OsYSL16 is Required for Preferential Cu Distribution to Floral Organs in Rice

OsMYB67 Knockout Promotes Rice Heading and Yield by Facilitating Copper Distribution in Panicles

Copper (Cu) is an essential micronutrient required for rice flowering and seed setting. Here, we identified that Cu-induced R2R3-MYB transcription factor, OsMYB67, acts as a negative regulator that controls rice heading and yield production by affecting Cu distribution in panicles. OsMYB67 was constitutively expressed, with the highest expression in the roots. OsMYB67 knockout did not affect plant growth, but significantly increased Cu concentrations in roots, shoots, and xylem sap at the seedling stage. At the reproductive stage, OsMYB67 mutants displayed an early heading phenotype, with significantly increased Cu distribution in panicles but decreased Cu distribution in leaves, whereas OsMYB67-overexpressing plants showed the opposite result. In addition, higher grain yield and Cu concentrations in seeds were observed in OsMYB67 mutants compared to the wild-type. The results of Y1H, transient co-expression, EMSA, in situ RT-PCR, and RT-qPCR showed that OsMYB67 directly binds to the promoter region of OsHMA9 and upregulates its expression. Significantly increased Cu concentrations were also observed in the roots, shoots, and seeds of oshma9 mutants, consistent with the results observed in OsMYB67 mutants. Interestingly, dramatically higher expression levels of OsATX1 and OsYSL16 were observed in the OsMYB67 mutants, which may contribute to the increased Cu distribution in the panicles.

Advances of the mechanism for copper tolerance in plants

Copper (Cu) is a vital trace element necessary for plants growth and development. It acts as a co-factor for enzymes and plays a crucial role in various physiological processes, including photosynthesis, respiration, antioxidant systems, and hormone signaling transduction. However, excessive amounts of Cu can disrupt normal physiological metabolism, thus hindering plant growth, development, and reducing yield. In recent years, the widespread abuse of Cu-containing fungicides and industrial Cu pollution has resulted in significant soil contamination. Therefore, it is of utmost importance to uncover the adverse effects of excessive Cu on plant growth and delve into the molecular mechanisms employed by plants to counteract the stress caused by excessive Cu. Recent studies have confirmed the inhibitory effects of excess Cu on mineral nutrition, chlorophyll biosynthesis, and antioxidant enzyme activity. This review systematically outlines the ways in which plants tolerate excessive Cu stress and summarizes them into eight Cu-tolerance strategies. Furthermore, it highlights the necessity for further research to comprehend the molecular regulatory mechanisms underlying the responses to excessive Cu stress.

Biofortifying multiple micronutrients and decreasing arsenic accumulation in rice grain simultaneously by expressing a mutant allele of OAS-TL gene

Rice grains typically contain relatively high levels of toxic arsenic (As) but low levels of essential micronutrients. Biofortification of essential micronutrients while decreasing As accumulation in rice would benefit human nutrition and health. We generated transgenic rice expressing a gain-of-function mutant allele astol1 driven by the OsGPX1 promoter. astol1 encodes a plastid-localized O-acetylserine (thiol) lyase (OAS-TL) with Ser189Asn substitution (OsASTOL1S189N), which enhances cysteine biosynthesis by forming an indissociable cysteine synthase complex with its partner serine acetyltransferase (SAT). The effects on growth, As tolerance, and nutrient and As accumulation in rice grain were evaluated in hydroponic, pot and field experiments. The expression of OsASTOL1S189N in pOsGPX1::astol1 transgenic lines enhanced SAT activity, sulphate uptake, biosynthesis of cysteine, glutathione, phytochelatins and nicotianamine, and enhanced tolerance to As. The expression of OsASTOL1S189N decreased As accumulation while increased the accumulation of multiple macronutrients (especially sulphur, nitrogen and potassium) and micronutrients (especially zinc and selenium) in rice grain in a pot experiment and two field experiments, and had little effect on plant growth and grain yield. Our study provides a new strategy to genetically engineer rice to biofortify multiple essential nutrients, reducing As accumulation in rice grain and enhancing As tolerance simultaneously.

