Expression of metallothionein genes in Arabidopsis

Weenun Bundithya, Purdue University

Abstract

Optimal concentrations of heavy metals are critical for plant growth and development. Some heavy metals are required as micronutrients while most of them are nonessential. In either case, excess amounts of heavy metals result in phytotoxicity. Plants are equipped with mechanisms to control cellular levels of heavy metals. Of recent interest are metal-binding peptides such as phytochelatin (PC) and metallothionein (MT). Mutant analyses indicate that PCs are essential for cadmium tolerance. A number of plant MT genes have been identified; however, their functions are not clearly understood. We have studied Arabidopsis MT genes regarding transcriptional expression during development and under metal stress in the wild type and PC-deficient cad1-3 mutant. Detailed studies on the spatial gene expression of MT genes were carried out in transgenic plants carrying MT promoter:GUS reporter genes. MT1 genes are highly expressed in the root and vasculature of aerial tissues. MT2a is expressed in the leaf and root tips while MT2b expression is associated with the vasculature throughout the plant. Expression of MT1 and MT2b is induced in senescing leaves and at the base of silique, respectively, whereas the expression of MT2a increases progressively during development. MT gene expression in the cad1-3 mutant is slightly different from the wild type in that transcript levels of MT1a and MT3 are much lower in the mutant under control and Cd stress, respectively. Cd also induces expression of MT1a and MT2a in cad1-3 plants compared to the wild type. Promoter analysis indicates that a Cu-responsive cis-acting element resides in the 190-bp MT2a promoter and is responsible for Cu-induced MT2a expression in cotyledons, leaves, and lateral root tips. Three new members of the Arabidopsis MT family, MT3, MT4a, and MT4b, have been characterized. RNA expression of MT3 occurs primarily in the leaf while that of the MT4s is restricted to the seed. DNA and RNA analyses also indicate that there are MT homologues in a metal hyperaccumulator, Thlaspi caerulescens, and in a related species, Thlaspi arvense , which is not a hyperaccumulator.

Degree

Ph.D.

Advisors

Goldsbrough, Purdue University.

Subject Area

Molecular biology

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