Characterization of a Novel Rice Metallothionein Gene Promoter: Its Tissue Specificity and Heavy Metal Responsiveness

The rice (Oryza sativa L.) metallothionein gene OsMT-I-4b has previously been identified as a type I MT gene. To elucidate the regulatory mechanism involved in its tissue specificity and abiotic induction, we isolated a 1 730 bp fragment of the OsMT-I-4b promoter region. Histochemical beta-glucuronidase (GUS) staining indicated a precise spacial and temporal expression pattern in transgenic Arabidopsis. Higher GUS activity was detected in the roots and the buds of flower stigmas, and relatively lower GUS staining in the shoots was restricted to the trichomes and hydathodes of leaves. No activity was observed in the stems and seeds. Additionally, in the root of transgenic plants, the promoter activity was highly upregulated by various environmental signals, such as abscisic acid, drought, dark, and heavy metals including Cu2+, Zn2+, Pb2+ and Al3+. Slight induction was observed in transgenic seedlings under salinity stress, or when treated with Co2+ and Cd2+. Promoter analysis of 5'-deletions revealed that the region -583/-1 was sufficient to drive strong GUS expression in the roots but not in the shoots. Furthermore, deletion analysis indicated important promoter regions containing different metal-responsive cis-elements that were responsible for responding to different heavy metals. Collectively, these findings provided important insight into the transcriptional regulation mechanisms of the OsMT-I-4b promoter, and the results also gave us some implications for the potential application of this promoter in plant genetic engineering.