Low Iron Supply Research Articles

Characterization of a Southern Ocean deep chlorophyll maximum: Response of phytoplankton to light, iron, and manganese enrichment

AbstractSouthern Ocean phytoplankton growth is limited by low iron (Fe) supply and irradiance, impacting the strength of the biological carbon pump. Unfavorable upper ocean conditions, such as low nutrient concentrations, can lead to the formation of deep chlorophyll or biomass maxima (DCM/DBM). While common in the Southern Ocean, these features remain understudied due to their subsurface location. To increase our understanding of their occurrence, we studied the responses of phytoplankton communities from a Southern Ocean DCM to increasing light, Fe, and manganese (Mn) levels. The DCM communities were light‐ and Fe‐limited, but light limitation did not increase phytoplankton Fe requirements. The greatest physiological responses were observed under combined Fe/light additions, which stimulated macronutrient drawdown, biomass production and the growth of large diatoms. Combined Mn/light additions induced subtle changes in Fe uptake rates and community composition, suggesting species‐specific Mn requirements. These results provide valuable information on Southern Ocean DCM phytoplankton physiology.

Iron Deficiency Leads to Chlorosis Through Impacting Chlorophyll Synthesis and Nitrogen Metabolism in Areca catechu L.

Deficiency of certain elements can cause leaf chlorosis in Areca catechu L. trees, which causes considerable production loss. The linkage between nutrient deficiency and chlorosis phenomenon and physiological defect in A. catechu remains unclear. Here, we found that low iron supply is a determinant for chlorosis of A. catechu seedling, and excessive iron supply resulted in dark green leaves. We also observed morphological characters of A. catechu seedlings under different iron levels and compared their fresh weight, chlorophyll contents, chloroplast structures and photosynthetic activities. Results showed that iron deficiency directly caused chloroplast degeneration and reduced chlorophyll synthesis in chlorosis leaves, while excessive iron treatment can increase chlorophyll contents, chloroplasts sizes, and inflated starch granules. However, both excessive and deficient of iron decreases fresh weight and photosynthetic rate in A. catechu seedlings. Therefore, we applied transcriptomic and metabolomic approaches to understand the effect of different iron supply to A. catechu seedlings. The genes involved in nitrogen assimilation pathway, such as NR (nitrate reductase) and GOGAT (glutamate synthase), were significantly down-regulated under both iron deficiency and excessive iron. Moreover, the accumulation of organic acids and flavonoids indicated a potential way for A. catechu to endure iron deficiency. On the other hand, the up-regulation of POD-related genes was assumed to be a defense strategy against the excessive iron toxicity. Our data demonstrated that A. catechu is an iron-sensitive species, therefore the precise control of iron level is believed to be the key point for A. catechu cultivation.

Melatonin alleviates iron stress by improving iron homeostasis, antioxidant defense and secondary metabolism in cucumber

Iron is an essential element for plants and animals; however, deficit or excess iron can result in stress on plants. In this study, we unveiled the mechanisms of shoot-based tolerance to iron toxicity by melatonin (N-acetyl-5-methoxytryptamine) in cucumber plants. Both low and high iron (Fe) supply in hydroponics decreased growth and biomass accumulation, induced chlorosis and oxidative stress, and reduced chlorophyll content, photosynthesis rate and transpiration rate in cucumber leaves. Notably, the negative effect of low-Fe was more profound than that of high-Fe treatment. However, exogenous melatonin application alleviated those inhibitions in growth, biochemical and physiological parameters, which entailed an elevation of endogenous melatonin content and a reduction of electrolyte leakage, reactive oxygen species accumulation and lipid peroxidation by improving the activity and transcripts of antioxidant enzymes and secondary metabolism-related enzymes, and concentrations of phenols and flavonoids under low and high iron conditions after melatonin treatment. Analysis of iron content in leaves and roots revealed that melatonin significantly increased the iron content under low-Fe conditions, but it decreased the same under high-Fe conditions. Moreover, melatonin increased the transcript levels of FRO2 and IRT1 under low-Fe, but it decreased those transcripts under high-Fe, suggesting that melatonin plays a dual role in iron uptake under low and high iron conditions. This study expands the stress ameliorative role of melatonin in plants and may have potential implications in agronomic management of crops in low and high iron prone soils.

