An assessment of the biotechnological use of hemoglobin modulation in cereals

Genetic resources and precise gene editing for targeted improvement of barley abiotic stress tolerance

Abiotic stresses, predominately drought, heat, salinity, cold, and waterlogging, adversely affect cereal crops. They limit barley production worldwide and cause huge economic losses. In barley, functional genes under various stresses have been identified over the years and genetic improvement to stress tolerance has taken a new turn with the introduction of modern gene-editing platforms. In particular, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) is a robust and versatile tool for precise mutation creation and trait improvement. In this review, we highlight the stress-affected regions and the corresponding economic losses among the main barley producers. We collate about 150 key genes associated with stress tolerance and combine them into a single physical map for potential breeding practices. We also overview the applications of precise base editing, prime editing, and multiplexing technologies for targeted trait modification, and discuss current challenges including high-throughput mutant genotyping and genotype dependency in genetic transformation to promote commercial breeding. The listed genes counteract key stresses such as drought, salinity, and nutrient deficiency, and the potential application of the respective gene-editing technologies will provide insight into barley improvement for climate resilience.

Nitric Oxide and Globin Glb1 Regulate Fusarium oxysporum Infection of Arabidopsis thaliana

Plants continuously interact with fungi, some of which, such as Fusarium oxysporum, are lethal, leading to reduced crop yields. Recently, nitric oxide (NO) has been found to play a regulatory role in plant responses to F. oxysporum, although the underlying mechanisms involved are poorly understood. In this study, we show that Arabidopsis mutants with altered levels of phytoglobin 1 (Glb1) have a higher survival rate than wild type (WT) after infection with F. oxysporum, although all the genotypes analyzed exhibited a similar fungal burden. None of the defense responses that were analyzed in Glb1 lines, such as phenols, iron metabolism, peroxidase activity, or reactive oxygen species (ROS) production, appear to explain their higher survival rates. However, the early induction of the PR genes may be one of the reasons for the observed survival rate of Glb1 lines infected with F. oxysporum. Furthermore, while PR1 expression was induced in Glb1 lines very early on the response to F. oxysporum, this induction was not observed in WT plants.

Over-expression of the barley Phytoglobin 1 (HvPgb1) evokes leaf-specific transcriptional responses during root waterlogging

Oxygen deprivation (hypoxia) in the root due to waterlogging causes profound metabolic changes in the aerial organs depressing growth and limiting plant productivity in barley (Hordeum vulgare L.). Genome-wide analyses in waterlogged wild type (WT) barley (cv. Golden Promise) plants and plants over-expressing the phytoglobin 1 HvPgb1 [HvPgb1(OE)] were performed to determine leaf specific transcriptional responses during waterlogging. Normoxic WT plants outperformed their HvPgb1(OE) counterparts for dry weight biomass, chlorophyll content, photosynthetic rate, stomatal conductance, and transpiration. Root waterlogging severely depressed all these parameters in WT plants but not in HvPgb1(OE) plants, which exhibited an increase in photosynthetic rate. In leaftissue, root waterlogging repressed genes encoding photosynthetic components and chlorophyll biosynthetic enzymes, while induced those of reactive oxygen species (ROS)-generating enzymes. This repression was alleviated in HvPgb1(OE) leaves which also exhibited an induction of enzymes participating in antioxidant responses. In the same leaves, the transcript levels of several genes participating in nitrogen metabolism were also higher relative to WT leaves. Ethylene levels were diminished by root waterlogging in leaves of WT plants, but not in HvPgb1(OE), which were enriched in transcripts of ethylene biosynthetic enzymes and ethylene response factors. Pharmacological treatments increasing the level or action of ethylene further suggested the requirement of ethylene in plant response to root waterlogging. In natural germplasm an elevation in foliar HvPgb1 between 16h and 24h of waterlogging occurred in tolerant genotypes but not in susceptible ones. By integrating morpho-physiological parameters with transcriptome data, this study provides a framework defining leaf responses to root waterlogging and indicates that the induction of HvPgb1 may be used as a selection tool to enhance resilience to excess moisture.

