Effects of different Fe supplies on mineral partitioning and remobilization during the reproductive development of rice (Oryza sativa L.)

Field study on the uptake pathways and their contributions to the accumulation of organophosphate esters, phthalates, and polycyclic aromatic hydrocarbons in upland rice

Plant uptake of organic contaminants generally occurs through either root, gas-phase foliar, or particle-phase foliar uptake. Understanding these pathways is essential for food-system practitioners to reduce human exposures, and to clean contaminated-sites with phytoremediation. Herein, we conducted a field-based experiment using an improved specific exposure chamber to elucidate the uptake pathways of organophosphate esters, phthalates, and polycyclic aromatic compounds, and quantitatively assessed their contributions to organic contaminant accumulations in field-grown rice. For most target compounds, all three uptake pathways (root, foliar gas, and foliar particle uptakes) contributed substantially to the overall contaminant burden in rice. Compounds with lower octanol-water partition coefficients (Kow) were more readily translocated from roots to leaves, and compounds with higher octanol-air partition coefficients (Koa) tended to enter rice leaves mostly through particle deposition. Most compounds were mostly stored in the inner leaves (55.3-98.2 %), whereas the relatively volatile compounds were more readily absorbed by the waxy layer and then transferred to the inner leaves. Air particle desorption was a key process regulating foliar uptake of low-volatility compounds. The results can help us to better understand and predict the environmental fate of those contaminants, and develop more effective management strategies for reducing their human exposure through food ingestion.

Nutrient accumulation and transcriptome patterns during grain development in rice

Rice is an important source of calories and mineral nutrients for more than half of the world's population. The accumulation of essential and toxic mineral elements in rice grain affects its nutritional quality and safety. However, the patterns and processes by which different elements progressively accumulate during grain filling remain largely unknown. In the present study, we investigated temporal changes in dry matter, elemental concentrations, and the transcriptome in the grain of field-grown rice. We also investigated the effects of seed setting rate and the position of the grain within the rice panicle on element accumulation. Three different patterns of accumulation were observed: (i) elements including K, Mn, B, and Ca showed an early accumulation pattern; (ii) dry matter and elements including N, P, S, Mg, Cu, Zn, Mo, As, and Cd showed a mid accumulation pattern; and (iii) elements such as Fe showed a gradual increase pattern. These different accumulation patterns can be explained by the differences in the biogeochemical behavior of the various elements in the soil, as well as differences in plant nutrient redistribution, gene expression, and the sink-source relationship. These results improve our knowledge of the dynamics of elemental accumulation in rice grain and are helpful for identification of functional genes mediating the translocation of elements to grain.

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.

A tale of two metals: Biofortification of rice grains with iron and zinc

Iron (Fe) and zinc (Zn) are essential micronutrients needed by virtually all living organisms, including plants and humans, for proper growth and development. Due to its capacity to easily exchange electrons, Fe is important for electron transport in mitochondria and chloroplasts. Fe is also necessary for chlorophyll synthesis. Zn is a cofactor for several proteins, including Zn-finger transcription factors and redox metabolism enzymes such as copper/Zn superoxide dismutases. In humans, Fe participates in oxygen transport, electron transport, and cell division whereas Zn is involved in nucleic acid metabolism, apoptosis, immunity, and reproduction. Rice (Oryza sativa L.) is one of the major staple food crops, feeding over half of the world's population. However, Fe and Zn concentrations are low in rice grains, especially in the endosperm, which is consumed as white rice. Populations relying heavily on rice and other cereals are prone to Fe and Zn deficiency. One of the most cost-effective solutions to this problem is biofortification, which increases the nutritional value of crops, mainly in their edible organs, without yield reductions. In recent years, several approaches were applied to enhance the accumulation of Fe and Zn in rice seeds, especially in the endosperm. Here, we summarize these attempts involving transgenics and mutant lines, which resulted in Fe and/or Zn biofortification in rice grains. We review rice plant manipulations using ferritin genes, metal transporters, changes in the nicotianamine/phytosiderophore pathway (including biosynthetic genes and transporters), regulators of Fe deficiency responses, and other mutants/overexpressing lines used in gene characterization that resulted in Fe/Zn concentration changes in seeds. This review also discusses research gaps and proposes possible future directions that could be important to increase the concentration and bioavailability of Fe and Zn in rice seeds without the accumulation of deleterious elements. We also emphasize the need for a better understanding of metal homeostasis in rice, the importance of evaluating yield components of plants containing transgenes/mutations under field conditions, and the potential of identifying genes that can be manipulated by gene editing and other nontransgenic approaches.

