Iron deficiency and the structure and physiology of maize chloroplasts

Iron Supplement-Enhanced Growth and Development of Hydrangea macrophylla In Vitro under Normal and High pH

Hydrangea macrophylla is a popular perennial ornamental shrub commercially grown as potted plants, landscape plants, and cut flowers. In the process of reproduction and production of ornamental plants, the absorption of nutrients directly determines the value of the ornamental plants. Hydrangea macrophylla is very sensitive to the content and absorption of the micronutrient iron (Fe) that affects growth of its shoots. However, the physiological activity of Fe as affected by deficiency or supplementation is unknown. This work aimed at preliminary exploring the relationship between Fe and photosynthesis, and also to find the most favorable iron source and level of pH for the growth of H. macrophylla. Two Fe sources, non-chelated iron sulfate (FeSO₄) and iron ethylenediaminetetraacetic acid (Fe-EDTA), were supplemented to the multipurpose medium with a final Fe concentration of 2.78 mg·L^-1. The medium without any Fe supplementation was used as the control. The pH of the agar-solidified medium was adjusted to either 4.70, 5.70, or 6.70, before autoclaving. The experiment was conducted in a culture room for 60 days with 25/18 °C day and night temperatures, and a 16-hour photoperiod provided at a light intensity of 50 mmol·m^-2·s^-1 photosynthetic photon flux density (PPFD) from white light-emitting diodes. Supplementary Fe increased the tissue Fe content, and leaves were greener with the medium pH of 4.70, regardless of the Fe source. Compared to the control, the number of leaves for plantlets treated with FeSO₄ and Fe-EDTA were 2.0 and 1.5 times greater, respectively. The chlorophyll, macronutrient, and micronutrient contents were the greatest with Fe-EDTA at pH 4.70. Furthermore, the Fe in the leaf affected the photosynthesis by regulating stomata development, pigment content, and antioxidant system, and also by adjusting the expression of genes related to Fe absorption, transport, and redistribution. Supplementation of Fe in a form chelated with EDTA along with a medium pH of 4.70 was found to be the best for the growth and development of H. macrophylla plantlets cultured in vitro.

Early-stage iron deficiency alters physiological processes and iron transporter expression, along with photosynthetic and oxidative damage to sorghum

Iron (Fe) starvation in Strategy II plants is a major nutritional problem causing severe visual symptoms and yield reductions. This prompted us to investigate the physiological and molecular consequences of Fe deficiency responses at an early stage in sorghum plants. The Fe-starved sorghum did not show shoot biomass reduction, but the root length, biomass, and chlorophyll synthesis were severely affected. The chlorophyll a fluorescence analysis showed that the quantum yield efficiency of PSII (Fv/Fm) and photosynthesis performance index (Pi_ABS) in young leaves significantly reduced in response to low Fe. Besides, Fe concentration in root and shoot significantly declined in Fe-starved plants relative to Fe-sufficient plants. Accordingly, this Fe reduction in tissues was accompanied by a marked decrease in PS-release in roots. The qPCR experiment showed the downregulation of SbDMAS2 (deoxymugineic acid synthase 2), SbNAS3 (nicotianamine synthase 3), and SbYSL1 (Fe-phytosiderophore transporter yellow stripe 1) in Fe-deprived roots, suggesting that decreased rhizosphere mobilization of Fe(III)-PS contributes to reduced uptake and long-distance transport of Fe. The cis-acting elements of these gene promoters are commonly responsive to abscisic acid and methyl jasmonate, while SbYSL1 additionally responsive to salicylic acid. Further, antioxidant defense either through metabolites or antioxidant enzymes is not efficient in counteracting oxidative damage in Fe-deprived sorghum. These findings may be beneficial for the improvement of sorghum genotypes sensitive to Fe-deficiency through breeding or transgenic approaches.

Ferroportin 3 is a dual-targeted mitochondrial/chloroplast iron exporter necessary for iron homeostasis in Arabidopsis

Mitochondria and chloroplasts are organelles with high iron demand that are particularly susceptible to iron-induced oxidative stress. Despite the necessity of strict iron regulation in these organelles, much remains unknown about mitochondrial and chloroplast iron transport in plants. Here, we propose that Arabidopsis ferroportin 3 (FPN3) is an iron exporter that is dual-targeted to mitochondria and chloroplasts. FPN3 is expressed in shoots, regardless of iron conditions, but its transcripts accumulate under iron deficiency in roots. fpn3 mutants cannot grow as well as the wild type under iron-deficient conditions and their shoot iron levels are lower compared with the wild type. Analyses of iron homeostasis gene expression in fpn3 mutants and inductively coupled plasma mass spectrometry (ICP-MS) measurements show that iron levels in the mitochondria and chloroplasts are increased relative to the wild type, consistent with the proposed role of FPN3 as a mitochondrial/plastid iron exporter. In iron-deficient fpn3 mutants, abnormal mitochondrial ultrastructure was observed, whereas chloroplast ultrastructure was not affected, implying that FPN3 plays a critical role in the mitochondria. Overall, our study suggests that FPN3 is essential for optimal iron homeostasis.

