Photodynamic effect of iron excess on photosystem II function in pea plants

MsNRAMP2 Enhances Tolerance to Iron Excess Stress in Nicotiana tabacum and MsMYB Binds to Its Promoter

Iron(Fe) is a trace metal element necessary for plant growth, but excess iron is harmful to plants. Natural resistance-associated macrophage proteins (NRAMPs) are important for divalent metal transport in plants. In this study, we isolated the MsNRAMP2 (MN_547960) gene from alfalfa, the perennial legume forage. The expression of MsNRAMP2 is specifically induced by iron excess. Overexpression of MsNRAMP2 conferred transgenic tobacco tolerance to iron excess, while it conferred yeast sensitivity to excess iron. Together with the MsNRAMP2 gene, MsMYB (MN_547959) expression is induced by excess iron. Y1H indicated that the MsMYB protein could bind to the "CTGTTG" cis element of the MsNRAMP2 promoter. The results indicated that MsNRAMP2 has a function in iron transport and its expression might be regulated by MsMYB. The excess iron tolerance ability enhancement of MsNRAMP2 may be involved in iron transport, sequestration, or redistribution.

Bioaccumulation and physiological traits qualify Pistia stratiotes as a suitable species for phytoremediation and bioindication of iron-contaminated water

Serious concerns have recently been raised regarding the association of Fe excess with neurodegenerative diseases in mammals and nutritional and oxidative disorders in plants. Therefore, the current study aimed to understand the physiological changes induced by Fe excess in Pistia stratiotes, a species often employed in phytoremediation studies. P. stratiotes were subjected to five concentrations of Fe: 0.038 (control), 1.0, 3.0, 5.0 and 7.0 mM. Visual symptoms of Fe-toxicity such as bronzing of leaf edges in 5.0 and 7.0 mM-grown plants were observed after 5 days. Nevertheless, no major changes were observed in photosynthesis-related parameters at this time-point. In contrast, plants growing for 10 days in high Fe concentrations showed decreased chlorophyll concentrations and lower net CO₂ assimilation rate. Notwithstanding, P. stratiotes accumulated high amounts of Fe, especially in roots (maximum of 10,000 µg g^-1 DW) and displayed a robust induction of the enzymatic antioxidant system. In conclusion, we demonstrated that P. stratiotes can be applied to clean up Fe-contaminated water, as the species displays high Fe bioaccumulation, mostly in root apoplasts, and can maintain physiological processes under Fe excess. Our results further revealed that by monitoring visual symptoms, P. stratiotes could be applied for bioindication purposes.

Iron counteracts zinc-induced toxicity in soybeans

Zinc (Zn) and iron (Fe) are essential micronutrients for all living organisms and the major targets for crop biofortification. However, when acquired in excess quantities, Zn and Fe can be toxic to plants. In this study, we examined the interaction between Zn and Fe in soybean plants under various Zn and Fe treatments. While the level of Zn accumulation increased with increasing Zn supplies, Zn content greatly decreased with rising Fe supplies. Moreover, Zn uptake rates were negatively correlated with Fe supplies. However, Fe accumulation was not greatly affected by elevating Zn supplies. Excess Zn supplies were found to induce typical Fe deficiency symptoms under low Fe conditions, which can be counteracted by increasing Fe supplies. Interestingly, leaf chlorosis caused by excess Zn and low Fe supplies was not directly associated with reduced total Fe content but likely associated with deleterious effects of excess Zn. The combination of high Zn and low Fe greatly activates FRO2 and FIT1 gene expression in soybean roots. Besides, Zn-Fe interaction influences the activities of antioxidative enzymes as well as the uptake, accumulation, and homeostasis of other essential micronutrients, such as copper and manganese in soybean plants. These findings provide new perspectives on Zn and Fe interaction and on heavy metal-induced Fe deficiency-like symptoms.

