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)