Iron Availability Influences the Tolerance of Southern Ocean Phytoplankton to Warming and Elevated Irradiance

The Southern Ocean is responsible for approximately 40% of oceanic carbon uptake through biological and physical processes. In the Southern Ocean, phytoplankton growth is limited by low iron (Fe) and light supply. Climate model projections for the Southern Ocean indicate that temperature, underwater irradiance and Fe supply are likely to change simultaneously in the future due to increasing anthropogenic carbon dioxide emissions. The individual effects of these environmental properties on phytoplankton physiology have been extensively researched, and culturing studies using Southern Ocean phytoplankton have shown that temperature and Fe will play a key role on setting growth under future conditions. To explore the potential responses of Southern Ocean phytoplankton to these environmental changes, we cultured the haptophyte Phaeocystis antarctica and the diatoms Chaetoceros flexuosus, Proboscia inermis, and Thalassiosira antarctica under two light and iron combinations and over a range of temperatures. Our study revealed that the thermal response curves of key Southern Ocean phytoplankton are diverse, with the highest growth rates measured at 5°C (the annual temperature range at the isolation sites is currently 1–4°C). Warming had species-specific effects on the photochemical efficiency of photosystem II (PSII; Fv/Fm), the functional absorption cross-section of PSII (σPSII), carbon:nitrogen ratio and cellular Chlorophyll a concentrations. Iron availability increased species’ ability to tolerate warmer conditions by increasing the upper limit for growth and subsequently increasing the thermal niche that each species inhabit.

A Cool, Nutrient‐Enriched Eastern Equatorial Pacific During the Mid‐Pleistocene Transition

AbstractThe emergence of high‐amplitude, low‐frequency glacial‐interglacial cycles during the mid‐Pleistocene climate transition (MPT; 800–1,200 ka) is associated with global cooling. In the eastern equatorial Pacific, sea surface temperatures cooled, and the upwelling‐induced cold tongue expanded significantly during the MPT. Here we use sedimentary records of iron, biogenic silica, and nutrient‐nitrogen consumption to evaluate biogeochemical changes hypothesized to accompany the cold tongue expansion. Our results suggest that the eastern equatorial Pacific of the MPT hosted surface waters with higher nitrate contents and biogenic silica production relative to the last 600 ka. Increased production occurred despite low iron supply. We attribute this to enhanced upwelling and nutrient enrichment of thermocline waters, both likely related to the northward migration of Southern Ocean fronts. The return of these fronts to their southward positions after the MPT may be associated with stronger drawdown of nutrients and, potentially, atmospheric CO2 in the Southern Ocean.

Gene Expression Profiling of Iron Deficiency Chlorosis Sensitive and Tolerant Soybean Indicates Key Roles for Phenylpropanoids under Alkalinity Stress.

Alkaline soils comprise 30% of the earth and have low plant-available iron (Fe) concentration, and can cause iron deficiency chlorosis (IDC). IDC causes soybean yield losses of $260 million annually. However, it is not known whether molecular responses to IDC are equivalent to responses to low iron supply. IDC tolerant and sensitive soybean lines provide a contrast to identify specific factors associated with IDC. We used RNA-seq to compare gene expression under combinations of normal pH (5.7) or alkaline pH (7.7, imposed by 2.5 mM bicarbonate, or pH 8.2 imposed by 5 mM bicarbonate) and normal (25 μM) or low (1 μM) iron conditions from roots of these lines. Thus, we were able to treat pH and Fe supply as separate variables. We also noted differential gene expression between IDC sensitive and tolerant genotypes in each condition. Classical iron uptake genes, including ferric-chelate reductase (FCR) and ferrous transporters, were upregulated by both Fe deficiency and alkaline stress, however, their gene products did not function well at alkaline pH. In addition, genes in the phenylpropanoid synthesis pathway were upregulated in both alkaline and low Fe conditions. These genes lead to the production of fluorescent root exudate (FluRE) compounds, such as coumarins. Fluorescence of nutrient solution increased with alkaline treatment, and was higher in the IDC tolerant line. Some of these genes also localized to previously identified QTL regions associated with IDC. We hypothesize that FluRE become essential at alkaline pH where the classical iron uptake system does not function well. This work could result in new strategies to screen for IDC tolerance, and provide breeding targets to improve crop alkaline stress tolerance.

