Expression of metallothionein genes in coffee leaves in response to the absence or excess of Cu and Zn

Abstract

Copper (Cu) and zinc (Zn) are plant nutrients; however, intracellular concentration must be kept within the sufficiency limits in order to adequately maintain metabolic functions. Like other organisms, plants have metallothioneins (MTs), which are cysteine-rich peptides that have been associated with metal homeostasis and detoxification because of their capacity to bind divalent metals. Nonetheless, the physiological roles of MTs are not completely understood and much is still unknown concerning their characterization in many higher plant species. In this study, MT expression was analyzed in leaves of Coffea arabica plants exposed to the absence or high levels of Cu and Zn in a nutrient solution, aiming to investigate the role of MTs in the maintenance of Cu and Zn homeostasis and in detoxification of excessive levels of these nutrients. At the same time, the studied MT sequences underwent phylogenetic analysis, showing that in C. arabica three of the studied MTs showed characteristics of three of the four different groups of plant MTs. It was observed that CaMT4 and CaMT15 belonged to group I but also presented characteristics of group II. CaMT3 showed protein characteristics of MT group II, and CaMT8 was positioned in type III MT clade. The expression levels of genes CaMT3, CaMT4, CaMT8, and CaMT15 were evaluated in leaves. The results indicated that MT type I, II, and III studied in C. arabica leaves are probably involved in Cu homeostasis and detoxification. However, further studies are needed to identify other MTs in coffee plants and specifically those involved in response to Zn, since the evaluation here showed a low response to excessive concentrations of this metal.