Iron-dependent modifications of the flower transcriptome, proteome, metabolome, and hormonal content in an Arabidopsis ferritin mutant

Cellular clarity: a logistic regression approach to identify root epidermal regulators of iron deficiency response

BACKGROUND

Plants respond to stress through highly tuned regulatory networks. While prior works identified master regulators of iron deficiency responses in A. thaliana from whole-root data, identifying regulators that act at the cellular level is critical to a more comprehensive understanding of iron homeostasis. Within the root epidermis complex molecular mechanisms that facilitate iron reduction and uptake from the rhizosphere are known to be regulated by bHLH transcriptional regulators. However, many questions remain about the regulatory mechanisms that control these responses, and how they may integrate with developmental processes within the epidermis. Here, we use transcriptional profiling to gain insight into root epidermis-specific regulatory processes.

RESULTS

Set comparisons of differentially expressed genes (DEGs) between whole root and epidermis transcript measurements identified differences in magnitude and timing of organ-level vs. epidermis-specific responses. Utilizing a unique sampling method combined with a mutual information metric across time-lagged and non-time-lagged windows, we identified relationships between clusters of functionally relevant differentially expressed genes suggesting that developmental regulatory processes may act upstream of well-known Fe-specific responses. By integrating static data (DNA motif information) with time-series transcriptomic data and employing machine learning approaches, specifically logistic regression models with LASSO, we also identified putative motifs that served as crucial features for predicting differentially expressed genes. Twenty-eight transcription factors (TFs) known to bind to these motifs were not differentially expressed, indicating that these TFs may be regulated post-transcriptionally or post-translationally. Notably, many of these TFs also play a role in root development and general stress response.

CONCLUSIONS

This work uncovered key differences in -Fe response identified using whole root data vs. cell-specific root epidermal data. Machine learning approaches combined with additional static data identified putative regulators of -Fe response that would not have been identified solely through transcriptomic profiles and reveal how developmental and general stress responses within the epidermis may act upstream of more specialized -Fe responses for Fe uptake.

Lipo-chitooligosaccharide and thuricin 17 act as plant growth promoters and alleviate drought stress in Arabidopsis thaliana

Lipo-chito-oligosaccharide (LCO-from Bradyrhizobium japonicum) and thuricin 17 (Th17-from Bacillus thuringiensis) are bacterial signal compounds from the rhizosphere of soybean that have been shown to enhance plant growth in a range of legumes and non-legumes. In this study, an attempt to quantify phytohormones involved in the initial hours after exposure of Arabidopsis thaliana to these compounds was conducted using UPLC-ESI-MS/MS. A petri-plate assay was conducted to screen for drought stress tolerance to PEG 8000 infusion and plant growth was studied 21-days post-stress. Arabidopsis thaliana plants grown in trays with drought stress imposed by water withhold were used for free proline determination, elemental analysis, and untargeted proteomics using LC-MS/MS studies. At 24 h post-exposure to the signal compounds under optimal growth conditions, Arabidopsis thaliana rosettes varied in their responses to the two signals. While LCO-treated rosettes showed a decrease in total IAA, cytokinins, gibberellins, and jasmonic acid, increases in ABA and SA was very clear. Th17-treated rosettes, on the other hand, showed an increase in IAA and SA. Both treatments resulted in decreased JA levels. Under severe drought stress imposed by PEG 8000 infusion, LCO and Th17 treatments were found to significantly increase fresh and dry weight over drought-stressed control plates, indicating that the presence of the signaling compounds decreased the negative effects experienced by the plants. Free proline content increased in LCO- and Th17-treated plants after water-withhold drought stress. Elemental analysis showed a significant increase in carbon percentage at the lower concentration of Th17. Untargeted proteomics revealed changes in the levels of drought-specific ribosomal proteins, glutathione S-transferase, late embryogenesis proteins, vegetative storage proteins 1 and 2, thaumatin-like proteins, and those related to chloroplast and carbon metabolism. The roles of some of these significantly affected proteins detected under drought stress are discussed.

Unravelling the molecular mechanism underlying drought stress response in chickpea via integrated multi-omics analysis

Drought stress affects growth and productivity significantly in chickpea. An integrated multi-omics analysis can provide a better molecular-level understanding of drought stress tolerance. In the present study, comparative transcriptome, proteome and metabolome analyses of two chickpea genotypes with contrasting responses to drought stress, ICC 4958 (drought-tolerant, DT) and ICC 1882 (drought-sensitive, DS), was performed to gain insights into the molecular mechanisms underlying drought stress response/tolerance. Pathway enrichment analysis of differentially abundant transcripts and proteins suggested the involvement of glycolysis/gluconeogenesis, galactose metabolism, and starch and sucrose metabolism in the DT genotype. An integrated multi-omics analysis of transcriptome, proteome and metabolome data revealed co-expressed genes, proteins and metabolites involved in phosphatidylinositol signaling, glutathione metabolism and glycolysis/gluconeogenesis pathways, specifically in the DT genotype under drought. These stress-responsive pathways were coordinately regulated by the differentially abundant transcripts, proteins and metabolites to circumvent the drought stress response/tolerance in the DT genotype. The QTL-hotspot associated genes, proteins and transcription factors may further contribute to improved drought tolerance in the DT genotype. Altogether, the multi-omics approach provided an in-depth understanding of stress-responsive pathways and candidate genes involved in drought tolerance in chickpea.

Dual specificity and target gene selection by the MADS-domain protein FRUITFULL

How transcription factors attain their target gene specificity and how this specificity may be modulated, acquiring different regulatory functions through the development of plant tissues, is an open question. Here we characterized different regulatory roles of the MADS-domain transcription factor FRUITFULL (FUL) in flower development and mechanisms modulating its activity. We found that the dual role of FUL in regulating floral transition and pistil development is associated with its different in vivo patterns of DNA binding in both tissues. Characterization of FUL protein complexes by liquid chromatography-tandem mass spectrometry and SELEX-seq experiments shows that aspects of tissue-specific target site selection can be predicted by tissue-specific variation in the composition of FUL protein complexes with different DNA binding specificities, without considering the chromatin status of the target region. This suggests a role for dynamic changes in FUL TF complex composition in reshaping the regulatory functions of FUL during flower development.