Interactive effects of salinity and iron deficiency on different rice genotypes

AbstractSalinity adversely affects plant growth, photosynthesis, and availability of nutrients including iron. Rice (Oryza sativa L.) is susceptible to soil salinity and highly prone to iron (Fe) deficiency due to lower release of Fe‐chelating compounds under saline conditions. In order to investigate the effects of salinity and low iron supply on growth, photosynthesis, and ionic composition of five rice genotypes (KS‐282, Basmati Pak, Shaheen Basmati, KSK‐434 and 99417), a solution culture experiment was conducted with four treatments (control, 50 mM NaCl, Fe‐deficient, and 50 mM NaCl + Fe‐deficient). Salinity and Fe deficiency reduced shoot and root growth, photosynthetic and transpiration rates, chlorophyll concentration, and stomatal conductance. The reduction in all these parameters was more in the interactive treatment of salinity and low Fe supply. Moreover, a significant increase in shoot and root Na+ with corresponding decrease in K+ and Fe concentrations was also observed in the combined salinity and Fe‐deficiency treatment. Among the tested genotypes, Basmati Pak was the most sensitive genotype both under salt stress and Fe deficiency. The genotype KS‐282 performed better than other genotypes under salinity stress alone, whereas Shaheen Basmati was the best genotype under Fe deficiency in terms of all the studied parameters.

Iron stores of breastfed infants during the first year of life.

The birth iron endowment provides iron for growth in the first months of life. We describe the iron endowment under conditions of low dietary iron supply. Subjects were infants participating in a trial of Vitamin D supplementation from 1 to 9 months. Infants were exclusively breastfed at enrollment but could receive complementary foods from 4 months but not formula. Plasma ferritin (PF) and transferrin receptor (TfR) were determined at 1, 2, 4, 5.5, 7.5, 9 and 12 months. At 1 month PF ranged from 38 to 752 µg/L and was only weakly related to maternal PF. PF declined subsequently and flattened out at 5.5 months. PF of females was significantly higher than PF of males except at 12 months. TfR increased with age and was inversely correlated with PF. PF and TfR tracked strongly until 9 months. Iron deficiency (PF < 10 µg/L) began to appear at 4 months and increased in frequency until 9 months. Infants with ID were born with low iron endowment. We concluded that the birth iron endowment is highly variable in size and a small endowment places infants at risk of iron deficiency before 6 months. Boys have smaller iron endowments and are at greater risk of iron deficiency than girls.

Overweight children have higher circulating hepcidin concentrations and lower iron status but have dietary iron intakes and bioavailability comparable with normal weight children