'Candidatus Phytoplasma solani' interferes with the distribution and uptake of iron in tomato

Background

‘Candidatus Phytoplasma solani’ is endemic in Europe and infects a wide range of weeds and cultivated plants. Phytoplasmas are prokaryotic plant pathogens that colonize the sieve elements of their host plant, causing severe alterations in phloem function and impairment of assimilate translocation. Typical symptoms of infected plants include yellowing of leaves or shoots, leaf curling, and general stunting, but the molecular mechanisms underlying most of the reported changes remain largely enigmatic. To infer a possible involvement of Fe in the host-phytoplasma interaction, we investigated the effects of ‘Candidatus Phytoplasma solani’ infection on tomato plants (Solanum lycopersicum cv. Micro-Tom) grown under different Fe regimes.

Results

Both phytoplasma infection and Fe starvation led to the development of chlorotic leaves and altered thylakoid organization. In infected plants, Fe accumulated in phloem tissue, altering the local distribution of Fe. In infected plants, Fe starvation had additive effects on chlorophyll content and leaf chlorosis, suggesting that the two conditions affected the phenotypic readout via separate routes. To gain insights into the transcriptional response to phytoplasma infection, or Fe deficiency, transcriptome profiling was performed on midrib-enriched leaves. RNA-seq analysis revealed that both stress conditions altered the expression of a large (> 800) subset of common genes involved in photosynthetic light reactions, porphyrin / chlorophyll metabolism, and in flowering control. In Fe-deficient plants, phytoplasma infection perturbed the Fe deficiency response in roots, possibly by interference with the synthesis or transport of a promotive signal transmitted from the leaves to the roots.

Conclusions

‘Candidatus Phytoplasma solani’ infection changes the Fe distribution in tomato leaves, affects the photosynthetic machinery and perturbs the orchestration of root-mediated transport processes by compromising shoot-to-root communication.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6062-x) contains supplementary material, which is available to authorized users.

Diurnal Changes in Transcript and Metabolite Levels during the Iron Deficiency Response of Rice

BACKGROUND

Rice (Oryza sativa L.) is highly susceptible to iron (Fe) deficiency due to low secretion levels of the mugineic acid (MA) family phytosiderophore (PS) 2'-deoxymugineic acid (DMA) into the rhizosphere. The low levels of DMA secreted by rice have proved challenging to measure and, therefore, the pattern of DMA secretion under Fe deficiency has been less extensively studied relative to other graminaceous monocot species that secrete high levels of PS, such as barley (Hordeum vulgare L.).

RESULTS

Gene expression and metabolite analyses were used to characterise diurnal changes occurring during the Fe deficiency response of rice. Iron deficiency inducible genes involved in root DMA biosynthesis and secretion followed a diurnal pattern with peak induction occurring 3-5 h after the onset of light; a result consistent with that of other Strategy II plant species such as barley and wheat. Furthermore, triple quadrupole mass spectrometry identified 3-5 h after the onset of light as peak time of DMA secretion from Fe-deficient rice roots. Metabolite profiling identified accumulation of amines associated with metal chelation, metal translocation and plant oxidative stress responses occurring with peak induction 10-12 h after the onset of light.

CONCLUSION

The results of this study confirmed that rice shares a similar peak time of Fe deficiency associated induction of DMA secretion compared to other Strategy II plant species but has less prominent daily fluctuations of DMA secretion. It also revealed metabolic changes associated with the remediation of Fe deficiency and mitigation of damage from resulting stress in rice roots. This study complements previous studies on the genetic changes in response to Fe deficiency in rice and constitutes an important advance towards our understanding of the molecular mechanisms underlying the rice Fe deficiency response.

Rootstock influence on iron uptake responses in Citrus leaves and their regulation under the Fe paradox effect

BACKGROUND AND AIMS

This work evaluates the regulation of iron uptake responses in Citrus leaves and their involvement in the Fe paradox effect.

