Debasis Chakrabarty ^*

• National Botanical Research Institute (CSIR), Lucknow, Uttar Pradesh, India

Introduction The intricate relationship between plants and their environment has become a focal pointfocus of research in recent years, particularly at the molecular and physiological levels. Plants, being sessile organisms, are constantly exposed to a range of environmental stressors, including toxic compounds like heavy metals (HMs). These HMs can significantly disrupt various physiological and metabolic processes in plants, negatively impacting theirplant growth, development, and productivity. There are several studies on the multilevel molecular mechanisms involved in plant responses to abiotic stress, but there is limited research specifically focused on responses to HMheavy metal exposure. Several researchers have identified significant molecular differences in plant responses to HMheavy metal stress (Xie et al., 2023, Gao et al., 2023)., butHowever, the specific molecular mechanisms by which heavy metals affect plants remain underexplored. Thus, Tthere is a critical need to integrate omic approaches—such as proteomics, transcriptomics, and genomics—to enhance our understanding of the physiological and molecular mechanisms underlying HMheavy metal responses in crops (Dubey et al., 2023, Tiwari et al., 2022). This includes summarizing key transcription factors (TFs) (Dutta et al., 2024), proteins, and miRNAs (Dubey et al., 2021) involved in HMheavy metal tolerance and exploring their potential in developing resistant crop varieties. Various genes and TFs have also been identified to play their role in metals stress adaptation. GSTU5 improves arsenic (As) tolerance in rice by chelating As in the root vacuole and limiting its translocation to shoot (Tiwari et al., 2022). Kidwai et al. (2018) reported OsPRX38 enhances plant tolerance to As stress by reducing As accumulation via lignin biosynthesis and activating signaling network of different antioxidant systems. OsGrx_C7, upregulated by arsenite (AsIII), enhanced rice tolerance by reducing grain As accumulation via regulating root growth and AsIII transport through aquaporin expression changes (Verma et al., 2020). miRNAs play a crucial role in plant responses to HM stress by influencing gene expression at both transcriptional and post-translational levels (Singh et al., 2021). They are known to play a central role in the transcriptional regulation of gene networks in response to metal stress. This is evident as many target genes of miRNAs encode transcription factors and proteins involved in metabolic processes and responses to metal stress. Different plant varieties exhibit differential miRNA expression patterns in response to exposure to various HMs, such as cadmium (Cd), chromium (Cr), As, lead (Pb), mercury (Hg), and aluminum (Al) (Dubey et al., 2021). For example, a study on cadmium stress reported the miR166-OsHB4 module regulates Cd accumulation and tolerance in rice. miR166 expression is downregulated in rice under Cd stress. Conversely, overexpressing miR166 enhances Cd tolerance, mitigates oxidative stress, and decreases Cd translocation from roots to shoots and accumulation in rice grains. Altered expression levels of the primary target of miR166, OsHB4 in rice plants exhibit corresponding changes in Cd tolerance (Ding et al., 2018). In addition to miRNAs, HM stress also activates various signaling pathways involving reactive oxygen species (ROS), nitric oxide (NO), hormones like auxin, cytokinin, and ethylene, as well as calcium and MAPK signaling (Tang et al., 2023; Singh et al., 2021). Abscisic acid (ABA) mediates the activation of the MAPK signaling pathway in response to Cd toxicity. The BvPYL9 protein serves as a key regulator within this cascade, contributing to the hormetic effects of low-level Cd exposure on sugar beet (Zhao et al., 2024). NO was discovered to regulate metal transporters, especially ABC, NIP, NRAMP and iron transporters, as well as stress-related genes such as GSTs, CytP450, GRXs, TFs, signaling, amino acids, hormone(s) and secondary metabolism genes involved in As detoxification (Singh et al., 2017). Cd induced ABA upregulates ABSCISIC ACID-INSENSITIVE5 (ABI5), a basic region/Leu zipper transcription factor that interacts with the R2R3-type MYB transcription factor, MYB49. This prevents the binding of MYB49 to the promoter of the downstream genes (bHLH38, bHLH101, HIPP22 and HIPP44) involved in Cd uptake, thus reducing Cd accumulation (Zhang et al., 2019). Recent research highlights epigenetic marks, particularly DNA methylation, as crucial regulators of abiotic stress-related gene expression, influencing the activity of stress-responsive genes. DNA methylation patterns are consistently associated with heavy metal (HM) exposure in plants, indicating that methylation plays a dual role in HM stress responses (Aina et al., 2004; Choi and Sano, 2007). Evidence suggests that methylation can protect against HM-induced DNA damage and regulate gene expression, with specific changes linked to transcriptional variations and transgenerational inheritance of stress tolerance (Gallo-Franco et al., 2000; Oono et al., 2016; Feng et al., 2016; Cong et al., 2019). Cong et al. (2024) revealed a strong correlation between elevated expression of Hg resistance genes and DNA hypomethylation in the putative promoter regions of these genes in a Hg-resistant rice line (RHg) derived from an OsMET1-2 (DNA methyltransferase-coding gene) heterozygous mutant. This finding indicates an epigenetic basis for mercury resistance in this plant. The decreased methylation levels likely facilitate greater accessibility of these genes to transcriptional machinery, leading to increased expression and enhanced mercury tolerance. Despite numerous