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

Physiological and molecular mechanisms of plant-root responses to iron toxicity

The chemical form and physiological activity of iron (Fe) in soil are dependent on soil pH and redox potential (Eh), and Fe levels in soils are frequently elevated to the point of causing Fe toxicity in plants, with inhibition of normal physiological activities and of growth and development. In this review, we describe how iron toxicity triggers important physiological changes, including nitric-oxide (NO)-mediated potassium (K+) efflux at the tips of roots and accumulation of reactive oxygen species (ROS) and reactive nitrogen (RNS) in roots, resulting in physiological stress. We focus on the root system, as the first point of contact with Fe in soil, and describe the key processes engaged in Fe transport, distribution, binding, and other mechanisms that are drawn upon to defend against high-Fe stress. We describe the root-system regulation of key physiological processes and of morphological development through signaling substances such as ethylene, auxin, reactive oxygen species, and nitric oxide, and discuss gene-expression responses under high Fe. We especially focus on studies on the physiological and molecular mechanisms in rice and Arabidopsis under high Fe, hoping to provide a valuable theoretical basis for improving the ability of crop roots to adapt to soil Fe toxicity.

Transcriptome profiling of barnyard millet (Echinochloa frumentacea L.) during grain development to reveal the genomic insights into iron accumulation

In the realm of food nutritional security, the development of mineral-rich grains assumes a pivotal role in combating malnutrition. Within the scope of the current investigation, we endeavoured to discern the transcripts accountable for the improved accumulation of grain-Fe within Indian barnyard millet. This pursuit entailed transcriptome sequencing of genotypes BAR-1433 (with high Fe content) and BAR-1423 (with low Fe content) during two distinct stages of spike development-spike emergence and milking stage. In the context of spike emergence, we identified a cohort of 895 up-regulated transcripts and 126 down-regulated transcripts that delineated the difference between the high and low grain-Fe genotypes. In contrast, during the milking stage, the tally of up-regulated transcripts reached 436, while down-regulated transcripts numbered 285. The transcripts that consistently ascended in both developmental stages underwent functional annotation, aligning their roles with nucleolar proteins, metal-nicotianamine transporters, ribonucleoprotein complexes, vinorine synthases, cellulose synthases, auxin response factors, embryogenesis abundant proteins, cytochrome c oxidases, and zinc finger BED domain-containing proteins. Meanwhile, a heterogeneous spectrum of transcripts exhibited differential expression and upregulation throughout the distinct stages. These transcripts encompassed various facets, such as ABC Transporter family proteins, Calcium-dependent kinase family, Ferritin, Metal ion binding, Iron-sulfur cluster binding, Cytochrome family, Zinc finger transcription factor family, Ferredoxin-NADP reductase type 1 family, Putative laccase, Multicopper oxidase family, and Terpene synthase family. To authenticate the reliability of these transcripts, six contigs representing probable functions, including metal transporters, iron sulfur coordination, metal ion binding, auxin-responsive GH3-like protein 2, and cytochrome P450 71B16, were harnessed for primer design. Subsequently, these primers were utilized in the validation process through qRT-PCR, with the outcomes aligning harmoniously with the transcriptome results. This study chronicles a constellation of genes linked to elevated iron content within barnyard millet, showcasing a proof of concept for leveraging transcriptome insights in marker-assisted selection to fortify barnyard millet with iron. This marks the inaugural comprehensive transcriptome analysis delineating transcripts associated with varying levels of grain-iron content during the panicle developmental stages within the barnyard millet paradigm.

Mechanistic insights into DNA damage recognition and checkpoint control in plants

The plant DNA damage response (DDR) pathway safeguards genomic integrity by rapid recognition and repair of DNA lesions that, if unrepaired, may cause genome instability. Most frequently, DNA repair goes hand in hand with a transient cell cycle arrest, which allows cells to repair the DNA lesions before engaging in a mitotic event, but consequently also affects plant growth and yield. Through the identification of DDR proteins and cell cycle regulators that react to DNA double-strand breaks or replication defects, it has become clear that these proteins and regulators form highly interconnected networks. These networks operate at both the transcriptional and post-transcriptional levels and include liquid-liquid phase separation and epigenetic mechanisms. Strikingly, whereas the upstream DDR sensors and signalling components are well conserved across eukaryotes, some of the more downstream effectors are diverged in plants, probably to suit unique lifestyle features. Additionally, DDR components display functional diversity across ancient plant species, dicots and monocots. The observed resistance of DDR mutants towards aluminium toxicity, phosphate limitation and seed ageing indicates that gaining knowledge about the plant DDR may offer solutions to combat the effects of climate change and the associated risk for food security.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

Over-expression of the BnVIT-L2 gene improves the lateral root development and biofortification under iron stress

The vacuolar iron transporter (VIT) family is responsible for absorbing and storing iron ions in vacuoles. Here, the BnVIT-L2 gene from Brassica napus has been cloned for the first time and was found to be expressed in multiple tissues and organs, induced by iron stress. The BnVIT-L2 protein is located in vacuolar membranes and has the ability to bind both iron and other bivalent metal ions. Over-expression of the BnVIT-L2 gene increased lateral root number and main root length, as well as chlorophyll and iron content in transgenic Arabidopsis plants (BnVIT-L2/At) exposed to iron stress, compared to wild type Col-0. Furthermore, over-expression of this gene improved the adaptability of transgenic B. napus plants (BnVIT-L2-OE) under iron stress. The regulation of plant tolerance under iron stress by BnVIT-L2 gene may involve in the signal of reactive oxygen species (ROS), as suggested by Ribosome profiling sequencing (Ribo-seq). This study provides a reference for investigating plant growth and biofortification under iron stress through the BnVIT-L2 gene.

Ferric reduction oxidase in Lilium pumilum affects plant saline-alkaline tolerance by regulating ROS homeostasis

Ferric reduction oxidase (FRO) plays important roles in biotic and abiotic stress. However, the function of ferric reduction oxidase from Lilium pumilum in response to NaHCO3 is unknown. Here we report the functional characterization of ferric reduction oxidase 7 in Lilium pumilum (LpFRO7) in stresses. Under NaHCO3 stress, the LpFRO7 overexpression lines exhibited lower accumulation of reactive oxygen species (ROS), higher activities in antioxidant enzyme (CAT, SOD and POD) and ferrite reductase, resulting in improved tolerance compared to the wild type (WT). In order to determine the functional network of LpFRO7, it was confirmed by EMSA assays, Yeast one-hybrid assays and Dual luciferase reporter assays that LpbHLH115 transcription factor can bind to the promoter of LpFRO7. Yeast two-hybrid assays, BiFC, and LCI assays were performed to prove that LpFRO7 can interact with LpTrx. Combining these findings, we concluded that LpFRO7 affects plant saline-alkaline tolerance by regulating ROS homeostasis.

Lateral root primordium: Formation, influencing factors and regulation

Roots are the primary determinants of water and nutrient uptake by plants. The structure of roots is largely determined by the repeated formation of new lateral roots (LR). A new lateral root primordium (LRP) is formed between the beginning and appearance of LR, which defines the organization and function of LR. Therefore, proper LRP morphogenesis is a crucial process for lateral root formation. The development of LRP is regulated by multiple factors, including hormone and environmental signals. Roots integrate signals and regulate growth and development. At the molecular level, many genes regulate the growth and development of root organs to ensure stable development plans, while also being influenced by various environmental factors. To gain a better understanding of the LRP formation and its influencing factors, this study summarizes previous research. The cell cycle involved in LRP formation, as well as the roles of ROS, auxin, other auxin-related plant hormones, and genetic regulation, are discussed in detail. Additionally, the effects of gravity, mechanical stress, and cell death on LRP formation are explored. Throughout the text unanswered or poorly understood questions are identified to guide future research in this area.