Obesity increases the risk for iron deficiency, but the underlying mechanism is unclear. It is possible that overweight individuals may have lower dietary iron intake and/or bioavailability. Alternatively, obesity-related inflammation may increase hepcidin concentrations and reduce iron availability. Circulating hepcidin levels have not been compared in normal weight vs overweight individuals. The objective of this study was to compare iron status, dietary iron intake and bioavailability, as well as circulating levels of hepcidin, leptin and interleukin-6 (IL-6), in overweight vs normal weight children. In 6-14-year-old normal and overweight children (n=121), we measured dietary iron intake, estimated iron bioavailability and determined body mass index s.d. scores (BMI-SDS). In all children (n=121), we measured fasting serum ferritin, soluble transferrin receptor (sTfR), C-reactive protein (CRP) and leptin; in a subsample, we measured IL-6 (n=68) and serum hepcidin (n=30). There were no significant differences in dietary iron intake or bioavailability comparing normal and overweight children. The prevalence of iron-deficient erythropoiesis (an increased sTfR concentration) was significantly higher in the overweight than in the normal weight children (20 vs 6%, P=0.022, with sTfR concentrations of 4.40+/-0.77 and 3.94+/-0.88 mg l(-1), respectively, P=0.010). Serum hepcidin levels were significantly higher in the overweight children (P=0.001). BMI-SDS significantly correlated with sTfR (P=0.009), serum hepcidin (P=0.005) and the three measures of subclinical inflammation, namely CRP (P<0.001), IL-6 (P<0.001) and leptin (P<0.001). In a multiple regression model, serum hepcidin was correlated with BMI-SDS (P=0.020) and body iron (P=0.029), but not with the inflammatory markers. Our findings indicate that there is reduced iron availability for erythropoiesis in overweight children and that this is unlikely due to low dietary iron supply but rather due to hepcidin-mediated reduced iron absorption and/or increased iron sequestration.

Iron limitation of the postbloom phytoplankton communities in the Iceland Basin

Measurements performed on a cruise within the central Iceland Basin in the high‐latitude (&gt;55°N) North Atlantic Ocean during late July to early September 2007 indicated that the concentration of dissolved iron (dFe) in surface waters was very low, with an average of 0.093 (&lt;0.010–0.218, n = 43) nM, while nitrate concentrations ranged from 2 to 5 μM and in situ chlorophyll concentrations ranged from 0.2 to 0.4 mg m−3. In vitro iron addition experiments demonstrated increased photosynthetic efficiencies (Fv/Fm) and enhanced chlorophyll accumulation in treatments amended with iron when compared to controls. Enhanced net growth rates for a number of phytoplankton taxa including the coccolithophore Emiliania huxleyi were also observed following iron addition. These results provide strong evidence that iron limitation within the postspring bloom phytoplankton community contributes to the observed residual macronutrient pool during summer. Low atmospheric iron supply and suboptimal Fe:N ratios in winter overturned deep water are suggested to result in the formation of this seasonal high‐nutrient, low‐chlorophyll (HNLC) condition, representing an inefficiency of the biological (soft tissue) carbon pump in the region.

Nitric oxide accumulation is required for molecular and physiological responses to iron deficiency in tomato roots

Iron is an essential and commonly limited nutrient for plants. To increase the uptake of iron during times of low iron supply, plants, except the grasses, activate a set of physiological and morphological responses in their roots that include iron reduction, soil acidification, Fe(II) transport and proliferation of root hairs. It is not known how root cells sense and transduce the changes that occur after the onset of iron deficiency. This work presents evidence that nitric oxide (NO) is produced rapidly in the root epidermis of tomato plants (Solanum lycopersicum) that are grown in iron-deficient conditions. The scavenging of NO prevented iron-deficiency-induced upregulation of the basic helix-loop-helix transcription factor FER, the ferric-chelate reductase LeFRO1 and the Fe(II) transporter LeIRT1 genes. On the other hand, exogenous application of the NO donor S-nitrosoglutathione enhanced the accumulation of FER, LeFRO1 and LeIRT1 mRNA in roots of iron-deficient plants. The activity of the root ferric-chelate reductase and the proliferation of root hairs induced by iron deficiency were stimulated by NO supplementation and suppressed by NO scavenging. Nitric oxide was ineffective in inducing iron-deficiency responses in the tomato fer mutant, which indicates that the FER protein is necessary to mediate the action of NO. Furthermore, NO supplementation improved plant growth under low iron supply, which suggests that NO is a key component of the regulatory mechanisms that control iron uptake and homeostasis in plants. In summary, the results of this investigation indicate that an increase in NO production is an early response of roots to iron deprivation that contributes to the improvement of iron availability by (i) modulating the expression of iron uptake-related genes and (ii) regulating the physiological and morphological adaptive responses of roots to iron-deficient conditions.