Rice Plants (Oryza sativa L.) under Cd Stress in Fe Deficiency Conditions

Due to the environment pollution by cadmium (Cd) near industrial metallurgic factories and the widespread use of phosphorus fertilizers, the problem of toxic Cd effect on plants is well discussed by many authors, but the phytotoxicity of Cd under iron (Fe) deficiency stress has not been sufficiently studied. The aim of the work was to study comprehensively the effect of Cd under Fe deficiency conditions on physiological, biochemical, and anatomical parameters of rice varieties, to identify varietal differences in plant response to the effect of double stress. Relative resistance and sensitivity to the joint effect of Cd and Fe deficiency stress rice varieties have been identified. Double stress decreased a linear growth and biomass accumulation of roots and shoots (by 36-50% and 33-46% and 32-56% and 32-48%, accordingly), content of photosynthetic pigments (Chla, Chlb, and carotenoids by 36-51%, 32-47%, and 64-78%, accordingly), and relative water content (by 18-26%). Proline content increased by 28-103% in all rice varieties, but to a lesser extent in sensitive varieties. The thickness of the lower and upper epidermis and the diameter of vascular bundles of leaves decreased by 18-50%, 46-60%, and 13-48%, accordingly. The thickness of the root endodermis and exodermis and diameter of the central cylinder mainly decreased. The thickness of the exodermis increased slightly by 7%, and the diameter of the central cylinder remained at the control level in resistant Madina variety while in sensitive Chapsari variety, these indicators decreased significantly by 50 and 45%, accordingly. Thus, the aggravation of adverse effect of Cd under Fe deficiency conditions and the varietal specificity of plants' response to double stress were shown. It creates the need for further study of these rice varieties using Fe to identify mechanisms for reducing the toxic effect of Cd on plants as well as the study of Fe and Cd transporter genes at the molecular level.

Nano-Scale Zero Valent Iron (nZVI) Priming Enhances Yield, Alters Mineral Distribution and Grain Nutrient Content of Oryza sativa L. cv. Gobindobhog: A Field Study

In recent decades, nano-scale zero valent iron is reported to have plant growth enhancement capacity under laboratory conditions, but till date, there is no report to highlight its effect on the growth and yield of field-grown plants. In this study, we have evaluated the potential of nZVI priming on rice yield. A two-year field study has been conducted with different concentrations (10, 20, 40, and 80 mg l^-1) of nZVI for seed priming. The efficacy of nanopriming was compared with the hydroprimed control set. Seeds were treated for 72 h and sown in nursery beds and after 30 days seedlings were transplanted in the field. Root anatomy and morphology were studied in 7 days old seedlings where no changes were found. RAPD analysis also confirmed that low doses of nZVI were not genotoxic. Nanoprimed plants also had broader leaves, higher growth, biomass, and tiller number than control plants. Maximum yield was obtained from the 20 mg l^-1 nZVI primed set (3.8 fold higher than untreated control) which is achieved primarily because of the increase in fertile tiller numbers (two fold higher than untreated control). Higher values of other agronomic parameters like growth rate, net assimilation rate proved that nZVI priming enhanced photosynthetic efficiency and helped in the proper storage of photo-assimilates. All these attributed to increased accumulation of phytochemicals like starch, soluble sugar, protein, lipid, phenol, riboflavin, thiamine, and ascorbic acid in the grains. The elemental analysis confirmed that nZVI priming also promoted higher accumulations of macro and micronutrients in grains. Thus, nanoprimed seeds showed better crop performance compared to the traditional hydropimed seeds. Hence, nZVI can be considered as 'pro-fertilizer' and can be used commercially as a seed treatment agent which is capable of boosting plant growth and yield along with minimum interference to the soil ecosystem.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00344-021-10335-0.

