The Effect of Cadmium on Plants in Terms of the Response of Gene Expression Level and Activity

Abstract

Cadmium (Cd) is a heavy metal that can cause damage to living organisms at different levels. Even at low concentrations, Cd can be toxic to plants, causing harm at multiple levels. As they are unable to move away from areas contaminated by Cd, plants have developed various defence mechanisms to protect themselves. Hyperaccumulators, which can accumulate and detoxify heavy metals more efficiently, are highly valued by scientists studying plant accumulation and detoxification mechanisms, as they provide a promising source of genes for developing plants suitable for phytoremediation techniques. So far, several genes have been identified as being upregulated when plants are exposed to Cd. These genes include genes encoding transcription factors such as iron-regulated transporter-like protein (ZIP), natural resistance associated macrophage protein (NRAMP) gene family, genes encoding phytochelatin synthases (PCs), superoxide dismutase (SOD) genes, heavy metal ATPase (HMA), cation diffusion facilitator gene family (CDF), Cd resistance gene family (PCR), ATP-binding cassette transporter gene family (ABC), the precursor 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and precursor 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) multigene family are also influenced. Thanks to advances in omics sciences and transcriptome analysis, we are gaining more insights into the genes involved in Cd stress response. Recent studies have also shown that Cd can affect the expression of genes related to antioxidant enzymes, hormonal pathways, and energy metabolism.

Keywords: Cadmium; genes; hyperaccumulators.

Figure 1

Transcription factors and chelators facilitate the entry of Cd into the plant. Upon Cd entry into the root cell cytosol, the cell activates defense mechanisms. Cd is partly removed by binding to PCs and transported to the vacuole by ABC transporters. HMAs are also involved in the transfer of Cd to vacuoles, as well as to the xylem and subsequently to other parts of the plant. In hyperaccumulators, this transfer to aerial parts takes place. These plants can detoxify or sequester Cd in their leaves. When exposed to Cd, the ROS signalling pathway is activated, involving MAPKs. This pathway, along with Ca-calmodulin, affects the expression of ERFs, MYB, WRKY, and other transcription factor genes. Cd impacts several genes within the cell, although the precise effects are not yet fully comprehended. Plant hormones, such as IAA, also affect gene expression.

Figure 2

Non-hyperaccumulators and hyperaccumulators differ in several ways. The most significant difference is that hyperaccumulators can transfer Cd to the aerial parts of the plant, allowing them to accumulate Cd not only in the roots but also in the leaves. Additionally, hyperaccumulators can take up much larger amounts of Cd than non-hyperaccumulators. Although both types of plants possess the same genes, the expression of these genes is more tightly regulated in hyperaccumulators when exposed to Cd.