GmABCG5, an ATP-binding cassette G transporter gene, is involved in the iron deficiency response in soybean

Iron deficiency is a major nutritional problem causing iron deficiency chlorosis (IDC) and yield reduction in soybean, one of the most important crops. The ATP-binding cassette G subfamily plays a crucial role in substance transportation in plants. In this study, we cloned the GmABCG5 gene from soybean and verified its role in Fe homeostasis. Analysis showed that GmABCG5 belongs to the ABCG subfamily and is subcellularly localized at the cell membrane. From high to low, GmABCG5 expression was found in the stem, root, and leaf of young soybean seedlings, and the order of expression was flower, pod, seed stem, root, and leaf in mature soybean plants. The GUS assay and qRT-PCR results showed that the GmABCG5 expression was significantly induced by iron deficiency in the leaf. We obtained the GmABCG5 overexpressed and inhibitory expressed soybean hairy root complexes. Overexpression of GmABCG5 promoted, and inhibition of GmABCG5 retarded the growth of soybean hairy roots, independent of nutrient iron conditions, confirming the growth-promotion function of GmABCG5. Iron deficiency has a negative effect on the growth of soybean complexes, which was more obvious in the GmABCG5 inhibition complexes. The chlorophyll content was increased in the GmABCG5 overexpression complexes and decreased in the GmABCG5 inhibition complexes. Iron deficiency treatment widened the gap in the chlorophyll contents. FCR activity was induced by iron deficiency and showed an extraordinary increase in the GmABCG5 overexpression complexes, accompanied by the greatest Fe accumulation. Antioxidant capacity was enhanced when GmABCG5 was overexpressed and reduced when GmABCG5 was inhibited under iron deficiency. These results showed that the response mechanism to iron deficiency is more actively mobilized in GmABCG5 overexpression seedlings. Our results indicated that GmABCG5 could improve the plant's tolerance to iron deficiency, suggesting that GmABCG5 might have the function of Fe mobilization, redistribution, and/or secretion of Fe substances in plants. The findings provide new insights into the ABCG subfamily genes in the regulation of iron homeostasis in plants.

Auxin-responsive ROS homeostasis genes display dynamic expression pattern during rice crown root primordia morphogenesis

Reactive oxygen species (ROS) are generated continuously as a by-product of aerobic metabolism in plants. While excessive ROS cause oxidative stresses in cells, they act as signaling molecules when maintained at an optimum concentration through the dynamic equilibrium of ROS metabolizing mechanisms to regulate growth, development and response to environmental stress. Auxin and its crosstalk with other signaling cascades are crucial for maintaining ROS homeostasis and orchestrating root architecture but dissecting the underlying mechanism requires detailed investigation at the molecular level. Rice fibrous root system is primarily composed of shoot-derived adventitious roots (also called crown roots). Here, we uncover auxin-ROS cross-talk during initiation and growth of rice roots. Potassium iodide treatment changes ROS levels that results in an altered rice root architecture. We reveal that auxin induction recover root growth and development defects by recouping level of hydrogen peroxide. By comparing global datasets previously generated by auxin induction and laser capture microdissection-RNA sequencing, we identify the redox-related antioxidants genes from peroxidase, glutathione reductase, glutathione S-transferase, and thioredoxin reductase families whose expression is regulated by the auxin signaling and also display dynamic expression patterns during crown root primordia morphogenesis. The auxin-mediated differential transcriptome data were validated by quantifying expression levels of a set of genes upon auxin induction. Further, in-depth spatio-temporal expression pattern analysis by RNA in situ hybridization shows the spatially restricted expression of selected genes in the developing crown root primordia. Together, our findings uncover molecular components of auxin-ROS crosstalk involved in root organogenesis.

Arabidopsis PDE1 confers phosphate-deficiency tolerance in primary root growth

KEY MESSAGE

PDE1 acts as a mediator of primary root growth in response to Pi deficiency. Phosphorus is commonly considered as a limiting nutrient for plant growth, which is mainly due to the immobility and uneven distribution of phosphate (Pi) in soils so that available Pi is not adequate in the rhizosphere. Although various mediators have been identified in Pi sensing and response, more details need to be uncovered in plant Pi-deficiency tolerance. Here, we isolated a mutant, termed pde1 (phosphate-deficiency sensitive 1), showing the hypersensitive Pi-deficiency-induced growth inhibition of primary roots. PDE1 encodes a hydroxyphenylpyruvate reductase with rare activity in vitro and repressed by Pi deficiency. Histochemical analysis displayed that Pi-deprived pde1 accumulated more Fe and reactive oxygen species (ROS) in primary roots than the wild type (WT). Addition of ferrozine, a Fe2+ chelator, or a ROS scavenger (e.g., thiourea and potassium iodide), alleviated the sensitivity of Pi-deficiency in pde1 primary roots. By contrast, pde1 showed reduced cotyledon expansion rate with treatment of H2O2 compared to WT. Taken together, these results suggested that PDE1 is responsible for regulating primary root growth in response to Pi deficiency, which is associated with ROS.

Heterologous Expression of a Ferritin Homologue Gene PpFer1 from Prunus persica Enhances Plant Tolerance to Iron Toxicity and H2O2 Stress in Arabidopsis thaliana

In plants, ferritin proteins play an important role in iron (Fe) storage which contributes to plant growth and development. However, the biological functions of ferritins in fruit trees are essentially unknown. In this study, three Ferritin genes were isolated from 'Zhentong No. 3' peach, which were named PpFer1-PpFer3. The expression levels of these genes were different in distinct tissues/organs. Notably, PpFer1 was the most abundantly expressed Ferritin family gene in all tested tissues of 'Zhentong No. 3' peach; its expression levels were significantly enhanced throughout the entire peach seedling under Fe toxicity and H2O2 stress, particularly in the leaves. In addition, over-expression of PpFer1 was effective in rescuing the retarded growth of Arabidopsis fer1-2 knockout mutant, embodied in enhanced fresh weight, primary root length, lateral root numbers, total root length, total leaf chlorophyll, stomatal conductance (Gs), net photosynthetic rate (Pn), transpiration rate, and tissue Fe concentration. This study provides insights into understanding the molecular mechanisms of Fe storage and sequestration in perennial fruit trees.

The response of Arabidopsis to the apocarotenoid β-cyclocitric acid reveals a role for SIAMESE-RELATED 5 in root development and drought tolerance

New regulatory functions in plant development and environmental stress responses have recently emerged for a number of apocarotenoids produced by enzymatic or nonenzymatic oxidation of carotenoids. β-Cyclocitric acid (β-CCA) is one such compound derived from β-carotene, which triggers defense mechanisms leading to a marked enhancement of plant tolerance to drought stress. We show here that this response is associated with an inhibition of root growth affecting both root cell elongation and division. Remarkably, β-CCA selectively induced cell cycle inhibitors of the SIAMESE-RELATED (SMR) family, especially SMR5, in root tip cells. Overexpression of the SMR5 gene in Arabidopsis induced molecular and physiological changes that mimicked in large part the effects of β-CCA. In particular, the SMR5 overexpressors exhibited an inhibition of root development and a marked increase in drought tolerance which is not related to stomatal closure. SMR5 up-regulation induced changes in gene expression that strongly overlapped with the β-CCA-induced transcriptomic changes. Both β-CCA and SMR5 led to a down-regulation of many cell cycle activators (cyclins, cyclin-dependent kinases) and a concomitant up-regulation of genes related to water deprivation, cellular detoxification, and biosynthesis of lipid biopolymers such as suberin and lignin. This was correlated with an accumulation of suberin lipid polyesters in the roots and a decrease in nonstomatal leaf transpiration. Taken together, our results identify the β-CCA-inducible and drought-inducible SMR5 gene as a key component of a stress-signaling pathway that reorients root metabolism from growth to multiple defense mechanisms leading to drought tolerance.

Multiple roles of wheat ferritin genes during stress treatment and TaFER5D-1 as a positive regulator in response to drought and salt tolerance

Ferritin not only regulates the plant's iron content but also plays a significant role in the plant's development and resistance to oxidative damage. However, the role of the FER family in wheat has not been systematically elucidated. In this study, 39 FERs identified from wheat and its ancestral species were clustered into two subgroups, and gene members from the same group contain relatively conservative protein models. The structural analyses indicated that the gene members from the same group contained relatively conserved protein models. The cis-acting elements and expression patterns analysis suggested that TaFERs might play an important role combating to abiotic and biotic stresses. In the transcriptional analysis, the TaFER5D-1 gene was found to be significantly up-regulated under drought and salt stresses and was, therefore, selected to further explore the biological functions Moreover, the GFP expression assay revealed the subcellular localization of TaFER5D-1 proteins in the chloroplast, nucleus, membrane and cytoplasm. Over-expression of TaFER5D-1 in transgenic Arabidopsis lines conferred greater tolerance to drought and salt stress. According to the qRT-PCR data, TaFER5D-1 gene over-expression increased the expression of genes related to root development (Atsweet-17 and AtRSL4), iron storage (AtVIT1 and AtYSL1), and stress response (AtGolS1 and AtCOR47). So it is speculated that TaFER5D-1 could improve stress tolerance by promoting root growth, iron storage, and stress-response ability. Thus, the current study provides insight into the role of TaFER genes in wheat.