METHODS

Experiments were performed in field-grown 'Navelina' trees grafted onto two Cleopatra mandarin × Poncirus trifoliata (L.) Raf. hybrids with different Fe-chlorosis symptoms: 030146 (non-chlorotic) and 030122 (chlorotic).

RESULTS

Chlorotic leaves were smaller than non-chlorotic ones for both dry weight (DW) and area basis, and exhibited marked photosynthetic state affection, but reduced catalase and peroxidase enzymatic activities. Although both samples had a similar total Fe concentration on DW, it was lower in chlorotic leaves when expressed on an area basis. A similar pattern was observed for the total Fe concentration in the apoplast and cell sap and in active Fe (Fe2+) concentration. FRO2 gene expression and ferric chelate reductase (FC-R) activity were also lower in chlorotic samples, while HA1 and IRT1 were more induced. Despite similar apoplasmic pH, K+/Ca2+ was higher in chlorotic leaves, and both citrate and malate concentrations in total tissue and apoplast fluid were lower.

CONCLUSION

(1) The rootstock influences Fe acquisition system in the leaf; (2) the increased sensitivity to Fe-deficiency as revealed by chlorosis and decreased biomass, was correlated with lower FC-R activity and lower organic acid level in leaf cells, which could cause a decreased Fe mobility and trigger other Fe-stress responses in this organ to enhance acidification and Fe uptake inside cells; and (3) the chlorosis paradox phenomenon in citrus likely occurs as a combination of a marked FC-R activity impairment in the leaf and the strong growth inhibition in this organ.

Hydrogen sulphide improves adaptation of Zea mays seedlings to iron deficiency

Hydrogen sulphide (H2S) is emerging as a potential molecule involved in physiological regulation in plants. However, whether H2S regulates iron-shortage responses in plants is largely unknown. Here, the role of H2S in modulating iron availability in maize (Zea mays L. cv Canner) seedlings grown in iron-deficient culture solution is reported. The main results are as follows: Firstly, NaHS, a donor of H2S, completely prevented leaf interveinal chlorosis in maize seedlings grown in iron-deficient culture solution. Secondly, electron micrographs of mesophyll cells from iron-deficient maize seedlings revealed plastids with few photosynthetic lamellae and rudimentary grana. On the contrary, mesophyll chloroplasts appeared completely developed in H2S-treated maize seedlings. Thirdly, H2S treatment increased iron accumulation in maize seedlings by changing the expression levels of iron homeostasis- and sulphur metabolism-related genes. Fourthly, phytosiderophore (PS) accumulation and secretion were enhanced by H2S treatment in seedlings grown in iron-deficient solution. Indeed, the gene expression of ferric-phytosiderophore transporter (ZmYS1) was specifically induced by iron deficiency in maize leaves and roots, whereas their abundance was decreased by NaHS treatment. Lastly, H2S significantly enhanced photosynthesis through promoting the protein expression of ribulose-1,5-bisphosphate carboxylase large subunit (RuBISCO LSU) and phosphoenolpyruvate carboxylase (PEPC) and the expression of genes encoding RuBISCO large subunit (RBCL), small subunit (RBCS), D1 protein (psbA), and PEPC in maize seedlings grown in iron-deficient solution. These results indicate that H2S is closely related to iron uptake, transport, and accumulation, and consequently increases chlorophyll biosynthesis, chloroplast development, and photosynthesis in plants.

Remodeling of membrane lipids in iron-starved Chlamydomonas

Chlamydomonas reinhardtii cells exposed to abiotic stresses (e.g. nitrogen, zinc, or phosphorus deficiency) accumulate triacylglycerols (TAG), which are stored in lipid droplets. Here, we report that iron starvation leads to formation of lipid droplets and accumulation of TAGs. This occurs between 12 and 24 h after the switch to iron-starvation medium. C. reinhardtii cells deprived of iron have more saturated fatty acid (FA), possibly due to the loss of function of FA desaturases, which are iron-requiring enzymes with diiron centers. The abundance of a plastid acyl-ACP desaturase (FAB2) is decreased to the same degree as ferredoxin. Ferredoxin is a substrate of the desaturases and has been previously shown to be a major target of the iron deficiency response. The increase in saturated FA (C16:0 and C18:0) is concomitant with the decrease in unsaturated FA (C16:4, C18:3, or C18:4). This change was gradual for diacylglyceryl-N,N,N-trimethylhomoserine (DGTS) and digalactosyldiacylglycerol (DGDG), whereas the monogalactosyldiacylglycerol (MGDG) FA profile remained stable during the first 12 h, whereas MGDG levels were decreasing over the same period of time. These changes were detectable after only 2 h of iron starvation. On the other hand, DGTS and DGDG contents gradually decreased until a minimum was reached after 24-48 h. RNA-Seq analysis of iron-starved C. reinhardtii cells revealed notable changes in many transcripts coding for enzymes involved in FA metabolism. The mRNA abundances of genes coding for components involved in TAG accumulation (diacylglycerol acyltransferases or major lipid droplet protein) were increased. A more dramatic increase at the transcript level has been observed for many lipases, suggesting that major remodeling of lipid membranes occurs during iron starvation in C. reinhardtii.