The oxidative damage of the Lagerstroemia indica chlorosis mutant gl1 involves in ferroptosis

Photooxidation is the major physiological performance of the Lagerstroemia indica chlorosis mutant gl1 under field conditions. The mechanisms of the progressive symptoms of oxidative damage from the lower older leaves to the upper mature leaves are complicated and still unclear. The aim of this work was to investigate the physiological mechanisms of oxidative stress from the perspective of the photosynthetic metabolites. The phytosynthetic metabolites of gl1 mutant changed significantly compared to wild type (WT) L. indica, such as by increasing phenolics, decreasing soluble sugar, protein and ascorbate, and redistributing antioxidant enzyme activities. The co-accumulation of phenolics and guaiacol-POD in gl1 mutant promote the removal of H₂O₂, as well the increase of phenoxyl radicals levels. Furthermore, the ion balance was significantly disturbed and Fe accumulated the most among these fluctuating nutrients in the leaves of gl1 mutant. The accumulated Fe was found neither in the chloroplasts nor in the cell wall of the leaves and became unshielded Fe, which favors the Fenton/Haber-Weiss reaction and stabilizes the phenoxyl radicals in metal complexation. The results suggested that the increase of phenolics and Fe accumulation were obviously involved in oxidative damage of gl1 mutant.

Fe toxicity in plants: Impacts and remediation

Fe is the fourth abundant element in the earth crust. Fe toxicity is not often discussed in plant science though it causes severe morphological and physiological disorders, including reduced germination percentage, interferes with enzymatic activities, nutritional imbalance, membrane damage, and chloroplast ultrastructure. It also causes severe toxicity to important biomolecules, which leads to ferroptotic cell death and induces structural changes in the photosynthetic apparatus, which results in retardation of carbon metabolism. However, some agronomic practices like soil remediation through chemicals, nutrients, and organic amendments and some breeding and genetic approaches can provide fruitful results in enhancing crop production in Fe-contaminated soils. Some quantitative trait loci have been reported for Fe tolerance in plants but the function of underlying genes is just emerging. Physiological and molecular mechanism of Fe uptake, translocation, toxicity, and remediation techniques are still under experimentation. In this review, the toxic effects of Fe on seed germination, carbon assimilation, water relations, nutrient uptake, oxidative damages, enzymatic activities, and overall plant growth and development have been discussed. The Fe dynamics in soil rhizosphere and role of remediation strategies, that is, biological, physical, and chemical, have also been described. Use of organic amendments, microbe, phytoremediation, and biological strategies is considered to be both cost and environment friendly for the purification of Fe-contaminated soil, while to ensure better crop yield and quality the manipulation of agronomic practices are suggested.

Morpho-physiological retardations due to iron toxicity involve redox imbalance rather than photosynthetic damages in tomato

Iron (Fe) toxicity is a major nutritional disorder that affects growth and yield in plants. Understanding the responses or damages due to Fe-toxicity may provide useful knowledge to improve tomato varieties. This study investigates the physiological and molecular responses in Fe-toxic tomato plants. The tomato plants were grown in separate hydroponic containers with two concentrations of Fe-EDTA (25 μM and 5 mM) in addition to the other nutrient elements. Fe-toxicity showed a severe reduction in growth parameters, which was accompanied by the increased electrolyte leakage and cell death in tomato. However, the SPAD score, quantum efficiency of PSII, and photosynthesis performance index did not show any changes in leaves, suggesting that damages due to Fe-toxicity are not related to the photosynthetic disturbance in tomato. The FCR (ferric chelate reductase) activity in root along with the Fe concentration in root and shoot significantly increased, being consistent with the upregulation of Fe-related genes (SlNramp1 and SlFRO1) in roots. It suggests that inefficiency to cope with elevated Fe is closely linked to Fe mobilization and uptake in roots of tomato. Consequently, this sensitive genotype was more prone to oxidative damages because of the inefficient antioxidant defense linked to antioxidant enzymes and metabolites. In conclusion, the growth retardation in Fe-toxic tomato is not related to photosynthetic inefficiency but highly associated with oxidative injuries in cells. These findings could be targeted in breeding or transgenic program to improve tomato plants sensitive to Fe toxicity.

How Does Rice Defend Against Excess Iron?: Physiological and Molecular Mechanisms