Bicarbonate rather than high pH in growth medium induced Fe-deficiency chlorosis in dwarfing rootstock quince A (Cydonia oblonga Mill.) but did not impair Fe nutrition of vigorous rootstock Pyrus betulifolia

Introduction

Quince A (Cydonia oblonga Mill.), a typical dwarfing rootstock in pear cultivation, is susceptible to iron (Fe) deficiency in calcareous soils. The aim of this study was to compare the strategies in Fe uptake and utilization in dwarfing rootstock quince A (low Fe efficiency) versus a typical vigorous rootstock Pyrus betulifolia (PB) with high Fe efficiency.

Methods

Quince A and PB were grown in nutrient solution (pH 6.3) for 4 weeks followed by three pH treatments: pH6.3, pH8.3a (adjusted with hydroxide) and pH8.3b (adjusted with bicarbonate). The Fe uptake and utilization indicators of the rootstocks were assessed at the onset of chlorosis symptoms (after 58 days of treatments).

Results and discussion

In contrast to PB, quince A exhibited Fe deficiency chlorosis under bicarbonate (pH8.3b). Bicarbonate stimulated the root proton secretion, inhibited root growth and ferric chelate reductase (FCR) activity in both PB and quince A, whereas high pH without bicarbonate (pH8.3a) stimulated only root proton release. Both species accumulated more Fe in roots under high pH treatments than under pH6.3, resulting in Fe sufficiency in leaves. Both high pH treatments increased the activity of leaf FCR in PB and quince A. However, extractable Fe(II) concentration in leaves was increased by high pH treatments in PB only. This study demonstrated that depressed Fe(III) reduction in leaves caused by bicarbonate rather than high pH explained Fe deficiency in quince A grown in bicarbonate-containing medium.

GWAS identifies candidate genes controlling adventitious rooting in Populus trichocarpa

Adventitious rooting (AR) is critical to the propagation, breeding, and genetic engineering of trees. The capacity for plants to undergo this process is highly heritable and of a polygenic nature; however, the basis of its genetic variation is largely uncharacterized. To identify genetic regulators of AR, we performed a genome-wide association study (GWAS) using 1148 genotypes of Populus trichocarpa. GWASs are often limited by the abilities of researchers to collect precise phenotype data on a high-throughput scale; to help overcome this limitation, we developed a computer vision system to measure an array of traits related to adventitious root development in poplar, including temporal measures of lateral and basal root length and area. GWAS was performed using multiple methods and significance thresholds to handle non-normal phenotype statistics and to gain statistical power. These analyses yielded a total of 277 unique associations, suggesting that genes that control rooting include regulators of hormone signaling, cell division and structure, reactive oxygen species signaling, and other processes with known roles in root development. Numerous genes with uncharacterized functions and/or cryptic roles were also identified. These candidates provide targets for functional analysis, including physiological and epistatic analyses, to better characterize the complex polygenic regulation of AR.

Iron Availability Influences Protein Carbonylation in Arabidopsis thaliana Plants

Protein carbonylation is an irreversible form of post-translational modification triggered by reactive oxygen species in animal and plant cells. It occurs either through the metal-catalyzed oxidation of Lys, Arg, Pro, and Thr side chains or the addition of α, β-unsaturated aldehydes and ketones to the side chains of Cys, Lys, and His. Recent genetic studies concerning plants pointed to an implication of protein carbonylation in gene regulation through phytohormones. However, for protein carbonylation to stand out as a signal transduction mechanism, such as phosphorylation and ubiquitination, it must be controlled in time and space by a still unknown trigger. In this study, we tested the hypothesis that the profile and extent of protein carbonylation are influenced by iron homeostasis in vivo. For this, we compared the profile and the contents of the carbonylated proteins in the Arabidopsis thaliana wild-type and mutant-deficient in three ferritin genes under normal and stress conditions. Additionally, we examined the proteins specifically carbonylated in wild-type seedlings exposed to iron-deficient conditions. Our results indicated that proteins were differentially carbonylated between the wild type and the triple ferritin mutant Fer1-3-4 in the leaves, stems, and flowers under normal growth conditions. The profile of the carbonylated proteins was also different between the wild type and the ferritin triple mutant exposed to heat stress, thus pointing to the influence of iron on the carbonylation of proteins. Consistent with this, the exposure of the seedlings to iron deficiency and iron excess greatly influenced the carbonylation of certain proteins involved in intracellular signal transduction, translation, and iron deficiency response. Overall, the study underlined the importance of iron homeostasis in the occurrence of protein carbonylation in vivo.

SEC14-GOLD protein PATELLIN2 binds IRON-REGULATED TRANSPORTER1 linking root iron uptake to vitamin E

Organisms require micronutrients, and Arabidopsis (Arabidopsis thaliana) IRON-REGULATED TRANSPORTER1 (IRT1) is essential for iron (Fe2+) acquisition into root cells. Uptake of reactive Fe2+ exposes cells to the risk of membrane lipid peroxidation. Surprisingly little is known about how this is avoided. IRT1 activity is controlled by an intracellular variable region (IRT1vr) that acts as a regulatory protein interaction platform. Here, we describe that IRT1vr interacted with peripheral plasma membrane SEC14-Golgi dynamics (SEC14-GOLD) protein PATELLIN2 (PATL2). SEC14 proteins bind lipophilic substrates and transport or present them at the membrane. To date, no direct roles have been attributed to SEC14 proteins in Fe import. PATL2 affected root Fe acquisition responses, interacted with ROS response proteins in roots, and alleviated root lipid peroxidation. PATL2 had high affinity in vitro for the major lipophilic antioxidant vitamin E compound α-tocopherol. Molecular dynamics simulations provided insight into energetic constraints and the orientation and stability of the PATL2-ligand interaction in atomic detail. Hence, this work highlights a compelling mechanism connecting vitamin E with root metal ion transport at the plasma membrane with the participation of an IRT1-interacting and α-tocopherol-binding SEC14 protein.

Impact of soybean-associated plant growth-promoting bacteria on plant growth modulation under alkaline soil conditions

Conventional strategies to manage iron (Fe) deficiency still present drawbacks, and more eco-sustainable solutions are needed. Knowledge on soybean-specific diversity and functional traits of their plant growth-promoting bacteria (PGPB) potentiates their applicability as bioinoculants to foster soybean performance under calcareous soil conditions. This work aimed to assess the efficacy of PGPB, retrieved from soybean tissues/rhizosphere, in enhancing plant growth and development as well as crop yield under alkaline soil conditions. Seventy-six bacterial strains were isolated from shoots (18%), roots (53%), and rhizosphere (29%) of soybean. Twenty-nine genera were identified, with Bacillus and Microbacterium being the most predominant. Based on distinct plant growth-promoting traits, the endophyte Bacillus licheniformis P2.3 and the rhizobacteria Bacillus aerius S2.14 were selected as bioinoculants. In vivo tests showed that soybean photosynthetic parameters, chlorophyll content, total fresh weight, and Fe concentrations were not significantly affected by bioinoculation. However, inoculation with B. licheniformis P2.3 increased pod number (33%) and the expression of Fe-related genes (FRO2, IRT1, F6'H1, bHLH38, and FER4), and decreased FC-R activity (45%). Moreover, bioinoculation significantly affected Mn, Zn, and Ca accumulation in plant tissues. Soybean harbors several bacterial strains in their tissues and in the rhizosphere with capacities related to Fe nutrition and plant growth promotion. The strain B. licheniformis P2.3 showed the best potential to be incorporated in bioinoculant formulations for enhancing soybean performance under alkaline soil conditions.