Iron is involved in the maintenance of circadian period length in Arabidopsis

The homeostasis of iron (Fe) in plants is strictly regulated to maintain an optimal level for plant growth and development but not cause oxidative stress. About 30% of arable land is considered Fe deficient because of calcareous soil that renders Fe unavailable to plants. Under Fe-deficient conditions, Arabidopsis (Arabidopsis thaliana) shows retarded growth, disordered chloroplast development, and delayed flowering time. In this study, we explored the possible connection between Fe availability and the circadian clock in growth and development. Circadian period length in Arabidopsis was longer under Fe-deficient conditions, but the lengthened period was not regulated by the canonical Fe-deficiency signaling pathway involving nitric oxide. However, plants with impaired chloroplast function showed long circadian periods. Fe deficiency and impaired chloroplast function combined did not show additive effects on the circadian period, which suggests that plastid-to-nucleus retrograde signaling is involved in the lengthening of circadian period under Fe deficiency. Expression pattern analyses of the central oscillator genes in mutants defective in CIRCADIAN CLOCK ASSOCIATED1/LATE ELONGATED HYPOCOTYL or GIGANTEA demonstrated their requirement for Fe deficiency-induced long circadian period. In conclusion, Fe is involved in maintaining the period length of circadian rhythm, possibly by acting on specific central oscillators through a retrograde signaling pathway.

Gene regulation of iron-deficiency responses is associated with carbon monoxide and heme oxydase 1 in Chlamydomonas reinhardtii

Carbon monoxide (CO) as an endogenous gaseous molecule regulates a variety of biological processes in animals. However, CO regulating nutrient stress responses in green alga is largely unknown. On the other hand, heme oxydase (HO1 as a rate-limiting enzyme of the first step for heme degration and to catalyze heme into biliverdin (BV), which is concomitant with releasing of CO and ferrous ions, probably participates in the process of CO-regulating response to nutrient stress in green alga. In this paper, we described an observation that CO could regulate iron-homeostasis in iron-starving Chlamydomonas reinhardtii. Exogenous CO at 8 µM was able to prevent the iron deficient-inducing chlorosis and improve chlorophyll accumulation. Expression pattern of FOX1, FTR1 and ferredoxin was up-regulated by CO exposure in iron-deficient mediam. treatment with external CO increasing iron accumulation in iron-deficient C. reinhardtii. Moreover, to get insights into the regulatory role of HO1, we constructed a transgenic alga overexpressing HO1 and HO1 knock-out mutants. The results show that there was no significant influence on chlorosis with HO1 overexpression of C. reinhardtii under iron-deficiency and the chlorophyll accumulation, and gene expression associated with iron deficiency of mutant were greatly improved. Otherwise, those results from HO1 knock-out mutants were opposite to HO1 overexpression mutants. Finally, CO exposure induced NO accumulation in cells. However, such an action could be blocked by NO scavenger cPTIO. These results indicate that CO/HO1 may play an important role in improving green algae adaptation to iron deficiency or cross-talking with NO under the iron deficiency.

Circadian clock adjustment to plant iron status depends on chloroplast and phytochrome function

Plant chloroplasts are not only the main cellular location for storage of elemental iron (Fe), but also the main site for Fe, which is incorporated into chlorophyll, haem and the photosynthetic machinery. How plants measure internal Fe levels is unknown. We describe here a new Fe-dependent response, a change in the period of the circadian clock. In Arabidopsis, the period lengthens when Fe becomes limiting, and gradually shortens as external Fe levels increase. Etiolated seedlings or light-grown plants treated with plastid translation inhibitors do not respond to changes in Fe supply, pointing to developed chloroplasts as central hubs for circadian Fe sensing. Phytochrome-deficient mutants maintain a short period even under Fe deficiency, stressing the role of early light signalling in coupling the clock to Fe responses. Further mutant and pharmacological analyses suggest that known players in plastid-to-nucleus signalling do not directly participate in Fe sensing. We propose that the sensor governing circadian Fe responses defines a new retrograde pathway that involves a plastid-encoded protein that depends on phytochromes and the functional state of chloroplasts.