studies, significant research gaps remain in understanding the epigenetic regulation of heavy metal and metalloid stress responses in plants. Further investigation is needed to clarify the specific mechanisms and effects of epigenetic modifications under such environmental stresses. The topic "Molecular Mechanisms of Metal Toxicity and Transcriptional/Post-transcriptional Regulation in Plant Model Systems" means the complexity and variability of plant responses to the presence of toxic metals in their environment. In terms of reducing plant growth, development, and productivity, metal toxicity has wide-ranging effects, mainly due to pollutants like Cd, Cr, and As. Therefore, to develop strategies for minimizing the adverse effects or improving plant resilience against metal stress, knowledge of the underlying molecular mechanisms of plant responses becomes very important. One of the prime objectives of this research topic is to dissect the molecular responses of plants towards metal toxicity through an understanding of transcriptional and post-transcriptional regulatory mechanisms, identifying key genes and regulatory networks involved in metal stress responses, and correlating metabolic and physiological changes with the former. Objective Despite advancements in identifying key components involved in HM tolerance and elucidating HM toxicity, numerous questions remain unanswered. Furthermore, different HMs can elicit distinct toxicity symptoms, and plants employ diverse defense strategies to counteract specific metal stressors. This research topic aims to address these knowledge gaps by investigating the regulatory mechanisms governing metal toxicity and the underlying molecular processes. By addressing these themes, this research topic aims to contribute to a deeper understanding of plant metal stress responses and provide valuable insights for developing strategies to mitigate the negative impacts of heavy metals on plant health and productivity. One such study investigates the effects of magnesium stress on flavonoid biosynthesis in sweet orange 'Newhall' peels. Flavonoids are important secondary metabolites considered to have benefits in nutrition, food, and medicine. However, not much is known about the molecular mechanisms regulating flavonoid biosynthesis under Mg stress. Scientists performed an integrated metabolome and transcriptome analysis of sweet orange peels under Mg-deficient and –sufficient conditions. Altogether, this investigation revealed high variability of flavonoid composition and increased the total flavonoid content upon Mg deficiency. The current study identified 1,533 secondary metabolites, of which 740 were flavonoids, and flavones constituted their main component. There were 17,897 differentially expressed genes enriched and participating in flavonoid pathways by transcriptomic analysis. Weighted Gene Correlation Network Analysis found six structural hub genes and ten transcription factor hub genes related to flavonoid biosynthesis, while CitCHS was the key gene controlling flavone synthesis. These results shed light on the metabolism of flavonoids under Mg stress and give more insight into the mechanisms at a molecular level, but they also put forward strategies for the cultivation of high-flavonoid plants (Xiong et al., 2023). The other important aspect of the research is related to the response of Ligusticum chuanxiong to cadmium stress. Cadmium is one of the highly toxic metals known to considerably hamper plant growth and productivity. The researchers pointed out that Cd stress inhibited the accumulation of biomass and root development but activated the antioxidant system of the plant. Basically, Cd accumulated in root tissues, distributing mainly in the soluble fraction and cell wall. Transcriptome analysis indicated that genes related to photosynthetic pathways were downregulated, while some genes encoding plant hormones and antioxidant systems responded positively to Cd regulation. Many genes known to be involved in cell wall modification were upregulated, which may indicate improvements in the potential of the root cell wall to sequester more Cd. It implicated key metal transport proteins, such as ATPases, MSR2, and HAM3, in Cd translocation from the apoplast to the cell membrane and pointed out the important role played by ABC transport proteins in intravesicular compartmentalization and efflux of Cd. These results bring into relief the minute molecular responses of L. Chuanxiong to Cd stress and underline the role of the antioxidant system and cell wall modifications in reducing Cd toxicity (Zhang et al., 2023) Another critical area of research is the toxicity of chromium (Cr) in plants, since Cr is a trace metal harmful to plants and a human carcinogen present in the environment due to industrial and anthropogenic activities. Cr(VI) is more toxic; it interferes with several physiological and metabolic plant pathways by increasing ROS activity. Plants developed a number of mechanisms of tolerance to Cr toxicity, which involved vacuolar absorption and accumulation of Cr, its immobilization with organic chelates, extraction by various transporters and ion channels. The key proteins for Cr sequestration are metallothioneins, phytochelatins, and glutathione-S-transferases. Several genes and transcription factors, including WRKY and AP2/ERF, take part in defense against Cr stress. It is now well established that OMICs approaches, including genomics, proteomics, and transcriptomics in metallomics, have contributed to great advances in the knowledge of Cr-stress tolerance in plants. This review exhaustively presents a model of Cr-plant interactions, detailing Cr uptake, translocation, and accumulation in plants, and emphasizes the potential of system biology and integrated