Liver specific disruption of Glutaredoxin 3 leads to iron accumulation and impaired cellular iron homeostasis

The role mammalian glutaredoxin 3 (Grx3) plays in iron homeostasis is poorly understood. Here we report the generation and characterization of a Grx3 liver-specific knockout (LKO) mouse strain. Grx3 LKO and WT mice had similar growth however, the LKO mice had elevated iron concentration and ROS production leading to impaired liver function and altered cytosolic and nuclear Fe-S cluster assembly. The expression of hepatic FTH1 and other iron homeostasis genes appeared to correlate with the elevation in iron concentration. Interestingly, this increase in hepatic FTH1 showed an inverse correlation with the abundance of autophagy pathway proteins. These findings suggest a crucial role for Grx3 in regulating hepatocyte iron homeostasis by controlling cellular storage protein turnover and recycling via the autophagy pathway.

Biofortification of common bean (Phaseolus vulgaris L.) with iron and zinc: Achievements and challenges

Micronutrient deficiencies (hidden hunger), particularly in iron (Fe) and zinc (Zn), remain one of the most serious public health challenges, affecting more than three billion people globally. A number of strategies are used to ameliorate the problem of micronutrient deficiencies and to improve the nutritional profile of food products. These include (i) dietary diversification, (ii) industrial food fortification and supplements, (iii) agronomic approaches including soil mineral fertilisation, bioinoculants and crop rotations, and (iv) biofortification through the implementation of biotechnology including gene editing and plant breeding. These efforts must consider the dietary patterns and culinary preferences of the consumer and stakeholder acceptance of new biofortified varieties. Deficiencies in Zn and Fe are often linked to the poor nutritional status of agricultural soils, resulting in low amounts and/or poor availability of these nutrients in staple food crops such as common bean. This review describes the genes and processes associated with Fe and Zn accumulation in common bean, a significant food source in Africa that plays an important role in nutritional security. We discuss the conventional plant breeding, transgenic and gene editing approaches that are being deployed to improve Fe and Zn accumulation in beans. We also consider the requirements of successful bean biofortification programmes, highlighting gaps in current knowledge, possible solutions and future perspectives.

Physiological and transcriptomic evidence of antioxidative system and ion transport in chromium detoxification in germinating seedlings of soybean

Chromium (Cr) toxicity impairs the productivity of crops and is a major threat to food security worldwide. However, the effect of Cr toxicity on seed germination and transcriptome of germinating seedlings of soybean crop has not been fully explored. In this study, two Cr-tolerant lines (J82, S125) and two Cr-sensitive ones (LD1, RL) were screened out of twenty-one soybean (Glycine max L.) genotypes based on seed germination rate, seed germinative energy, seed germination index, and growth of germinating seedlings under 50 mg L^-1 Cr treatment. We found that Cr stress inhibits the growth of soybean seed germinating seedlings due to the Cr-induced overaccumulation of reactive oxygen species (ROS). Significantly different levels of element contents, antioxidant enzyme activities, malondialdehyde content were observed in the four soybean genotypes with contrasting Cr tolerance. Further, a total of 13,777 differentially expressed genes (DEGs) were identified in transcriptomic sequencing and 1298 DEGs in six gene modules were found highly correlated with the physiological traits by weighted correlation network analysis (WGCNA) analysis. The DEGs encoding antioxidant enzymes, transcription factors, and ion transporters are proposed to confer Cr tolerance in soybean germinating seedlings by reducing the uptake and translocation of Cr, decreasing the level of ROS, and keeping the osmotic balance in soybean germinating seedings. In conclusion, our study provided a molecular regulation network on soybean Cr tolerance at seed germinating stage and identified candidate genes for molecular breeding of low Cr accumulation soybean cultivars.

Iron nanoparticles induced the growth and physio-chemical changes in Kobresia capillifolia seedlings

Iron nanoparticles (NPs) priming is known to affect the seed germination and seedling growth in many plants. However, whether it has an important role in stimulating the growth of perennial Qinghai-Tibet Plateau plants remains unclear. In this study, the effects of seed priming with different concentrations of nFe2O3 and FeCl3 (10, 50, 100, 500, and 1000 mg L-1) on seed germination, plant growth, photosystem, antioxidant enzyme activities, root morphology, and biomass distribution of Kobresia capillifolia were evaluated under laboratory conditions. The results showed that compared with treatment materials, concentration had more significant effects on K. capillifolia development. There was no significant impact on germination rate were discovered under all treatments, but decreased the seed mildew rate at 100 mg L-1 nFe2O3. Compare with control, Fe-based priming significantly decreased root biomass. All Fe-based treatments increased rubisco activity of leaves, and significantly enhanced Pn at ranged from 10 to 100 mg L-1. Meanwhile, chlorophyll contents were decreased, the chloroplasts were swollen, and thylakoids were disorganized under all Fe treatments. Iron-based priming significantly enhanced SOD, POD, and CAT activities in Kobresia roots. In conclusion, the thick cuticle-covered seed coat of K. capillifolia postponed the penetration of FeNPs into seeds, so FeNPs priming had a weak impact on seed germination. The sustainable release of Fe ions from FeNPs and the uptake of Fe ions by roots affected the physiology, biochemistry and morphology of K. capillifolia. The findings of this study provide an in-depth understanding of how FeNPs impact the alpine meadow plant, K. capillifolia.

ZmRop1 participates in maize defense response to the damage of Spodoptera frugiperda larvae through mediating ROS and soluble phenol production

As plant-specific molecular switches, Rho-like GTPases (Rops) are vital for plant survival in response to biotic and abiotic stresses. However, their roles in plant defense response to phytophagous insect's damage are largely unknown. In this study, the expression levels of nine maize RAC family genes were analyzed after fall armyworm (FAW) larvae infestation. Among the analyzed genes, ZmRop1 was specifically and highly expressed, and its role in maize response to FAW larvae damage was studied. The results showed that upon FAW larvae infestation, salicylic acid and methyl jasmonate treatment ZmRop1 gene transcripts were all down-regulated. However, upon mechanical injury, the expression level of ZmRop1 was up-regulated. Overexpression of ZmRop1 gene in maize plants could improve maize plant resistance to FAW larvae damage. Conversely, silencing of ZmRop1 increased maize plant susceptibility to FAW larvae damage. The analysis of the potential anti-herbivore metabolites, showed that ZmRop1 promoted the enzyme activities of catalase, peroxidase and the expression levels of ZmCAT, ZmPOD, ZmRBOHA and ZmRBOHB, thereby enhancing the reactive oxygen species (ROS) production, including the content of O^2- and H₂O₂. In addition, overexpression or silencing of ZmRop1 could have influence on the content of the total soluble phenol through mediating the activity of polyphenol oxidase. In summary, the results illuminated our understanding of how ZmRop1 participate in maize defense response to FAW larvae damage as a positive regulator through mediating ROS production and can be used as a reference for the green prevention and control of FAW larvae.

Proteomic analysis of the meristematic root zone in contrasting genotypes reveals new insights in drought tolerance in rice

Drought is responsible for major losses in rice production. Root tips contain meristematic and elongation zones that play major roles in determination of root traits and adaptive strategies to drought. In this study we analysed two contrasting genotypes of rice: IR64, a lowland, drought-susceptible, and shallow-rooting genotype; and Azucena, an upland, drought-tolerant, and deep-rooting genotype. Samples were collected of root tips of plants grown under control and water deficit stress conditions. Quantitative proteomics analysis resulted in the identification of 7294 proteins from the root tips of IR64 and 6307 proteins from Azucena. Data are available via ProteomeXchange with identifier PXD033343. Using a Partial Least Square Discriminant Analysis on 4170 differentially abundant proteins, 1138 statistically significant proteins across genotypes and conditions were detected. Twenty two enriched biological processes showing contrasting patterns between two genotypes in response to stress were detected through gene ontology enrichment analysis. This included identification of novel proteins involved in root elongation with specific expression patterns in Azucena, including four Expansins and seven Class III Peroxidases. We also detected an antioxidant network and a metallo-sulfur cluster assembly machinery in Azucena, with roles in reactive oxygen species and iron homeostasis, and positive effects on root cell cycle, growth and elongation.

