IDENTIFICATION AND CHARACTERIZATION OF SEVERAL NEW MEMBERS OF THE ZIP FAMILY OF METAL ION TRANSPORTERS IN MEDICAGO TRUNCATULA

Authors: LOPEZ-MILLAN, ANA - BAYLOR COLLEGE OF MED; ELLIS, DANIELLE - BAYLOR COLLEGE OF MED; Grusak, Michael

Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2004
Publication Date: 3/15/2004
Citation: Lopez-Millan, A.F., Ellis, D.R., Grusak, M.A. 2004. Identification and characterization of several new members of the zip family of metal ion transporters in medicago truncatula. Plant Molecular Biology. 54: 583-596.

Interpretive Summary: Zinc, iron, and manganese are important micronutrient metals that are required by all organisms. Plant foods can serve as dietary sources of all these metals for humans; however, metal concentrations vary between different foods, and sometimes can be quite low. Because we would like to enhance the metal content of commonly consumed agronomic food crops, we have been studying the processes responsible for moving these three micronutrients into and through the plant, and especially to the edible portions of the plant. In this work, we used a member of the legume family and identified six new genes that encode proteins involved in metal movement. These genes belong to the ZIP family of membrane transport proteins. Experiments also were conducted to determine which metals (zinc, iron, manganese) could be transported by each of these proteins. The discovery of these new genes will provide us with useful tools to determine how the acquisition, movement, and distribution of micronutrient metals are regulated within crop plants.

Technical Abstract: To broaden our understanding of micronutrient metal transport in plants, we have identified cDNAs for six new metal transporters in the model legume Medicago truncatula. All of the predicted proteins have high similarity to the ZIP protein family, and have been designated MtZIP1, MtZIP3, MtZIP4, MtZIP5, MtZIP6 and MtZIP7. The six predicted proteins ranged from 350 to 372 amino acids in length; sequence analysis revealed that all proteins contained eight transmembrane domains and the highly conserved ZIP signature motif. Most of the proteins also exhibited a histidine-rich region in the variable sequence between transmembrane domains III and IV. When MtZIPs were transformed into appropriate metal-uptake defective yeast mutants and grown on metal-limited media, MtZIP1, MtZIP5 and MtZIP6 proteins restored yeast growth on Zn-limited media, MtZIP4 and MtZIP7 proteins restored yeast growth on Mn-limited media, and MtZIP3, MtZIP5 and MtZIP6 proteins restored yeast growth on Fe-limited media. Therefore, we conclude that these proteins function as metal transporters in Medicago truncatula. The expression pattern for each gene was studied by semi-quantitative RT-PCR in roots and leaves from plants grown under various metal supplies. MtZIP1 transcripts were only detected in Zn-deficient roots and leaves. MtZIP3 and MtZIP4 expression was down regulated in leaves from Mn- and Fe-deficient plants and appeared to be upregulated under Zn-deficient conditions in both roots and leaves. MtZIP5 was upregulated in leaves under Zn and Mn deficiency. The expression of MtZIP6 and MtZIP7 was unaffected by the metal supply. Characterizing these proteins in a single organism will allow us to understand the interplay between various ZIP genes, and the role they play in the regulation/execution of plant metal homeostasis.