Genetic determinants of micronutrient traits in graminaceous crops to combat hidden hunger

KEY MESSAGE

Improving the nutritional content of graminaceous crops is imperative to ensure nutritional security, wherein omics approaches play pivotal roles in dissecting this complex trait and contributing to trait improvement. Micronutrients regulate the metabolic processes to ensure the normal functioning of the biological system in all living organisms. Micronutrient deficiency, thereby, can be detrimental that can result in serious health issues. Grains of graminaceous crops serve as an important source of micronutrients to the human population; however, the rise in hidden hunger and malnutrition indicates an insufficiency in meeting the nutritional requirements. Improving the elemental composition and nutritional value of the graminaceous crops using conventional and biotechnological approaches is imperative to address this issue. Identifying the genetic determinants underlying the micronutrient biosynthesis and accumulation is the first step toward achieving this goal. Genetic and genomic dissection of this complex trait has been accomplished in major cereals, and several genes, alleles, and QTLs underlying grain micronutrient content were identified and characterized. However, no comprehensive study has been reported on minor cereals such as small millets, which are rich in micronutrients and other bioactive compounds. A comparative narrative on the reports available in major and minor Graminaceae species will illustrate the knowledge gained from studying the micronutrient traits in major cereals and provides a roadmap for dissecting this trait in other minor species, including millets. In this context, this review explains the progress made in studying micronutrient traits in major cereals and millets using omics approaches. Moreover, it provides insights into deploying integrated omics approaches and strategies for genetic improvement in micronutrient traits in graminaceous crops.

Comparative physiological, biochemical and transcriptomic analysis of hexaploid wheat (T. aestivum L.) roots and shoots identifies potential pathways and their molecular regulatory network during Fe and Zn starvation

The combined effect of iron (Fe) and zinc (Zn) starvation on their uptake and transportation and the molecular regulatory networks is poorly understood in wheat. To fill this gap, we performed a comprehensive physiological, biochemical and transcriptome analysis in two bread wheat genotypes, i.e. Narmada 195 and PBW 502, differing in inherent Fe and Zn content. Compared to PBW 502, Narmada 195 exhibited increased tolerance to Fe and Zn withdrawal by significantly modulating the critical physiological and biochemical parameters. We identified 25 core genes associated with four key pathways, i.e. methionine cycle, phytosiderophore biosynthesis, antioxidant and transport system, that exhibited significant up-regulation in both the genotypes with a maximum in Narmada 195. We also identified 26 microRNAs targeting 14 core genes across the four pathways. Together, core genes identified can serve as valuable resources for further functional research for genetic improvement of Fe and Zn content in wheat grain.

Nano-scale zero valent iron modulates Fe/Cd transporters and immobilizes soil Cd for production of Cd free rice

In recent decades, nanoscale zero valent iron (nZVI) has been found to be a promising approach for heavy metal remediation. This study is the first report highlighting the role of nZVI to ameliorate Cadmium (Cd) stress in rice along with its effects in expressions of transporter genes, agronomic parameters and grain nutrient status. Initially, 3 concentration of Cd (10, 50, 250 μM) and nZVI (50, 100, 200 mg L^-1) were selected. PCA analysis based on growth parameters, photosynthetic pigment contents and lipid peroxidation rate confirmed that 100 mg L^-1 nZVI was most suitable for remediation of 10 μM Cd. It was evident that, nZVI can alleviate Cd-induced toxic effects by enhancing antioxidant defense mechanisms and other physiological processes in plants. nZVI treated rice seedlings also showed upregulation of phytochelatins which aided in Cd chelation within vacuoles. Study of root morphology with scanning electron microscopy and ROS imaging with confocal microscopy confirmed that nZVI could alleviate oxidative stress due to Cd uptake. In nZVI treated rice seedlings, gene expressions of iron (Fe) transporters (like, IRT1,IRT2,YSL2,YSL15) which are responsible for both Fe and Cd uptake were significantly down-regulated whereas, OsVIT1 and OsCAX4 genes were over expressed which lead to sequestration of Cd in vacuoles. Cd localization assay with leadmium proved that Cd translocation was reduced with nZVI treatment. To further validate our findings a pot experiment was carried out where it was found that nZVI could immobilize Cd in soil prevented accumulation of Cd in rice grains in addition to improving yield.