Copper stress in rice: Perception, signaling, bioremediation and future prospects

Copper (Cu) is an indispensable micronutrient for plants, animals, and microorganisms and plays a vital role in different physiological processes. However, excessive Cu accumulation in agricultural soil, often through anthropogenic action, poses a potential risk to plant health and crop productivity. This review article provided a comprehensive overview of the available information regarding Cu dynamics in agricultural soils, major sources of Cu contamination, factors influencing its mobility and bioavailability, and mechanisms of Cu uptake and translocation in rice plants. This review examined the impact of Cu toxicity on the germination, growth, and photosynthesis of rice plants. It also highlighted molecular mechanisms underlying Cu stress signaling and the plant defense strategy, involving chelation, compartmentalization, and antioxidant responses. This review also identified significant areas that need further research, such as Cu uptake mechanism in rice, Cu signaling process, and the assessment of Cu-polluted paddy soil and rice toxicity under diverse environmental conditions. The development of rice varieties with reduced Cu accumulation through comprehensive breeding programs is also necessary. Regulatory measures, fungicide management, plant selection, soil and environmental investigation are recommended to prevent Cu buildup in agricultural lands to achieve sustainable agricultural goals.

The transcription factor OsSPL9 endows rice with copper deficiency resilience

Copper (Cu) is a crucial micronutrient essential for the growth and development of plants. Rice exhibits remarkable resistance to Cu deficiency, but the underlying molecular mechanisms are not well understood. In this study, we reveal that the plant's ability to withstand Cu deficiency is orchestrated by a transcription factor known as OsSPL9. We have demonstrated that OsSPL9 functions as a central regulator of Cu homeostasis. Disrupting OsSPL9 through knockout significantly reduced the plant's tolerance to Cu deficiency. As a result, the spl9 mutants exhibited reduced Cu accumulation in their shoots when compared with wild-type plants. This reduction was linked to a disruption in the transport of Cu from older leaves to younger ones. Furthermore, we show that OsSPL9 directly bound to GTAC motifs in the promoters of key genes involved in Cu uptake and transport, as well as Cu-miRNAs, and enhanced their transcription under Cu-deficient conditions. Overall, our findings shed light on the molecular basis of rice resilience to Cu deficiency stress and place the transcription factor OsSPL9 as a master regulator of this response.

Transcriptome-wide m6A methylation profile reveals tissue specific regulatory networks in switchgrass (Panicum virgatum L.) under cadmium stress

The heavy metal cadmium (Cd), known for its high toxicity, poses a grave threat to human health through the food chain. N6-methyladenosine (m6A), the most abundant internal modification, regulates plant adaptation to various adversities, yet the panorama of m6A modifications in switchgrass under cadmium stress remains elusive. This study examines the physiological responses of switchgrass roots and shoots exposed to 50 μM CdCl2, alongside an overview of transcriptome-wide m6A methylation patterns. After cadmium treatment, methylation modifications are primarily enriched near stop codons and the 3'UTR region, with a negative correlation between m6A modification and gene expression levels. In shoots, approximately 58 % of DEGs with m6A modifications show upregulation in expression and decrease in m6A peaks, including zinc transporter 4-like (ZIP4). In roots, about 43 % of DEGs with m6A modifications exhibit downregulation in expression and increase in m6A peaks, such as the ABC transporter family member (ABCG25). We further validate the m6A enrichment, gene expression and mRNA stability of ZIP4 in response to Cd treatment. The results suggest that the negative correlation of m6A enrichment and gene expression is due to altered mRNA stability. Our study establishes an m6A regulatory network governing cadmium transport in switchgrass roots and shoots, offering new avenues for candidate gene manipulation in phytoremediation applications of heavy metal pollution.