Diacylglycerol Acyltransferase 3(DGAT3) Is Responsible for the Biosynthesis of Unsaturated Fatty Acids in Vegetative Organs of Paeonia rockii

'Diacylglycerol acyltransferase (DGAT)' acts as a key rate-limiting enzyme that catalyzes the final step of the de novo biosynthesis of triacylglycerol (TAG). The study was to characterize the function of the DGAT3 gene in Paeonia rockii, which is known for its accumulation of high levels of unsaturated fatty acids (UFAs). We identified a DGAT3 gene which encodes a soluble protein that is located within the chloroplasts of P. rockii. Functional complementarity experiments in yeast demonstrated that PrDGAT3 restored TAG synthesis. Linoleic acid (LA, C18:2) and α-linolenic acid (ALA, C18:3) are essential unsaturated fatty acids that cannot be synthesized by the human body. Through the yeast lipotoxicity test, we found that the yeast cell density was largely increased by adding exogenous LA and, especially, ALA to the yeast medium. Further ectopic transient overexpression in Nicotiana benthamiana leaf tissue and stable overexpression in Arabidopsis thaliana indicated that PrDGAT3 significantly enhanced the accumulation of the TAG and UFAs. In contrast, we observed a significant decrease in the total fatty acid content and in several major fatty acids in PrDGAT3-silenced tree peony leaves. Overall, PrDGAT3 is important in catalyzing TAG synthesis, with a substrate preference for UFAs, especially LA and ALA. These results suggest that PrDGAT3 may have practical applications in improving plant lipid nutrition and increasing oil production in plants.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Fe and Zn stress induced gene expression analysis unraveled mechanisms of mineral homeostasis in common bean (Phaseolus vulgaris L.)

Iron (Fe) and zinc (Zn) stress significantly affects fundamental metabolic and physiological processes in plants that results in reduction of plant growth and development. In the present study, common bean variety; Shalimar French Bean-1 (SFB-1) was used as an experimental material. Four different MGRL media i.e. normal MGRL medium (Control), media without Fe (0-Fe), media without Zn (0-Zn) and media with excess Zn (300-Zn) were used for growing seeds of SFB-1 under in vitro condition for three weeks under optimum conditions. Three week old shoot and root tissues were harvested from the plants grown in these four different in vitro conditions and were, subjected to Fe and Zn estimation. Further, extraction of total RNA for differential gene expression of ten candidate genes selected based on our in silico investigation and their classification, phylogeny and expression pattern was unraveled. Expression analysis of three candidate genes (OPT3, NRAMP2 and NRAMP3) in roots revealed possible cross talk among Fe/Zn stress that was further confirmed by observing less accumulation of Fe in roots under both these conditions. However, we observed, higher accumulation of Fe in shoots under 0-Fe condition compared to control that suggests precise sensing for priority based compartmentalization and partitioning leading to higher accumulation of Fe in shoots. Furthermore, the expression analysis of IRT1, FRO1 and Ferritin 1 genes under Fe/Zn stress suggested their role in uptake/transport and signaling of Fe and Zn, whereas the expression of ZIP2, NRAMP1, HA2 and GLP1 genes were highly responsive to Zn in Phaseolus vulgaris. The identified genes highly responsive to Fe and Zn stress condition can be potential candidates for overcoming mineral stress in dicot crop plants.

Iron insufficiency in floral buds impairs pollen development by disrupting tapetum function

Reduction of crop yield due to iron (Fe) deficiency has always been a concern in agriculture. How Fe insufficiency in floral buds affects pollen development remains unexplored. Here, plants transferred to Fe-deficient medium at the reproductive stage had reduced floral Fe content and viable pollen and showed a defective pollen outer wall, all restored by supplying floral buds with Fe. A comparison of differentially expressed genes (DEGs) in Fe-deficient leaves, roots, and anthers suggested that changes in several cellular processes were unique to anthers, including increased lipid degradation. Co-expression analysis revealed that ABORTED MICROSPORES (AMS), DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1, and BASIC HELIX-LOOP-HELIX 089/091/010 encode key upstream transcription factors of Fe deficiency-responsive DEGs involved in tapetum function and development, including tapetal ROS homeostasis, programmed cell death, and pollen outer wall formation-related lipid metabolism. Analysis of RESPIRATORY-BURST OXIDASE HOMOLOG E (RBOHE) gain- and loss-of-function under Fe deficiency indicated that RBOHE- and Fe-dependent regulation cooperatively control anther reactive oxygen species levels and pollen development. Since DEGs in Fe-deficient anthers were not significantly enriched in genes related to mitochondrial function, the changes in mitochondrial status under Fe deficiency, including respiration activity, density, and morphology, were probably because the Fe amount was insufficient to maintain proper mitochondrial protein function in anthers. To sum up, Fe deficiency in anthers may affect Fe-dependent protein function and impact upstream transcription factors and their downstream genes, resulting in extensively impaired tapetum function and pollen development.

Physio-biochemical and molecular assessment of Iron (Fe2+) toxicity responses in contrasting indigenous aromatic Joha rice cultivars of Assam, India

Iron (Fe) toxicity is one of the major abiotic stresses which limits the yield of lowland rice. This study aims to investigate the physiological, biochemical, and molecular aspects of two contrasting aromatic Joha rice, viz., Keteki and Kola Joha of Assam. Oxidative damage caused due to Fe²⁺ toxicity was quantitatively determined. Fe²⁺ toxicity in the growth medium increases the level of ROS and anti-oxidative enzyme activity. Along with the aforementioned damage caused due to Fe²⁺ toxicity, chlorophyll content decreases in both the rice varieties. Detection of Fe³⁺ and Fe²⁺ was also conducted by Perls' Prussian and Turnbull blue method, respectively. In addition, spectrophotometric quantification of Fe²⁺ was determined by 2, 2'-Bipyridyl (Bpy). Above 2.5 mM, Fe²⁺ toxicity was found to be lethal in rice seedlings affecting their total growth and biomass. Gene expression analysis of iron-regulated transporter 1 (OsIRT1), Yellow Stripe-Like 15 (OsYSL15), and ferritin 1 (OsFer1) revealed the differential gene expression over a time period of Fe²⁺ toxicity. Our study suggested that the different parameters which are considered here can be helpful for the better understanding of how aromatic Joha rice performed under Fe²⁺ toxicity which can also help to reveal broader aspects that how gene players are involved in the iron homeostasis mechanism in Joha rice in coming future.