Main nitric oxide (NO) hallmarks to relieve arsenic stress in higher plants

Arsenic (As) is a toxic metalloid that adversely affects plant growth, and poses severe risks to human health. It induces disturbance to many physiological and metabolic pathways such as nutrient, water and redox imbalance, abnormal photosynthesis and ATP synthesis and loss of membrane integrity. Nitric oxide (NO) is a free radical molecule endogenously generated in plant cells which has signalling properties. Under As-stress, the endogenous NO metabolism is significantly affected in a clear connection with the metabolism of reactive oxygen species (ROS) triggering nitro-oxidative stress. However, the exogenous NO application provides beneficial effects under As-stress conditions which can relieve oxidative damages by stimulating the antioxidant systems, regulation of the expression of the transporter and other defence-related genes, modification of root cell wall composition or the biosynthesis of enriched sulfur compounds such phytochelatins (PCs). This review aims to provide up-to-date information on the key NO hallmarks to relieve As-stress in higher plants. Furthermore, it will be analyzed the diverse genetic engineering techniques to increase the endogenous NO content which could open new biotechnological applications, especially in crops under arsenic stress.

The effect of phytoglobin overexpression on the plant proteome during nonhost response of barley (Hordeum vulgare) to wheat powdery mildew (Blumeria graminis f. sp. tritici)

Nonhost resistance, a resistance of plant species against all nonadapted pathogens, is considered the most durable and efficient immune system in plants. To increase our understanding of the response of barley plants to infection by powdery mildew, Blumeria graminis f. sp. tritici, we used quantitative proteomic analysis (LC-MS/MS). We compared the response of two genotypes of barley cultivar Golden Promise, wild type (WT) and plants with overexpression of phytoglobin (previously hemoglobin) class 1 (HO), which has previously been shown to significantly weaken nonhost resistance. A total of 8804 proteins were identified and quantified, out of which the abundance of 1044 proteins changed significantly in at least one of the four comparisons ('i' stands for 'inoculated')- HO/WT and HOi/WTi (giving genotype differences), and WTi/WT and HOi/HO (giving treatment differences). Among these differentially abundant proteins (DAP) were proteins related to structural organization, disease/defense, metabolism, transporters, signal transduction and protein synthesis. We demonstrate that quantitative changes in the proteome can explain physiological changes observed during the infection process such as progression of the mildew infection in HO plants that was correlated with changes in proteins taking part in papillae formation and preinvasion resistance. Overexpression of phytoglobins led to modification in signal transduction prominently by dramatically reducing the number of kinases induced, but also in the turnover of other signaling molecules such as phytohormones, polyamines and Ca²⁺. Thus, quantitative proteomics broaden our understanding of the role NO and phytoglobins play in barley during nonhost resistance against powdery mildew.

Transcriptional and Metabolic Changes Associated with Phytoglobin Expression during Germination of Barley Seeds

To understand how the class 1 phytoglobin is involved in germination process via the modulation of the nitric oxide (NO) metabolism, we performed the analysis of physiological and molecular parameters in the embryos of transgenic barley (Hordeum vulgare L. cv Golden Promise) plants differing in expression levels of the phytoglobin (Pgb1) gene during the first 48 h of germination. Overexpression of Pgb1 resulted in a higher rate of germination, higher protein content and higher ATP/ADP ratios. This was accompanied by a lower rate of NO emission after radicle protrusion, as compared to the wild type and downregulating line, and a lower rate of S-nitrosylation of proteins in the first hours postimbibition. The rate of fermentation estimated by the expression and activity of alcohol dehydrogenase was significantly higher in the Pgb1 downregulating line, the same tendency was observed for nitrate reductase expression. The genes encoding succinate dehydrogenase and pyruvate dehydrogenase complex subunits were more actively expressed in embryos of the seeds overexpressing Pgb1. It is concluded that Pgb1 expression in embryo is essential for the maintenance of redox and energy balance before radicle protrusion, when seeds experience low internal oxygen concentration and exerts the effect on metabolism during the initial development of seedlings.

The Hypoxic Proteome and Metabolome of Barley (Hordeum vulgare L.) with and without Phytoglobin Priming

