Lichens are an excellent model to study the bioaccumulation of heavy metals but limited information is available on the molecular mechanisms occurring during bioaccumulation. We investigated the changes of the lichen proteome during exposure to constant concentrations of mercury. We found that most of changes involves proteins of the photosynthetic pathway, such as the chloroplastic photosystem I reaction center subunit II, the oxygen-evolving protein and the chloroplastic ATP synthase β-subunit. This suggests that photosynthesis is a target of the toxic effects of mercury. These findings are also supported by changes in the content of photosynthetic pigments (chlorophyll a and b, and β-carotene). Alterations to the photosynthetic machinery also reflect on the structure of thylakoid membranes of algal cells. Response of lichens to mercury also involves stress-related proteins (such as Hsp70) but not cytoskeletal proteins. Results suggest that lichens adapt to mercury exposure by changing the metabolic production of energy.

Ionic liquids (ILs) are extensively used in various fields, posing a potential threat in the ecosystem because of their high stability, excellent solubility, and biological toxicity. In this study, the toxicity mechanism of three ILs, 1-octyl-3-methylimidazolium chloride ([C₈MIM]Cl), 1-decyl-3-methylimidazolium chloride ([C₁₀MIM]Cl), and 1-dodecyl-3-methylimidazolium chloride ([C₁₂MIM]Cl) on Arabidopsis thaliana were revealed. Reactive oxygen species (ROS) level increased with higher concentration and longer carbon chain length of ILs, which led to the increase of malondialdehyde (MDA) content and antioxidase activity, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX) and peroxidase (POD) activities. SOD, CAT, and GPX activities decreased in high ILs concentration due to the excessive ROS. Differentially expressed protein was analyzed based on Gene ontology (GO) and KEGG pathways analysis. 70, 45, 84 up-regulated proteins, and 72, 104, 79 down-regulated proteins were identified in [C₈MIM]Cl, [C₁₀MIM]Cl, and [C₁₂MIM]Cl treatment, respectively (fold change ≥ 1.5 with ≥95% confidence). Cellular aldehyde metabolic process, mitochondrial and mitochondrial respiratory chains, glutathione transferase and oxidoreductase activity were enriched as up-regulated proteins as the defense mechanism of A. thaliana to resist external stresses. Chloroplast, photosynthetic membrane and thylakoid, structural constituent of ribosome, and transmembrane transport were enriched as the down-regulated protein. Compared with the control, 8 and 14 KEGG pathways were identified forup-regulated and down-regulated proteins, respectively, in three IL treatments. Metabolic pathways, carbon metabolism, biosynthesis of amino acids, porphyrin and chlorophyll metabolism were significantly down-regulated. The GO terms annotation demonstrated the oxidative stress response and effects on photosynthesis of A. thaliana in ILs treatment from biological process, cellular component, and molecular function categories.

Mangroves have been subject to more metal contamination, including cadmium (Cd). This study evaluated if a relatively short Cd exposure may induce metabolic, morphological and ultrastructural cell disturbance in Avicennia schaueriana. Cd induced evident constraints to seedlings since there was reduction in leaf gas exchanges and the plants did not survive for more than 10 days at a higher Cd exposure in controlled conditions. The highest Cd accumulation was observed in roots and gradually less in stem and leaves. Cadmium induced lignin deposition was observed in xylem cells of all vegetative organs. Intense sclerification in xylem cells, endoderm and change in the hypoderm organization were also detected. Cadmium clearly induced chloroplast deformities with ruptures of its membranes, thylakoids and core and provoked cytoplasm disorganization. These metal constraints under natural conditions for long term can lead to the accumulation of cellular and metabolic damages and jeopardize seedlings establishment and local biodiversity.