Functional characterization of an novel acyl-CoA:diacylglycerol acyltransferase 3-3 (CsDGAT3-3) gene from Camelina sativa

Diacylglycerol acyltransferases (DGAT) catalyze the final committed step of de novo biosynthesis of triacylglycerol (TAG) in plant seeds. This study was to functionally characterize DGAT3 genes in Camelina sativa, an important oil crops accumulating high levels of unsaturated fatty acids (UFAs) in seeds. Three camelina DGAT3 genes (CsDGAT3-1, CsDGAT3-2 and CsDGAT3-3) were identified, and the encoded proteins were predicted to be cytosolic-soluble proteins present as a homodimer containing the 2Fe-2S domain. They had divergent expression patterns in various tissues, suggesting that they may function in tissue-specific manner with CsDGAT3-1 in roots, CsDGAT3-2 in flowers and young seedlings, and CsDGAT3-3 in developing seeds. Functional complementation assay in yeast demonstrated that CsDGAT3-3 restored TAG synthesis. TAG content and UFAs, particularly eicosenoic acid (EA, 20:1n-9) were largely increased by adding exogenous UFAs in the yeast medium. Further heterogeneously transient expression in N. benthamiana leaves and seed-specific expression in tobacco seeds indicated that CsDGAT3-3 significantly enhanced oil and UFA accumulation with much higher level of EA. Overall, CsDGAT3-3 exhibited a strong abilty catalyzing TAG synthesis and high substrate preference for UFAs, especially for 20:1n-9. The present data provide new insights for further understanding oil biosynthesis mechanism in camelina seeds, indicating that CsDGAT3-3 may have practical applications for increasing both oil yield and quality.

The Geomagnetic Field (GMF) Modulates Nutrient Status and Lipid Metabolism during Arabidopsis thaliana Plant Development

The Geomagnetic field (GMF) is a typical component of our planet. Plant perception of the GMF implies that any magnetic field (MF) variation would induce possible metabolic changes. In this work was we assessed the role of the GMF on Arabidopsis thaliana Col0 mineral nutrition and lipid metabolism during plant development. We reduced the local GMF (about 40 μT) to Near Null Magnetic Field (NNMF, about 30 nT) to evaluate the effects of GMF on Arabidopsis in a time-course (from rosette to seed-set) experiment by studying the lipid content (fatty acids, FA; and surface alkanes, SA) and mineral nutrients. The expression of selected genes involved in lipid metabolism was assessed by Real-Time PCR (qPCR). A progressive increase of SA with carbon numbers between 21 and 28 was found in plants exposed to NNMF from bolting to flowering developmental stages, whereas the content of some FA significantly (p < 0.05) increased in rosette, bolting and seed-set developmental stages. Variations in SA composition were correlated to the differential expression of several Arabidopsis 3-ketoacyl-CoAsynthase (KCS) genes, including KCS1, KCS5, KCS6, KCS8, and KCS12, a lipid transfer protein (LTPG1) and a lipase (LIP1). Ionomic analysis showed a significant variation in some micronutrients (Fe, Co, Mn and Ni) and macronutrients (Mg, K and Ca) during plant development of plants exposed to NNMF. The results of this work show that A. thaliana responds to variations of the GMF which are perceived as is typical of abiotic stress responses.

Genome-Wide Transcriptomic Analysis Reveals a Regulatory Network of Oxidative Stress-Induced Flowering Signals Produced in Litchi Leaves

Litchi is an important subtropical fruit tree that requires an appropriately low temperature to trigger floral initiation. Our previous studies have shown that reactive oxygen species (ROS) are involved in litchi flowering. To identify oxidative stress-induced flowering related genes in leaves, ‘Nuomici’ potted trees were grown at medium low-temperature conditions (18/13 °C for day/night, medium-temperature). The trees were treated with the ROS generator methyl viologen dichloride hydrate (MV) as the MV-generated ROS treatment (MM, medium-temperature plus MV) and water as the control treatment (M, medium-temperature plus water). Sixteen RNA-sequencing libraries were constructed, and each library generated more than 5,000,000 clean reads. A total of 517 differentially expressed genes (DEGs) were obtained. Among those DEGs, plant hormone biosynthesis and signal transduction genes, ROS-specific transcription factors, such as AP2/ERF and WRKY genes, stress response genes, and flowering-related genes FLOWERING LOCUS T1 (FT1) and FLOWERING LOCUS T2 (FT2) were significantly enriched. Then, as a confirmatory experiment, the potted trees were uniformly sprayed with MV, N,N’-dimethylthiourea (DMTU, ROS scavenger) plus MV, and water at medium-temperature. The results showed that the MV-generated ROS promoted flowering and changed related gene expression, but these effects were repressed by DMTU treatment. The results of our studies indicate that ROS could promote flowering and partly bypass chilling for litchi flowering.

Cadmium sulfide quantum dots impact Arabidopsis thaliana physiology and morphology

The differential mechanisms of CdS QDs (Quantum Dots) and Cd ion toxicity to Arabidopsis thaliana (L.) Heynh were investigated. Plants were exposed to 40 and 60 mg L^-1 for CdS QDs and 76.9 and 115.2 mg L^-1 CdSO₄·7H₂O and toxicity was evaluated at 5, 20, 35 (T5, T20, T35) days after exposure. Oxidative stress upon exposure was evaluated by biochemical essays targeting non-enzymatic oxidative stress physiological parameters, including respiration efficiency, total chlorophylls, carotenoids, ABTS and DPPH radicals reduction, total phenolics, GSH redox state, lipid peroxidation. Total Cd in plants was measured with AAS. Root and leaf morphology and element content were assessed in vivo utilizing low-vacuum Environmental Scanning Electron Microscopy (ESEM) with X-ray microanalysis (EDX). This integrated approach allowed identification of unique nanoscale CdS QDs toxicity to the plants that was distinct from CdSO₄ exposure. The analyses highlighted that CdS QDs and Cd ions effects are modulated by the developmental stage of the plant, starting from T20 till T35 the plant development was modulated by the treatments, in particular CdS QDs induced early flowering. Both treatments induced Fe accumulation in roots, but at different intensities, while CdS QDs was associated with Mn increase into plant leaf. CdSO₄ elicited higher levels of oxidative stress compared with QDs, especially the former treatment caused more intense respiration damages and reduction in chlorophyll and carotenoids than the latter. The two types of treatments impact differently on root and leaf morphology.