The circadian clock of Arabidopsis is found to be hardwired to cellular iron levels, with chloroplasts playing a central role in iron sensing.

Overexpression of AtFRO6 in transgenic tobacco enhances ferric chelate reductase activity in leaves and increases tolerance to iron-deficiency chlorosis

The Arabidopsis gene FRO6(AtFRO6) encodes ferric chelate reductase and highly expressed in green tissues of plants. We have expressed the gene AtFRO6 under the control of a 35S promoter in transgenic tobacco plants. High-level expression of AtFRO6 in transgenic plants was confirmed by northern blot analysis. Ferric reductase activity in leaves of transgenic plants grown under iron-sufficient or iron-deficient conditions is 2.13 and 1.26 fold higher than in control plants respectively. The enhanced ferric reductase activity led to increased concentrations of ferrous iron and chlorophyll, and reduced the iron deficiency chlorosis in the transgenic plants, compared to the control plants. In roots, the concentration of ferrous iron and ferric reductase activity were not significantly different in the transgenic plants compared to the control plants. These results suggest that FRO6 functions as a ferric chelate reductase for iron uptake by leaf cells, and overexpression of AtFRO6 in transgenic plants can reduce iron deficiency chlorosis.

Overexpression of AtFRO6 in transgenic tobacco enhances ferric chelate reductase activity in leaves and increases tolerance to iron-deficiency chlorosis

The Arabidopsis gene FRO6(AtFRO6) encodes ferric chelate reductase and highly expressed in green tissues of plants. We have expressed the gene AtFRO6 under the control of a 35S promoter in transgenic tobacco plants. High-level expression of AtFRO6 in transgenic plants was confirmed by northern blot analysis. Ferric reductase activity in leaves of transgenic plants grown under iron-sufficient or iron-deficient conditions is 2.13 and 1.26 fold higher than in control plants respectively. The enhanced ferric reductase activity led to increased concentrations of ferrous iron and chlorophyll, and reduced the iron deficiency chlorosis in the transgenic plants, compared to the control plants. In roots, the concentration of ferrous iron and ferric reductase activity were not significantly different in the transgenic plants compared to the control plants. These results suggest that FRO6 functions as a ferric chelate reductase for iron uptake by leaf cells, and overexpression of AtFRO6 in transgenic plants can reduce iron deficiency chlorosis.

Carbon monoxide improves adaptation of Arabidopsis to iron deficiency

Carbon monoxide (CO) is an endogenous gaseous molecule and regulates a variety of biological processes in animals. However, whether CO regulates nutrient stress responses in plants is largely unknown. In this paper, we described an observation that CO can regulate iron-homeostasis in iron-starved Arabidopsis. Exogenous CO at 50 microm was able to prevent the iron deficient-induced chlorosis and improve chlorophyll accumulation. Expression of AtIRT1, AtFRO2, AtFIT1 and AtFER1 was up-regulated by CO exposure in iron-deficient seedlings. CO-regulated iron homeostasis could also be found in monocot maize and green alga Chlamydomonas reinhardtii. Treatment with external CO increased iron accumulation in iron-deficient Arabidopsis and C. reinhardtii, and restored leaf greening in Maize ys1 and ys3 mutants (defective in Fe uptake). Moreover, endogenous CO level was increased in Arabidopsis under iron-deficiency. Finally, CO exposure induced NO accumulation in root tips. However, such an action could be blocked by NO scavenger cPTIO. These results indicate that CO may play an important role in improving plant adaptation to iron deficiency or cross-talking with NO under the iron deficiency.

