Promoter structure and activity of type 1 rice metallothionein-like gene

Methods for metal chelation in plant homeostasis: Review

Metal uptake, transport and storage in plants depend on specialized ligands with closely related functions. Individual studies differing by species, nutrient availability, tissue type, etc. are not comprehensive enough to understand plant metal homeostasis in its entirety. A thorough review is required that distinguishes the role of ligands directly involved in chelation from the myriad of plant responses to general stress. Distinguishing between the functions of metal chelating compounds is the primary focus of this review; reactive oxygen species mediation and other aspects of metal homeostasis are also discussed. High molecular weight ligands (polysaccharides, phytochelatin, metallothionein), low molecular weight ligands (nicotianamine, histidine, secondary metabolites) and select studies which demonstrate the complex nature of plant metal homeostasis are explored.

Isolation and characterization of three cadmium-inducible promoters from Oryza sativa

Cadmium (Cd) is an important soil pollutant. Developing genetically engineered crops might be a feasible strategy for Cd decontamination and damage prevention. Both genes and promoters are critical for the effective construction of genetically modified plants. Although many functional genes for Cd tolerance and accumulation have been identified, few reports have focused on plant Cd-inducible promoters. Here, we identified three Cd-inducible genes in the rice genome: two tau class glutathione S-transferase (GSTU) genes, OsGSTU5 and OsGSTU37, and an HSP20/alpha crystallin family protein gene, OsHSP18.6. The promoter sequences were isolated and tested in transgenic rice lines using a GUSplus reporter gene. All of the promoters exhibited low background expression under normal conditions and could be strongly induced by Cd stress. Although their strength was comparable to that of the constitutive OsACTIN promoter under Cd stress, their time-dependent expression patterns under both short- and long-term Cd exposure were markedly different. The responses of the three promoters to other heavy metals were also examined. Furthermore, heavy metal-responsive cis elements in the promoters were computationally analyzed, and regions determining the Cd stress response were analyzed using a series of truncations. Our results indicate that the three Cd-inducible rice promoters described herein could potentially be used in applications aimed at improving heavy metal tolerance in crops or for the bio-monitoring of environmental contamination.

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 β-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.

The oil palm metallothionein promoter contains a novel AGTTAGG motif conferring its fruit-specific expression and is inducible by abiotic factors

The 1,053-bp promoter of the oil palm metallothionein gene (so-called MSP1) and its 5' deletions were fused to the GUS reporter gene, and analysed in transiently transformed oil palm tissues. The full length promoter showed sevenfold higher activity in the mesocarp than in leaves and 1.5-fold more activity than the CaMV35S promoter in the mesocarp. The 1,053-bp region containing the 5' untranslated region (UTR) gave the highest activity in the mesocarp, while the 148-bp region was required for minimal promoter activity. Two positive regulatory regions were identified at nucleotides (nt) -953 to -619 and -420 to -256 regions. Fine-tune deletion of the -619 to -420 nt region led to the identification of a 21-bp negative regulatory sequence in the -598 to -577 nt region, which is involved in mesocarp-specific expression. Gel mobility shift assay revealed a strong interaction of the leaf nuclear extract with the 21-bp region. An AGTTAGG core-sequence within this region was identified as a novel negative regulatory element controlling fruit-specificity of the MSP1 promoter. Abscisic acid (ABA) and copper (Cu(2+)) induced the activity of the promoter and its 5' deletions more effectively than methyl jasmonate (MeJa) and ethylene. In the mesocarp, the full length promoter showed stronger inducibility in response to ABA and Cu(2+) than its 5' deletions, while in leaves, the -420 nt fragment was the most inducible by ABA and Cu(2+). These results suggest that the MSP1 promoter and its regulatory regions are potentially useful for engineering fruit-specific and inducible gene expression in oil palm.

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 MT (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 MT 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.