Draft genome of Tanacetum cinerariifolium, the natural source of mosquito coil

Pyrethrum (Tanacetum cinerariifolium), which is a perennial Asteraceae plant with white daisy-like flowers, is the original source of mosquito coils and is known for the biosynthesis of the pyrethrin class of natural insecticides. However, the molecular basis of the production of pyrethrins by T. cinerariifolium has yet to be fully elucidated. Here, we present the 7.1-Gb draft genome of T. cinerariifolium, consisting of 2,016,451 scaffolds and 60,080 genes predicted with high confidence. Notably, analyses of transposable elements (TEs) indicated that TEs occupy 33.84% of the genome sequence. Furthermore, TEs of the sire and oryco clades were found to be enriched in the T. cinerariifolium-specific evolutionary lineage, occupying a total of 13% of the genome sequence, a proportion approximately 8-fold higher than that in other plants. InterProScan analysis demonstrated that biodefense-related toxic proteins (e.g., ribosome inactivating proteins), signal transduction-related proteins (e.g., histidine kinases), and metabolic enzymes (e.g., lipoxygenases, acyl-CoA dehydrogenases/oxygenases, and P450s) are also highly enriched in the T. cinerariifolium genome. Molecular phylogenetic analysis detected a variety of enzymes with genus-specific multiplication, including both common enzymes and others that appear to be specific to pyrethrin biosynthesis. Together, these data identify possible novel components of the pyrethrin biosynthesis pathway and provide new insights into the unique genomic features of T. cinerariifolium.

Different expression pattern of flowering pathway genes contribute to male or female organ development during floral transition in the monoecious weed Ambrosia artemisiifolia L. (Asteraceae)

The highly allergenic and invasive weed Ambrosia artemisiifolia L. is a monoecius plant with separated male and female flowers. The genetic regulation of floral morphogenesis is a less understood field in the reproduction biology of this species. Therefore the objective of this work was to investigate the genetic control of sex determination during floral organogenesis. To this end, we performed a genome-wide transcriptional profiling of vegetative and generative tissues during the plant development comparing wild-growing and in vitro cultivated plants. RNA-seq on Illumina NextSeq 500 platform with an integrative bioinformatics analysis indicated differences in 80 floral gene expressions depending on photoperiodic and endogenous initial signals. Sex specificity of genes was validated based on RT-qPCR experiments. We found 11 and 16 uniquely expressed genes in female and male transcriptomes that were responsible particularly to maintain fertility and against abiotic stress. High gene expression of homologous such as FD, FT, TFL1 and CAL, SOC1, AP1 were characteristic to male and female floral meristems during organogenesis. Homologues transcripts of LFY and FLC were not found in the investigated generative and vegetative tissues. The repression of AP1 by TFL1 homolog was demonstrated in male flowers resulting exclusive expression of AP2 and PI that controlled stamen and carpel formation in the generative phase. Alterations of male and female floral meristem differentiation were demonstrated under photoperiodic and hormonal condition changes by applying in vitro treatments.

VcFT-induced mobile florigenic signals in transgenic and transgrafted blueberries

FLOWERING LOCUS T (FT) can promote early flowering in annual species, but such role has not been well demonstrated in woody species. We produced self and reciprocal grafts involving non-transgenic blueberry (NT) and transgenic blueberry (T) carrying a 35S-driven blueberry FT (VcFT-OX). We demonstrated that the transgenic VcFT-OX rootstock promoted flowering of non-transgenic blueberry scions in the NT (scion):T (rootstock) grafts. We further analyzed RNA-Seq profiles and six groups of phytohormones in both NT:T and NT:NT plants. We observed content changes of several hormone metabolites, in a descending order, in the transgenic NT:T, non-transgenic NT:T, and non-transgenic NT:NT leaves. By comparing differential expression transcripts (DETs) of these tissues in relative to their control, we found that the non-transgenic NT:T leaves had many DETs shared with the transgenic NT:T leaves, but very few with the transgenic NT:T roots. Interestingly, a number of these shared DETs belong to hormone pathway genes, concurring with the content changes of hormone metabolites in both transgenic and non-transgenic leaves of the NT:T plants. These results suggest that phytohormones induced by VcFT-OX in the transgenic leaves might serve as part of the signals that resulted in early flowering in both transgenic plants and the non-transgenic NT:T scions.

A multielement compound fertilizer used polydopamine and sodium carboxymethyl starch matrices as coatings

In this work, a multielement compound fertilizer (MCF) was fabricated using ammonium zinc phosphate (AZP) as kernel nutrient element, and polydopamine (Pdop) film as inner coating. Besides, sodium carboxymethyl starch (SCS) was proposed as a constituent in AZP@Pdop mixture due to its adhesion, gelling and swelling abilities, to prepare well dispersed suspensions and consolidate the single-coated fertilizer. What's more, iron (Fe), the vital microelement for the growth of crops and alleviating the leaf chlorosis, was chelated by the carboxylate groups of SCS, contributing to make the outer coating compact. The release behavior showed that zinc (Zn), phosphorus (P) and Fe reached 60% cumulative release in 30 days, and the use efficiency of nutrients for corn was about 60%. In summary, this work provides a novel approach to improve the utilization efficiency and prolong duration of the MCF, which might have a potential application in agronomics.

Arabidopsis thaliana DGAT3 is a [2Fe-2S] protein involved in TAG biosynthesis

Acyl-CoA:diacylglycerol acyltransferases 3 (DGAT3) are described as plant cytosolic enzymes synthesizing triacylglycerol. Their protein sequences exhibit a thioredoxin-like ferredoxin domain typical of a class of ferredoxins harboring a [2Fe-2S] cluster. The Arabidopsis thaliana DGAT3 (AtDGAT3; At1g48300) protein is detected in germinating seeds. The recombinant purified protein produced from Escherichia coli, although very unstable, exhibits DGAT activity in vitro. A shorter protein version devoid of its N-terminal putative chloroplast transit peptide, Δ46AtDGAT3, was more stable in vitro, allowing biochemical and spectroscopic characterization. The results obtained demonstrate the presence of a [2Fe-2S] cluster in the protein. To date, AtDGAT3 is the first metalloprotein described as a DGAT.