A soil bacterium regulates plant acquisition of iron via deficiency-inducible mechanisms

Despite the abundance of iron in nature, it is the third most limiting nutrient for plants due to its minimal solubility in most soils. While certain soil microbes produce chelating agents that enhance the solubility of iron, the effectiveness of such siderophores in the assimilation of iron by plants is debated. With an increasing understanding that select soil microbes play a signaling role in activating growth and stress responses in plants, the question arises as to whether such symbionts regulate iron assimilation. Here we report a previously unidentified mechanism in which the growth-promoting bacterium Bacillus subtilis GB03 activates the plant's own iron acquisition machinery to increase assimilation of metal ions in Arabidopsis. Mechanistic studies reveal that GB03 transcriptionally up-regulates the Fe-deficiency-induced transcription factor 1 (FIT1), which is necessary for GB03-induction of ferric reductase FRO2 and the iron transporter IRT1. In addition, GB03 causes acidification of the rhizosphere by enhancing root proton release and by direct bacterial acidification, thereby facilitating iron mobility. As a result, GB03-exposed plants have elevated endogenous iron levels as well as increased photosynthetic capacity compared with water-treated controls. In contrast, loss-of-function fit1-2 mutants are compromised in terms of enhanced iron assimilation and photosynthetic efficiency triggered by GB03. In all studies reported herein, a physical partition separating roots from bacterial media precludes non-volatile microbial siderophores from contributing to GB03-stimulated iron acquisition. These results demonstrate the potential of microbes to control iron acquisition in plants and emphasize the sophisticated integration of microbial signaling in photosynthetic regulation.

Iron deficiency in rice shoots: identification of novel induced genes using RDA and possible relation to leaf senescence

Rice plants are highly susceptible to Fe-deficiency. Under nutrient deprivation, plant cells undergo extensive metabolic changes for their continued survival. To provide further insight into the pathways induced during Fe-deficiency, rice seedlings were grown for 3, 6 and 9 days in the presence or absence of Fe. Using RDA (Representational Difference Analysis), sequences of 32 induced genes in rice shoots under Fe-deficiency were identified. About 30% of the sequences found have been previously reported as responsive to other abiotic and even biotic stresses. However, this is the first report that indicates their relation to Fe deprivation. Differential expression of selected genes was confirmed by semi-quantitative RT-PCR analysis. The identification of classical senescence-related sequences, such as lipase EC 3.1.1.-, ubiquitin-conjugating enzyme EC 6.3.2.19, beta-Glucosidase EC 3.2.1.21 and cysteine synthase EC 2.5.1.47, besides the higher accumulation of total soluble sugars prior to the decrease of total chlorophyll content in Fe-deficient leaves, indicate that sugar accumulation may be one of the factors leading to premature leaf senescence induced by Fe-deficiency.

Down co-regulation of light absorption, photochemistry, and carboxylation in Fe-deficient plants growing in different environments

The regulation of photosynthesis through changes in light absorption, photochemistry, and carboxylation efficiency has been studied in plants grown in different environments. Iron deficiency was induced in sugar beet (Beta vulgaris L.) by growing plants hydroponically in controlled growth chambers in the absence of Fe in the nutrient solution. Pear (Pyrus communis L.) and peach (Prunus persica L. Batsch) trees were grown in field conditions on calcareous soils, in orchards with Fe deficiency-chlorosis. Gas exchange parameters were measured in situ with actual ambient conditions. Iron deficiency decreased photosynthetic and transpiration rates, instantaneous transpiration efficiencies and stomatal conductances, and increased sub-stomatal CO(2) concentrations in the three species investigated. Photosynthesis versus CO(2) sub-stomatal concentration response curves and chlorophyll fluorescence quenching analysis revealed a non-stomatal limitation of photosynthetic rates under Fe deficiency in the three species investigated. Light absorption, photosystem II, and Rubisco carboxylation efficiencies were down-regulated in response to Fe deficiency in a coordinated manner, optimizing the use of the remaining photosynthetic pigments, electron transport carriers, and Rubisco.

Nitric oxide improves internal iron availability in plants

Iron deficiency impairs chlorophyll biosynthesis and chloroplast development. In leaves, most of the iron must cross several biological membranes to reach the chloroplast. The components involved in the complex internal iron transport are largely unknown. Nitric oxide (NO), a bioactive free radical, can react with transition metals to form metal-nitrosyl complexes. Sodium nitroprusside, an NO donor, completely prevented leaf interveinal chlorosis in maize (Zea mays) plants growing with an iron concentration as low as 10 microM Fe-EDTA in the nutrient solution. S-Nitroso-N-acetylpenicillamine, another NO donor, as well as gaseous NO supply in a translucent chamber were also able to revert the iron deficiency symptoms. A specific NO scavenger, 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, blocked the effect of the NO donors. The effect of NO treatment on the photosynthetic apparatus of iron-deficient plants was also studied. Electron micrographs of mesophyll cells from iron-deficient maize plants revealed plastids with few photosynthetic lamellae and rudimentary grana. In contrast, in NO-treated maize plants, mesophyll chloroplast appeared completely developed. NO treatment did not increase iron content in plant organs, when expressed in a fresh matter basis, suggesting that root iron uptake was not enhanced. NO scavengers 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and methylene blue promoted interveinal chlorosis in iron-replete maize plants (growing in 250 microM Fe-EDTA). Even though results support a role for endogenous NO in iron nutrition, experiments did not establish an essential role. NO was also able to revert the chlorotic phenotype of the iron-inefficient maize mutants yellow stripe1 and yellow stripe3, both impaired in the iron uptake mechanisms. All together, these results support a biological action of NO on the availability and/or delivery of metabolically active iron within the plant.

