Traversing the Links between Heavy Metal Stress and Plant Signaling

Abstract

Plants confront multifarious environmental stresses widely divided into abiotic and biotic stresses, of which heavy metal stress represents one of the most damaging abiotic stresses. Heavy metals cause toxicity by targeting crucial molecules and vital processes in the plant cell. One of the approaches by which heavy metals act in plants is by over production of reactive oxygen species (ROS) either directly or indirectly. Plants act against such overdose of metal in the environment by boosting the defense responses like metal chelation, sequestration into vacuole, regulation of metal intake by transporters, and intensification of antioxidative mechanisms. This response shown by plants is the result of intricate signaling networks functioning in the cell in order to transmit the extracellular stimuli into an intracellular response. The crucial signaling components involved are calcium signaling, hormone signaling, and mitogen activated protein kinase (MAPK) signaling that are discussed in this review. Apart from signaling components other regulators like microRNAs and transcription factors also have a major contribution in regulating heavy metal stress. This review demonstrates the key role of MAPKs in synchronously controlling the other signaling components and regulators in metal stress. Further, attempts have been made to focus on metal transporters and chelators that are regulated by MAPK signaling.

Keywords: MAPKs; calcium signaling; chelators; heavy metals; hormones; metal transporters; metallothioneins; microRNAs.

Figure 1

Metal detection, plant signaling, and sequestration. Different transporters are involved in metal ion uptake. Elevated level of heavy metals triggers different signaling modules which transmit the signals inside cell, thus triggering defense response. The toxicity of these metals inside the cell is sequestered by metal chelators like phytochellatins and metallothionines. The chelated metals are then ultimately transported to the vacuoles with the help of metal transporters present on the vacuole membrane. PC, phytochelatins; MT, metallothionines; GSH, Glutathiones.

Figure 2

Crosstalk of signaling pathways and its ultimate response in heavy metal stress. This figure displays the involvement of several signaling components working during metal stress. Sensing of significant level of heavy metals by plants initiates signaling network causing activation of various metal responsive transcription factors. These transcription factors (TFs) regulate the expression of metal responsive and other stress related genes ultimately helping the plant to counteract stressed situation. These stress related genes are mainly metal transporters, phytochellatins, metallothionine, antioxidant genes, and miRNA genes (MIR genes). The ROS produced in response to metal stress either by respiratory burst oxidase homolog (RBOH) activity or by alteration in electron transport is also known to activate signal transduction. This figure also exhibits the crosstalk between different signaling modules and the feedback regulation of MAPK cascade by miRNA. P = phosphorylated.

Figure 3

Differential expression of heavy metal responsive microRNAs in plants. The figure represents the data taken from genome wide study of differentially expressing miRNAs in different plant species. Green color and red color indicates up regulated and down regulated miRNAs respectively.