Diversity and distribution of plant metallothioneins: a review of structure, properties and functions.

More than 30 years have passed since the discovery of the first plant metallothionein in wheat embryos, from which the emergence of a uniquely diverse metallothionein family with a fascinating array of structural nuances and molecular properties has been witnessed. Metallothioneins are not only constitutively expressed, but the production of different types of plant metallothionein is also stimulated by a myriad of endogenous and exogenous agents in both a temporally and spatially regulated manner. This ubiquitous, yet discrete expression of metallothioneins not only signifies their importance for plant survival and development, but also suggests a functional divergence for the individual plant metallothionein subfamilies. Understanding why one type of plant metallothionein has more advantageous structural and metal binding attributes over another for a given biological process is a crucial piece in the puzzle of assigning physiological functions to these proteins. In this review, we discuss how in vivo and in vitro studies have advanced our understanding of the structure-property-function relationship for the plant metallothionein family. In particular, we highlight the progress that has been made for the Type 4 plant metallothioneins.