The effects of CuO NPs and bulk Cu at 0–1000 mg L⁻¹ on the growth, photosynthesis and biochemical parameters were investigated in 30-day-old rice plants grown hydroponically. ICP-OES measurements showed that CuO NPs released ≤ 1 mg L⁻¹ of Cu²⁺ ions compared with ≤ 81 mg L⁻¹ by bulk Cu at their highest concentration. Both treatments showed that growth, photo-phosphorylation and carbon dioxide assimilation declined considerably. Bulk particles caused oxidative stress whereas NP showed no such effect. Electromicrographs showed that CuO NPs accumulated in chloroplasts resulting in destacking and distortions of thylakoid membranes while bulk Cu showed no such behaviour. Results suggest that NP affected the growth by accumulation in non-ionic form in chloroplasts causing damage to thylakoid membrane without oxidative damage, whereas the bulk Cu affected the growth by causing oxidative damage as a result of release of Cu²⁺ ions without affecting the ultrastructure of the chloroplasts.

Seasonal variations of chloroplast thylakoids and plasma membrane ultrastructure and changes in some biochemical parameters (e.g., metal accumulation, photosynthetic pigments, carbohydrates, lipid peroxidation, and electrolyte leakage) were studied in fronds of Salvinia minima plants exposed to increasing concentrations of Cr(VI) in both winter and summer. Disorganization of stacked (grana) and unstacked (stroma lamellae) thylakoids was greater in winter chloroplasts than in summer chloroplasts. Plasma membrane was less affected than thylakoids. Photosynthetic pigments, lipid peroxidation, soluble sugars, and starch were affected differently in winter and summer. Our results suggest that much greater ultrastructural alterations and changes in metabolite levels occurring in winter fronds are produced by higher oxidative stress resulting from the interactive effect between low temperature, low solar irradiance, and Cr(VI) toxicity, rather than from metal accumulation per se. Seasonal differences occurring in chloroplast ultrastructure and metabolite concentrations were discussed in relation to metabolic implications. Evaluated parameters represent a relevant approach to enhance knowledge on performance and fitness of plants exposed to heavy metals under fluctuating environmental conditions. This work also indicates that selection of suitable macrophytes to remove Cr(VI) requires an additional analyzing focus on structural and metabolic interactions that occur in plants exposed to heavy metals in contrasting seasons.

Exogenous application of melatonin to plants is a promising approach for assisted phytoremediation of soil lead (Pb). In this study, we investigated the effects of foliar applications of melatonin to mature bermudagrass (Cynodon dactylon (L.) Pers.), a fast-growing perennial with potential as a non-hyperaccumulator plant for Pb phytoremediation. Following exposure to Pb (3000 mg kg⁻¹) for 30 days, decreases in biomass and chlorophyll production, degradation of thylakoid membranes, reduced photosynthesis and PSII (reaction center of photosystem II) efficiency, and elevated oxidative stress were found. Foliar applications of melatonin to Pb-stressed bermudagrass mitigated these negative effects, restoring photosynthetic pigments and chloroplast ultrastructure, subsequently improving photosynthesis and photochemistry efficiency of PSII. Exogenous melatonin also eliminated the excessive accumulations of reactive oxygen species (ROS) and methylglyoxal (MG) which associated with cellular redox homeostasis by improving ascorbic acid (AsA) and reduced glutathione (GSH) contents, redox status of GSH/GSSG (oxidative glutathione), and key enzymes activities in both AsA-GSH and glyoxalase systems. Ultimately, treating bermudagrass plants with exogenous melatonin elevated biomass production and disproportionally greater Pb translocation to roots and senescent leaves. This collectively resulted in 21% greater recovery of Pb compared to Pb-stressed bermudagrass lacking melatonin application. Overall, results from this study demonstrated the beneficial roles of melatonin for improving the effectiveness of bermudagrass as a non-hyperaccumulator plant for soil Pb phytoremediation.

