Sulfur-related alleviation mechanisms of selenium toxicity in plants

Abstract

Selenium and sulfur present a strict relationship in plants and soils, affecting their uptake and accumulation. Thus, addressing this interaction is important to understand selenium nutrition and toxicity in plants, which could influence crop nutrition and production. Here, we aimed to evaluate the impact of selenate exposure on rice plants grown under different sulfate supplies at tillering and grain ripening phases. We studied the effects of varying selenate and sulfate doses on rice plants grown hydroponically or in soils with varying contents of clay. We also performed selenate sorption assays to evaluate the influence of clay content and sulfur doses on selenate adsorption and desorption in soils. Sulfate supply alleviated selenate toxicity in both short-term and long-term experiments. Selenate treatment up-regulated the expression of sulfate transporters, leading to increased sulfur contents in rice seedlings, which enhanced the antioxidant system and alleviated selenate toxicity. However, this enhanced mechanism is absent in seedlings grown under a low sulfur supply. Moreover, soil clay contents strongly influenced selenate availability. A high clay content promoted a higher selenate adsorption capacity, resulting in lower selenium contents in shoots and grains and the absence of toxicity symptoms. In contrast, a low clay content increased the availability of selenate added, which can favor the biofortification of crops. However, high selenate doses caused growth and yield impairment in rice cultivated in low-clay-content soil, with higher selenium concentrations in shoots and grains, potentially increasing the risk of selenate toxicity for humans and animals. In addition, we studied the influence of selenium treatment on sulfur and selenium metabolisms and plant growth of Arabidopsis and broccoli with different concentrations of glutathione, a key molecule of sulfur metabolism and plant detoxification. The selenate treatment decreased the glutathione contents in plant tissues. The pad2-1 plants (a glutathione-deficient Arabidopsis mutant) exhibited lower selenate tolerance and higher sulfate transporters (AtSULTR1;1 and AtSULTR1;2) gene expression on roots compared to Arabidopsis wild-type (WT), even exhibiting similar selenium and sulfur concentrations on shoots and roots. However, the reduced glutathione (GSH) supply alleviated the selenate toxicity and partially inhibited the sulfate transporters expression, indicating that both selenate effects are directly linked to glutathione metabolism. Conversely, the selenite did not present clear relation with glutathione or BoSULTR1;1 and BoSULTR1;2 in broccoli plants, while selenate decreased the glutathione contents and inhibited the growth of broccoli with lower glutathione concentrations severely.