Identification and functional characterization of a novel selenocysteine methyltransferase from Brassica juncea L

Organic selenium (Se), specifically Se-methylselenocysteine (MeSeCys), has demonstrated potential effects in human disease prevention including cancer and the emerging ameliorating effect on Alzheimer's disease. In plants, selenocysteine methyltransferase (SMT) is the key enzyme responsible for MeSeCys formation. In this study, we first isolated a novel SMT gene, designated as BjSMT, from the genome of a known Se accumulator, Brassica juncea L. BjSMT shows high sequence (amino acid) similarity with its orthologues from Brassica napus and Brassica oleracea var. oleracea, which can use homocysteine (HoCys) and selenocysteine (SeCys) as substrates. Similar to its closest homologues, BjSMT also possesses a conserved Thr187 which is involved in transferring a methyl group to HoCys by donating a hydrogen bond, suggesting that BjSMT can methylate both HoCys and SeCys substrates. Using quantitative real-time PCR (qRT-PCR) technology and BjSMT-transformed tobacco (Nicotiana tabacum) plants, we observed how BjSMT responds to selenite [Se(IV)] and selenate [Se(VI)] stress in B. juncea, and how the phenotypes of BjSMT-overexpressing tobacco cultured under selenite stress are affected. BjSMT expression was nearly undetectable in the B. juncea plant without Se exposure, but in the plant leaves it can be rapidly and significantly up-regulated upon a low level of selenite stress, and enormously up-regulated upon selenate treatment. Overexpression of BjSMT in tobacco substantially enhanced tolerance to selenite stress manifested as significantly higher fresh weight, plant height, and chlorophyll content than control plants. In addition, transgenic plants exhibited low glutathione peroxidase activity in response to a lower dose of selenite stress (with a higher dose of selenite stress resulting in a high activity response) compared with the controls. Importantly, the BjSMT-transformed tobacco plants accumulated a high level of Se upon selenite stress, and the plants also had significantly increased MeSeCys production potential in their leaves. This first study of B. juncea SMT demonstrates its potential applications in crop MeSeCys biofortification and phytoremediation of Se pollution.