Overexpression of phytoglobins (formerly plant hemoglobins) increases the survival rate of plant tissues under hypoxia stress by the following two known mechanisms: (1) scavenging of nitric oxide (NO) in the phytoglobin/NO cycle and (2) mimicking ethylene priming to hypoxia when NO scavenging activates transcription factors that are regulated by levels of NO and O2 in the N-end rule pathway. To map the cellular and metabolic effects of hypoxia in barley (Hordeum vulgare L., cv. Golden Promise), with or without priming to hypoxia, we studied the proteome and metabolome of wild type (WT) and hemoglobin overexpressing (HO) plants in normoxia and after 24 h hypoxia (WT24, HO24). The WT plants were more susceptible to hypoxia than HO plants. The chlorophyll a + b content was lowered by 50% and biomass by 30% in WT24 compared to WT, while HO plants were unaffected. We observed an increase in ROS production during hypoxia treatment in WT seedlings that was not observed in HO seedlings. We identified and quantified 9694 proteins out of which 1107 changed significantly in abundance. Many proteins, such as ion transporters, Ca2+-signal transduction, and proteins related to protein degradation were downregulated in HO plants during hypoxia, but not in WT plants. Changes in the levels of histones indicates that chromatin restructuring plays a role in the priming of hypoxia. We also identified and quantified 1470 metabolites, of which the abundance of >500 changed significantly. In summary the data confirm known mechanisms of hypoxia priming by ethylene priming and N-end rule activation; however, the data also indicate the existence of other mechanisms for hypoxia priming in plants.

Nitric oxide in plant-fungal interactions

Whilst many interactions with fungi are detrimental for plants, others are beneficial and result in improved growth and stress tolerance. Thus, plants have evolved sophisticated mechanisms to restrict pathogenic interactions while promoting mutualistic relationships. Numerous studies have demonstrated the importance of nitric oxide (NO) in the regulation of plant defence against fungal pathogens. NO triggers a reprograming of defence-related gene expression, the production of secondary metabolites with antimicrobial properties, and the hypersensitive response. More recent studies have shown a regulatory role of NO during the establishment of plant-fungal mutualistic associations from the early stages of the interaction. Indeed, NO has been recently shown to be produced by the plant after the recognition of root fungal symbionts, and to be required for the optimal control of mycorrhizal symbiosis. Although studies dealing with the function of NO in plant-fungal mutualistic associations are still scarce, experimental data indicate that different regulation patterns and functions for NO exist between plant interactions with pathogenic and mutualistic fungi. Here, we review recent progress in determining the functions of NO in plant-fungal interactions, and try to identify common and differential patterns related to pathogenic and mutualistic associations, and their impacts on plant health.

Phytoglobin overexpression promotes barley growth in the presence of enhanced level of atmospheric nitric oxide

To investigate the effect of high atmospheric NO concentrations on crop plants and the role of phytoglobins under these conditions, we performed a long-term study on barley 'Golden Promise' wild type (WT), class 1 phytoglobin knockdown (HvPgb1.1-) and class 1 phytoglobin overexpression (HvPgb1.1+) lines. Plants were cultivated with nitrogen-free nutrient solution during the entire growth period and were fumigated with different NO concentration (ambient, 800, 1500, and 3000 ppb). Analysis of fresh weight, stem number, chlorophyll content, and effective quantum yield of PSII showed that NO fumigation promoted plant growth and tillering significantly in the HvPgb1.1+ line. After 80 d of NO fumigation, dry matter weight, spikes number, kernel number, and plant kernel weight were significantly increased in HvPgb1.1+ plants with increasing NO concentration. In contrast, yield decreased in WT and HvPgb1.1- plants the higher the NO level. Application of atmospheric 15NO and 15NO2 demonstrated NO specificity of phytoglobins. 15N from 15NO could be detected in RNA, DNA, and proteins of barley leaves and the 15N levels were significantly higher in HvPgb1.1+ plants in comparison with HvPgb1.1- and WT plants. Our results demonstrate that overexpression of phytoglobins allows plants to more efficiently use atmospheric NO as N source.

Quantum Cascade Lasers-Based Detection of Nitric Oxide

Despite the established importance of nitric oxide (NO) in many physiological and molecular processes in plants, most methods for quantifying NO are open to criticism This reflects the differing methods either lacking specificity or sensitivity, or even from an undue dependence of results on experimental conditions (i.e., chemical concentrations, pH, etc.). In this chapter we describe a protocol to measure gaseous NO produced by a biological sample using quantum cascade laser (QCL)-based spectroscopy. This technique is based on absorption of the laser light by the NO molecules which have been passed from a biological sample into an optical s cell that is equipped with two mirrors placed at both ends. This design greatly increases the interaction path length with the NO molecules due to multiple reflections of the light coupled inside the cell. Thus, the method is able to provide online, in planta measurements of the dynamics of NO production, being highly selective and sensitive (down to ppbv levels;1 ppbv = part per billion by volume mixing ratio = 1:10^-9).

