Plant Responses to Heavy Metal Stress: Molecular Mechanisms, Transcriptomic Regulation, and Multi-Omics Integration in Crop Improvement

Heavy-metal contamination poses a serious threat to plant growth, productivity, and resilience, as it causes complex changes in cellular homeostasis, redox balance, element transport, and metabolism. This review summarizes current understanding of the molecular mechanisms by which plants adapt to heavy-metal stress, with a particular focus on transcriptional and omics approaches. The main detoxification and defense mechanisms are discussed, including chelation and vacuolar sequestration, antioxidant regulation, hormonal signaling, and ion transport. Particular attention is paid to the role of MAPK cascades and transcription factors in coordinating the expression of genes involved in metal transport, maintenance of redox homeostasis, and detoxification. Analysis of transcriptomic studies shows that the plant response is characterized by pronounced temporal, tissue-specific, and genotypic specificity and involves the reprogramming of genes involved in transport, antioxidant defense, secondary metabolism, and signaling pathways. At the same time, transcript levels do not always reflect the functional state of proteins and metabolic processes. Therefore, the integration of transcriptomics with genomics, epigenomics, proteomics, metabolomics, and single-cell and spatial approaches combined with functional validation is proposed as a key direction that may facilitate the identification of robust molecular markers and functionally validated genes for improving crop tolerance to heavy-metal contamination.