## 4 Discussion
### Effect of Se Supplementation on Surforaphane Content in Broccoli  
Sulforaphane level after autolysis is largely dependent on the precursors of 4 MSOB content and MYR activity [66]. The sulforaphane content was undetected in roots, largely attributed to the extremely low activity of MYRs in broccoli roots [67]. The study showed that sulforaphane content, MYR activity, and _MYR_ transcript level in fresh broccoli leaves were not affected by Se treatment. However, the opposite results occurred in other studies, which showed that Se supplementation via foliar spraying activates the MYR activity, ultimately increasing sulforaphane levels in broccoli spruts and florets [68,69]. This discrepancy could be caused by the different effects of spraying versus hydroponic conditions, growth periods, and Se forms or concentrations. Interestingly, the study showed that Se addition via hydroponics significantly enhances the MYR activity and the _MYR_ transcript level in broccoli roots. On the other hand, some studies reported that the ESP enzyme could catalyze the 4 MSOB into sulforaphane nitrile, reducing sulforaphane levels. Unlike sulforaphane, sulforaphane nitrile does not possess anti-cancer prosperities [70]. Results revealed that sulforaphane content in young leaves was not substantially changed under the Se treatment, probably due to the comparatively higher _ESP_ expression level than the control conditions. A similar trend was observed in broccoli sprouts under Ca\({}^{2+}\) channel blocker stress [48]. The conversion efficiency from the precursor compound 4 MSOB to sulforaphane was low, and the sulforaphane was easily degraded during the extraction process [71], which probably resulted in an upward trend of sulforaphane level that does not look as large as the 4 MSOB content. For humans ingestion, consuming broccoli with more of the 4 MSOB content will produce more sulforaphane with anti-cancer activity because humans can completely hydrolyze the 4 MSOB compound to sulforaphane in the body [72].  
Based on these results, we propose a simple working model of Se-induced regulation of GSLs and 4 MSOB accumulations in broccoli tissues, a better understanding of the molecular mechanism of Se in regulating GSL, and 4 MSOB biosynthesis (Figure 7). When broccoli roots were exposed to Se conditions, the GSLs contents were remarkably increased in broccoli roots, possibly due to the increase of total sulfur content and the up-regulated expression of GSLs biosynthesis genes. The 4 MSOB concentration in fresh broccoli leaves was enhanced significantly, increased by 67% with Se supplementation because of the comparatively higher transcript levels of Met-derived GSLs biosynthesis genes than the control conditions. Another possibility was that exogenous Se-induced up-regulated expressions of \(GTRI/2\) genes in broccoli roots and senescent leaves promote the translocation of the 4 MSOB from other tissues to young leaves. The sulforaphane level in fresh leaves of broccoli was not significantly increased by Se treatment, possibly because Se-induced up-regulated expression of \(ESP\) gene in fresh leaves undermined the conversion efficiency from the precursor compound 4 MSOB to sulforaphane.