Expression of the Tobacco Non-symbiotic Class 1 Hemoglobin Gene Hb1 Reduces Cadmium Levels by Modulating Cd Transporter Expression Through Decreasing Nitric Oxide and ROS Level in Arabidopsis

Hemoglobin (Hb) proteins are ubiquitous in plants, and non-symbiotic class 1 hemoglobin (Hb1) is involved in various biotic and abiotic stress responses. Here, the expression of the tobacco (Nicotiana tabacum) hemoglobin gene NtHb1 in Arabidopsis (Arabidopsis thaliana) showed higher cadmium (Cd) tolerance and lower accumulations of Cd, nitric oxide (NO), and reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂). NtHb1-expressing Arabidopsis exhibited a reduced induction of NO levels in response to Cd, suggesting scavenging of NO by Hb1. In addition, transgenic plants had reduced accumulation of ROS and increased activities of antioxidative enzymes (catalase, superoxide dismutase, and glutathione reductase) in response to Cd. While the expression of the Cd exporters ABC transporter (PDR8) and Ca²⁺/H⁺ exchangers (CAXs) was increased, that of the Cd importers iron responsive transporter 1 (IRT1) and P-type 2B Ca²⁺ ATPase (ACA10) was reduced in response to Cd. When Col-0 plants were treated with the NO donor sodium nitroprusside (SNP) and H₂O₂, the expression pattern of Cd transporters (PDR8, CAX3, IRT1, and ACA10) was reversed, suggesting that NtHb1 expression decreased the Cd level by regulating the expression of Cd transporters via decreased NO and ROS. Correspondingly, NtHb1-expressing Arabidopsis showed increased Cd export. In summary, the expression of NtHb1 reduces Cd levels by regulating Cd transporter expression via decreased NO and ROS levels in Arabidopsis.

Expression of phytoglobin affects nitric oxide metabolism and energy state of barley plants exposed to anoxia

Class 1 plant hemoglobins (phytoglobins) are upregulated during low-oxygen stress and participate in metabolism and cell signaling via modulation of the levels of nitric oxide (NO). We studied the effects of overexpression and knockdown of the class 1 phytoglobin gene in barley (Hordeum vulgare L.) under low-oxygen stress. The overexpression of phytoglobin reduced the amount of NO released, while knockdown significantly stimulated NO emission. It has previously been shown that NO inhibits aconitase activity, so decreased aconitase activity in knockdown plants acts as a biomarker for high internal NO levels. The overexpression of phytoglobin corresponded to higher ATP/ADP ratios, pyrophosphate levels and aconitase activity under anoxia, while knockdown of phytoglobin resulted in the increased level of protein nitrosylation, elevation of alcohol dehydrogenase and nitrosoglutathione reductase activities. The overexpressing plants showed various signs of stunted growth under normoxia, but were the only type to germinate and survive under hypoxia. The results show that overexpression of phytoglobin protects plant cells via NO scavenging and improves their low-oxygen stress survival. However, it may not be useful for cereal crop improvement since it comes with a significant interference with normoxic NO signalling pathways.

Reduced nitric oxide levels during drought stress promote drought tolerance in barley and is associated with elevated polyamine biosynthesis

Nitric oxide (NO) is a key messenger in plant stress responses but its exact role in drought response remains unclear. To investigate the role of NO in drought response we employed transgenic barley plants (UHb) overexpressing the barley non-symbiotic hemoglobin gene HvHb1 that oxidizes NO to NO3-. Reduced NO production under drought conditions in UHb plants was associated with increased drought tolerance. Since NO biosynthesis has been related to polyamine metabolism, we investigated whether the observed drought-related NO changes could involve polyamine pathway. UHb plants showed increases in total polyamines and in particular polyamines such as spermidine. These increases correlated with the accumulation of the amino acid precursors of polyamines and with the expression of specific polyamine biosynthesis genes. This suggests a potential interplay between NO and polyamine biosynthesis during drought response. Since ethylene has been linked to NO signaling and it is also related to polyamine metabolism, we explored this connection. In vivo ethylene measurement showed that UHb plants significantly decrease ethylene production and expression of aminocyclopropane-1-carboxylic acid synthase gene, the first committed step in ethylene biosynthesis compared with wild type. These data suggest a NO-ethylene influenced regulatory node in polyamine biosynthesis linked to drought tolerance/susceptibility in barley.