Iron (Fe) is an essential nutrient for all living organisms but can lead to cytotoxicity when present in excess. Fe toxicity often occurs in rice grown in submerged paddy fields with low pH, leading dramatical increases in ferrous ion concentration, disrupting cell homeostasis and impairing growth and yield. However, the underlying molecular mechanisms of Fe toxicity response and tolerance in plants are not well characterized yet. Microarray and genome-wide association analyses have shown that rice employs four defense systems to regulate Fe homeostasis under Fe excess. In defense 1, Fe excess tolerance is implemented by Fe exclusion as a result of suppression of genes involved in Fe uptake and translocation such as OsIRT1, OsYSL2, OsTOM1, OsYSL15, OsNRAMP1, OsNAS1, OsNAS2, OsNAAT1, OsDMAS1, and OsIRO2. The Fe-binding ubiquitin ligase, HRZ, is a key regulator that represses Fe uptake genes in response to Fe excess in rice. In defense 2, rice retains Fe in the root system rather than transporting it to shoots. In defense 3, rice compartmentalizes Fe in the shoot. In defense 2 and 3, the vacuolar Fe transporter OsVIT2, Fe storage protein ferritin, and the nicotinamine synthase OsNAS3 mediate the isolation or detoxification of excess Fe. In defense 4, rice detoxifies the ROS produced within the plant body in response to excess Fe. Some OsWRKY transcription factors, S-nitrosoglutathione-reductase variants, p450-family proteins, and OsNAC4, 5, and 6 are implicated in defense 4. These knowledge will facilitate the breeding of tolerant crops with increased productivity in low-pH, Fe-excess soils.

How Does Rice Defend Against Excess Iron?: Physiological and Molecular Mechanisms

Iron (Fe) is an essential nutrient for all living organisms but can lead to cytotoxicity when present in excess. Fe toxicity often occurs in rice grown in submerged paddy fields with low pH, leading dramatical increases in ferrous ion concentration, disrupting cell homeostasis and impairing growth and yield. However, the underlying molecular mechanisms of Fe toxicity response and tolerance in plants are not well characterized yet. Microarray and genome-wide association analyses have shown that rice employs four defense systems to regulate Fe homeostasis under Fe excess. In defense 1, Fe excess tolerance is implemented by Fe exclusion as a result of suppression of genes involved in Fe uptake and translocation such as OsIRT1, OsYSL2, OsTOM1, OsYSL15, OsNRAMP1, OsNAS1, OsNAS2, OsNAAT1, OsDMAS1, and OsIRO2. The Fe-binding ubiquitin ligase, HRZ, is a key regulator that represses Fe uptake genes in response to Fe excess in rice. In defense 2, rice retains Fe in the root system rather than transporting it to shoots. In defense 3, rice compartmentalizes Fe in the shoot. In defense 2 and 3, the vacuolar Fe transporter OsVIT2, Fe storage protein ferritin, and the nicotinamine synthase OsNAS3 mediate the isolation or detoxification of excess Fe. In defense 4, rice detoxifies the ROS produced within the plant body in response to excess Fe. Some OsWRKY transcription factors, S-nitrosoglutathione-reductase variants, p450-family proteins, and OsNAC4, 5, and 6 are implicated in defense 4. These knowledge will facilitate the breeding of tolerant crops with increased productivity in low-pH, Fe-excess soils.

Impact of calcium and magnesium substituted strontium nano-hexaferrite on mineral uptake, magnetic character, and physiology of barley (Hordeum vulgare L.)

In this study, calcium and magnesium substituted strontium nano-hexaferrites (Sr_0.96Mg_0.02Ca_0.02Fe₁₂O₁₉, SrMgCa nano-HF) were synthesized by the sol-gel auto-combustion method and their impact on the nutrient uptake, magnetic character and physiology of barley (Hordeum vulgare L.), a crop plant, was investigated. Structural, microstructural, and magnetic properties of nano-HF were evaluated by using vibrating sample magnetometry (VSM), X-ray diffraction (XRD), scanning electron microscopy (SEM) along with energy-dispersive X-ray (EDX) and elemental mapping techniques. Plants were hydroponically exposed to nano-HF (ranging from 125 to 1000 mg/L) for three weeks. Results showed that the SrMgCa nano-HF application enhanced germination rate (about 20%), tissue growth (about 38%), biomass (about 20%), soluble protein content (about 41%), and chlorophyll pigments (about 33-42%) when compared to the untreated control. In general, the plants showed the highest growth achievement at 125 or 250 mg/L of nano-HF treatment. However, higher doses diminished the growth parameters. Element concentrations and magnetic behavior analyses of plant parts proved that SrMgCa nano-HF with a size of 42.4 nm are up-taken by the plant roots and lead to increase in iron, calcium, magnesium, and strontium contents of leaves, which were about 20, 18, 3, and 60 times higher in 500 mg/L nano-HF-treated leaves than those of control, respectively. Overall, this study shows for the first time that the four elements have been internalized into the plant body through the application of substituted nano-HF. These findings suggest that mineral-substituted nanoparticles can be incorporated into plant breeding programs for the i) enhancement of seed germination and ii) treatment of plants by fighting with mineral deficiencies.