Role of Heme Oxygenase-1 in Dual Stress Response of Herbicide and Micronutrient Fe in Arabidopsis thaliana

Increasingly intensive agricultural practices are leading not only to herbicide contamination but also to nutritional stress on nontarget plants. This study evaluated the role of heme oxygenase-1 (HO-1) in the dual stress response of herbicide dichlorprop and micronutrient Fe in Arabidopsis thaliana. Our results revealed that co-treatment with 20 μM zinc protoporphyrin (a specific inhibitor of HO-1) reduced the activity of HO-1 by 21.6%, Fe²⁺ content by 19.8%, and MDA content by 20.0%, reducing abnormal iron aggregation and oxidative stress in response to the herbicide compared to treatment with (R)-dichloroprop alone, which has herbicidal activity. Thus, free Fe²⁺ released from HO-1 mediated dichlorprop-induced oxidative stress in the Fenton reaction and affected aberrant Fe aggregation, which also had an enantioselective effect. This study contributes to an in-depth understanding of the toxicity mechanism of herbicides under nutrient stresses, thus providing new strategies to control the environmental risks of herbicides.

The Arabidopsis ATP-Binding Cassette E protein ABCE2 is a conserved component of the translation machinery

ATP-Binding Cassette E (ABCE) proteins dissociate cytoplasmic ribosomes after translation terminates, and contribute to ribosome recycling, thus linking translation termination to initiation. This function has been demonstrated to be essential in animals, fungi, and archaea, but remains unexplored in plants. In most species, ABCE is encoded by a single-copy gene; by contrast, Arabidopsis thaliana has two ABCE paralogs, of which ABCE2 seems to conserve the ancestral function. We isolated apiculata7-1 (api7-1), the first viable, hypomorphic allele of ABCE2, which has a pleiotropic morphological phenotype reminiscent of mutations affecting ribosome biogenesis factors and ribosomal proteins. We also studied api7-2, a null, recessive lethal allele of ABCE2. Co-immunoprecipitation experiments showed that ABCE2 physically interacts with components of the translation machinery. An RNA-seq study of the api7-1 mutant showed increased responses to iron and sulfur starvation. We also found increased transcript levels of genes related to auxin signaling and metabolism. Our results support for the first time a conserved role for ABCE proteins in translation in plants, as previously shown for the animal, fungal, and archaeal lineages. In Arabidopsis, the ABCE2 protein seems important for general growth and vascular development, likely due to an indirect effect through auxin metabolism.

Altered metal distribution in the sr45-1 Arabidopsis mutant causes developmental defects

The plant serine/arginine-rich (SR) splicing factor SR45 plays important roles in several biological processes, such as splicing, DNA methylation, innate immunity, glucose regulation, and abscisic acid signaling. A homozygous Arabidopsis sr45-1 null mutant is viable, but exhibits diverse phenotypic alterations, including delayed root development, late flowering, shorter siliques with fewer seeds, narrower leaves and petals, and unusual numbers of floral organs. Here, we report that the sr45-1 mutant presents an unexpected constitutive iron deficiency phenotype characterized by altered metal distribution in the plant. RNA-Sequencing highlighted severe perturbations in metal homeostasis, the phenylpropanoid pathway, oxidative stress responses, and reproductive development. Ionomic quantification and histochemical staining revealed strong iron accumulation in the sr45-1 root tissues accompanied by iron starvation in aerial parts. Mis-splicing of several key iron homeostasis genes, including BTS, bHLH104, PYE, FRD3, and ZIF1, was observed in sr45-1 roots. We showed that some sr45-1 developmental abnormalities can be complemented by exogenous iron supply. Our findings provide new insight into the molecular mechanisms governing the phenotypes of the sr45-1 mutant.

B3 Transcription Factors Determine Iron Distribution and FERRITIN Gene Expression in Embryo but Do Not Control Total Seed Iron Content

Iron is an essential micronutrient for humans and other organisms. Its deficiency is one of the leading causes of anemia worldwide. The world health organization has proposed that an alternative to increasing iron content in food is through crop biofortification. One of the most consumed part of crops is the seed, however, little is known about how iron accumulation in seed occurs and how it is regulated. B3 transcription factors play a critical role in the accumulation of storage compounds such as proteins and lipids. Their role in seed maturation has been well characterized. However, their relevance in accumulation and distribution of micronutrients like iron remains unknown. In Arabidopsis thaliana and other plant models, three master regulators belonging to the B3 transcription factors family have been identified: FUSCA3 (FUS3), LEAFY COTYLEDON2 (LEC2), and ABSCISIC ACID INSENSITIVE 3 (ABI3). In this work, we studied how seed iron homeostasis is affected in B3 transcription factors mutants using histological and molecular approaches. We determined that iron distribution is modified in abi3, lec2, and fus3 embryo mutants. For abi3-6 and fus3-3 mutant embryos, iron was less accumulated in vacuoles of cells surrounding provasculature compared with wild type embryos. lec2-1 embryos showed no difference in the pattern of iron distribution in hypocotyl, but a dramatic decrease of iron was observed in cotyledons. Interestingly, for the three mutant genotypes, total iron content in dry mutant seeds showed no difference compared to wild type. At the molecular level, we showed that genes encoding the iron storage ferritins proteins are misregulated in mutant seeds. Altogether our results support a role of the B3 transcription factors ABI3, LEC2, and FUS3 in maintaining iron homeostasis in Arabidopsis embryos.

Bacterial-type ferroxidase tunes iron-dependent phosphate sensing during Arabidopsis root development

Access to inorganic phosphate (Pi), a principal intermediate of energy and nucleotide metabolism, profoundly affects cellular activities and plant performance. In most soils, antagonistic Pi-metal interactions restrict Pi bioavailability, which guides local root development to maximize Pi interception. Growing root tips scout the essential but immobile mineral nutrient; however, the mechanisms monitoring external Pi status are unknown. Here, we show that Arabidopsis LOW PHOSPHATE ROOT 1 (LPR1), one key determinant of Fe-dependent Pi sensing in root meristems, encodes a novel ferroxidase of high substrate specificity and affinity (apparent K_M ∼ 2 μM Fe²⁺). LPR1 typifies an ancient, Fe-oxidizing multicopper protein family that evolved early upon bacterial land colonization. The ancestor of streptophyte algae and embryophytes (land plants) acquired LPR1-type ferroxidase from soil bacteria via horizontal gene transfer, a hypothesis supported by phylogenomics, homology modeling, and biochemistry. Our molecular and kinetic data on LPR1 regulation indicate that Pi-dependent Fe substrate availability determines LPR1 activity and function. Guided by the metabolic lifestyle of extant sister bacterial genera, we propose that Arabidopsis LPR1 monitors subtle concentration differentials of external Fe availability as a Pi-dependent cue to adjust root meristem maintenance via Fe redox signaling and cell wall modification. We further hypothesize that the acquisition of bacterial LPR1-type ferroxidase by embryophyte progenitors facilitated the evolution of local Pi sensing and acquisition during plant terrestrialization.

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Arabidopsis thaliana LPR1 multicopper oxidase typifies a novel ferroxidase cohort

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Fe availability tunes LPR1-dependent root responses to phosphate (Pi) limitation

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LPR1 specificity links Fe-Pi interactions to root Pi sensing via redox cycling

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Streptophyte ancestors acquired LPR1-type ferroxidase from soil bacteria by HGT

Metal crossroads in plants: modulation of nutrient acquisition and root development by essential trace metals

The metals iron, zinc, manganese, copper, molybdenum, and nickel are essential for the growth and development of virtually all plant species. Although these elements are required at relatively low amounts, natural factors and anthropogenic activities can significantly affect their availability in soils, inducing deficiencies or toxicities in plants. Because essential trace metals can shape root systems and interfere with the uptake and signaling mechanisms of other nutrients, the non-optimal availability of any of them can induce multi-element changes in plants. Interference by one essential trace metal with the acquisition of another metal or a non-metal nutrient can occur prior to or during root uptake. Essential trace metals can also indirectly impact the plant's ability to capture soil nutrients by targeting distinct root developmental programs and hormone-related processes, consequently inducing largely metal-specific changes in root systems. The presence of metal binding domains in many regulatory proteins also enables essential trace metals to coordinate nutrient uptake by acting at high levels in hierarchical signaling cascades. Here, we summarize the known molecular and cellular mechanisms underlying trace metal-dependent modulation of nutrient acquisition and root development, and highlight the importance of considering multi-element interactions to breed crops better adapted to non-optimal trace metal availabilities.