Integrative analysis of hexaploid wheat roots identifies signature components during iron starvation

Iron (Fe) is an essential micronutrient for all organisms. In crop plants, Fe deficiency can decrease crop yield significantly; however, our current understanding of how major crops respond to Fe deficiency remains limited. Herein, the effect of Fe deprivation at both the transcriptomic and metabolic level in hexaploid wheat was investigated. Genome-wide gene expression reprogramming was observed in wheat roots subjected to Fe starvation, with a total of 5854 genes differentially expressed. Homoeologue and subgenome-specific analysis unveiled the induction-biased contribution from the A and B genomes. In general, the predominance of genes coding for nicotianamine synthase, yellow stripe-like transporters, metal transporters, ABC transporters, and zinc-induced facilitator-like protein was noted. Expression of genes related to the Strategy II mode of Fe uptake was also predominant. Our transcriptomic data were in agreement with the GC-MS analysis that showed the enhanced accumulation of various metabolites such as fumarate, malonate, succinate, and xylofuranose, which could be contributing to Fe mobilization. Interestingly, Fe starvation leads to a significant temporal increase of glutathione S-transferase at both the transcriptional level and enzymatic activity level, which indicates the involvement of glutathione in response to Fe stress in wheat roots. Taken together, our result provides new insight into the wheat response to Fe starvation at the molecular level and lays the foundation to design new strategies for the improvement of Fe nutrition in crops.

Phytosiderophores determine thresholds for iron and zinc accumulation in biofortified rice endosperm while inhibiting the accumulation of cadmium

Nicotianamine (NA) and 2'-deoxymugenic acid (DMA) are metal-chelating ligands that promote the accumulation of metals in rice endosperm, but it is unclear how these phytosiderophores regulate the levels of different metals and limit their accumulation. In this study, transgenic rice plants producing high levels of NA and DMA accumulated up to 4-fold more iron (Fe) and 2-fold more zinc (Zn) in the endosperm compared with wild-type plants. The distribution of Fe and Zn in vegetative tissues suggested that both metals are sequestered as a buffering mechanism to avoid overloading the seeds. The buffering mechanism involves the modulation of genes encoding metal transporters in the roots and aboveground vegetative tissues. As well as accumulating more Fe and Zn, the endosperm of the transgenic plants accumulated less cadmium (Cd), suggesting that higher levels of Fe and Zn competitively inhibit Cd accumulation. Our data show that although there is a strict upper limit for Fe (~22.5 µg g-1 dry weight) and Zn (~84 µg g-1 dry weight) accumulation in the endosperm, the careful selection of strategies to increase endosperm loading with essential minerals can also limit the accumulation of toxic metals such as Cd, thus further increasing the nutritional value of rice.

Phenolic acids, anthocyanins, proanthocyanidins, antioxidant activity, minerals and their correlations in non-pigmented, red, and black rice

Soluble-free, soluble-conjugated, insoluble-bound phenolics and antioxidant activity, flavonoid (TFC), proanthocyanidins (TPAC), anthocyanins and minerals of fifteen whole rice grains with different colors were investigated. Soluble-free protocatechuic and vanillic acids were only quantified in black rice, which had the most quantities. Non-pigmented rice had no detectable conjugated protocatechuic and 2,5-dihydroxybenzoic acids both of which were found in black and red rice, respectively. The main bound phenolic acids were ferulic and p-coumaric, as well as 2,5-dihydroxybenzoic in red rice and protocatechuic and vanillic acids in black rice. Soluble-conjugated phenolics, TFC, and anthocyanins were negatively correlated with L^∗, b^∗, C and H° values. TPAC was positively correlated with a^∗ (P<0.01). Protocatechuic, vanillic, syringic and ferulic acids were associated with TPC and antioxidant activity in the soluble-conjugated fraction while protocatechuic and ferulic acid were correlated with those in the insoluble-bound fraction. Principal component analysis divided samples into non-pigmented, red and black rice groups.

