Three Brassica rapa metallothionein genes are differentially regulated under various stress conditions

The expression profiles of three Brassica rapa metallothionein genes (BrMT 1-3) were determined in 7-day-old seedlings exposed to various exogenous factors including plant hormones, heavy metals and abiotic stresses. BrMT1, BrMT2, and BrMT3 were representatives of MT gene type 1, type 2, and type 3, respectively, according to their cysteine alignment. BrMT2 showed a relatively higher basal expression level compared to BrMT1 and BrMT3 under normal conditions. The BrMT1 transcript was markedly increased by various factors including ethephon, polyethylene glycol and hydrogen peroxide, with no down-regulation evident. On the contrary, BrMT2 expression was down-regulated by abscisic acid, salicylic acid, and methyl jasmonate. Heavy metals did not increase BrMT2 expression. BrMT3 expression was only marginally and non-significantly up- and down-regulated by the stress conditions tested. Promoter regions of BrMT1 and BrMT2 display different cis-acting elements supporting the different responses of both genes against various stresses. The results demonstrate the differential regulation of BrMT1-3 by various plant exogenous factors, and indicate the utility of the BrMT1 promoter as a multiple stress inducible promoter.