Plant iron nutrition: the long road from soil to seeds

Iron (Fe) is an essential plant micronutrient since many cellular processes including photosynthesis, respiration, and the scavenging of reactive oxygen species depend on adequate Fe levels; however, non-complexed Fe ions can be dangerous for cells, as they can act as pro-oxidants. Hence, plants possess a complex homeostatic control system for safely taking up Fe from the soil and transporting it to its various cellular destinations, and for its subcellular compartmentalization. At the end of the plant's life cycle, maturing seeds are loaded with the required amount of Fe needed for germination and early seedling establishment. In this review, we discuss recent findings on how the microbiota in the rhizosphere influence and interact with the strategies adopted by plants to take up iron from the soil. We also focus on the process of seed-loading with Fe, and for crop species we also consider its associated metabolism in wild relatives. These two aspects of plant Fe nutrition may provide promising avenues for a better comprehension of the long pathway of Fe from soil to seeds.

Heme oxygenase-nitric oxide crosstalk-mediated iron homeostasis in plants under oxidative stress

Plant growth under abiotic stress conditions significantly enhances intracellular generation of reactive oxygen species (ROS). Oxidative status of plant cells is directly affected by the modulation of iron homeostasis. Among mammals and plants, heme oxygenase-1 (HO-1) is a well-known antioxidant enzyme. It catalyzes oxygenation of heme, thereby producing Fe²⁺, CO and biliverdin as byproducts. The antioxidant potential of HO-1 is primarily due to its catalytic reaction byproducts. Biliverdin and bilirubin possess conjugated π-electrons which escalate the ability of these biomolecules to scavenge free radicals. CO also enhances the ROS scavenging ability of plants cells by upregulating catalase and peroxidase activity. Enhanced expression of HO-1 in plants under oxidative stress accompanies sequestration of iron in specialized iron storage proteins localized in plastids and mitochondria, namely ferritin for Fe³⁺ storage and frataxin for storage of Fe-S clusters, respectively. Nitric oxide (NO) crosstalks with HO-1 at multiple levels, more so in plants under oxidative stress, in order to maintain intracellular iron status. Formation of dinitrosyl-iron complexes (DNICs) significantly prevents Fenton reaction during oxidative stress. DNICs also release NO upon dissociation in target cells over long distance in plants. They also function as antioxidants against superoxide anions and lipidic free radicals. A number of NO-modulated transcription factors also facilitate iron homeostasis in plant cells. Plants facing oxidative stress exhibit modulation of lateral root formation by HO-1 through NO and auxin-dependent pathways. The present review provides an in-depth analysis of the structure-function relationship of HO-1 in plants and mammals, correlating them with their adaptive mechanisms of survival under stress.

MxRop1-MxrbohD1 interaction mediates ROS signaling in response to iron deficiency in the woody plant Malus xiaojinensis

Iron (Fe) deficiency affects crop production and quality. Rho of plants (ROPs) involves in multiple physiological processes in plants. While it has not been well characterized under Fe deficiency, especially in perennial woody plants. In our study, we cloned ROP homologous gene MxRop1 from Malus xiaojinenesis, then overexpressed it in Arabidopsis, showing enhanced plant tolerance to Fe deficiency, which demonstrated its gene function during this stress. Overexpression of MxRop1 also increased reactive oxygen species (ROS) levels. Moreover, active state of MxRop1 (CA-MxRop1) interacted with N-terminal region of MxrbohD1, one ROS synthesis gene. When MxrbohD1 was overexpressed in apple calli, it showed significantly increased H₂O₂ content, fresh weight and FCR activity, while ROS inhibitor application dramatically inhibited FCR activity, demonstrating ROS produced by MxrbohD1 regulated Fe deficiency responses. Furthermore, using Agrobacterium rhizogenes transformation, MxrbohD1 was overexpressed in apple roots, with increased expression of Fe deficiency-induced genes and increased root FCR activity. Under Fe deficiency, it exhibited slight leaf yellowing phenotype. Co-expression of CA-MxRop1 and MxrbohD1 significantly induced ROS generation. Finally, we proposed that MxRop1 interacted with MxrbohD1 to modulate ROS mediated Fe deficiency adaptive responses in Malus xiaojinensis, which will provide a guidance of cultivation of Fe-deficiency tolerant apple plant.

Cross-Talks Between Macro- and Micronutrient Uptake and Signaling in Plants

In nature, land plants as sessile organisms are faced with multiple nutrient stresses that often occur simultaneously in soil. Nitrogen (N), phosphorus (P), sulfur (S), zinc (Zn), and iron (Fe) are five of the essential nutrients that affect plant growth and health. Although these minerals are relatively inaccessible to plants due to their low solubility and relative immobilization, plants have adopted coping mechanisms for survival under multiple nutrient stress conditions. The double interactions between N, Pi, S, Zn, and Fe have long been recognized in plants at the physiological level. However, the molecular mechanisms and signaling pathways underlying these cross-talks in plants remain poorly understood. This review preliminarily examined recent progress and current knowledge of the biochemical and physiological interactions between macro- and micro-mineral nutrients in plants and aimed to focus on the cross-talks between N, Pi, S, Zn, and Fe uptake and homeostasis in plants. More importantly, we further reviewed current studies on the molecular mechanisms underlying the cross-talks between N, Pi, S, Zn, and Fe homeostasis to better understand how these nutrient interactions affect the mineral uptake and signaling in plants. This review serves as a basis for further studies on multiple nutrient stress signaling in plants. Overall, the development of an integrative study of multiple nutrient signaling cross-talks in plants will be of important biological significance and crucial to sustainable agriculture.

Fe acquisition at the crossroad of calcium and reactive oxygen species signaling

Due to its redox properties, iron is both essential and toxic. Therefore, soil iron availability variations pose a significant problem for plants. Recent evidence suggests that calcium and reactive oxygen species coordinate signaling events related to soil iron acquisition. Calcium was found to affect directly IRT1-mediated iron import through the lipid-binding protein EHB1 and to trigger a CBL-CIPK-mediated signaling influencing the activity of the key iron-acquisition transcription factor FIT. In parallel, under prolonged iron deficiency, reactive oxygen species both inhibit FIT function and depend on FIT through the function of the catalase CAT2. We discuss the role of calcium and reactive oxygen species signaling in iron acquisition, with post-translational mechanisms influencing the localization and activity of iron-acquisition regulators and effectors.

Optimal Distribution of Iron to Sink Organs via Autophagy Is Important for Tolerance to Excess Zinc in Arabidopsis

Zinc (Zn) is nutritionally an essential metal element, but excess Zn in the environment is toxic to plants. Autophagy is a major pathway responsible for intracellular degradation. Here, we demonstrate the important role of autophagy in adaptation to excess Zn stress. We found that autophagy-defective Arabidopsis thaliana (atg2 and atg5) exhibited marked excess Zn-induced chlorosis and growth defects relative to wild-type (WT). Imaging and biochemical analyses revealed that autophagic activity was elevated under excess Zn. Interestingly, the excess Zn symptoms of atg5 were alleviated by supplementation of high levels of iron (Fe) to the media. Under excess Zn, in atg5, Fe starvation was especially severe in juvenile true leaves. Consistent with this, accumulation levels of Fe3+ near the shoot apical meristem remarkably reduced in atg5. Furthermore, excision of cotyledons induced severe excess Zn symptoms in WT, similar to those observed in atg5.Our data suggest that Fe3+ supplied from source leaves (cotyledons) via autophagy is distributed to sink leaves (true leaves) to promote healthy growth under excess Zn, revealing a new dimension, the importance of heavy-metal stress responses by the intracellular recycling.