The metal ion transporter IRT1 is necessary for iron homeostasis and efficient photosynthesis in Arabidopsis thaliana

The mutants irt1-1 and irt1-2 of Arabidopsis thaliana were identified among a collection of T-DNA-tagged lines on the basis of a decrease in the effective quantum yield of photosystem II. The mutations responsible interfere with expression of IRT1, a nuclear gene that encodes the metal ion transporter IRT1. In irt1 mutants, photosensitivity and chlorophyll fluorescence parameters, as well as abundance and composition of the photosynthetic apparatus, are significantly altered. Additional effects of the mutation under greenhouse conditions, including chlorosis and a drastic reduction in growth rate and fertility, are compatible with a deficiency in iron transport. Propagation of irt1 plants on media supplemented with additional quantities of iron salts restores almost all aspects of wild-type behaviour. The irt2-1 mutant, which carries an En insertion in the highly homologous IRT2 gene of Arabidopsis thaliana, was identified by reverse genetics and shows no symptoms of iron deficiency. This, together with the finding that irt1-1 can be complemented by 35S::IRT1 but not by 35S::IRT2, demonstrates that, although the products of the two genes are closely related, only AtIRT1 is required for iron homeostasis under physiological conditions.

Early iron deficiency stress response in leaves of sugar beet

Iron nutrient deficiency was investigated in leaves of hydroponically grown sugar beets (Beta vulgaris) to determine how ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) gene expression is affected when thylakoid components of photosynthesis are diminished. Rubisco polypeptide content was reduced by 60% in severely iron-stressed leaves, and the reduction was directly correlated to chlorophyll content. The concentration of Rubisco protein in iron-stressed leaves was found to be regulated by availability of mRNAs, and CO2 fixation by Rubisco was reduced from 45 mumol CO2 m-2 s-1 in extracts from iron-sufficient leaves to 20 mumol CO2 m-2 s-1 in extracts from severely stressed leaves. The rate of CO2 fixation was directly correlated to leaf chlorophyll content. Rubisco in iron-sufficient control leaves was 59% activated, whereas in severely stressed leaves grown under the same light, Rubisco was 43% activated. RNA synthesis was reduced by about 50% in iron-deficient leaves, but 16S and 25S rRNA and ctDNA were essentially unaffected by iron stress.

The role of iron in phytoplankton photosynthesis, and the potential for iron-limitation of primary productivity in the sea

Iron supply has been suggested to influence phytoplankton biomass, growth rate and species composition, as well as primary productivity in both high and low NO3 (-) surface waters. Recent investigations in the equatorial Pacific suggest that no single factor regulates primary productivity. Rather, an interplay of bottom-up (i.e., ecophysiological) and top-down (i.e., ecological) factors appear to control species composition and growth rates. One goal of biological oceanography is to isolate the effects of single factors from this multiplicity of interactions, and to identify the factors with a disproportionate impact. Unfortunately, our tools, with several notable exceptions, have been largely inadequate to the task. In particular, the standard technique of nutrient addition bioassays cannot be undertaken without introducing artifacts. These so-called 'bottle effects' include reducing turbulence, isolating the enclosed sample from nutrient resupply and grazing, trapping the isolated sample at a fixed position within the water column and thus removing it from vertical movement through a light gradient, and exposing the sample to potentially stimulatory or inhibitory substances on the enclosure walls. The problem faced by all users of enrichment experiments is to separate the effects of controlled nutrient additions from uncontrolled changes in other environmental and ecological factors. To overcome these limitations, oceanographers have sought physiological or molecular indices to diagnose nutrient limitation in natural samples. These indices are often based on reductions in the abundance of photosynthetic and other catalysts, or on changes in the efficiency of these catalysts. Reductions in photosynthetic efficiency often accompany nutrient limitation either because of accumulation of damage, or impairment of the ability to synthesize fully functional macromolecular assemblages. Many catalysts involved in electron transfer and reductive biosyntheses contain iron, and the abundances of most of these catalysts decline under iron-limited conditions. Reductions of ferredoxin or cytochrome f content, nitrate assimilation rates, and dinitrogen fixation rates are amongst the diagnostics that have been used to infer iron limitation in some marine systems. An alternative approach to diagnosing iron-limitation uses molecules whose abundance increases in response to iron-limitation. These include cell surface iron-transport proteins, and the electron transfer protein flavodoxin which replaces the Fe-S protein ferredoxin in many Fe-deficient algae and cyanobacteria.