Signaling through reactive oxygen and nitrogen species is differentially modulated in sunflower seedling root and cotyledon in response to various nitric oxide donors and scavengers<sup/>

Sodium nitroprusside (SNP), diethylenetriamine NONOate (DETA), S-nitroso-n-acetyl-D,L- penicillamine (SNAP), and 4-(p-methoxyphenyl)-1,3,2- Oxathiazolylium-5-olate (CAY) exhibit differential NO releasing ability in aqueous solution and hemoglobin is a more efficient NO quencher than cPTIO in solution. DETA releases 16% more NO compared with SNP in solution. Various NO donors (SNP, DETA, SNAP, and CAY) also bring about a differential but concentration-dependent increase in endogenous NO in seedling cotyledons and roots. Two-day old, dark-grown seedling roots exhibit 95%, 77%, 59% and 45% increase in NO content in presence of each of 500 µM of DETA, SNAP, CAY and SNP, respectively, relative to control. NO accumulation in the tissue system as a response to NO donors is reflected in terms of corresponding peroxynitrite accumulation. Release of cyanide and free iron as byproducts of SNP dissociation in solution limits its usefulness as an NO donor. SNP leads to profuse ROS generation in sunflower seedling roots. Light is not a pre-requisite for NO generation from SNP. Present work also demonstrates the usefulness of hemoglobin over cPTIO as NO scavenger. Hemoglobin brings about increasing NO quenching with its increasing concentration from 2.5 to 10 µM. Greater sensitivity of the root system to the NO donor/scavenger treatments is evident, it being in direct contact with the molecules in the incubation/ growth medium. This differential effect does not seem to be significantly transmitted to the cotyledons (long-distance signaling).

Phytoglobin expression influences soil flooding response of corn plants

Background and Aims Excess water is a limiting factor for crop productivity. Under conditions of full submergence or flooding, plants can experience prolonged oxygen depletion which compromises basic physiological and biochemical processes. Severe perturbations of the photosynthetic machinery with a concomitant decline in photosynthetic potential as a result of elevated levels of reactive oxygen species (ROS) are the major consequences of water excess. Phytoglobins (Pgbs) are ubiquitous proteins induced by several types of stress which affect plant response by modulating nitric oxide. Methods Maize plants overexpressing or downregulating two Pgb genes were subjected to soil flooding for 10 d and their performance was estimated by measuring several gas exchange parameters including photosynthetic rate. Above-ground tissue was utilized to localize ROS and to measure the expression and activities of major antioxidant enzymes. Key Results Relative to the wild type, flooded plants overexpressing Pgb genes retained a greater photosynthetic rate and enhanced activity of several antioxidant enzymes. These plants also exhibited high levels of ascorbic acid and reduced ROS staining. This was in contrast to flooded plants downregulating Pgb genes and characterized by the lowest photosynthetic rates and reduced expression and activities of many antioxidant enzymes. Conclusions Induction of Pgb genes alleviates flooding stress by limiting ROS-induced damage and ensuring a sustained photosynthetic rate. This is achieved through improvements of the ascorbate antioxidant status including an enrichment of the ascorbate pool via de novo and recycling mechanisms, and increased activities of several ROS-scavenging enzymes.

Overexpression of spinach non-symbiotic hemoglobin in Arabidopsis resulted in decreased NO content and lowered nitrate and other abiotic stresses tolerance

A class 1 non-symbiotic hemoglobin family gene, SoHb, was isolated from spinach. qRT-PCR showed that SoHb was induced by excess nitrate, polyethylene glycol, NaCl, H₂O₂, and salicylic acid. Besides, SoHb was strongly induced by application of nitric oxide (NO) donor, while was suppressed by NO scavenger, nitrate reductase inhibitor, and nitric oxide synthase inhibitor. Overexpression of SoHb in Arabidopsis resulted in decreased NO level and sensitivity to nitrate stress, as shown by reduced root length, fresh weight, the maximum photosystem II quantum ratio of variable to maximum fluorescence (Fv/Fm), and higher malondialdehyde contents. The activities and gene transcription of superoxide dioxidase, and catalase decreased under nitrate stress. Expression levels of RD22, RD29A, DREB2A, and P5CS1 decreased after nitrate treatment in SoHb-overexpressing plants, while increased in the WT plants. Moreover, SoHb-overexpressing plants showed decreased tolerance to NaCl and osmotic stress. In addition, the SoHb-overexpression lines showed earlier flower by regulating the expression of SOC, GI and FLC genes. Our results indicated that the decreasing NO content in Arabidopsis by overexpressing SoHb might be responsible for lowered tolerance to nitrate and other abiotic stresses.