Increased senescence-associated gene expression and lipid peroxidation induced by iron deficiency in rice roots

Iron deficiency is among the most common nutritional disorders in plants. Low iron supply causes decreased root growth and even plant death. However, there are no reports about the specific pathways that lead Fe-deficient roots to senescence and death. To investigate the molecular mechanisms that regulate rice roots response to Fe-deficiency, rice seedlings were grown for 3, 6 and 9 days in the presence or absence of Fe. Sequences of 28 induced genes in rice roots under Fe-deficiency were identified by representational difference analysis (RDA). About 40% of these sequences have been previously reported as senescence-related. Differential expression of selected genes was confirmed by semi-quantitative RT-PCR analysis. Classical senescence-related sequences, such as MYB and WRKY transcription factors, cysteine protease, ubiquitin-conjugating enzyme, lipid transfer protein, fatty acid hydroxylase, beta-glucosidase and cytochrome P450 oxydoreductase were identified. Fe-deficiency also resulted in decreased dry weight, increased lipid peroxidation (detected by TBA and histochemical methods) as well as evident membrane damage in Fe-deficient roots. Taken together, the results indicate that Fe-deficiency in roots is linked to typical senescence pathways, associated with lipid peroxidation.

A Transcription Factor Cascade Involving Fep1 and the CCAAT-Binding Factor Php4 Regulates Gene Expression in Response to Iron Deficiency in the Fission Yeast Schizosaccharomyces pombe

We have identified genes encoding candidate proteins involved in iron storage (pcl1+), the tricarboxylic acid cycle (sdh4+), and iron-sulfur cluster assembly (isa1+) that are negatively regulated in response to iron deprivation. Promoter deletion and site-directed mutagenesis permitted identification of a new cis-regulatory element in the promoter region of the pcl1+ gene. This cis-acting regulatory sequence containing the pentanucleotide sequence CCAAT is responsible for transcriptional repression of pcl1+ under low iron supply conditions. In Schizosaccharomyces pombe, the CCAAT-binding factor is a heteromeric DNA-binding complex that contains three subunits, designated Php2, Php3, and Php5. Inactivation of the php2+ locus negatively affects the transcriptional competency of pcl1+. A fourth subunit, designated Php4, is not essential for the transcriptional activation of target genes under basal and iron-replete conditions. We demonstrate that, in response to iron-limiting conditions, Php4 is required for down-regulation of pcl1+, sdh4+, and isa1+ mRNA levels. In vivo RNase protection studies reveal that the expression of php4+ is negatively regulated by iron and that this regulated expression requires a functional fep1+ gene. The results of these studies reveal that Fep1 represses php4+ expression in response to iron. In contrast, when iron is scarce, Fep1 becomes inactive and php4+ is expressed to act as a regulatory subunit of the CCAAT-binding factor that is required to block pcl1+, sdh4+, and isa1+ gene transcription.

Metal dependence and intracellular regulation of the bidirectional NiFe-hydrogenase in Synechocystis sp. PCC 6803

Investigating the regulation of the cyanobacterial bidirectional hydrogenase on the molecular level is a prerequisite to learn more about the details of its function and to enhance biohydrogen production. Several deletion mutants of histidine kinase genes ( hik) of Synechocystis were analysed concerning their hydrogenase activity. hik7 showed no hydrogenase activity at all. It is supposed that the decreased activity is due to an improper processing of the enzyme. Furthermore, the effect of iron deficiency on the bidirectional hydrogenase was investigated on the level of transcription and enzyme activity. Low iron supply elicited low hydrogenase activity but a high promoter activity. Genetic and phenotypic differences have been repeatedly described for different wild-type (WT) strains of Synechocystis. Here, we report the identification of a substrain that has a severely reduced hydrogenase activity compared to the strain used throughout this study. The hydrogenase activity of the substrain was found to be partly complemented by the addition of nickel. Thus, revealing an inhibited nickel uptake into the cell or insertion into the hydrogenase. It emphasizes the importance of the right choice of WT strain for investigations concerning the hydrogen production capacities.