Silicon alleviates the impairments of iron toxicity on the rice photosynthetic performance via alterations in leaf diffusive conductance with minimal impacts on carbon metabolism

Iron (Fe) toxicity is often observed in lowland rice (Oryza sativa L.) plants, disrupting cell homeostasis and impairing growth and crop yields. Silicon (Si) can mitigate the effects of Fe excess on rice by decreasing tissue Fe concentrations, but no information exists whether Si could prevent the harmful effects of Fe toxicity on the photosynthesis and carbon metabolism. Two rice cultivars with contrasting abilities to tolerate Fe excess were hydroponically grown under two Fe levels (25 μM or 5 mM) and amended or not with Si (0 or 2 mM). Fe toxicity caused decreases in net photosynthetic rate (A), particularly in the sensitive cultivar. These decreases were correlated with reductions in stomatal (g_s) and mesophyll (g_m) conductances, as well as with increasing photorespiration. Photochemical (e.g. electron transport rate) and biochemical (e.g., maximum RuBisCO carboxylation capacity and RuBisCO activity) parameters of photosynthesis, and activities of a range of carbon metabolism enzymes, were minimally, if at all, affected by the treatments. Si attenuated the decreases in A by presumably reducing the Fe content. In fact, A as well as g_s and g_m, correlated significantly with leaf Fe contents. In summary, our data suggest a remarkable metabolic homeostasis under Fe toxicity, and that Si attenuated the impairments of Fe excess on the photosynthetic apparatus by affecting the leaf diffusive conductance with minimal impacts on carbon metabolism.

Genotype Variation in Rice (Oryza sativa L.) Tolerance to Fe Toxicity Might Be Linked to Root Cell Wall Lignification

Iron (Fe) is an essential element to plants, but can be harmful if accumulated to toxic concentrations. Fe toxicity can be a major nutritional disorder in rice (Oryza sativa) when cultivated under waterlogged conditions, as a result of excessive Fe solubilization of in the soil. However, little is known about the basis of Fe toxicity and tolerance at both physiological and molecular level. To identify mechanisms and potential candidate genes for Fe tolerance in rice, we comparatively analyzed the effects of excess Fe on two cultivars with distinct tolerance to Fe toxicity, EPAGRI 108 (tolerant) and BR-IRGA 409 (susceptible). After excess Fe treatment, BR-IRGA 409 plants showed reduced biomass and photosynthetic parameters, compared to EPAGRI 108. EPAGRI 108 plants accumulated lower amounts of Fe in both shoots and roots compared to BR-IRGA 409. We conducted transcriptomic analyses of roots from susceptible and tolerant plants under control and excess Fe conditions. We found 423 up-regulated and 92 down-regulated genes in the susceptible cultivar, and 42 up-regulated and 305 down-regulated genes in the tolerant one. We observed striking differences in root gene expression profiles following exposure to excess Fe: the two cultivars showed no genes regulated in the same way (up or down in both), and 264 genes were oppositely regulated in both cultivars. Plants from the susceptible cultivar showed down-regulation of known Fe uptake-related genes, indicating that plants are actively decreasing Fe acquisition. On the other hand, plants from the tolerant cultivar showed up-regulation of genes involved in root cell wall biosynthesis and lignification. We confirmed that the tolerant cultivar has increased lignification in the outer layers of the cortex and in the vascular bundle compared to the susceptible cultivar, suggesting that the capacity to avoid excessive Fe uptake could rely in root cell wall remodeling. Moreover, we showed that increased lignin concentrations in roots might be linked to Fe tolerance in other rice cultivars, suggesting that a similar mechanism might operate in multiple genotypes. Our results indicate that changes in root cell wall and Fe permeability might be related to Fe toxicity tolerance in rice natural variation.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Mechanistic understanding of iron toxicity tolerance in contrasting rice varieties from Africa: 1. Morpho-physiological and biochemical responses

