Changes in Sulforaphane and Selenocysteine Methyltransferase Transcript Levels in Broccoli Treated with Sodium Selenite

Broccoli (Brassica oleracea L. var. italica planck) has been highly valued because of its nutrient content, which has been attributed to both sulforaphane, a sulfur-containing compound, and selenium enrichment ability. The sulforaphane synthesis may be affected by Se/S metabolism because sulfate and selenate share the same initial pathway for uptake in plants. Selenocysteine methyltransferase (SMT) plays a critical role in the Se/S metabolism system. In the present work, we analyzed the SMT expression level and sulforaphane content in transgenic broccoli under the conditions of overexpression and RNAi knockdown of SMT under normal and selenium-stressed conditions. The relative expression value of SMT in the overexpression line is 13.93 % higher than that in the untransformed control. Interestingly, the sulforaphane content of the overexpression line was 14.09 % lower after selenium treatment, while that of the empty vector (pCAMBIA1301) transformed and untransformed plants was 59.04 and 66.56 % lower than that of the non-seleniumtreated, respectively. In the SMT-RNAi line, which has a relative expression value for SMT 15.60 % lower than that of the untransformed, we cannot detect sulforaphane after selenium treatment. These results showed that SMT plays a key role in sulforaphane synthesis in a selenium-rich environment. Specifically, overexpression of SMT decreases the negative effect of selenium on sulforaphane synthesis, while knockdown of SMT by RNAi enhances the negative effect.