Iron-Mediated Control of the Basic Helix-Loop-Helix Protein FER, a Regulator of Iron Uptake in Tomato

Root iron mobilization genes are induced by iron deficiency downstream of an unknown signaling mechanism. The FER gene, encoding a basic helix-loop-helix domain protein and putative transcription factor, is required for induction of iron mobilization genes in roots of tomato (Lycopersicon esculentum). To study upstream regulatory events of FER action, we examined the control of FER gene and FER protein expression in response to iron nutritional status. We analyzed expression of the FER gene and FER protein in wild-type plants, in mutant plants with defects in iron uptake regulation, and in 35S transgenic plants that overexpressed the FER gene. An affinity-purified antiserum directed against FER epitopes was produced that recognized FER protein in plant protein extracts. We found that the FER gene and FER protein were consistently down-regulated in roots after generous (100 mum, physiologically optimal) iron supply compared to low (0.1 mum) and sufficient (10 mum) iron supply. FER gene and FER protein expression were also occasionally down-regulated at sufficient compared to low iron supply. Analysis of FER protein expression in FER overexpression plants, as well as cellular protein localization studies, indicated that FER was down-regulated by high iron at the posttranscriptional level. The FER protein was targeted to plant nuclei and showed transcriptional activation in yeast (Saccharomyces cerevisiae). FER protein regulation in the iron accumulation mutant chloronerva indicated that FER protein expression was not directly controlled by signals derived from iron transport. We conclude that FER is able to affect transcription in the nucleus and its action is controlled by iron supply at multiple regulatory levels.

FRU ( BHLH029) is required for induction of iron mobilization genes in Arabidopsis thaliana

Iron mobilization responses are induced by low iron supply at transcriptional level. In tomato, the basic helix-loop-helix gene FER is required for induction of iron mobilization. Using molecular-genetic techniques, we analyzed the function of BHLH029, named FRU ( FER-like regulator of iron uptake), the Arabidopsis thaliana homolog of the tomato FER gene. The FRU gene was mainly expressed in roots in a cell-specific pattern and induced by iron deficiency. FRU mutant plants were chlorotic, and the FRU gene was found necessary for induction of the essential iron mobilization genes FRO2 (ferric chelate reductase gene) and IRT1 (iron-regulated transporter gene). Overexpression of FRU resulted in an increase of iron mobilization responses at low iron supply. Thus, the FRU gene is a mediator in induction of iron mobilization responses in Arabidopsis, indicating that regulation of iron uptake is conserved in dicot species.

Differential involvement of the IDRS cis-element in the developmental and environmental regulation of the AtFer1 ferritin gene from Arabidopsis.

Four different ferritin genes have been identified in Arabidopsis thaliana, namely AtFer1, 2, 3 and 4. AtFer1, which strongly accumulates in leaves treated with excess iron, contains in its promoter an Iron- Dependent Regulatory Sequence (IDRS). The IDRS sequence is responsible for repression of AtFer1 transcription under conditions of low iron supply. Arabidopsis plants transformed with a 1,400-bp AtFer1 promoter, with either a wild-type or a mutated IDRS fused to the beta-glucuronidase (GUS) reporter gene, enabled us to analyze the activity of the AtFer1 promoter in different tissues as well as during age-dependent or dark-induced senescence. Our results show that IDRS mediates AtFer1 expression during dark-induced senescence while it does not affect AtFer1 expression during age-dependent senescence or in young seedlings. Photoinhibition promoted either by high light or chilling temperature, or wounding, does not activate the AtFer1 promoter. In contrast, AtFer2, AtFer3, AtFer4 transcript abundances are increased in response to photoinhibition and AtFer3 transcript abundance is increased upon wounding. Taken together, our results indicate that other cis-elements, different from the IDRS, regulate the territory-specific or developmental expression of AtFer1 gene. Expression of this gene appears insensitive to some of the environmental stresses tested, which instead up-regulate other members of the Arabidopsis ferritin gene family.

The tomato fer gene encoding a bHLH protein controls iron-uptake responses in roots.