OMICS approaches in enhancing plants' Cr-stress tolerance (Abdullah et al., 2024). Another study investigates the bioremediation potential of Gracilaria bailinae in Cd-contaminated waters, hence macroalgae being efficient tools for bioremediation. In this respect, absorption and accumulation capacity for Cd by G. bailinae were assessed based on physiological and biochemical analyses in the event of Cd exposure coupled with transcriptomic analysis. The study showed that G. At low Cd concentration, bailinae showed stable growth; however, the higher Cd concentrations had striking impacts on the plantlet growth and activities of antioxidant enzymes. A lot of DEGs involved in peptidase activity, ion transport, and metabolism were identified through transcriptome analysis. Under low Cd stress, the overexpressions of the DEGs related to histidine metabolism and the antioxidant pathway significantly promoted cell wall regeneration and enhanced the activities of antioxidant enzymes. bailinae to Cd stress and underscoring its potential for bioremediation in Cd-contaminated waters (Li et al., 2024). Another area of interest is the interrelationship between the plant growth-promoting rhizobacterium Staphylococcus arlettae and Helianthus annuus under arsenic (As) stress. As stress significantly diminishes the relative growth rate and net assimilation rate of H. annuus. In the presence of S. arlettae, which exhibits tolerance to As, there is an enhanced plant growth in As-contaminated media. S. Arlettae helps to transform As into more accessible forms to plants, increasing its uptake and accumulation. The bacterium enhances plant enzymatic antioxidant systems that include superoxide dismutase, peroxidase, ascorbate peroxidase, catalase, and also non-enzymatic antioxidants such as flavonoids and phenolics, glutathione. Moreover, S. arlettae induces the production of osmolytes like proline and sugars, which mitigate water loss and maintain cellular osmotic balance under As-induced stress. Malonaldehyde content is also reduced, and the electrolyte leakage is minimized by the bacterium, counteracting the As toxicity. The findings underscore that S. arlettae may have potential for mitigating plant growth against As toxicity and favoring the growth of the plants in contaminated environments (Qadir et al., 2024). Another study in potato (Solanum tuberosum) addresses phosphorus deficiency and aluminium toxicity in acidic soils, which act as a significant constraint on crop yield. In this study, members of the StPHR gene family from potato have been identified, among which StPHR1 is the key regulator responding to phosphorus deficiency and aluminum toxicity. Further bioinformatics analysis revealed that the expression level of StPHR1 was highly induced in potato roots under stress. Subcellular localization, GUS staining, heterologous overexpression, and protein interaction studies experimentally confirmed StPHR1's regulatory function in the nucleus to mediate resistance to both stresses. Heterologous expression of StPHR1 in Arabidopsis resulted in a growth phenotype resistant to aluminum toxicity and phosphorus deficiency with reduced Al content in roots. It also identified an interaction between StPHR1 and StALMT6 for applications in improving potato resistance against nutrient deficiencies and toxic metal stress in acidic soils (Zhang et al., 2024). Overall, these studies further our current understanding of the molecular mechanisms underlying metal toxicity and transcriptional/posttranscriptional regulation in plant model systems. They illustrate the intricate interplay of metabolic pathways, gene expression, and stress responses that can be used in the design of strategies to improve resilience and plant use in environmental bioremediation. It would allow the identification of regulatory networks and key mechanisms that control plant response to metal stress, the modulation of which may mitigate adverse effects of metal toxicity and thus help in sustainable agriculture and the environment. Future research areas Future research on plant responses to metal stress should prioritize several key areas to enhance our understanding and improve agricultural outcomes. First, integrating multi-omics approaches—combining genomics, transcriptomics, proteomics, and metabolomics—can provide a comprehensive view of the complex biological responses to metal exposure. This holistic perspective is essential for identifying critical pathways and molecular mechanisms involved in metal stress responses. Second, elucidating epigenetic mechanisms is crucial, as these processes can regulate gene expression without altering the underlying DNA sequence. Understanding how epigenetic modifications, such as DNA methylation and histone modifications, influence plant responses to metal stress can reveal new strategies for enhancing tolerance. Exploring plant-microbe interactions also holds promise, as beneficial microbes can play a significant role in mitigating metal toxicity and enhancing plant resilience. Research in this area can lead to innovative biotechnological applications that leverage these interactions for crop improvement. Furthermore, developing targeted crop improvement strategies, including breeding programs that incorporate tolerance traits identified through omics and epigenetic studies, can help create varieties better suited for metal-contaminated soils. Finally, addressing the implications of metal toxicity for human health is critical, as heavy metals can accumulate in the food chain, posing risks to consumers. By focusing on these areas, we can deepen our understanding of plant metal stress responses, improve crop tolerance, and mitigate the negative impacts of heavy metals on both agricultural productivity and human health.