Physiological and metabolome changes during anther development in wheat (Triticum aestivum L.)

This study used cytology, cytochemistry, and non-targeted metabolomics to investigate the distribution characteristic of polysaccharides, lipids, and all the metabolites present during five wheat (Triticum aestivum L.) anther developmental stages to provide insights into wheat anther development. Anthers were collected from the tetrad through trinucleate stages, and 1.5% (w/v) acetocarmine and 4',6-diamidino-2-phenylindole staining were used to confirm the developmental stage and visualize the nuclei, respectively. Polysaccharides and lipids were detected by staining with periodic acid-Schiff and Sudan Black B, respectively. The integrated optical density of the tapetum and microspores were calculated using IPP6.0 software. Furthermore, the metabolites were identified by gas chromatograph system coupled with a Pegasus HT time-of-flight mass spectrometer (GC-TOF-MS). The results indicated that the interior and exterior surface cells of anthers are orderly. Pollen was rich in numerous nutrient substances (e.g., lipids, insoluble carbohydrates, and others), and formed a normal sperm cell that contained three nuclei, i.e., one vegetative nuclei and two reproductive nuclei in the mature pollen. Semi-thin sectioning indicated that the tapetum cells degraded progressively from the tetrad to late uninucleate stage and disappeared from the bi-to trinucleate stages. Moreover, nutrient substances (lipids and insoluble carbohydrates) accumulated, were synthesized in the pollen, and gradually increased from the tetrad to trinucleate stages. Finally, the metabolomics results identified that 146 metabolites were present throughout the wheat anther developmental stages. Principal component analysis, hierarchical cluster analysis, and metabolite-metabolite correlation revealed distinct dynamic changes in metabolites. The metabolism of organic acids, amino acids, sugars, fatty acids, amines, polyols, and nucleotides were interrelated and involved in the tricarboxylic acid (TCA) cycle and glycolysis. Furthermore, their interactions were revealed using an integrated metabolic map, which indicated that the TCA cycle and glycolysis were very active during anther development to provide the required energy for anther and pollen development. Our study provides valuable insights into the mechanisms of substance metabolism in wheat anthers and can be used for possible application by metabolic engineers for the improvement of cell characteristics or creating new compounds and molecular breeders in improving pollen fertility or creating the ideal male sterile line, to improve wheat yield per unit area to address global food security.

A biodegradable Fe-fertilizer with high mechanical property and sustainable release for potential agriculture and horticulture applications

A low-cost and biodegradable Fe-fertilizer bead with sustained release behaviors and excellent mechanical strength for potential agriculture applications.

A RhABF2/Ferritin module affects rose (Rosa hybrida) petal dehydration tolerance and senescence by modulating iron levels

Plants often develop the capacity to tolerate moderate and reversible environmental stresses, such as drought, and to re-establish normal development once the stress has been removed. An example of this phenomenon is provided by cut rose (Rosa hybrida) flowers, which experience typical reversible dehydration stresses during post-harvest handling after harvesting at the bud stages. The molecular mechanisms involved in rose flower dehydration tolerance are not known, however. Here, we characterized a dehydration- and abscisic acid (ABA)-induced ferritin gene (RhFer1). Dehydration-induced free ferrous iron (Fe²⁺ ) is preferentially sequestered by RhFer1 and not transported outside of the petal cells, to restrict oxidative stresses during dehydration. Free Fe²⁺ accumulation resulted in more serious oxidative stresses and the induction of genes encoding antioxidant enzyme in RhFer1-silenced petals, and poorer dehydration tolerance was observed compared with tobacco rattle virus (TRV) controls. We also determined that RhABF2, an AREB/ABF transcription factor involved in the ABA signaling pathway, can activate RhFer1 expression by directly binding to its promoter. The silencing of RhABF2 decreased dehydration tolerance and disrupted Fe homeostasis in rose petals during dehydration, as did the silencing of RhFer1. Although both RhFer1 and Fe transporter genes are induced during flower natural senescence in plants, the silencing of RhABF2 or RhFer1 accelerates the petal senescence processes. These results suggest that the regulatory module RhABF2/RhFer1 contributes to the maintenance of Fe levels and enhances dehydration tolerance through the action of RhFer1 locally sequestering free Fe²⁺ under dehydration conditions, and plays synergistic roles with transporter genes during flower senescence.

Directing iron transport in dicots: regulation of iron acquisition and translocation

Iron is essential for plant growth and development, but excess iron is cytotoxic. While iron is abundant in soil, it is often a limiting nutrient for plant growth. Consequentially, plants have evolved mechanisms to tightly regulate iron uptake, trafficking and storage. Recent work has contributed to a more comprehensive picture of iron uptake, further elucidating molecular and physiological processes that aid in solubilization of iron and modulation of the root system architecture in response to iron availability. Recent progress in understanding the regulators of the iron deficiency response and iron translocation from root to shoots, and especially to seeds are noteworthy. The molecular bases of iron sensing and signaling are gradually emerging, as well.

Molybdenum and iron mutually impact their homeostasis in cucumber (Cucumis sativus) plants

Molybdenum (Mo) and iron (Fe) are essential micronutrients required for crucial enzyme activities in plant metabolism. Here we investigated the existence of a mutual control of Mo and Fe homeostasis in cucumber (Cucumis sativus). Plants were grown under single or combined Mo and Fe starvation. Physiological parameters were measured, the ionomes of tissues and the ionomes and proteomes of root mitochondria were profiled, and the activities of molybdo-enzymes and the synthesis of molybdenum cofactor (Moco) were evaluated. Fe and Mo were found to affect each other's total uptake and distribution within tissues and at the mitochondrial level, with Fe nutritional status dominating over Mo homeostasis and affecting Mo availability for molybdo-enzymes in the form of Moco. Fe starvation triggered Moco biosynthesis and affected the molybdo-enzymes, with its main impact on nitrate reductase and xanthine dehydrogenase, both being involved in nitrogen assimilation and mobilization, and on the mitochondrial amidoxime reducing component. These results, together with the identification of > 100 proteins differentially expressed in root mitochondria, highlight the central role of mitochondria in the coordination of Fe and Mo homeostasis and allow us to propose the first model of the molecular interactions connecting Mo and Fe homeostasis.