The DnaJ proteins DJA6 and DJA5 are essential for chloroplast iron-sulfur cluster biogenesis

Fe-S clusters are ancient, ubiquitous and highly essential prosthetic groups for numerous fundamental processes of life. The biogenesis of Fe-S clusters is a multistep process including iron acquisition, sulfur mobilization, and cluster formation. Extensive studies have provided deep insights into the mechanism of the latter two assembly steps. However, the mechanism of iron utilization during chloroplast Fe-S cluster biogenesis is still unknown. Here we identified two Arabidopsis DnaJ proteins, DJA6 and DJA5, that can bind iron through their conserved cysteine residues and facilitate iron incorporation into Fe-S clusters by interactions with the SUF (sulfur utilization factor) apparatus through their J domain. Loss of these two proteins causes severe defects in the accumulation of chloroplast Fe-S proteins, a dysfunction of photosynthesis, and a significant intracellular iron overload. Evolutionary analyses revealed that DJA6 and DJA5 are highly conserved in photosynthetic organisms ranging from cyanobacteria to higher plants and share a strong evolutionary relationship with SUFE1, SUFC, and SUFD throughout the green lineage. Thus, our work uncovers a conserved mechanism of iron utilization for chloroplast Fe-S cluster biogenesis.

Genome-resolved metagenomics reveals role of iron metabolism in drought-induced rhizosphere microbiome dynamics

Recent studies have demonstrated that drought leads to dramatic, highly conserved shifts in the root microbiome. At present, the molecular mechanisms underlying these responses remain largely uncharacterized. Here we employ genome-resolved metagenomics and comparative genomics to demonstrate that carbohydrate and secondary metabolite transport functionalities are overrepresented within drought-enriched taxa. These data also reveal that bacterial iron transport and metabolism functionality is highly correlated with drought enrichment. Using time-series root RNA-Seq data, we demonstrate that iron homeostasis within the root is impacted by drought stress, and that loss of a plant phytosiderophore iron transporter impacts microbial community composition, leading to significant increases in the drought-enriched lineage, Actinobacteria. Finally, we show that exogenous application of iron disrupts the drought-induced enrichment of Actinobacteria, as well as their improvement in host phenotype during drought stress. Collectively, our findings implicate iron metabolism in the root microbiome's response to drought and may inform efforts to improve plant drought tolerance to increase food security.

Adaptation of Arabidopsis halleri to extreme metal pollution through limited metal accumulation involves changes in cell wall composition and metal homeostasis

Metallophytes constitute powerful models for the study of metal homeostasis, adaptation to extreme environments and the evolution of naturally selected traits. Arabidopsis halleri is a pseudometallophyte which shows constitutive zinc/cadmium (Zn/Cd) tolerance and Zn hyperaccumulation but high intraspecific variability in Cd accumulation. To examine the molecular basis of the variation in metal tolerance and accumulation, ionome, transcriptome and cell wall glycan array profiles were compared in two genetically close A. halleri populations from metalliferous and nonmetalliferous sites in Northern Italy. The metallicolous population displayed increased tolerance to and reduced hyperaccumulation of Zn, and limited accumulation of Cd, as well as altered metal homeostasis, compared to the nonmetallicolous population. This correlated well with the differential expression of transporter genes involved in trace metal entry and in Cd/Zn vacuolar sequestration in roots. Many cell wall-related genes were also more highly expressed in roots of the metallicolous population. Glycan array and histological staining analyses demonstrated that there were major differences between the two populations in terms of the accumulation of specific root pectin and hemicellulose epitopes. Our results support the idea that both specific cell wall components and regulation of transporter genes play a role in limiting accumulation of metals in A. halleri at contaminated sites.

Reactive oxygen species coordinate the transcriptional responses to iron availability in Arabidopsis

Reactive oxygen species play a central role in the regulation of plant responses to environmental stress. Under prolonged iron (Fe) deficiency, increased levels of hydrogen peroxide (H2O2) initiate signaling events, resulting in the attenuation of Fe acquisition through the inhibition of FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT). As this H2O2 increase occurs in a FIT-dependent manner, our aim was to understand the processes involved in maintaining H2O2 levels under prolonged Fe deficiency and the role of FIT. We identified the CAT2 gene, encoding one of the three Arabidopsis catalase isoforms, as regulated by FIT. CAT2 loss-of-function plants displayed severe susceptibility to Fe deficiency and greatly increased H2O2 levels in roots. Analysis of the Fe homeostasis transcription cascade revealed that H2O2 influences the gene expression of downstream regulators FIT, BHLH genes of group Ib, and POPEYE (PYE); however, H2O2 did not affect their upstream regulators, such as BHLH104 and ILR3. Our data shows that FIT and CAT2 participate in a regulatory loop between H2O2 and prolonged Fe deficiency.

Using natural variation to understand plant responses to iron availability

Iron bioavailability varies dramatically between soil types across the globe. This has given rise to high levels of natural variation in plant iron responses, allowing members of even a single species to thrive across a wide range of soil types. In recent years we have seen the use of genome-wide association analysis to identify natural variants underlying plant responses to changes in iron availability in both Arabidopsis and important crop species. These studies have provided insights into which genes have been important in shaping local adaptation to iron availability in different plant species and have allowed the discovery of novel regulators and mechanisms, not previously identified using mutagenesis approaches. Furthermore, these studies have allowed the identification of markers that can be used to accelerate breeding of future elite varieties with increased resilience to iron stress and improved nutritional quality. The studies highlighted here show that, in addition to studying plant responses to iron alone, it is important to consider these responses within the context of plant nutrition more broadly and to also consider iron regulation in relation to additional traits of agronomic importance such as yield and disease resistance.

Highlighting reactive oxygen species as multitaskers in root development

Reactive oxygen species (ROS) are naturally produced by several redox reactions during plant regular metabolism such as photosynthesis and respiration. Due to their chemical properties and high reactivity, ROS were initially described as detrimental for cells during oxidative stress. However, they have been further recognized as key players in numerous developmental and physiological processes throughout the plant life cycle. Recent studies report the important role of ROS as growth regulators during plant root developmental processes such as in meristem maintenance, in root elongation, and in lateral root, root hair, endodermis, and vascular tissue differentiation. All involve multifaceted interplays between steady-state levels of ROS with transcriptional regulators, phytohormones, and nutrients. In this review, we attempt to summarize recent findings about how ROS are involved in multiple stages of plant root development during cell proliferation, elongation, and differentiation.

Reactive Oxygen Species Link Gene Regulatory Networks During Arabidopsis Root Development

Plant development under altered nutritional status and environmental conditions and during attack from invaders is highly regulated by plant hormones at the molecular level by various signaling pathways. Previously, reactive oxygen species (ROS) were believed to be harmful as they cause oxidative damage to cells; however, in the last decade, the essential role of ROS as signaling molecules regulating plant growth has been revealed. Plant roots accumulate relatively high levels of ROS, and thus, maintaining ROS homeostasis, which has been shown to regulate the balance between cell proliferation and differentiation at the root tip, is important for proper root growth. However, when the balance is disturbed, plants are unable to respond to the changes in the surrounding conditions and cannot grow and survive. Moreover, ROS control cell expansion and cell differentiation processes such as root hair formation and lateral root development. In these processes, the transcription factor-mediated gene expression network is important downstream of ROS. Although ROS can independently regulate root growth to some extent, a complex crosstalk occurs between ROS and other signaling molecules. Hormone signals are known to regulate root growth, and ROS are thought to merge with these signals. In fact, the crosstalk between ROS and these hormones has been elucidated, and the central transcription factors that act as a hub between these signals have been identified. In addition, ROS are known to act as important signaling factors in plant immune responses; however, how they also regulate plant growth is not clear. Recent studies have strongly indicated that ROS link these two events. In this review, we describe and discuss the role of ROS signaling in root development, with a particular focus on transcriptional regulation. We also summarize the crosstalk with other signals and discuss the importance of ROS as signaling molecules for plant root development.

