Abstract:
Due to their stationery nature, plants are exposed to a diverse range of biotic and abiotic stresses, of which heavy metals stress poses as one of the most detrimental abiotic stresses, targeting crucial and vital processes. Heavy metals instigate the over-production of reactive oxygen species (ROS), and in order to mitigate the adverse effects of ROS, plants induce multiple defence mechanisms. Besides the negative implications of overproduction of ROS, these molecules play a multitude of signaling roles in plants, acting as a central player in the complex signaling network of cells. One of the signaling mechanisms it is involved in is the mitogen-activated protein kinase (MAPK) cascade, a signaling pathway used to transduce extracellular stimuli into intracellular responses. Plant MAPKs have been implicated in signaling of stresses, phytohormones and cell cycle cues. However, the influence of various heavy metals on MAPKs activation has not been well documented. In this review, we will attempt to address and summarize several aspects related to various heavy metal-induced ROS signaling, how these signals activate the MAPK cascade and the downstream transcription factors that instigates the plants response to these heavy metals. Moreover, we will highlight a modern research methodology that could characterize the novel genes associated with MAPKs and their roles in heavy metal stress.
Keywords: Heavy metal, reactive oxygen species, MAPK, transcription factor, signaling pathway, stress response
Introduction
The emergence of rapid urbanization coupled with various anthropogenic activities such as modern agricultural and farm management practice (phosphate-based fertilizer and metallo-pesticide applications and hard water and wastewater irrigations) is the leading cause of environmental and human health deterioration (Thakur et al., 2022; Rai et al., 2019). This is due to their major contribution of excess metals, especially heavy metals (HM), to soil, water bodies and air environment (Thakur et al., 2022; Rai et al., 2019). Although, heavy metals naturally occur within the Earth’s crust (Keyster et al., 2020), the aforementioned activities have been pumping excess amounts of HM into environments and ecosystems, causing them reach threshold levels or to surpass their permissible concentrations (Table 1 ) within various environmental systems (Thakur et al., 2022; Rai et al., 2019). Heavy metals are referred to as such owing to their high densities, atomic weight and/or number (Oladoye et al., 2022). Heavy metals such as cadmium, chromium, lead, arsenic, and mercury are classified among the most toxic metals (Keyster et al., 2020) and therefore they are required to be assessed and controlled within the various environments (Table 1 ). HM are able to cause damaging effects on an extensive variety of ecosystems through food chains and water, mainly because these metal ions are prone to dissolve in water, bioaccumulate in aquatic animals or crops (Oladoye et al., 2022). Hence, they cause notable threats in agricultural fields, living organism, animals, and humans (Xiang et al., 2021; Shao et al., 2022). The toxic heavy metals accumulate in animal and human bones, gastrointestinal tract, liver, lungs, and kidneys, leading to diseases such as cancer, diabetes, emphysema, liver damage, osteoporosis, renal tubular damage, and kidney stone formation (Hossein-Khannazer et al. 2020; Wang et al. 2021).
Within plants, the processes of seed germination, plant growth and development are significantly affected by HM-toxicity, rendering basic plant functions dysfunctional at the physiological (chlorosis, reduction of stomatal density and conductance, inhibition of carbon fixation, reduction in root respiration, transpiration and leaf relative water content, reduction of chlorophyll content and photosynthetic efficiency, decline yield), biochemical (destruction and damage to plant membranes and membranous organelles, altered protein patterning, suppression of antioxidant activity, dysfunctional cellular osmoregulation, and reactive oxygen species (ROS) scavenging system) and molecular level (reduction of crop nutritional value, uptake and transportation of mineral elements, inhibited storage protein catabolism) (Huybrechts et al. 2020; Hussain et al. 2021; Zhou et al. 2021; Wu et al. 2022). However, plants have evolved signaling systems to regulate responses, in order to adapt or acclimatize to HM-stresses (Keyster et al., 2020), therefore it is important to understand these systems and how they are regulated under HM-stress. However, in plants the relationship between HM-induced ROS production and their regulation of the MAPKs cascade is controversial (Lui et al., 2019). Therefore, the review focuses on the mechanism of HM-uptake into root cells to induce ROS production, and how this induction activates the MAPK cascade. Furthermore, the review wants to unravel which downstream transcription factors are activated in order to instigate a HM-response in plants.
Table 1. Permissible and threshold concentrations of various heavy metals in soil. (Singh et al., 2010; Al-boghdady et al., 2019; Adagunodo et al., 2018)