Residues in the Distal Heme Pocket of Arabidopsis Non-Symbiotic Hemoglobins: Implication for Nitrite Reductase Activity

It is well-established that plant hemoglobins (Hbs) are involved in nitric oxide (NO) metabolism via NO dioxygenase and/or nitrite reductase activity. The ferrous-deoxy Arabidopsis Hb1 and Hb2 (AHb1 and AHb2) have been shown to reduce nitrite to NO under hypoxia. Here, to test the hypothesis that a six- to five-coordinate heme iron transition might mediate the control of the nitrite reduction rate, we examined distal pocket mutants of AHb1 and AHb2 for nitrite reductase activity, NO production and spectroscopic features. Absorption spectra of AHbs distal histidine mutants showed that AHb1 mutant (H69L) is a stable pentacoordinate high-spin species in both ferrous and ferric states, whereas heme iron in AHb2 mutant (H66L) is hexacoordinated low-spin with Lys69 as the sixth ligand. The bimolecular rate constants for nitrite reduction to NO were 13.3 ± 0.40, 7.3 ± 0.5, 10.6 ± 0.8 and 171.90 ± 9.00 M⁻¹·s⁻¹ for AHb1, AHb2, AHb1 H69L and AHb2 H66L, respectively, at pH 7.4 and 25 °C. Consistent with the reductase activity, the amount of NO detected by chemiluminescence was significantly higher in the AHb2 H66L mutant. Our data indicate that nitrite reductase activity is determined not only by heme coordination, but also by a unique distal heme pocket in each AHb.

Jasmonic acid is a downstream component in the modulation of somatic embryogenesis by Arabidopsis Class 2 phytoglobin

Previous studies have shown that the beneficial effect of suppression of the Arabidopsis phytoglobin 2 gene, PGB2, on somatic embryogenesis occurs through the accumulation of nitric oxide (NO) within the embryogenic cells originating from the cultured explant. NO activates the expression of Allene oxide synthase (AOS) and Lipoxygenase 2 (LOX2), genes encoding two key enzymes of the jasmonic acid (JA) biosynthetic pathway, elevating JA content within the embryogenic tissue. The number of embryos in the single aos1-1 mutant and pgb2-aos1-1 double mutant declined, and was not rescued by increasing levels of NO stimulating embryogenesis in wild-type tissue. NO also influenced JA responses by up-regulating PLANT DEFENSIN 1 (PDF1) and JASMONATE-ZIM-PROTEIN (JAZ1), as well as down-regulating MYC2. The NO and JA modulation of MYC2 and JAZ1 controlled embryogenesis. Ectopic expression of JAZ1 or suppression of MYC2 promoted the formation of somatic embryos, while repression of JAZ1 and up-regulation of MYC2 reduced the embryogenic performance. Sustained expression of JAZ1 induced the transcription of several indole acetic acid (IAA) biosynthetic genes, resulting in higher IAA levels in the embryogenic cells. Collectively these data fit a model integrating JA in the PGB2 regulation of Arabidopsis embryogenesis. Suppression of PGB2 increases JA through NO. Elevated levels of JA repress MYC2 and induce JAZ1, favoring the accumulation of IAA in the explants and the subsequent production of somatic embryos.

Micromilling enhances iron bioaccessibility from wholegrain wheat

Cereals constitute important sources of iron in human diet; however, much of the iron in wheat is lost during processing for the production of white flour. This study employed novel food processing techniques to increase the bioaccessibility of naturally occurring iron in wheat. Iron was localized in wheat by Perl's Prussian blue staining. Soluble iron from digested wheat flour was measured by a ferrozine spectrophotometric assay. Iron bioaccessibility was determined using an in vitro simulated peptic-pancreatic digestion, followed by measurement of ferritin (a surrogate marker for iron absorption) in Caco-2 cells. Light microscopy revealed that iron in wheat was encapsulated in cells of the aleurone layer and remained intact after in vivo digestion and passage through the gastrointestinal tract. The solubility of iron in wholegrain wheat and in purified wheat aleurone increased significantly after enzymatic digestion with Driselase, and following mechanical disruption using micromilling. Furthermore, following in vitro simulated peptic-pancreatic digestion, iron bioaccessibility, measured as ferritin formation in Caco-2 cells, from micromilled aleurone flour was significantly higher (52%) than from whole aleurone flour. Taken together our data show that disruption of aleurone cell walls could increase iron bioaccessibility. Micromilled aleurone could provide an alternative strategy for iron fortification of cereal products.