Chloroplast: structural and functional alteration under nickel stress and tolerance mechanism via the omics approach

Nickel stress induces significant structural and functional alterations in chloroplasts, impairing photosynthetic efficiency. Omics-based approaches provide deeper insights into tolerance mechanisms and help identify key pathways involved in stress adaptation. Chloroplasts are crucial cell organelles for photosynthesis, metabolic regulation, and redox homeostasis. The function of chloroplasts is negatively regulated by various abiotic stresses, including nickel (Ni²⁺). Ni²⁺ poses a significant challenge due to its dual properties. At trace amounts, it serves as an essential micronutrient (particularly for nitrogen metabolism), while above the threshold level it exhibits toxic effects. Rapid industrialization and anthropogenic activities lead to the enhancement of Ni²⁺ concentration in the environment, often exceeding permissible limits. Accumulation of excess Ni²⁺ causes numerous alterations in the biological system. In this review, we critically emphasize the Ni²⁺-induced disruption of chloroplast ultrastructure, pigment stability, photosynthetic efficiency, and redox balance. Under stress conditions, chloroplasts act as primary stress sensors, triggering adaptive responses. To combat such stress, chloroplasts contain self-tolerance mechanisms through different adaptive strategies. This review provides the first comprehensive understanding of the chloroplast stress tolerance mechanisms using "omics" study. Omics are highly effective approaches that associated with gene expression, protein synthesis, and metabolite accumulation. Various approaches; proteomics, genomics, metabolomics, transcriptomics, and ionomics are actively involved in omics. During the omics study, especially genomics and transcriptomics, upregulated genes and transcripts are converted into functional components, which remains a major controversy. This review also summarizes adaptive molecular mechanisms of chloroplast stress tolerance via stress-related proteins, osmoprotectants, chelating agents, and antioxidant systems. In conclusion, chloroplasts possess self-tolerance mechanisms that enable them to overcome Ni²⁺-induced stress conditions.