The Understanding of the Plant Iron Deficiency Responses in Strategy I Plants and the Role of Ethylene in This Process by Omic Approaches

Iron (Fe) is an essential plant micronutrient but is toxic in excess. Fe deficiency chlorosis is a major constraint for plant growth and causes severe losses of crop yields and quality. Under Fe deficiency conditions, plants have developed sophisticated mechanisms to keep cellular Fe homeostasis via various physiological, morphological, metabolic, and gene expression changes to facilitate the availability of Fe. Ethylene has been found to be involved in the Fe deficiency responses of plants through pharmacological studies or by the use of ethylene mutants. However, how ethylene is involved in the regulations of Fe starvation responses remains not fully understood. Over the past decade, omics approaches, mainly focusing on the RNA and protein levels, have been used extensively to investigate global gene expression changes under Fe-limiting conditions, and thousands of genes have been found to be regulated by Fe status. Similarly, proteome profiles have uncovered several hallmark processes that help plants adapt to Fe shortage. To find out how ethylene participates in the Fe deficiency response and explore putatively novel regulators for further investigation, this review emphasizes the integration of those genes and proteins, derived from omics approaches, regulated both by Fe deficiency, and ethylene into a systemic network by gene co-expression analysis.

Iron homeostasis in Arabidopsis thaliana: transcriptomic analyses reveal novel FIT-regulated genes, iron deficiency marker genes and functional gene networks

BACKGROUND

FIT (FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR) is the central regulator of iron uptake in Arabidopsis thaliana roots. We performed transcriptome analyses of six day-old seedlings and roots of six week-old plants using wild type, a fit knock-out mutant and a FIT over-expression line grown under iron-sufficient or iron-deficient conditions. We compared genes regulated in a FIT-dependent manner depending on the developmental stage of the plants. We assembled a high likelihood dataset which we used to perform co-expression and functional analysis of the most stably iron deficiency-induced genes.

RESULTS

448 genes were found FIT-regulated. Out of these, 34 genes were robustly FIT-regulated in root and seedling samples and included 13 novel FIT-dependent genes. Three hundred thirty-one genes showed differential regulation in response to the presence and absence of FIT only in the root samples, while this was the case for 83 genes in the seedling samples. We assembled a virtual dataset of iron-regulated genes based on a total of 14 transcriptomic analyses of iron-deficient and iron-sufficient wild-type plants to pinpoint the best marker genes for iron deficiency and analyzed this dataset in depth. Co-expression analysis of this dataset revealed 13 distinct regulons part of which predominantly contained functionally related genes.

CONCLUSIONS

We could enlarge the list of FIT-dependent genes and discriminate between genes that are robustly FIT-regulated in roots and seedlings or only in one of those. FIT-regulated genes were mostly induced, few of them were repressed by FIT. With the analysis of a virtual dataset we could filter out and pinpoint new candidates among the most reliable marker genes for iron deficiency. Moreover, co-expression and functional analysis of this virtual dataset revealed iron deficiency-induced and functionally distinct regulons.

Cytokinin Determines Thiol-Mediated Arsenic Tolerance and Accumulation

The presence of arsenic in soil and water is a constant threat to plant growth in many regions of the world. Phytohormones act in the integration of growth control and stress response, but their role in plant responses to arsenic remains to be elucidated. Here, we show that arsenate [As(V)], the most prevalent arsenic chemical species in nature, causes severe depletion of endogenous cytokinins (CKs) in the model plant Arabidopsis (Arabidopsis thaliana). We found that CK signaling mutants and transgenic plants with reduced endogenous CK levels showed an As(V)-tolerant phenotype. Our data indicate that in CK-depleted plants exposed to As(V), transcript levels of As(V)/phosphate-transporters were similar or even higher than in wild-type plants. In contrast, CK depletion provoked the coordinated activation of As(V) tolerance mechanisms, leading to the accumulation of thiol compounds such as phytochelatins and glutathione, which are essential for arsenic sequestration. Transgenic CK-deficient Arabidopsis and tobacco lines show a marked increase in arsenic accumulation. Our findings indicate that CK is an important regulatory factor in plant adaptation to arsenic stress.

Comparative expression profiling reveals a role of the root apoplast in local phosphate response

BACKGROUND

Plant adaptation to limited phosphate availability comprises a wide range of responses to conserve and remobilize internal phosphate sources and to enhance phosphate acquisition. Vigorous restructuring of root system architecture provides a developmental strategy for topsoil exploration and phosphate scavenging. Changes in external phosphate availability are locally sensed at root tips and adjust root growth by modulating cell expansion and cell division. The functionally interacting Arabidopsis genes, LOW PHOSPHATE RESPONSE 1 and 2 (LPR1/LPR2) and PHOSPHATE DEFICIENCY RESPONSE 2 (PDR2), are key components of root phosphate sensing. We recently demonstrated that the LOW PHOSPHATE RESPONSE 1 - PHOSPHATE DEFICIENCY RESPONSE 2 (LPR1-PDR2) module mediates apoplastic deposition of ferric iron (Fe(3+)) in the growing root tip during phosphate limitation. Iron deposition coincides with sites of reactive oxygen species generation and triggers cell wall thickening and callose accumulation, which interfere with cell-to-cell communication and inhibit root growth.

RESULTS

We took advantage of the opposite phosphate-conditional root phenotype of the phosphate deficiency response 2 mutant (hypersensitive) and low phosphate response 1 and 2 double mutant (insensitive) to investigate the phosphate dependent regulation of gene and protein expression in roots using genome-wide transcriptome and proteome analysis. We observed an overrepresentation of genes and proteins that are involved in the regulation of iron homeostasis, cell wall remodeling and reactive oxygen species formation, and we highlight a number of candidate genes with a potential function in root adaptation to limited phosphate availability. Our experiments reveal that FERRIC REDUCTASE DEFECTIVE 3 mediated, apoplastic iron redistribution, but not intracellular iron uptake and iron storage, triggers phosphate-dependent root growth modulation. We further highlight expressional changes of several cell wall-modifying enzymes and provide evidence for adjustment of the pectin network at sites of iron accumulation in the root.