Further insights into the role of bHLH121 in the regulation of iron homeostasis in Arabidopsis thaliana

Iron (Fe) is an important micronutrient for plant growth and development but any excess of Fe is toxic because of the Fe-dependent generation of reactive oxygen species (ROS). Thus, Fe homeostasis must be tightly regulated. In Arabidopsis thaliana, a cascade of transcription factors has been identified as involved in the regulation of this process by modulating the expression of genes related to Fe uptake, transport, and storage. Recently, it was demonstrated that in response to Fe deficiency, bHLH121/URI (UPSTREAM REGULATOR OF IRT1) directly activates the expression of several genes involved in this regulatory network. It was also shown that bHLH121 interacts with ILR3 (bHLH105) and its homologs. Herein it is shown that bHLH121 is necessary for the expression of the main markers of the plant responses to Fe excess, the ferritin genes (i.e. FER1, FER3, and FER4). bHLH121 regulates ferritin genes expression by directly binding to their promoters, at the same locus than the ILR3-PYE repressive complex. Therefore, this study highlight that bHLH121, PYE, and ILR3 form a chain of antagonistic switches that regulate the expression of ferritin genes. The implication of this finding is discussed.

Ethylene promotes seed iron storage during Arabidopsis seed maturation via ERF95 transcription factor

Because Iron (Fe) is an essential element, Fe storage in plant seeds is necessary for seedling establishment following germination. However, the mechanisms controlling seed Fe storage during seed development remain largely unknown. Here we reveal that an ERF95 transcription factor regulates Arabidopsis seed Fe accumulation. We show that expression of ERF95 increases during seed maturation, and that lack of ERF95 reduces seed Fe accumulation, consequently increasing sensitivity to Fe deficiency during seedling establishment. Conversely, overexpression of ERF95 has the opposite effects. We show that lack of ERF95 decreases abundance of FER1 messenger RNA in developing seed, which encodes Fe-sequestering ferritin. Accordingly, a fer1-1 loss-of-function mutation confers reduced seed Fe accumulation, and suppresses ERF95-promoted seed Fe accumulation. In addition, ERF95 binds to specific FER1 promoter GCC-boxes and transactivates FER1 expression. We show that ERF95 expression in maturing seed is dependent on EIN3, the master transcriptional regulator of ethylene signaling. While lack of EIN3 reduces seed Fe content, overexpression of ERF95 rescues Fe accumulation in the seed of ein3 loss-of-function mutant. Finally, we show that ethylene production increases during seed maturation. We conclude that ethylene promotes seed Fe accumulation during seed maturation via an EIN3-ERF95-FER1-dependent signaling pathway.

Alteration of iron responsive gene expression in Arabidopsis glutaredoxin S17 loss of function plants with or without iron stress

Iron (Fe) is a mineral nutrient and a metal cofactor essential for plants. Iron limitation can have detrimental effects on plant growth and development, while excess iron inside plant cells leads to oxidative damage. As a result, plants have evolved complex regulatory networks to respond to fluctuations in cellular iron concentrations. The mechanisms that regulate these responses however, are not fully understood. Heterologous expression of an Arabidopsis thaliana monothiol glutaredoxin S17 (GRXS17) suppresses the over-accumulation of iron in the Saccharomyces cerevisiae Grx3/Grx4 mutant and disruption of GRXS17 causes plant sensitivity to exogenous oxidants and iron deficiency stress. GRXS17 may act as an important regulator in the plant's ability to respond to iron deficiency stress and maintain redox homeostasis. Here, we extend this investigation by analyzing iron-responsive gene expression of the Fer-like iron deficiency-induced transcription factor (FIT) network (FIT, IRT1, FRO1, and FRO2) and the bHLH transcription factor POPEYE (PYE) network (PYE, ZIF1, FRO3, NAS4, and BTS) in GRXS17 KO plants and wildtype controls grown under iron sufficiency and deficiency conditions. Our findings suggest that GRXS17 is required for tolerance to iron deficiency, and plays a negative regulatory role under conditions of iron sufficiency.

Deregulated High Affinity Copper Transport Alters Iron Homeostasis in Arabidopsis

The present work describes the effects on iron homeostasis when copper transport was deregulated in Arabidopsis thaliana by overexpressing high affinity copper transporters COPT1 and COPT3 (COPT^OE ). A genome-wide analysis conducted on COPT1^OE plants, highlighted that iron homeostasis gene expression was affected under both copper deficiency and excess. Among the altered genes were those encoding the iron uptake machinery and their transcriptional regulators. Subsequently, COPT^OE seedlings contained less iron and were more sensitive than controls to iron deficiency. The deregulation of copper (I) uptake hindered the transcriptional activation of the subgroup Ib of basic helix-loop-helix (bHLH-Ib) factors under copper deficiency. Oppositely, copper excess inhibited the expression of the master regulator FIT but activated bHLH-Ib expression in COPT^OE plants, in both cases leading to the lack of an adequate iron uptake response. As copper increased in the media, iron (III) was accumulated in roots, and the ratio iron (III)/iron (II) was increased in COPT^OE plants. Thus, iron (III) overloading in COPT^OE roots inhibited local iron deficiency responses, aimed to metal uptake from soil, leading to a general lower iron content in the COPT^OE seedlings. These results emphasized the importance of appropriate spatiotemporal copper uptake for iron homeostasis under non-optimal copper supply. The understanding of the role of copper uptake in iron metabolism could be applied for increasing crops resistance to iron deficiency.

GSNOR provides plant tolerance to iron toxicity via preventing iron-dependent nitrosative and oxidative cytotoxicity

Iron (Fe) is essential for life, but in excess can cause oxidative cytotoxicity through the generation of Fe-catalyzed reactive oxygen species. It is yet unknown which genes and mechanisms can provide Fe-toxicity tolerance. Here, we identify S-nitrosoglutathione-reductase (GSNOR) variants underlying a major quantitative locus for root tolerance to Fe-toxicity in Arabidopsis using genome-wide association studies and allelic complementation. These variants act largely through transcript level regulation. We further show that the elevated nitric oxide is essential for Fe-dependent redox toxicity. GSNOR maintains root meristem activity and prevents cell death via inhibiting Fe-dependent nitrosative and oxidative cytotoxicity. GSNOR is also required for root tolerance to Fe-toxicity throughout higher plants such as legumes and monocots, which exposes an opportunity to address crop production under high-Fe conditions using natural GSNOR variants. Overall, this study shows that genetic or chemical modulation of the nitric oxide pathway can broadly modify Fe-toxicity tolerance.

How plants deal with iron toxicity is still unclear. Here, the authors reveal that S-nitrosoglutathione-reductase (GSNOR) provides tolerance to iron toxicity by preventing iron-dependent nitrosative and oxidative cytotoxicity in Arabidopsis, legumes, and rice.

The bHLH transcription factor gene AtUPB1 regulates growth by mediating cell cycle progression in Arabidopsis

Plant growth, development and interaction with the environment involve the action of transcription factor. bHLH proteins play an essential and often conserved role in the plant kingdom. However, bHLH proteins that participate in the cell division process are less well known. Here, we report that the bHLH transcription factor gene AtUPB1 is involved in mediating cell cycle progression and root development. In yeast cells, AtUPB1 inhibits cells proliferation and the cells had increased numbers of nuclei. UPB1 overexpression decreased the expression of the cell division marker CYCB1-1, and CDKA1 expression could overcome the defect of UPB1 overexpression. Moreover, UPB1 could directly bind to the promoter region of the SIM and SMR1 genes to regulate cell cycle. These results support a new role for AtUPB1 regulating root meristem development by mediating the expression of SIM/SMR1 genes.

Keep talking: crosstalk between iron and sulfur networks fine-tunes growth and development to promote survival under iron limitation

Plants are capable of synthesizing all the molecules necessary to complete their life cycle from minerals, water, and light. This plasticity, however, comes at a high energetic cost and therefore plants need to regulate their economy and allocate resources accordingly. Iron-sulfur (Fe-S) clusters are at the center of photosynthesis, respiration, amino acid, and DNA metabolism. Fe-S clusters are extraordinary catalysts, but their main components (Fe2+ and S2-) are highly reactive and potentially toxic. To prevent toxicity, plants have evolved mechanisms to regulate the uptake, storage, and assimilation of Fe and S. Recent advances have been made in understanding the cellular economy of Fe and S metabolism individually, and growing evidence suggests that there is dynamic crosstalk between Fe and S networks. In this review, we summarize and discuss recent literature on Fe sensing, allocation, use efficiency, and, when pertinent, its relationship to S metabolism. Our future perspectives include a discussion about the open questions and challenges ahead and how the plant nutrition field can come together to approach these questions in a cohesive and more efficient way.