# An Integrated Metabolomics Study of Glucosinolate Metabolism in Different Brassicaceae Genera
## 1 Introduction  
Glucosinolates are unique and prevalent secondary metabolites found in the order Brassicales [1]. This order includes the economically important family Brassicaceae, consisting of many common vegetables such as broccoli, cabbage, Chinese cabbage, radiishes, watercress, and rocket [1]. Glucosinolates act as precursors for compounds with anti-carcinogenic properties [2]. Epidemiological studies show that a diet rich in broccoli and other cruciferous vegetables reduces the risk of cancer [3; 4; 5; 6; 7]. In plants, glucosinolate breakdown pathways are known to have protective roles in both abiotic and biotic stress. During cell breakdown, endogenous \(\beta\)-glucosidase, also called myosinase enzymes, get mixed with the glucosinolates in the cell, hydrolyzing the thioglucidic bond [8]. The reaction can form isothiyanates, which can be converted into nitriles, epithionitriles, or organic thiocyanates. Most of these compounds can evaporate and enter intact cells because most of them are volatile and lipophilic [9]. Many Brassicaceae plants have been found to have differences in the composition and content of glucosinolate metabolites [10]. The exact role of these glucosinolate metabolites remains unknown. However, isothi cyanates specifically have been shown to be toxic to many organisms that may be harmful to plants, such as insects, microorganisms, and nematodes [11; 12; 13]. In aboveground plants, the roots are known to contain higher concentrations of a larger variety of glucosinolates than shoots [14]. The differences between different parts of the plant are, at least partly, due to the effectiveness of certain types of breakdown products in the soil. Constant high pathogen pressure on roots has also been suggested as a factor that could explain why glucosinolate levels in roots are reported to be more stable than in shoots [14]. Recent studies suggest that glucosinolates may have certain roles within undamaged cells, which could be linked to signaling and defense against pathogens [9; 15]. Despite the complexity caused by genome duplication, investigations have started to shed light on the genetic variation and gene function in glucosinolate metabolism [16; 17; 18; 19]. Combining this information with metabolomics could prove to be important in understanding the regulation of the biosynthetic pathways and complex roles of glucosinolates and their breakdown products in plants. Understanding of glucosinolate metabolism can lead to significant improvements in breeding vegetables that have beneficial qualities [10]. An example of using traditional breeding to achieve this is the Beneforte broccoli, where segments of the genome were introressed from a wild variety of Brassica villosa [20]. This led to 2.5-3 times glucoraphanin content due to the enhancement of sulfate assimilation and modifications in partitioning among sulfur-containing metabolites. In the future, systematic approaches in breeding that combine understanding of the metabolic pathways, availability of precursors, and the regulation of biosynthesis will benefit from metabolomics studies that generated large datasets across the complex network of glucosinolate metabolism. A large number of studies have been conducted on glucosinolates in Brassicaceae vegetables with the aim of ultimately introducing more beneficial compounds into the diet. However, differences in extraction and analysis methods as well as experimental protocols can lead to data incomparability among different experiments [21]. Some studies have also shown that growth, harvesting, and storage conditions can affect the metabolite content [22; 23; 24; 25; 26]. More comprehensive information is therefore needed across different Brassica vegetables when considering consumption and diet.