Iron (Fe) is a fundamental element involved in various plant metabolic processes. However, when Fe uptake is excessive, it becomes toxic to the plant and disrupts cellular homeostasis. The aim of this study was to determine the physiological and biochemical mechanisms underlying tolerance to Fe toxicity in contrasting rice varieties adapted to African environments. Four varieties (CK801 and Suakoko 8 (tolerant), Supa and IR64 (sensitive)) selected from our previous work were analysed in more detail, and the first part of this study reports morphological, physiological and biochemical responses induced by Fe toxicity in these four varieties. Morphological (shoot length, root length, number of lateral roots), physiological (photosynthesis rate, stomatal conductance, transpiration rate, fluorescence, relative water content and cell membrane stability) and biochemical (tissue Fe, chlorophyll pigments, soluble sugars, protein and starch) traits were measured, as appropriate, on both shoot and root tissues and at different time points during the stress period. Fe toxicity significantly (P≤0.05) reduced growth and metabolism of all the four varieties. Tolerant varieties showed more lateral roots than the sensitive ones, under Fe toxic conditions as well as higher photosynthesis rate, chlorophyll content and cell membrane stability. Strong dilution of Fe concentration in cells was identified, as one of the additional tolerance mechanisms used by CK801, whereas Suakoko 8 mainly used strong mobilisation of carbohydrates at the early stage of the stress period to anticipate metabolite shortage. Traits associated with Fe toxicity tolerance in this study could be specifically targeted in trait-based breeding programs of superior lowland rice varieties tolerant of Fe toxicity.

Ecophysiological responses to excess iron in lowland and upland rice cultivars

Iron (Fe) is an essential nutrient for plants but under high concentrations, such as that found naturally in clay and waterlogged soils, its toxic effect can limit production. This study aimed to investigate the stress tolerance responses exhibited by different rice cultivars. Both lowland and upland cultivars were grown under excess Fe and hypoxic conditions. Lowland cultivars showed higher Fe accumulation in roots compared with upland cultivars suggesting the use of different strategies to tolerate excess Fe. The upland Canastra cultivar displayed a mechanism to limit iron translocation from roots to the shoots, minimizing leaf oxidative stress induced by excess Fe. Conversely, the cultivar Curinga invested in the increase of R1/A, as an alternative drain of electrons. However, the higher iron accumulation in the leaves, was not necessarily related to high toxicity. Nutrient uptake and/or utilization mechanisms in rice plants are in accordance with their needs, which may be defined in relation to crop environments. Alterations in the biochemical parameters of photosynthesis suggest that photosynthesis in rice under excess Fe is primarily limited by biochemical processes rather than by diffusional limitations, particularly in the upland cultivars. The electron transport rate, carboxylation efficiency and electron excess dissipation by photorespiration demonstrate to be good indicators of iron tolerance. Altogether, these chemical and molecular patterns suggests that rice plants grown under excess Fe exhibit gene expression reprogramming in response to the Fe excess per se and in response to changes in photosynthesis and nutrient levels to maintain growth under stress.

Comparative transcriptomics of rice plants under cold, iron, and salt stresses

Abiotic stresses such as salinity, iron toxicity, and low temperatures are the main limiting factors of rice (Oryza sativa L.) yield. The elucidation of the genes involved in responses to these stresses is extremely important to understand the mechanisms that confer tolerance, as well as for the development of cultivars adapted to these conditions. In this study, the RNA-seq technique was used to compare the transcriptional profile of rice leaves (cv. BRS Querência) in stage V3, exposed to cold, iron, and salt stresses for 24 h. A range of 41 to 51 million reads was aligned, in which a total range of 88.47 to 89.21 % was mapped in the reference genome. For cold stress, 7905 differentially expressed genes (DEGs) were observed, 2092 for salt and 681 for iron stress; 370 of these were common to the three DEG stresses. Functional annotation by software MapMan demonstrated that cold stress usually promoted the greatest changes in the overall metabolism, and an enrichment analysis of overrepresented gene ontology (GO) terms showed that most of them are contained in plastids, ribosome, and chloroplasts. Saline stress induced a more complex interaction network of upregulated overrepresented GO terms with a relatively low number of genes compared with cold stress. Our study demonstrated a high number of differentially expressed genes under cold stress and a greater relationship between salt and iron stress levels. The physiological process most affected at the molecular level by the three stresses seems to be photosynthesis.

Mechanisms of metal toxicity in plants

Metal toxicity in plants is still a global problem for the environment, agriculture and ultimately human health.