Dealing with iron metabolism in rice: from breeding for stress tolerance to biofortification

Iron is a well-known metal. Used by humankind since ancient times in many different ways, this element is present in all living organisms, where, unfortunately, it represents a two-way problem. Being an essential block in the composition of different proteins and metabolic pathways, iron is a vital component for animals and plants. That is why iron deficiency has a severe impact on the lives of different organisms, including humans, becoming a major concern, especially in developing countries where access to adequate nutrition is still difficult. On the other hand, this metal is also capable of causing damage when present in excess, becoming toxic to cells and affecting the whole organism. Because of its importance, iron absorption, transport and storage mechanisms have been extensively investigated in order to design alternatives that may solve this problem. As the understanding of the strategies that plants use to control iron homeostasis is an important step in the generation of improved plants that meet both human agricultural and nutritional needs, here we discuss some of the most important points about this topic.

The memory of iron stress in strawberry plants

To provide information towards optimization of strategies to treat Fe deficiency, experiments were conducted to study the responses of Fe-deficient plants to the resupply of Fe. Strawberry (Fragaria × ananassa Duch.) was used as model plant. Bare-root transplants of strawberry (cv. 'Diamante') were grown for 42 days in Hoagland's nutrient solutions without Fe (Fe0) and containing 10 μM of Fe as Fe-EDDHA (control, Fe10). For plants under Fe0 the total chlorophyll concentration of young leaves decreased progressively on time, showing the typical symptoms of iron chlorosis. After 35 days the Fe concentration was 6% of that observed for plants growing under Fe10. Half of plants growing under Fe0 were then Fe-resupplied by adding 10 μM of Fe to the Fe0 nutrient solution (FeR). Full Chlorophyll recovery of young leaves took place within 12 days. Root ferric chelate-reductase activity (FCR) and succinic and citric acid concentrations increased in FeR plants. Fe partition revealed that FeR plants expressively accumulated this nutrient in the crown and flowers. This observation can be due to a passive deactivation mechanism of the FCR activity, associated with continuous synthesis of succinic and citric acids at root level, and consequent greater uptake of Fe.

Association of Increased Grain Iron and Zinc Concentrations with Agro-morphological Traits of Biofortified Rice

Biofortification of rice (Oryza sativa L.) with micronutrients is widely recognized as a sustainable strategy to alleviate human iron (Fe) and zinc (Zn) deficiencies in developing countries where rice is the staple food. Constitutive overexpression of the rice nicotianamine synthase (OsNAS) genes has been successfully implemented to increase Fe and Zn concentrations in unpolished and polished rice grain. Intensive research is now needed to couple this high-micronutrient trait with high grain yields. We investigated associations of increased grain Fe and Zn concentrations with agro-morphological traits of backcross twice second filial (BC₂F₂) transgenic progeny carrying OsNAS1 or OsNAS2 overexpression constructs under indica/japonica and japonica/japonica genetic backgrounds. Thirteen agro-morphological traits were evaluated in BC₂F₂ transgenic progeny grown under hydroponic conditions. Concentrations of eight mineral nutrients (Fe, Zn, copper, manganese, calcium, magnesium, potassium, and phosphorus) in roots, stems/sheaths, non-flag leaves, flag leaves, panicles, and grain were also determined. A distance-based linear model (DistLM) was utilized to extract plant tissue nutrient predictors accounting for the largest variation in agro-morphological traits differing between transgenic and non-transgenic progeny. Overall, the BC₂F₂ transgenic progeny contained up to 148% higher Fe and 336% higher Zn concentrations in unpolished grain compared to non-transgenic progeny. However, unpolished grain concentrations surpassing 23 μg Fe g^-1 and 40 μg Zn g^-1 in BC₂F₂indica/japonica progeny, and 36 μg Fe g^-1 and 56 μg Zn g¹ in BC₂F₂japonica/japonica progeny, were associated with significant reductions in grain yield. DistLM analyses identified grain-Zn and panicle-magnesium as the primary nutrient predictors associated with grain yield reductions in the indica/japonica and japonica/japonica background, respectively. We subsequently produced polished grain from high-yield BC₂F₂ transgenic progeny carrying either the OsNAS1 or OsNAS2 overexpression constructs. The OsNAS2 overexpressing progeny had higher percentages of Fe and Zn in polished rice grain compared to the OsNAS1 overexpressing progeny. Results from this study demonstrate that genetic background has a major effect on the development of Fe and Zn biofortified rice. Moreover, our study shows that high-yielding rice lines with Fe and Zn biofortified polished grain can be developed by OsNAS2 overexpression and monitoring for Zn overaccumulation in the grain.

