The GUS reporter-aided analysis of the promoter activities of a rice metallothionein gene reveals different regulatory regions responsible for tissue-specific and inducible expression in transgenic Arabidopsis

To gain a better understanding of the regulatory mechanism of plant metallothionein (MT) genes, a chimeric expression unit consisting of the beta-glucuronidase (gusA) reporter gene under the control of a 1,324 bp fragment of the rice NIT (ricMT) promoter was introduced into Arabidopsis via Agrobacterium tumefaciens. The strongest histochemical staining for GUS activity was observed in the cotyledons and hypocotyls of the transgenic seedlings and in the stigma, filaments and anthers of young and mature flowers, and especially in the wounded tissues of transgenic plants. In contrast, a relatively low level of reporter gene expression was seen in the young roots of transgenic seedlings and no GUS activity was detected in the stems, seeds and leaves, but GUS activity was observed in cotyledons and the first two true leaves. Promoter analysis of 5' deletions further identified several important regions responsible for organ-specific expression including roots, flowers and wound induction, light and ABA, Cu and Zn responses. These findings demonstrate that a 1,324 bp fragment of the rice MT promoter performs a complicated transcriptional regulation with clearly functional regions in a model plant, and provide an important insight into the transcriptional regulation mechanisms that operate the temporal- and spatial-specific expression and stress responses of the rice NIT gene. These results suggest that the ricMT promoter and its functional regions are potentially useful in genetic engineering of plants to express the desired genes whose products are preferentially needed in roots, flowers and wound induction.