Iron excess affects rice photosynthesis through stomatal and non-stomatal limitations

Iron toxicity is the most important stressor of rice in many lowland environments worldwide. Rice cultivars differ widely in their ability to tolerate excess iron. A physiological evaluation of iron toxicity in rice was performed using non-invasive photosynthesis, photorespiration and chlorophyll a fluorescence imaging measurements and chlorophyll content determination by SPAD. Four rice cultivars (BR IRGA 409; BR IRGA 412; BRA 041171 and BRA 041152) from the Brazilian breeding programs were used. Fe(2+) was supplied in the nutrient solution as Fe-EDTA (0.019, 4, 7 and 9 mM). Increases in shoot iron content due to Fe(2+) treatments led to changes in most of the non-invasive physiological variables assessed. The reduction in rice photosynthesis can be attributed to stomatal limitations at moderate Fe(2+) doses (4mM) and both stomatal and non-stomatal limitations at higher doses. Photorespiration was an important sink for electrons in rice cultivars under iron excess. A decreased chlorophyll content and limited photochemical ability to cope with light excess were characteristic of the more sensitive and iron accumulator cultivars (BRA 041171 and BRA 041152). Chlorophyll fluorescence imaging revealed a spatial heterogeneity of photosynthesis under excessive iron concentrations. The results showed the usefulness of non-invasive physiological measurements to assess differences among cultivars. The contributions toward understanding the rice photosynthetic response to toxic levels of iron in the nutrient solution are also discussed.

Photoinhibition of Photosystem II

Photoinhibition of Photosystem II (PSII) is the light-induced loss of PSII electron-transfer activity. Although photoinhibition has been studied for a long time, there is no consensus about its mechanism. On one hand, production of singlet oxygen ((1)O(2)) by PSII has promoted models in which this reactive oxygen species (ROS) is considered to act as the agent of photoinhibitory damage. These chemistry-based models have often not taken into account the photophysical features of photoinhibition-like light response and action spectrum. On the other hand, models that reproduce these basic photophysical features of the reaction have not considered the importance of data about ROS. In this chapter, it is shown that the evidence behind the chemistry-based models and the photophysically oriented models can be brought together to build a mechanism that confirms with all types of experimental data. A working hypothesis is proposed, starting with inhibition of the manganese complex by light. Inability of the manganese complex to reduce the primary donor promotes recombination between the oxidized primary donor and Q(A), the first stable quinone acceptor of PSII. (1)O(2) production due to this recombination may inhibit protein synthesis or spread the photoinhibitory damage to another PSII center. The production of (1)O(2) is transient because loss of activity of the oxygen-evolving complex induces an increase in the redox potential of Q(A), which lowers (1)O(2) production.

Combined transcriptomic and physiological approaches reveal strong differences between short- and long-term response of rice (Oryza sativa) to iron toxicity

Ferrous iron toxicity is a mineral disorder frequently occurring under waterlogged soils where rice is cultivated. To decipher the main metabolic pathways involved in rice response to iron excess, seedlings have been exposed to 125 mg L(-1) FeSO(4) for 3 weeks. A combined transcriptomic, biochemical and physiological study has been performed after short-term (3 d) or long-term (3 weeks) exposure to iron in order to elucidate the strategy of stress adaptation with time. Our results showed that short- and long-term exposure involved a very different response in gene expression regarding both the number and function. A larger number of genes were up- or down-regulated after 3 d than after 3 weeks of iron treatment; these changes also occurred in shoot even though no significant difference in iron concentration was recorded. Those modifications in gene expression after 3 d affected not only genes involved in hormonal signalling but also genes involved in C-compound and carbohydrate metabolism, oxygen and electron transfer, oxidative stress, and iron homeostasis and transport. Modification in some gene expression can be followed by modification in corresponding metabolic products and physiological properties, or differed in time for some others, underlying the importance of an integrated study.

Impact of moderate Fe excess under Cd stress on the photosynthetic performance of poplar (Populus jacquemontiana var. glauca cv. Kopeczkii)