Iron deficiency is among the most common nutritional disorders in plants. To cope with low iron supply, plants with the exception of the Gramineae increase the solubility and uptake of iron by inducing physiological and developmental alterations including iron reduction, soil acidification, Fe(II) transport and root-hair proliferation (strategy I). The chlorotic tomato fer mutant fails to activate the strategy I. It was shown previously that the fer gene is required in the root. Here, we show that fer plants exhibit root developmental phenotypes after low and sufficient iron nutrition indicating that FER acts irrespective of iron supply. Mutant fer roots displayed lower Leirt1 expression than wild-type roots. We isolated the fer gene by map-based cloning and demonstrate that it encodes a protein containing a basic helix-loop-helix domain. fer is expressed in a cell-specific pattern at the root tip independently from iron supply. Our results suggest that FER may control root physiology and development at a transcriptional level in response to iron supply and thus may be the first identified regulator for iron nutrition in plants.

Nitric oxide mediates iron-induced ferritin accumulation in Arabidopsis.

Nitric oxide (NO) is a signaling molecule that plays a critical role in the activation of innate immune and inflammatory responses in animals. During the last few years, NO has also been detected in several plant species and the increasing number of reports on its function in plants have implicated NO as an important effector of growth, development and defense. Analogously to animals, NO has been recently shown to inhibit tobacco aconitase. This suggests that NO may elevate free iron levels in the cells by converting tobacco cytoplasmic aconitase into a mRNA binding protein that negatively regulates accumulation of ferritin. We investigated the possible role of NO as a regulator of ferritin levels in Arabidopsis and found that the NO-donor sodium nitroprusside (SNP) induces accumulation of ferritin both at mRNA and protein level. Iron is not necessary for this NO-mediated ferritin transcript accumulation, since SNP is still able to induce the accumulation of ferritin transcript in Arabidopsis suspension cultures pre-treated with the iron chelants DFO or ferrozine. However, NO is required for iron-induced ferritin accumulation, as the NO scavenger CPTIO prevents ferritin transcript accumulation in Arabidopsis suspension cultures treated with iron. The pathway is ser/thr phosphatase-dependent and necessitates protein synthesis; furthermore, NO mediates ferritin regulation through the IDRS sequence of the Atfer1 promoter responsible for transcriptional repression under low iron supply. NO, by acting downstream of iron in the induction of ferritin transcript accumulation is therefore a key signaling molecule for regulation of iron homeostasis in plants.

Modulation of the root epidermal phenotype by hormones, inhibitors and iron regime

Patterning of epidermal cells is subject to genetic regulation but also influenced by environmental stimuli. To adapt to unfavorable environmental conditions plants have developed various mechanisms to increase the plasma membrane's surface area of epidermal root cells, for example through the formation of root hairs and differentiation of rhizodermal transfer cells. Mechanisms controlling cell fate speciation in the rhizodermis were investigated by application of hormones and hormone antagonists. In addition, the effect of Fe deficiency on root epidermal patterning and Fe(III)-reduction activity was examined. In the iron-hyperaccumulating pea mutants dgl and brz and in the Arabidopsis mutant man1 Fe(III)-reduction activity was found to be up-regulated under both high and low iron supply. In contrast, morphological responses such as the development of transfer cells and extranumerary root hairs was repressed by a high iron concentration in the external medium. All morphological responses can be mimicked by exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or the auxin analog 2,4-dichlorophenoxyacetic acid (2,4-D). Conversely, Fe(III)-reduction rates were not influenced or only slightly affected by the hormone treatment. Application of inhibitors of ethylene synthesis, ethylene action or auxin transport was effective only in inhibiting the formation of extra root hairs, indicating that these hormones are not required for transfer cell formation or expression of Fe(III) reduction. These data suggest that the Fe reductase induced by iron stress does not depend on the formation of transfer cells and further imply separate regulatory pathways for the two responses. The data are compatible with a model in which root reduction activity is modulated by a shoot-borne signal coordinating iron uptake with the shoot demand, while the epidermal phenotype is primarily dependent on the intracellular iron concentration of root cells.