Zinc is an essential micronutrient for all living organisms. When facing a shortage in zinc supply, plants adapt by enhancing the zinc uptake capacity. The molecular regulators controlling this adaptation are not known. We present the identification of two closely related members of the Arabidopsis thaliana basic-region leucinezipper (bZIP) transcription factor gene family, bZIP19 and bZIP23, that regulate the adaptation to low zinc supply. They were identified, in a yeast-one-hybrid screening, to associate to promoter regions of the zinc deficiency-induced ZIP4 gene of the Zrt-and Irtrelated protein (ZIP) family of metal transporters. Although mutation of only one of the bZIP genes hardly affects plants, we show that the bzip19 bzip23 double mutant is hypersensitive to zinc deficiency. Unlike the wild type, the bzip19 bzip23 mutant is unable to induce the expression of a small set of genes that constitutes the primary response to zinc deficiency, comprising additional ZIP metal transporter genes. This set of target genes is characterized by the presence of one or more copies of a 10-bp imperfect palindrome in their promoter region, to which both bZIP proteins can bind. The bZIP19 and bZIP23 transcription factors, their target genes, and the characteristic cis zinc deficiency response elements they can bind to are conserved in higher plants. These findings are a significant step forward to unravel the molecular mechanism of zinc homeostasis in plants, allowing the improvement of zinc bio-fortification to alleviate human nutrition problems and phytoremediation strategies to clean contaminated soils.biofortification | zinc homeostasis regulation | plant nutrition | abiotic stress | adaptation Z inc is an essential cofactor for many transcription factors, protein interaction domains, and enzymes in plants (1). Plants are thought to control zinc homeostasis by using a tightly regulated network of zinc status sensors and signal transducers controlling the coordinated expression of proteins involved in zinc acquisition from soil, mobilization between organs and tissues, and sequestration within cellular compartments (2). Although candidate genes for the required proteins such as zinc transporters and chelator biosynthesizing enzymes are found, no regulator of such network was ever identified in plants.Zinc influx facilitators, members of the ZIP family of metal transporters, are thought to play a major role in zinc uptake in plants (3). In Arabidopsis there are 15 ZIP genes (4), with ZIP1, ZIP2, ZIP3, and IRT3 functionally characterized as zinc uptake transporters (3, 5). Gene expression analysis has shown that approximately half of the ZIP genes are induced in response to zinc deficiency (3,(5)(6)(7)(8). The ZIP4 gene in particular is strongly induced upon shortage in zinc supply (3,(6)(7)(8).We focused on the promoter of this zinc-deficiency-responsive gene as the starting point for unraveling the regulation of the zinc homeostasis network in plants. By using DNA fragments of the zincdeficiency-responsive Arabidopsi...

Summary• Here, Thlaspi caerulescens populations from contrasting soil types (serpentine, calamine and nonmetalliferous) were characterized with regard to tolerance, uptake and translocation of zinc (Zn), cadmium (Cd) and nickel (Ni) in hydroponic culture.• Results showed that high-level tolerances were apparently metal-specific and confined to the metals that were enriched at toxic levels in the soil at the population site.• With regard to metal accumulation, results suggested that, unlike Zn hyperaccumulation, Cd and Ni hyperaccumulation were not constitutive at the species level in T. caerulescens .• In general, the populations under study exhibited a pronounced uncorrelated and metal-specific variation in uptake, root to shoot translocation, and tolerance of Zn, Cd and Ni. The distinct intraspecific variation of these characters provides excellent opportunities for further genetic and physiological dissection of the hyperaccumulation trait.

Heavy metal hyperaccumulation in plants is an intriguing and poorly understood phenomenon. Transmembrane metal transporters are assumed to play a key role in this process. We describe the cloning and isolation of three zinc transporter cDNAs from the Zn hyperaccumulator Thlaspi caerulescens. The ZTP1 gene is highly similar to the Arabidopsis ZAT gene. Of the other two, one is most probably an allele of the recently cloned ZNT1 gene from T. caerulescens (Pence et al; Proceedings of the National Academy of Science USA 97, 4956–4960, 2000). The second, called ZNT2, is a close homologue of ZNT1. All three zinc transporter genes show increased expression in T. caerulescens compared with the non‐hyperaccumulator congener T. arvense, suggesting an important role in heavy metal hyperaccumulation. ZNT1 and ZNT2 are predominantly expressed in roots and ZTP1 is mainly expressed in leaves but also in roots. In T. arvense, ZNT1 and ZNT2 are exclusively expressed under conditions of Zn deficiency. Their expression in T. caerulescens is barely Zn‐responsive, suggesting that Zn hyperaccumulation might rely on a decreased Zn‐induced transcriptional downregulation of these genes. ZTP1 expression was higher in plants from calamine soil than in plants from serpentine or normal soil. The calamine plants were also the most Zn tolerant, suggesting that high ZTP1 expression might contribute to Zn tolerance.

SummaryStudying heavy metal hyperaccumulation is becoming more and more interesting for ecological, evolutionary, nutritional, and environmental reasons. One model species, especially in the era of high throughput genomics, transcriptomics, proteomics and metabolomics technologies, would be very advantageous. Although there are several hyperaccumulator species known, there is no single model species yet. The Zn, Cd and Ni hyperaccumulator species Thlaspi caerulescens has been studied to a great extent, especially for Zn and Cd hyperaccumulation and tolerance. Its physiological, morphological and genetic characteristics, and its close relationship to Arabidopsis thaliana , the general plant reference species, make it an excellent candidate to be the plant heavy metal hyperaccumulation model species.

Heavy metal hyperaccumulation in plants is an intriguing and poorly understood phenomenon. Transmembrane metal transporters are assumed to play a key role in this process. We describe the cloning and isolation of three zinc transporter cDNAs from the Zn hyperaccumulator Thlaspi caerulescens. The ZTP1 gene is highly similar to the Arabidopsis ZAT gene. Of the other two, one is most probably an allele of the recently cloned ZNT1 gene from T. caerulescens (Pence et al; Proceedings of the National Academy of Science USA 97, 4956-4960, 2000). The second, called ZNT2, is a close homologue of ZNT1. All three zinc transporter genes show increased expression in T. caerulescens compared with the non-hyperaccumulator congener T. arvense, suggesting an important role in heavy metal hyperaccumulation. ZNT1 and ZNT2 are predominantly expressed in roots and ZTP1 is mainly expressed in leaves but also in roots. In T. arvense, ZNT1 and ZNT2 are exclusively expressed under conditions of Zn deficiency. Their expression in T. caerulescens is barely Zn-responsive, suggesting that Zn hyperaccumulation might rely on a decreased Zn-induced transcriptional downregulation of these genes. ZTP1 expression was higher in plants from calamine soil than in plants from serpentine or normal soil. The calamine plants were also the most Zn tolerant, suggesting that high ZTP1 expression might contribute to Zn tolerance.