Source-Sink Communication: Regulated by Hormone, Nutrient, and Stress Cross-Signaling

Communication between source organs (exporters of photoassimilates) and sink organs (importers of fixed carbon) has a pivotal role in carbohydrate assimilation and partitioning during plant growth and development. Plant productivity is enhanced by sink strength and source activity, which are regulated by a complex signaling network encompassing sugars, hormones, and environmental factors. However, key components underlying the signaling pathways that regulate source-sink communication are only now beginning to be discovered. Here, we discuss recent advances in our understanding of the molecular mechanisms regulating sugar mobilization during seed development and seedling establishment in cereals, which provide the majority of nutrition for humans. Insights into these mechanisms may provide strategies for improving crop productivity.

Cultivar variability of iron uptake mechanisms in rice (Oryza sativa L.)

Rice (Oryza sativa L.) is the most important staple food in the world. It is rich in genetic diversity and can grow in a wide range of environments. Iron (Fe) deficiency is a major abiotic stress in crop production and in aerobic soils, where Fe forms insoluble complexes, and is not readily available for uptake. To cope with Fe deficiency, plants developed mechanisms for Fe uptake, and although rice was described as a Strategy II plant, recent evidence suggests that it is capable of utilizing mechanisms from both Strategies. The main objective of this work was to compare two cultivars, Bico Branco (japonica) and Nipponbare (tropical japonica), to understand if the regulation of Fe uptake mechanisms could be cultivar (cv.)dependent. Plants of both cultivars were grown under Fe-deficient and -sufficient conditions and physiological and molecular responses to Fe deficiency were evaluated. Bico Branco cv. developed more leaf chlorosis and was more susceptible to Fe deficiency, retaining more nutrients in roots, than Nipponbare cv., which translocated more nutrients to shoots. Nipponbare cv. presented higher levels of Fe reductase activity, which was significantly up-regulated by Fe deficiency, and had higher expression levels of the Strategy I-OsFRO2 gene in roots, while Bico Branco cv. induced more genes involved in Strategy II.These new findings show that rice cultivars have different responses to Fe deficiency and that the induction of Strategy I or II may be rice cultivar-dependent, although the utilization of the reduction mechanisms seems to be an ubiquitous advantage.

Over-expression of the MxIRT1 gene increases iron and zinc content in rice seeds

Iron and zinc are essential in plant and human nutrition. Iron deficiency has been one of the causes of human mortality, especially in developing countries with high rice consumption. MxIRT1 is a ferrous transporter that has been screened from an iron-efficient genotype of the apple tree, Malus xiaojinensis Cheng et Jiang. In order to produce Fe-biofortified rice with MxIRT1 to solve the Fe-deficiency problem, plant expression vectors of pCAMBIA1302-MxIRT1:GFP and pCAMBIA1302-anti MxIRT1:GFP were constructed that led to successful production of transgenic rice. The transgenic plant phenotypes showed that the expression of endogenous OsIRT1 was suppressed by anti-MxIRT1 in antisense lines that acted as an opposing control, while sense lines had a higher tolerance under Zn- and Fe-deficient conditions. The iron and zinc concentration in T3 seeds increased by three times in sense lines when compared to the wild type. To understand the MxIRT1 cadmium uptake, the MxIRT1 cadmium absorption trait was compared with AtIRT1 and OsIRT1 in transgenic rice protoplasts, and it was found that MxIRT1 had the lowest Cd uptake capacity. MxIRT1 transgenic tobacco-cultured bright yellow-2 (BY-2) cells and rice lines were subjected to different Fe conditions and the results from the non-invasive micro-test technique showed that iron was actively transported compared to cadmium as long as iron was readily available in the environment. This suggests that MxIRT1 is a good candidate gene for plant Fe and Zn biofortification.

