, and Gly II which ultimately reduced the MDA and H(2)O(2) levels. However, in most cases, pretreatment with 50 μM Se showed better results compared to pretreatment with 100 μM Se. The results indicate that the exogenous application of Se at low concentrations increases the tolerance of plants to Cd-induced oxidative damage by enhancing their antioxidant defense and MG detoxification systems.

Selenium pretreatment upregulates the antioxidant defense and methylglyoxal detoxification system and confers enhanced tolerance to drought stress in rapeseed seedlings

In order to observe the possible regulatory role of selenium (Se) in relation to the changes in ascorbate (AsA) glutathione (GSH) levels and to the activities of antioxidant and glyoxalase pathway enzymes, rapeseed (Brassica napus) seedlings were grown in Petri dishes. A set of 10-day-old seedlings was pretreated with 25 μM Se (Sodium selenate) for 48 h. Two levels of drought stress (10% and 20% PEG) were imposed separately as well as on Se-pretreated seedlings, which were grown for another 48 h. Drought stress, at any level, caused a significant increase in GSH and glutathione disulfide (GSSG) content; however, the AsA content increased only under mild stress. The activity of ascorbate peroxidase (APX) was not affected by drought stress. The monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR) activity increased only under mild stress (10% PEG). The activity of dehydroascorbate reductase (DHAR), glutathione S-transferase (GST), glutathione peroxidase (GPX), and glyoxalase I (Gly I) activity significantly increased under any level of drought stress, while catalase (CAT) and glyoxalase II (Gly II) activity decreased. A sharp increase in hydrogen peroxide (H(2)O(2)) and lipid peroxidation (MDA content) was induced by drought stress. On the other hand, Se-pretreated seedlings exposed to drought stress showed a rise in AsA and GSH content, maintained a high GSH/GSSG ratio, and evidenced increased activities of APX, DHAR, MDHAR, GR, GST, GPX, CAT, Gly I, and Gly II as compared with the drought-stressed plants without Se. These seedlings showed a concomitant decrease in GSSG content, H(2)O(2), and the level of lipid peroxidation. The results indicate that the exogenous application of Se increased the tolerance of the plants to drought-induced oxidative damage by enhancing their antioxidant defense and methylglyoxal detoxification systems.

Selenium-induced up-regulation of the antioxidant defense and methylglyoxal detoxification system reduces salinity-induced damage in rapeseed seedlings

The present study investigates the regulatory role of exogenous selenium (Se) in the antioxidant defense and methylglyoxal (MG) detoxification systems in rapeseed seedlings exposed to salt stress. Twelve-day-old seedlings, grown in Petri dishes, were supplemented with selenium (25 μM Na(2)SeO(4)) and salt (100 and 200 mM NaCl) separately and in combination, and further grown for 48 h. The ascorbate (AsA) content of the seedlings decreased significantly with increased salt stress. The amount of reduced glutathione (GSH) and glutathione disulfide (GSSG) increased with an increase in the level of salt stress, while the GSH/GSSG ratio decreased. In addition, the ascorbate peroxidase (APX) and glutathione S-transferase (GST) activity increased significantly with increased salt concentration (both at 100 and 200 mM NaCl), while glutathione peroxidase (GPX) activity increased only at moderate salt stress (100 mM NaCl). Glutathione reductase (GR) activity remained unchanged at 100 mM NaCl, while it was decreased under severe (200 mM NaCl) salt stress. Monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II) activities decreased upon the imposition of salt stress, whereas a sharp decrease of these activities was observed under severe salt stress (200 mM NaCl). Concomitant increases in the levels of H(2)O(2) and lipid peroxidation (MDA) were also measured. Exogenous Se treatment alone had little effect on the non-enzymatic and enzymatic components. However, further investigation revealed that Se treatment had a synergistic effect: in salt-stressed seedlings, it increased the AsA and GSH contents; GSH/GSSG ratio; and the activities of APX, MDHAR, DHAR, GR, GST, GPX, CAT, Gly I, and Gly II. As a result, addition of Se in salt-stressed seedlings led to a reduction in the levels of H(2)O(2) and MDA as compared to salt stress alone. These results suggest that the exogenous application of Se rendered the plants more tolerant to salt stress-induced oxidative damage by enhancing their antioxidant defense and MG detoxification systems.

Effects of nitrate supply site on selenite uptake by rice roots

Selenite uptake by rice roots is regulated by many factors. The goal of this paper was to study the effects of the nitrate supply site on selenite uptake by rice roots. Using the excised-root experiment system, we found that, due to anion-anion interactions, nitrate can partly block the selenite uptake by rice roots by local action (a corporate supply of nitrate and selenite in the same portion). Using a split-root experiment system (whole-plant level), we found that (1) nitrate can also partly block selenite uptake by rice (roots and shoots) by local action; and that (2) under long-distance action (separate supply of nitrate and selenite in two different portions of the root), nitrate can partly facilitate selenite uptake by rice (roots and shoots), probably by regulating root glutathione content. Thus, our results suggest that nitrate can inhibit or facilitate selenite uptake by rice roots depending on the nitrate supply site.