Limiting Factors in Photosynthesis: III. Effects of Iron Nutrition on the Activities of Three Regulatory Enzymes of Photosynthetic Carbon Metabolism

When Fe was withheld from sugar beets (Beta vulgaris L. CV F58-44H1), the activities of NADP-glyceraldehyde-3-phosphate dehydrogenase, fructose 1,6-bisphosphatase, and ribulose 5-phosphate kinase were not diminished, while chlorophyll per area was decreased by 75%. On resupplying Fe, chlorophyll per area increased to control levels within 5 days, whereas the activities of the three enzymes remained approximately constant. These results support the view advanced earlier (Terry 1980 Plant Physiol 65: 114-120) that the photosynthetic effects of Fe deprivation are mediated by changes in the lamellar components of chloroplasts and not by changes in stromal enzymes involved in photosynthetic carbon reduction.

Limiting Factors in Photosynthesis: II. IRON STRESS DIMINISHES PHOTOCHEMICAL CAPACITY BY REDUCING THE NUMBER OF PHOTOSYNTHETIC UNITS

It has been proposed that Fe stress may be used in the study of limiting factors in photosynthesis as an experimental means of varying photochemical capacity in vivo (Plant Physiol 1980 65: 114-120). In this paper the effect of Fe stress on photosynthetic unit number, size, and composition was investigated by measuring P(700), cytochrome (Cyt) f, chlorophyll (Chl) a, and Chl b in sugar beet leaves. The results show that when Fe stress reduced Chl per unit area by 80% (from 60 to 12 micrograms per square centimeter), it decreased the number of P(700) molecules per unit area by 88% and Cyt f per unit area by 86%; over the same range the Chl to P(700) ratio increased by 37% but there was no significant change in the Chl to Cyt f ratio. These data suggest that Fe stress decreases photochemical capacity and Chl per unit area by diminishing the number of photosynthetic units per unit leaf area.The ratio of Chl a to Chl b did not change with Fe stress. This suggests that the proportion of light-harvesting Chl a/b-protein complex within the photosynthetic unit remained constant. Electron microscopy of chloroplasts revealed that the decrease in the number of photosynthetic units which occurred during Fe stress was accompanied by a reduction in the number of granal and stromal lamellae per chloroplast and by a reduction in the number of thylakoids per granum.

Limiting Factors in Photosynthesis: I. USE OF IRON STRESS TO CONTROL PHOTOCHEMICAL CAPACITY IN VIVO

The possibility of using Fe stress as an experimental tool in the study of limiting factors was explored. Results show that Fe stress decreased the chlorophyll (Chl) a, Chl b, carotene, and xanthophyll content of leaves of sugar beets (Beta vulgaris L.) and that the maximum rate of photosynthetic CO(2) uptake (P(max)) per unit area was linearly related to Chl (a + b) per unit area. Measurements of noncyclic ATP formation by isolated chloroplasts at light saturation indicate that photosynthetic electron transport capacity decreased concomitantly with pigment content under Fe stress.Iron stress decreased Chl per chloroplast but had no effect on the number of leaf cells per unit area, average leaf cell volume, number of chloroplasts per unit area, or leaf soluble protein per unit area. Average chloroplast volume, protein N per chloroplast, and ribulose bisphosphate carboxylase activity were diminished by Fe stress but to a lesser extent than Chl per chloroplast. The reduction in pigment concentration with Fe stress led to a relatively small decrease in light absorption, the fraction of incident light absorbed remaining high (49%) even at very low leaf Chl contents. There was no apparent change in the quantum yield of attached leaves at low irradiances, but at high irradiances, the capacity to convert absorbed light to chemical energy was greatly diminished in Fe-stressed leaves.THE RESULTS SUGGEST: (a) that P(max) per unit area are decreased linearly with Chl per unit area because of a decrease in photochemical capacity rather than a change in light absorption; and (b) that the effect of Fe stress may be sufficiently specific for it to be used as an experimental tool for the control and study of photochemical capacity in vivo.