Genotypic variation in partitioning of dry matter and manganese between source and sink organs of rice under manganese stress

Genetic variability in dry matter and manganese partitioning between source and sink organs was the key mechanism for Mn efficient rice genotypes to cope with Mn stress. Considerable differences exist among cereal genotypes to cope manganese (Mn) deficiency, but the underlying mechanisms are poorly understood. Minimal information regarding partitioning and/or remobilization of dry matter and Mn between source and sink organs exists in rice genotypes differing in Mn efficiency. The present study was aimed to assess the growth dynamics in terms of dry matter and Mn remobilization in the whole plant (leaves and tillers as source and panicles and grains as sink) during the grain development in diverse rice genotypes. The efficient genotypes accumulated higher dry matter than inefficient genotypes under low Mn level. The translocation index i.e., uptake in grain/total uptake was 0.11 in efficient genotype (PR 116) and 0.04 in inefficient genotypes (PR 111). The efficient genotype had higher grain Mn utilization efficiency of 0.71 in comparison to 0.48 of inefficient genotype indicating that in efficient genotype, Mn in grain produces more dry matter than inefficient genotypes. The efficient genotypes also had higher flag leaf area and nitrate reductase activity. The source of efficient genotypes contributed to a greater extent to developing sink but further mobilization to grain was hindered by panicle. The panicle of inefficient genotypes had higher per cent of Mn uptake than efficient genotypes indicating that Mn was least mobilized from panicle to grain in inefficient genotypes. The lower per cent uptake of Mn in efficient genotypes indicated that Mn was mobilized from panicle to developing grain and this led to higher Mn translocation index in grain of efficient genotypes. The uptake partitioning revealed that source of all genotypes mobilized the Mn towards the sink to almost same extent but it was the panicle where highest per cent uptake per plant was in inefficient genotypes and lowest in efficient genotypes. The lowest per cent uptake in panicle of efficient genotypes revealed that it supported developing grain to have highest translocation index.

Expression patterns of QTL based and other candidate genes in Madhukar × Swarna RILs with contrasting levels of iron and zinc in unpolished rice grains

BACKGROUND

Identifying QTLs/genes for iron and zinc in rice grains can help in biofortification programs. Genome wide mapping showed 14 QTLs for iron and zinc concentration in unpolished rice grains of F7 RILs derived from Madhukar × Swarna. One line (HL) with high Fe and Zn and one line (LL) with low Fe and Zn in unpolished rice were compared with each other for gene expression using qPCR. 7 day old seedlings were grown in Fe+ and Fe- medium for 10 days and RNA extracted from roots and shoots to determine the response of 15 genes in Fe- conditions.

RESULTS

HL showed higher upregulation than LL in shoots but LL showed higher upregulation than HL in roots. YSL2 was upregulated only in HL roots and YSL15 only in HL shoots and both up to 60 fold under Fe- condition. IRT2 and DMAS1 were upregulated 100 fold and NAS2 1000 fold in HL shoot. NAS2, IRT1, IRT2 and DMAS1 were upregulated 40 to 100 fold in LL roots. OsZIP8, OsNAS3, OsYSL1 and OsNRAMP1 which underlie major Fe QTL showed clear allelic differences between HL and LL for markers flanking QTL. The presence of iron increasing QTL allele in HL was clearly correlated with high expression of the underlying gene. OsZIP8 and OsNAS3 which were within major QTL with increasing effect from Madhukar were 8 fold and 4 fold more expressed in HL shoot than in LL shoot. OsNAS1, OsNAS2, OsNAS3, OsYSL2 and OsYSL15 showed 1.5 to 2.5 fold upregulation in flag leaf of HL when compared with flag leaf of Swarna.

CONCLUSION

HL and LL differed in root length, Fe concentration and expression of several genes under Fe deficiency. The major distinguishing genes were NAS2, IRT2, DMAS1, and YSL15 in shoot and NAS2, IRT1, IRT2, YSL2, and ZIP8 in roots. The presence of iron increasing QTL allele in HL at marker locus close to genes also increased upregulation in HL.

