Nickel-Induced Differential Expression of Metallothioneins and Phytochelatin Synthase 1 in Arabidopsis thaliana: Organ-Specific Responses

Metallothioneins (MTs) and phytochelatins (PCs) are small Cys-rich proteins with low molecular mass responsible for detoxifying heavy metals in cells. Arabidopsis thaliana expresses eight metallothionein genes and two types of PCS; however, there is still a need to acquire more knowledge regarding their individual responses to some heavy metals. Thus, it was intended to study the expression of AtMT- and AtPCS1-encoding genes in response to high levels of nickel in wild-type A. thaliana. Seeds of A. thaliana were placed in MS medium supplemented with increasing concentrations of Ni-0 mg L-1, 2.5 mg L-1, 5 mg L-1, 7.5 mg L-1, and 10 mg L-1. After 21 days of exposure, the expression of the AtMTs (1A, 1B, 1C, 2A, 2B, and 3) and AtPCS1 was analysed through RT-qPCR in different plant organs: roots, young leaves, and mature leaves. The concentrations of photosynthetic pigments, hydrogen peroxide, and reduced glutathione were also evaluated, but no significant changes were observed. The gene expression analysis showed that the seven genes reacted differentially to the varying concentrations of Ni and in an organ-specific way. It was noted that in roots, the expression of AtMT1A, AtMT1C, and AtMT3 increased starting with the 2.5 mg L-1 treatment. At the same time, the response in the leaves fluctuated more as AtMT1B and AtMT1C increased in young leaves with concentrations higher than 7.5 and 2.5 mg L-1, respectively, with the remaining genes analysed having their expressions decreased starting with 7.5 mg L-1 of Ni. In mature leaves, AtMT1A increased, while AtMT2A, AtMT2B, and AtPCS1 decreased with Ni concentrations starting from 7.5 mg L-1. These results strongly suggest that the increase in the expression of AtMT1B, AtMT1C, and AtMT3 in the roots significantly reduced Ni toxicity, contributing to its local accumulation and buffering its translocation to the shoots. The overall reduction in the expression of MTs and PCS1 in leaves may be linked to the active participation of MT1A in mature leaves, while young leaves depended on the increased production of MT1B and MT1C to deal with the high amount of Ni present therein. These results contribute further knowledge to the understanding of the defence mechanisms of plants against high levels of Ni regarding the participation of MTs and PCS1.