CONCLUSION

Our study reveals new aspects of the elaborate interplay between phosphate starvation responses and changes in iron homeostasis. The results emphasize the importance of apoplastic iron redistribution to mediate phosphate-dependent root growth adjustment and suggest an important role for citrate in phosphate-dependent apoplastic iron transport. We further demonstrate that root growth modulation correlates with an altered expression of cell wall modifying enzymes and changes in the pectin network of the phosphate-deprived root tip, supporting the hypothesis that pectins are involved in iron binding and/or phosphate mobilization.

Proteomic insights into floral biology

The flower is the most important biological structure for ensuring angiosperms reproductive success. Not only does the flower contain critical reproductive organs, but the wide variation in morphology, color, and scent has evolved to entice specialized pollinators, and arguably mankind in many cases, to ensure the successful propagation of its species. Recent proteomic approaches have identified protein candidates related to these flower traits, which has shed light on a number of previously unknown mechanisms underlying these traits. This review article provides a comprehensive overview of the latest advances in proteomic research in floral biology according to the order of flower structure, from corolla to male and female reproductive organs. It summarizes mainstream proteomic methods for plant research and recent improvements on two dimensional gel electrophoresis and gel-free workflows for both peptide level and protein level analysis. The recent advances in sequencing technologies provide a new paradigm for the ever-increasing genome and transcriptome information on many organisms. It is now possible to integrate genomic and transcriptomic data with proteomic results for large-scale protein characterization, so that a global understanding of the complex molecular networks in flower biology can be readily achieved. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

ZINC FINGER OF ARABIDOPSIS THALIANA12 (ZAT12) Interacts with FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) Linking Iron Deficiency and Oxidative Stress Responses

Plants grown under iron (Fe)-deficient conditions induce a set of genes that enhance the efficiency of Fe uptake by the roots. In Arabidopsis (Arabidopsis thaliana), the central regulator of this response is the basic helix-loop-helix transcription factor FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT). FIT activity is regulated by protein-protein interactions, which also serve to integrate external signals that stimulate and possibly inhibit Fe uptake. In the search of signaling components regulating FIT function, we identified ZINC FINGER OF ARABIDOPSIS THALIANA12 (ZAT12), an abiotic stress-induced transcription factor. ZAT12 interacted with FIT, dependent on the presence of the ethylene-responsive element-binding factor-associated amphiphilic repression motif. ZAT12 protein was found expressed in the root early differentiation zone, where its abundance was modulated in a root layer-specific manner. In the absence of ZAT12, FIT expression was upregulated, suggesting a negative effect of ZAT12 on Fe uptake. Consistently, zat12 loss-of-function mutants had higher Fe content than the wild type at sufficient Fe. We found that under Fe deficiency, hydrogen peroxide (H2O2) levels were enhanced in a FIT-dependent manner. FIT protein, in turn, was stabilized by H2O2 but only in the presence of ZAT12, showing that H2O2 serves as a signal for Fe deficiency responses. We propose that oxidative stress-induced ZAT12 functions as a negative regulator of Fe acquisition. A model where H2O2 mediates the negative regulation of plant responses to prolonged stress might be applicable to a variety of stress conditions.

Integrative Approaches to Enhance Understanding of Plant Metabolic Pathway Structure and Regulation

Huge insight into molecular mechanisms and biological network coordination have been achieved following the application of various profiling technologies. Our knowledge of how the different molecular entities of the cell interact with one another suggests that, nevertheless, integration of data from different techniques could drive a more comprehensive understanding of the data emanating from different techniques. Here, we provide an overview of how such data integration is being used to aid the understanding of metabolic pathway structure and regulation. We choose to focus on the pairwise integration of large-scale metabolite data with that of the transcriptomic, proteomics, whole-genome sequence, growth- and yield-associated phenotypes, and archival functional genomic data sets. In doing so, we attempt to provide an update on approaches that integrate data obtained at different levels to reach a better understanding of either single gene function or metabolic pathway structure and regulation within the context of a broader biological process.

Analysis of Arabidopsis thaliana atfer4-1, atfh and atfer4-1/atfh mutants uncovers frataxin and ferritin contributions to leaf ionome homeostasis

Ferritins are iron-storage proteins involved in the environmental and developmental control of the free iron pool within cells. Plant ferritins are targeted to mitochondria as well as to chloroplasts. AtFer4 is the Arabidopsis thaliana ferritin isoform that can be also targeted to mitochondria. Frataxin is a mitochondrial protein whose role is essential for plants; lack of AtFH frataxin causes early embryo-lethality in Arabidopsis. Because of that, the Arabidopsis atfh KO mutant is propagated in heterozygosis. For exploring the functional interaction between frataxin and ferritin, Arabidopsis double mutant atfer4-1/atfh was isolated and its physiological parameters were measured, as well as its ionome profile, together with those of both atfer4 and atfh single mutants, in different conditions of Fe supply. Impairment of both ferritin and frataxin did not lead to any effect on mitochondrial respiration. However, ionomics revealed that the content of macro- and microelements, occurring when the nutritional Fe supply changes, were altered in the mutants analysed. These results suggest that both ferritin and frataxin can contribute to the composition of the leaf ionome and also confirm ionomics as an excellent tool for detecting alterations in the plant's physiology.

Involvement of Iron-Containing Proteins in Genome Integrity in Arabidopsis Thaliana

The Arabidopsis genome encodes numerous iron-containing proteins such as iron-sulfur (Fe-S) cluster proteins and hemoproteins. These proteins generally utilize iron as a cofactor, and they perform critical roles in photosynthesis, genome stability, electron transfer, and oxidation-reduction reactions. Plants have evolved sophisticated mechanisms to maintain iron homeostasis for the assembly of functional iron-containing proteins, thereby ensuring genome stability, cell development, and plant growth. Over the past few years, our understanding of iron-containing proteins and their functions involved in genome stability has expanded enormously. In this review, I provide the current perspectives on iron homeostasis in Arabidopsis, followed by a summary of iron-containing protein functions involved in genome stability maintenance and a discussion of their possible molecular mechanisms.

