## 4 Discussion  
### Se-Induced Total Sulfur and GSLs Accumulation in Broccoli Roots  
GSLs are a diverse group of nitrogen- and sulfur-rich specialized metabolites whose contents represent up to 30% of the total sulfur content of plant tissues [51]. Thus, GSLs concentrations may depend on the sulfur status of the entire plant [44]. In the study, the sulfur content in fresh leaves was significantly higher than in broccoli roots and senescent leaves under the control treatment. This result is in accordance with a previous study, which showed that the young growing tissues are the main sulfur sinks, and their sulfur content exhibits a downward trend after full expansion [52]. Furthermore, GSLs contents in fresh leaves were the highest, followed by that in broccoli roots and senescent leaves under the normal conditions. The same results were shown in Chinese cabbage [53]. Broccoli is a typical Selenium (Se) and Sulfur (S) accumulator, while both Se and S belong to chalcogen elements with similar chemical characteristics [54]. Presumably, Se supplementation could influence sulfur uptake and GSLs metabolism in plants. Based on results, exogenous Se treatment considerably increases the sulfur and GSLs contents in broccoli roots. This result is similar to Se addition in boosting the accumulation of total sulfur and GSLs contents in _Brassica oleracea_[38] and radish roots [41]. Accumulation of the GSLs contents is mainly dependent on the sulfur status of plants and transcriptional activity of GSLs biosynthesis genes [17; 34]. The study showed that the sulfur content in broccoli roots was greatly induced through activating _SULTRs_ expressions by Se treatment. However, the sulfur content of broccoli leaves was decreased under Se treatment. This result may be due to the substantial repression of _SULTR2.1_ expression in fresh leaves and senescent leaves by Se treatment. The role of _SULTR2.1_ is mainly responsible for the translocation of the sulfur from the roots to aboveground tissues [55; 56]. Consequently, exogenous Se treatment promotes the overaccumulation of sulfur and GSLs contents in broccoli roots, while probably inhibiting the partitioning and translocation of the sulfur from broccoli roots to the shoots. On the other hand, the activation of gene expression, such as _MAMI_, _BCAT4_, and _CYP9F1_, was the consequence of an increase in GSLs accumulation in the Se-treated cabbage [42]. Se treatment could promote GSLs production in broccoli florets by stimulating the transcriptional activity of _ST5b_ (_SOT18_) gene, which was involved in the Met-derived GSLs biosynthesis pathway [47]. In addition, the expression of _CYP79B2_ and _CYP83B1_ genes were induced by Se treatment. This led to an increase of Trp-derived  
Figure 6: Effect of Se on the genes expressions related to GSLs transport and 4 MSOB degradation. Different letters indicate significant differences compared to roots under control.  
GSLs contents in radish seedlings, especially at 7 and 14 days after germination (DAG) [40]. Results revealed that the Se-induced increase of GSLs contents in broccoli roots was largely attributed to the up-regulation expressions of the GSLs biosynthesis genes.