Increased emissions of fluoride into the atmosphere contribute to reducing the sustainability of agricultural systems worldwide. In order to improve the understanding of the factors behind such phenomenon, varieties of citrus (Citrus spp.), Valencia sweet-orange, Ponkan mandarin, and Lisbon lemon and coffee (Coffea spp.), Obatã, Catuai, and Apoatã, were treated with fluoride nebulization. The trees were exposed to nebulization for 60 min inside a chamber by using medium (0.04 mol L⁻¹) and high (0.16 mol L⁻¹) doses of fluoridic acid (HF) during three nonconsecutive days in a single week, for a total of 26 days of exposure during the experiment. Sixty days after beginning nebulization, we evaluated leaf gas exchange, (ultra)structural organization, tree growth, and fluoride and nutrient concentrations in plant tissue. Photosynthesis and leaf dry mass of citrus and coffee varieties were affected differently by fluoride toxicity, and based on the tolerance index (relative leaf dry mass of control versus leaf dry mass of trees treated with 0.16 mol L⁻¹ HF), the order of sensitivity for the varieties of each species was as follows: for citrus, lemon > mandarin > sweet-orange; and for coffee, Apoatã > Catuaí > Obatã. The ability of the trees to control fluoride absorption most likely explained this contrast in sensitivity among varieties because both photosynthesis and leaf growth were negatively correlated with leaf fluoride concentration. Although disorganization of the thylakoids, degeneration of vascular cells, and disruption of the middle lamella occurred in leaves of all varieties exposed to fluoride, the more severe damage was observed in those with greater sensitivity to the pollutant (i.e., lemon and Apoatã coffee). Taken together, these results provided insights into the factors that explain poor performance of citrus and coffee trees under fluoride pollution and also revealed the traits driving the tolerance of these crops such a limiting condition, which included a combination of the following: (i) reduced fluoride absorption, (ii) increased photosynthesis, and (iii) improved maintenance of the ultrastructural organization of leaves.

Several studies have previously reported that nanomaterial uptake and toxicity in plants are species dependent. However, the differences between photosynthetic pathways, C3 and C4, following nanomaterial exposure are poorly understood. In the current work, wheat and rice, two C3 pathway species are compared to amaranth and maize, which utilize the C4 photosynthetic mechanism. These plants were cultured in soils which were spiked with CuO, Ag, TiO₂, MWCNT, and FLG nanomaterials. Overall, the C4 plant exhibited higher resilience to NM stress than C3 plants. In particular, significant differences were observed in chlorophyll contents with rice returning a 40.9–54.2% decrease compared to 3.5–15.1% for maize. Fv/Fm levels were significantly reduced by up to 51% in rice whereas no significant reductions were observed in amaranth and maize. Furthermore, NM uptake in the C3 species was greater than that in C4 plants, a trend that was also seen in metal concentration. TEM results showed that CuO NPs altered the chloroplast thylakoid structure in rice leaves and a large number of CuO NPs were observed in the vascular sheath cells. In contrast, there were no significant changes in the chloroplasts in the vascular sheath and no significant CuO NPs were found in maize leaves. This study was the first to systematically characterize the effect of metal and carbon-based nanomaterials in soil on C3 and C4 plants, providing a new perspective for understanding the impact of nanomaterials on plants.

Cadmium (Cd) is a metal known for its genotoxicity and cytotoxicity, much concerned for its potential environmental and human health impacts. This study evaluates the toxic effect of Cd in Calophyllum brasiliense plants. The plants were cultivated for 30 days in full nutrient solution in order to adapt, and for 15 days in nutrient solution without Cd or with 4, 8, 16, and 32 μmol Cd L⁻¹. Anatomical analysis of the leaf showed no significant effects of Cd on epidermal thickness in abaxial and adaxial sides, palisade, and spongy parenchyma. Contrastingly, changes were noticed in the ultrastructural level in the leaf mesophyll cells as rupture of the membrane of chloroplasts and disorganization of the thylakoid membranes, in starch grains and in mitochondria with rupture of the membrane and invagination of the nuclear membrane. Electron dense materials into cells of the cortex and vascular bundle were also observed. In the cells of the root system, the observed ultrastructural changes were disruption of the cell wall and electron dense material deposition in the cortex cells and vascular region. Cd accumulated in roots with low translocation into shoot. Cd toxicity also affected the photosynthetic activity, inducing stomatal closure and photosynthetic assimilation reduction and the instantaneous carboxylation efficiency, drastically reducing the leaf transpiration. The nutrient content in the stem and root was variable, according to Cd increase in nutrient solution. Based on the experimental evidence, it can be concluded that C. brasiliense has potential to bioconcentrate high Cd levels in the root system.