Brassica juncea and the Se-hyperaccumulator Stanleya pinnata exhibit a different pattern of chromium and selenium accumulation and distribution while activating distinct oxidative stress-response signatures.

Soils high in chromium and selenium exist in some countries, like China, India and the US. In the forms of chromate and selenate, these elements can compete during uptake by plants and lead to secondary effects on the absorption of the essential nutrient sulfur. In this study, we evaluated the potential of Brassica juncea and the Se-hyperaccumulator Stanleya pinnata to take-up and store chromium and selenium when applied individually or jointly, the effect on sulfur content, and the plant antioxidant responses. The aim is to advise the best use of these species in phytotechnologies. Plants were grown hydroponically with 50 µM chromate, 50 µM selenate and equimolar concentrations of both elements (50 µM chromate + 50 µM selenate). Our results suggest that B. juncea and S. pinnata possess transport systems with different affinity for chromate and selenate. The joint application of chromate and selenate restricted the accumulation of both elements, but the reduction of selenate uptake by chromate was more evident in B. juncea. On the other hand, selenate decreased chromium accumulation in B. juncea, whereas in S. pinnata such effect was evident only in roots. B. juncea plants stored more chromium and selenium than S. pinnata due to the higher biomass produced, but less selenium when treated with both elements. Chromate and selenate decreased sulfur accumulation in both species, but B. juncea was more sensitive to their toxicity when applied individually, as revealed by increased lipid peroxidation, hydrogen peroxide content in roots and antioxidant enzyme activity. This species can still be efficient for chromium and selenium phytoextraction as these elements in soil are less available than in hydroponics. In soils high in both elements, or low in selenium, S. pinnata is preferred for selenium phytoextraction and the biomass could be used for crop biofortification due its negligible chromium content.

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.

Accumulation of selenium in Ulva sp. and effects on morphology, ultrastructure and antioxidant enzymes and metabolites.

The impact of selenium (Se) on Ulva sp., a green macroalga naturally growing in the Venice Lagoon, was investigated. The alga was provided for 10 days with concentrations of selenate (Na(2)SeO(4)) ranging from 0 to 100 µM. Se accumulation in the algal biomass was linearly related to the selenate dose and this relationship was not affected by the high sulfate concentration measured in the seawater. The amount of Se measured in the alga was always relatively low and not hazardous to algal consumers. However, Se induced the formation of hydrogen peroxide (H(2)O(2)) in Ulva sp. and, as a result, the activity of antioxidant enzymes (superoxide dismutase, SOD, and catalase, CAT) and the amount of antioxidant metabolites (phenols, flavonoids and carotenoids) increased, even when selenate was supplied to the macroalga at low concentration (2.5 µM). This indicated that different components of the antioxidant defence system played a pivotal role in overcoming oxidative damage by Se in the macroalga, and explained the lack of morphological and ultrastructural alterations in Ulva sp. exposed to selenate.