Cadmium interference with Fe nutrition has a strong impact on the development and efficiency of the photosynthetic apparatus. To shed more light on the interaction between Fe and Cd, it was studied how iron given in moderate excess under Cd stress affects the development and functioning of chlorophyll-protein complexes. Poplar plants grown in hydroponics up to four-leaf stage were treated with 10 μM Cd(NO₃)₂ in the presence of 50 μM Fe([III])-citrate as iron supply (5xFe + Cad) for two weeks. Though leaf area growth was inhibited similarly to that of Cad (10 μM Cd(NO₃)₂ + 10 μM Fe([III])-citrate) plants, chlorophyll content, ¹⁴CO₂ fixation and quenching parameters calculated from PAM fluorescence induction measurements were control-like in 5xFe+Cad leaves. Increased chloroplast iron content (measured photometrically by the bathophenanthroline disulfonate method) without changes in the iron and cadmium content of leaves (determined by inductively coupled plasma mass spectrometry) pointed out that a key factor in the observed protection of photosynthesis is the iron-excess-induced redistribution of iron in the leaf. However, the chlorophyll a/b ratio and the chlorophyll-protein pattern obtained by Deriphat PAGE remained similar to that of Cad leaves. The decreased amount of PSII core and PSI in mature and developing leaves, respectively, refers to developmental stage-dependent remodelling of thylakoids in the presence of Cd. The results underline not only the beneficial effect of iron excess under Cd stress, but also refer to the importance of a proper Fe/Cd ratio and light environment to avoid its possible harmful effects.

Identification of Fe-excess-induced genes in rice shoots reveals a WRKY transcription factor responsive to Fe, drought and senescence

Fe participates in several important reactions in plant metabolism. However, Fe homeostasis in plants is not completely understood, and molecular studies on Fe-excess stress are scarce. Rice (Oryza sativa L. ssp. indica) is largely cultivated in submerged conditions, where the extremely reductive environment can lead to severe Fe overload. In this work, we used representational difference analysis (RDA) to isolate sequences up-regulated in rice shoots after exposure to Fe-excess. We isolated 24 sequences which have putative functions in distinct cellular processes, such as transcription regulation (OsWRKY80), stress response (OsGAP1, DEAD-BOX RNA helicase), proteolysis (oryzain-α, rhomboid protein), photosynthesis (chlorophyll a/b binding protein), sugar metabolism (β glucosidase) and electron transport (NADH ubiquinone oxireductase). We show that the putative WRKY transcription factor OsWRKY80 is up-regulated in rice leaves, stems and roots after Fe-excess treatment. This up-regulation is also observed after dark-induced senescence and drought stress, indicating that OsWRKY80 could be a general stress-responsive gene. To our knowledge, this is the first report of an Fe-excess-induced transcription factor in plants.

Photosynthesis and oxidative stress in the restinga plant species Eugenia uniflora L. exposed to simulated acid rain and iron ore dust deposition: potential use in environmental risk assessment

The Brazilian sandy coastal plain named restinga is frequently subjected to particulate and gaseous emissions from iron ore factories. These gases may come into contact with atmospheric moisture and produce acid rain. The effects of the acid rain on vegetation, combined with iron excess in the soil, can lead to the disappearance of sensitive species and decrease restinga biodiversity. The effects of iron ore dust deposition and simulated acid rain on photosynthesis and on antioxidant enzymes were investigated in Eugenia uniflora, a representative shrub species of the restinga. This study aimed to determine the possible utility of this species in environmental risk assessment. After the application of iron ore dust as iron solid particulate matter (SPM(Fe)) and simulated acid rain (pH 3.1), the 18-month old plants displayed brown spots and necrosis, typical symptoms of iron toxicity and injuries caused by acid rain, respectively. The acidity of the rain intensified leaf iron accumulation, which reached phytotoxic levels, mainly in plants exposed to iron ore dust. These plants showed the lowest values for net photosynthesis, stomatal conductance, transpiration, chlorophyll a content and electron transport rate through photosystem II (PSII). Catalase and superoxide dismutase activities were decreased by simulated acid rain. Peroxidase activity and membrane injury increased following exposure to acid rain and simultaneous SPM(Fe) application. Eugenia uniflora exhibited impaired photosynthetic and antioxidative metabolism in response to combined iron and acid rain stresses. This species could become a valuable tool in environmental risk assessment in restinga areas near iron ore pelletizing factories. Non-invasive evaluations of visual injuries, photosynthesis and chlorophyll a fluorescence, as well as invasive biochemical analysis could be used as markers.