The metabolomics of oxidative stress

Oxidative stress resulting from increased availability of reactive oxygen species (ROS) is a key component of many responses of plants to challenging environmental conditions. The consequences for plant metabolism are complex and manifold. We review data on small compounds involved in oxidative stress, including ROS themselves and antioxidants and redox buffers in the membrane and soluble phases, and we discuss the wider consequences for plant primary and secondary metabolism. While metabolomics has been exploited in many studies on stress, there have been relatively few non-targeted studies focused on how metabolite signatures respond specifically to oxidative stress. As part of the discussion, we present results and reanalyze published datasets on metabolite profiles in catalase-deficient plants, which can be considered to be model oxidative stress systems. We emphasize the roles of ROS-triggered changes in metabolites as potential oxidative signals, and discuss responses that might be useful as markers for oxidative stress. Particular attention is paid to lipid-derived compounds, the status of antioxidants and antioxidant breakdown products, altered metabolism of amino acids, and the roles of phytohormone pathways.

Iron- and ferritin-dependent reactive oxygen species distribution: impact on Arabidopsis root system architecture

Iron (Fe) homeostasis is integrated with the production of reactive oxygen species (ROS), and distribution at the root tip participates in the control of root growth. Excess Fe increases ferritin abundance, enabling the storage of Fe, which contributes to protection of plants against Fe-induced oxidative stress. AtFer1 and AtFer3 are the two ferritin genes expressed in the meristematic zone, pericycle and endodermis of the Arabidopsis thaliana root, and it is in these regions that we observe Fe stained dots. This staining disappears in the triple fer1-3-4 ferritin mutant. Fe excess decreases primary root length in the same way in wild-type and in fer1-3-4 mutant. In contrast, the Fe-mediated decrease of lateral root (LR) length and density is enhanced in fer1-3-4 plants due to a defect in LR emergence. We observe that this interaction between excess Fe, ferritin, and root system architecture (RSA) is in part mediated by the H2O2/O2·- balance between the root cell proliferation and differentiation zones regulated by the UPB1 transcription factor. Meristem size is also decreased in response to Fe excess in ferritin mutant plants, implicating cell cycle arrest mediated by the ROS-activated SMR5/SMR7 cyclin-dependent kinase inhibitors pathway in the interaction between Fe and RSA.

Correlation analysis of proteins responsive to Zn, Mn, or Fe deficiency in Arabidopsis roots based on iTRAQ analysis

For discovering the functional correlation between the identified and quantified proteins by iTRAQ analysis, here we propose a correlation analysis method with cosine correlation coefficients as a powerful tool. iTRAQ analysis is a quantitative proteomics approach that enables identification and quantification of a large number of proteins. In order to obtain proteins responsive to Zn, Mn, or Fe mineral deficiency, we conducted iTRAQ analysis using a microsomal fraction of protein extractions from Arabidopsis root tissues. We identified and quantified 730 common proteins in three biological replicates with less than 1 % false discovery rate. To determine the role of these proteins in tolerating mineral deficiencies and their relation to each other, we calculated cosine correlation coefficients and represented the outcomes on a correlation map for visual understanding of functional relations among the identified proteins. Functionally similar proteins were gathered into the same clusters. Interestingly, a cluster of proteins (FRO2, IRT1, AHA2, PDR9/ABCG37, and GLP5) highly responsive to Fe deficiency was identified, which included both known and unknown novel proteins involved in tolerating Fe deficiency. We propose that the correlation analysis with the cosine correlation coefficients is a powerful method for finding important proteins of interest to several biological processes through comprehensive data sets.

A high-resolution gene expression map of the Arabidopsis shoot meristem stem cell niche

The shoot apical meristem (SAM) acts as a reservoir for stem cells. The central zone (CZ) harbors stem cells. The stem cell progenitors differentiate in the adjacent peripheral zone and in the rib meristem located just beneath the CZ. The SAM is further divided into distinct clonal layers: the L1 epidermal, L2 sub-epidermal and L3 layers. Collectively, SAMs are complex structures that consist of cells of different clonal origins that are organized into functional domains. By employing fluorescence-activated cell sorting, we have generated gene expression profiles of ten cell populations that belong to different clonal layers as well as domains along the central and peripheral axis. Our work reveals that cells in distinct clonal layers exhibit greater diversity in gene expression and greater transcriptional complexity than clonally related cell types in the central and peripheral axis. Assessment of molecular functions and biological processes reveals that epidermal cells express genes involved in pathogen defense: the L2 layer cells express genes involved in DNA repair pathways and telomere maintenance, and the L3 layers express transcripts involved in ion balance and salt tolerance besides photosynthesis. Strikingly, the stem cell-enriched transcriptome comprises very few hormone-responsive transcripts. In addition to providing insights into the expression profiles of hundreds of transcripts, the data presented here will act as a resource for reverse genetic analysis and will be useful in deciphering molecular pathways involved in cell type specification and their functions.

Root iron uptake efficiency of Ulmus laevis and U. minor and their distribution in soils of the Iberian Peninsula

The calcifuge and calcicole character of wild plants has been related to nutrient availability shortages, including iron (Fe)-deficiency. Surprisingly, just a few studies examined the relation between root Fe uptake and plant distribution in different soil types. We assessed the root Fe acquisition efficiency of two Ulmus species with calcareous (Ulmus minor) and siliceous (U. laevis) soil distribution patterns in the Iberian Peninsula. Seedlings of both elm species were grown hydroponically with different Fe concentrations during 6 weeks. Plant physiological responses to Fe-limiting conditions were evaluated as were the ferric reductase activity and proton (H(+)) extrusion capacity of the roots. Iron deprived elm seedlings of both species were stunted and suffered severe Fe-chlorosis symptoms. After Fe re-supply leaf chlorophyll concentrations rose according to species-dependent patterns. While U. minor leaves and seedlings re-greened evenly, U. laevis did so along the nerves of new growing leaves. U. minor had a higher root ferric reductase activity and H(+)-extrusion capability than U. laevis and maintained a better nutrient balance when grown under Fe-limiting conditions. The two elm species were found to have different Fe acquisition efficiencies which may be related to their natural distribution in calcareous and siliceous soils of the Iberian Peninsula.