## 5 Conclusions  
In the present study, the study showed that exogenous Se treatment exerts a significant effect on the GSLs and 4 MSOB accumulation in young broccoli plants, and this effect has tissue-specificity. The GSLs contents in broccoli roots increased 2 times after Se treatment. The 4 MSOB concentration and sulforaphane level in fresh leaves were increased by 67% and 30% with Se supplementation, respectively. Furthermore, some key genes involved in sulfate assimilation and transport, GSLs biosynthesis, and transport were induced strongly upon Se exposure. In conclusion, the exogenous supplementation of Se via root application mediates the biosynthesis and translocation of the GSLs and 4 MSOB compounds, and has different effects among the broccoli tissues.  
Figure 7: Model of Se-induced regulation of GSLs and 4 MSOB contents in broccoli roots and leaves. Se, 50 uM Na\({}_{2}\)SeO\({}_{3}\) treatment; S, total sulfur; AGS, Met-derived GSLs; IGS, Trp-derived GSLs; 4 MSOB, glucoraphanin; SF, sulforaphane; MYR, myrosinase activity. (We suggested the reader refer to the web version of this article to interpret the colors used in the figure legend.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12094101/s1](https://www.mdpi.com/article/10.3390/app12094101/s1)]([https://www.mdpi.com/article/10.3390/app12094101/s1](https://www.mdpi.com/article/10.3390/app12094101/s1)). Table S1: List of primer sequences of genes used for RT-qPCR analysis; Table S2: The desulfoGSLs identified in the broccoli plants using UPLC-MS analysis; Table S3: The recovery rate of Sinigrin (internal standard) in this study; Table S4: The calculation formula of the GSLs quantification; Figure S1: Simplified scheme of Met-derived Glucosinolates (GSLs) biosynthesis, Glucoraphanin hydrolysis and Trp-derived GSLs biosynthesis pathways. (A) represents the main biosynthetic pathway of Met-derived GSLs and glucoraphanin degradation pathways; (B) represents the main synthetic pathway of Trp-derived GSLs. Figure S2: Effect of incubation time on myrosinase activity in different tissues of broccoli. (A) represents the myrosinase activity after 30 min incubation time; (B) represents the myrosinase activity after 15 min incubation time.  
**Author Contributions:** [PERSON]. ([PERSON]) planned and executed the hydroponic experiments and performed with MYR activity measurement, HPLC, and RT-qPCR analysis with [PERSON] and [PERSON]; [PERSON]. ([PERSON]), [PERSON], [PERSON] ([PERSON]) and [PERSON] analyzed these results; [PERSON]. ([PERSON]), wrote the manuscript using the contributions of all authors. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Natural Science Foundation of China (Nos. 32072585, 32072568, 31772325, and 31902023), the Natural Science Foundation of Hunan province (Nos. 2021 JJ30324, 2021 JJ30345), the Natural Science Foundation of Changsha city (No. kq2007016), the Postgraduate Scientific Research Innovation Project of Hunan Province (CX20190503), and the Outstanding Youth Project of Educational Department of Hunan Province (20B275).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this study are presented in the article/Supplementary data. Further inquiries can be directed to the corresponding authors.  
**Conflicts of Interest:** The authors declare no conflict of interest.