Evaluation of constitutive iron reductase (AtFRO2) expression on mineral accumulation and distribution in soybean (Glycine max. L)

Iron is an important micronutrient in human and plant nutrition. Adequate iron nutrition during crop production is central for assuring appropriate iron concentrations in the harvestable organs, for human food or animal feed. The whole-plant movement of iron involves several processes, including the reduction of ferric to ferrous iron at several locations throughout the plant, prior to transmembrane trafficking of ferrous iron. In this study, soybean plants that constitutively expressed the AtFRO2 iron reductase gene were analyzed for leaf iron reductase activity, as well as the effect of this transgene's expression on root, leaf, pod wall, and seed mineral concentrations. High Fe supply, in combination with the constitutive expression of AtFRO2, resulted in significantly higher concentrations of different minerals in roots (K, P, Zn, Ca, Ni, Mg, and Mo), pod walls (Fe, K, P, Cu, and Ni), leaves (Fe, P, Cu, Ca, Ni, and Mg) and seeds (Fe, Zn, Cu, and Ni). Leaf and pod wall iron concentrations increased as much as 500% in transgenic plants, while seed iron concentrations only increased by 10%, suggesting that factors other than leaf and pod wall reductase activity were limiting the translocation of iron to seeds. Protoplasts isolated from transgenic leaves had three-fold higher reductase activity than controls. Expression levels of the iron storage protein, ferritin, were higher in the transgenic leaves than in wild-type, suggesting that the excess iron may be stored as ferritin in the leaves and therefore unavailable for phloem loading and delivery to the seeds. Also, citrate and malate levels in the roots and leaves of transgenic plants were significantly higher than in wild-type, suggesting that organic acid production could be related to the increased accumulation of minerals in roots, leaves, and pod walls, but not in the seeds. All together, these results suggest a more ubiquitous role for the iron reductase in whole-plant mineral accumulation and distribution.

Autophagy as a possible mechanism for micronutrient remobilization from leaves to seeds

Seed formation is an important step of plant development which depends on nutrient allocation. Uptake from soil is an obvious source of nutrients which mainly occurs during vegetative stage. Because seed filling and leaf senescence are synchronized, subsequent mobilization of nutrients from vegetative organs also play an essential role in nutrient use efficiency, providing source-sink relationships. However, nutrient accumulation during the formation of seeds may be limited by their availability in source tissues. While several mechanisms contributing to make leaf macronutrients available were already described, little is known regarding micronutrients such as metals. Autophagy, which is involved in nutrient recycling, was already shown to play a critical role in nitrogen remobilization to seeds during leaf senescence. Because it is a non-specific mechanism, it could also control remobilization of metals. This article reviews actors and processes involved in metal remobilization with emphasis on autophagy and methodology to study metal fluxes inside the plant. A better understanding of metal remobilization is needed to improve metal use efficiency in the context of biofortification.

kNACking on heaven's door: how important are NAC transcription factors for leaf senescence and Fe/Zn remobilization to seeds?

Senescence is a coordinated process where a plant, or a part of it, engages in programmed cell death to salvage nutrients by remobilizing them to younger tissues or to developing seeds. As Fe and Zn deficiency are the two major nutritional disorders in humans, increased concentration of these nutrients through biofortification in cereal grains is a long-sought goal. Recent evidences point to a link between the onset of leaf senescence and increased Fe and Zn remobilization. In wheat, one member of the NAC (NAM, ATAF, and CUC) transcription factor (TF) family (NAM-B1) has a major role in the process, probably regulating key genes for the early onset of senescence, which results in higher Fe and Zn concentrations in grains. In rice, the most important staple food for nearly half of the world population, the NAM-B1 ortholog does not have the same function. However, other NAC proteins are related to senescence, and could be playing roles on the same remobilization pathway. Thus, these genes are potential tools for biofortification strategies in rice. Here we review the current knowledge on the relationship between senescence, Fe and Zn remobilization and the role of NAC TFs, with special attention to rice. We also propose a working model for OsNAC5, which would act on the regulation of nicotianamine (NA) synthesis and metal-NA remobilization.