Irradiance regulates genotype-specific responses of Rhizobium-nodulated soybean to increasing iron and two manganese concentrations in solution culture

The growth of soybean plants were examined when subjected to three contrasting irradiance levels and to various combinations of nutrient solution Fe and Mn concentrations. Two Rhizobium-nodulated soybean genotypes (PI 227557 and Biloxi), which had been previously found to differ in their growth response to various Fe and Mn solutions, were studied. Both genotypes displayed the poorest growth, nodulation and the lowest chlorophyll and nodule ureide concentration at high irradiance (HI), regardless of the solution Fe and Mn concentrations. However, the genotypes differed under HI in their accumulation of Fe. For solution concentrations greater than 13 microM, PI 227557 accumulated up to 1200 microg Feg(-1) leaf dry wt mainly in the form of ferritin crystals within chloroplasts. In contrast, leaf Fe concentrations in Biloxi only reached 300 microg Feg(-1) dry wt and there were no ferritin crystals. Also, in PI 227557 HI induced more severe distortions in leaf cells and nodule ultrastructure than in Biloxi. Based on its poor growth under HI, PI 227557 could be categorized as an Fe-inefficient genotype prone to undergo photoinhibition at HI, in spite of the ferritin crystals in the chloroplasts. Enhanced growth, nodulation, chlorophyll and ureide concentrations in nodules as well as leaf ureide catabolism occurred in both genotypes grown at moderate irradiance (MI) in Fe solutions from 13 to 60 microM supplied with 20 microM Mn. At low irradiance (LI), plant growth and nodulation were lower than at MI values, but higher than those of plants at HI. Irradiance and solution Fe concentration did not alter leaf Cu and Zn concentration in either genotype, with the higher concentrations of these two elements detected in Biloxi. Solutions with Fe concentrations greater than 100 microM were deleterious for both genotypes at all irradiances. Low Fe and high Mn concentrations in leaves was bound to result in the best growth at HI.

Effects of simulated deposition of acid mist and iron ore particulate matter on photosynthesis and the generation of oxidative stress in Schinus terebinthifolius Radii and Sophora tomentosa L

Particulate matter is a natural occurrence in the environment, but some industries, such as the iron ore sector, can raise the total amount of particles in the atmosphere. This industry is primarily a source of iron and sulfur dioxide particulates. The effects of the pollutants from the iron ore industries on representatives of restinga vegetation in a Brazilian coastal ecosystem were investigated using physiological and biochemical measures. Two species, Schinus terebinthifolius and Sophora tomentosa, were exposed to simulated deposition of acid mist and iron ore particulate matter in acrylic chambers in a greenhouse. Parameters such as gas exchange, fluorescence emission, chlorophyll content, total iron content, antioxidant enzyme activity and malondialdehyde content were assessed in order to evaluate the responses of the two species. Neither treatment was capable of inducing oxidative stress in S. terebinthifolius. Nevertheless, the deposition of iron ore particulates on this species increased chlorophyll content, the maximum quantum efficiency of photosystem II and the electron transport rate, while iron content was unaltered. On the other hand, S. tomentosa showed a greater sensitivity to the treatments. Plants of S. tomentosa that were exposed to acid mist had a decrease in photosynthesis, while the deposition of iron particulate matter led to an increase in iron content and membrane permeability of the leaves. The activities of antioxidant enzymes, such as catalases and superoxide dismutase, were enhanced by both treatments. The results suggested that the two restinga species use different strategies to overcome the stressful conditions created by the deposition of particulate matter, either solid or wet. It seems that while S. terebinthifolius avoided stress, S. tomentosa used antioxidant enzyme systems to partially neutralize oxidative stress. The findings also point to the potential use of S. tomentosa as a biomarker species under field conditions.

Iron ore industry emissions as a potential ecological risk factor for tropical coastal vegetation

In the coastal zone of the Espírito Santo state, Brazil, fragments of restinga, which form a natural ecosystem, share their space with an increasing number of iron ore industries. The iron ore dust and SO(2) originating from the industry processing activities can interfere with the vegetation of the adjacent ecosystems at various levels. This study was undertaken in order to evaluate the effects of industry emissions on representative members of the restinga flora, by measuring physiological and phenological parameters. Foliar samples of Ipomoea pes caprae, Canavalia rosea, Sophora tomentosa, and Schinus terebinthifolius were collected at three increasing distances from an ore industry (1.0, 5.0, and 15.0 km), and were assessed for their dust deposition, chlorophyll, and Fe content. Phenological monitoring was focused on the formation of shoots, flowers, and fruits and was also performed throughout the course of a year. The results showed that the edaphic characteristics and the mineral constitutions of the plants were affected by industry emissions. In addition, the chlorophyll content of the four species increased with proximity to the industry. Phenological data revealed that the reproductive effort, as measured by fruit production, was affected by emissions and S. tomentosa was the most affected species. The use of an integrative approach that combines biochemical and ecological data indicates that the restinga flora is under stress due to industry emissions, which on a long-term basis may put the ecosystem at risk.