Effects of Selenium Supplementation on Glucosinolate Biosynthesis in Broccoli

Selenium (Se)-enriched
broccoli has health-beneficial selenium-containing
compounds, but it may contain reduced amounts of chemopreventive glucosinolates.
To investigate the basis by which Se treatment influences glucosinolate
levels, we treated two broccoli cultivars with 25 μM Na2SeO4. We found that Se supplementation suppressed
the accumulation of total glucosinolates, particularly glucoraphanin,
the direct precursor of a potent anticancer compound, in broccoli
florets and leaves. We showed that the suppression was not associated
with plant sulfur nutrition. The levels of the glucosinolate precursors
methionine and phenylalanine as well as the expression of genes involved
in glucosinolate biosynthesis were greatly decreased following Se
supplementation. Comparative proteomic analysis identified proteins
in multiple metabolic and cellular processes that were greatly affected
and detected an enzyme affecting methionine biosynthesis that was
reduced in the Se-biofortified broccoli. These results indicate that
Se-conferred glucosinolate reduction is associated with negative effects
on precursor amino acid biosynthesis and glucosinolate-biosynthetic-gene
expression and provide information for a better understanding of glucosinolate
accumulation in response to Se supplementation in broccoli.