Interactive Effects of Heavy Metals and Co-Occurring Abiotic Stresses in Plants: Shared Response Mechanisms and Mitigation Approaches

Heavy metal (HM) contamination of soils frequently occurs in combination with other environmental stressors, resulting in a cumulative effect that interferes with plant physiology, biochemistry, and development in a more severe and complex manner than the individual stressors alone. While there are many reviews that consider abiotic stress combinations broadly, this review focuses specifically on combined HM + abiotic stress conditions and classifies in terms of the effect on symptoms or induced defense mechanisms as additive, synergistic (exacerbated) or even antagonistic (counteracted). It is revealed that the combined stress response may depend on the way that each of the stresses presents itself to plant tissues, i.e., whether it is a chemical compound whose accumulation into roots may be regulated or a stress whose application is more general, such as heat stress or flooding (localized versus systemic, respectively). Combined localized stresses such as HMs and hydrocarbons tend to elicit synergistic responses, while localized + systemic stresses are more variable, tending toward antagonistic, such as HM and drought. Modern analysis tools indicate an overlap in or induction of unique response pathways that may alter depending on the conditions (intensity, duration and timing of exposure; greenhouse versus field conditions). The latest findings in mitigation techniques, ranging from chemical amendments and biological treatments, such as biochar and phytohormones, respectively, to genetic engineering, as also summarized. It is revealed that biochar and genetic approaches are both the most-widely used and the most effective.