Glucosinolate profile and specifier protein activity determine the glucosinolate hydrolysis product formation in kohlrabi (Brassica oleracea var. gongylodes) in a tissue-specific way

Glucosinolates, commonly found in Brassica vegetables, are hydrolyzed by myrosinase to form bioactive isothiocyanates unless specifier proteins redirect the degradation to less bioactive nitriles and epithionitriles. Here, the tissue-specific impact of specifier proteins on the outcome of glucosinolate hydrolysis in nine kohlrabi tissues was investigated. Glucosinolates and their hydrolysis product profiles, epithiospecifier protein and myrosinase activity and protein abundance patterns of key glucosinolate biosynthesis, transport and hydrolysis proteins were determined and correlated to the metabolites in the kohlrabi tissues. Leaf tissues were rich in glucoraphanin, while bulb tissues contained more glucoerucin. Moreover, a higher proportion of isothiocyanates was formed in leaf stalk, bulb peel, stem and root, tissues with relatively higher epithiospecifier modifier 1 abundance and a higher ratio of myrosinase activity to ESP activity. This study shows that tissue-specific glucosinolate hydrolysis is mediated by glucosinolate biosynthesis and profiles, myrosinase activity and specifier protein as well as modifier protein abundance.