Plant Defense Strategies and Biomarkers against Heavy Metal-Induced Stress: A Comprehensive Review

Plants respond to environmental pollutants and experience several abiotic stresses, among which heavy metal stress has been a serious concern in the global scientific community due to its yield-limiting effects on crop plants. Heavy metals intrude into the plant defense system and interfere with the cellular machinery, leading to metal toxicity and resulting in plant growth inhibition or death. Plants employ several counterbalance strategies, such as the formation of phytochelatins or metallothionein metal complexes, or vacuolar sequestration of ligand–metal complexes, etc., to combat heavy metal stress. Additionally, microbes present in the rhizospheric region share a special relationship with plants and immobilize heavy metals to improve plant health. Thus, the precise detection of heavy metals in adjoining environments is crucial to develop strategic defense strategies for sustainable agriculture. In this context, plant-based biomarkers have evolved as a promising approach. This review sheds light on heavy metal stress, various defense strategies employed by plants, and potential biomarkers used to detect heavy metal stresses and tries to draw a possible roadmap toward smart and sustainable agriculture.