Metal Transport Systems in Plants

Plants take up metals, including essential micronutrients [iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn)] and the toxic heavy metal cadmium (Cd), from soil and accumulate these metals in their edible parts, which are direct and indirect intake sources for humans. Multiple transporters belonging to different families are required to transport a metal from the soil to different organs and tissues, but only a few of them have been fully functionally characterized. The transport systems (the transporters required for uptake, translocation, distribution, redistribution, and their regulation) differ with metals and plant species, depending on the physiological roles, requirements of each metal, and anatomies of different organs and tissues. To maintain metal homeostasis in response to spatiotemporal fluctuations of metals in soil, plants have developed sophisticated and tightly regulated mechanisms through the regulation of transporters at the transcriptional and/or posttranscriptional levels. The manipulation of some transporters has succeeded in generating crops rich in essential metals but low in Cd accumulation. A better understanding of metal transport systems will contribute to better and safer crop production.

Rice transcription factor OsWRKY37 positively regulates flowering time and grain fertility under copper deficiency

Efficient uptake, translocation, and distribution of Cu to rice (Oryza sativa) spikelets is crucial for flowering and yield production. However, the regulatory factors involved in this process remain unidentified. In this study, we isolated a WRKY transcription factor gene induced by Cu deficiency, OsWRKY37, and characterized its regulatory role in Cu uptake and transport in rice. OsWRKY37 was highly expressed in rice roots, nodes, leaf vascular bundles, and anthers. Overexpression of OsWRKY37 promoted the uptake and root-to-shoot translocation of Cu in rice under -Cu condition but not under +Cu condition. While mutation of OsWRKY37 significantly decreased Cu concentrations in the stamen, the root-to-shoot translocation and distribution ratio in brown rice affected pollen development, delayed flowering time, decreased fertility, and reduced grain yield under -Cu condition. yeast one-hybrid, transient co-expression and EMSAs, together with in situ RT-PCR and RT-qPCR analysis, showed that OsWRKY37 could directly bind to the upstream promoter region of OsCOPT6 (copper transporter) and OsYSL16 (yellow stripe-like protein) and positively activate their expression levels. Analyses of oscopt6 mutants further validated its important role in Cu uptake in rice. Our study demonstrated that OsWRKY37 acts as a positive regulator involved in the uptake, root-to-shoot translocation, and distribution of Cu through activating the expression of OsCOPT6 and OsYSL16, which is important for pollen development, flowering, fertility, and grain yield in rice under Cu deficient conditions. Our results provide a genetic strategy for improving rice yield under Cu deficient condition.

Molecular Mechanisms of Plant Responses to Copper: From Deficiency to Excess

Copper (Cu) is an essential nutrient for plant growth and development. This metal serves as a constituent element or enzyme cofactor that participates in many biochemical pathways and plays a key role in photosynthesis, respiration, ethylene sensing, and antioxidant systems. The physiological significance of Cu uptake and compartmentalization in plants has been underestimated, despite the importance of Cu in cellular metabolic processes. As a micronutrient, Cu has low cellular requirements in plants. However, its bioavailability may be significantly reduced in alkaline or organic matter-rich soils. Cu deficiency is a severe and widespread nutritional disorder that affects plants. In contrast, excessive levels of available Cu in soil can inhibit plant photosynthesis and induce cellular oxidative stress. This can affect plant productivity and potentially pose serious health risks to humans via bioaccumulation in the food chain. Plants have evolved mechanisms to strictly regulate Cu uptake, transport, and cellular homeostasis during long-term environmental adaptation. This review provides a comprehensive overview of the diverse functions of Cu chelators, chaperones, and transporters involved in Cu homeostasis and their regulatory mechanisms in plant responses to varying Cu availability conditions. Finally, we identified that future research needs to enhance our understanding of the mechanisms regulating Cu deficiency or stress in plants. This will pave the way for improving the Cu utilization efficiency and/or Cu tolerance of crops grown in alkaline or Cu-contaminated soils.

Fe(III) transporter OsYSL15 may play a key role in the uptake of Cr(III) in rice (Oryza sativa L.)

Due to the widely discharge of chromium (Cr) by mining and smelting industries, etc., contamination of paddy soils and rice has become serious problems. Therefore it is crucial to explore how rice takes up Cr. Cr(III) is the most common Cr form in the long-term water flooding paddy soils. Here, we demonstrate that OsYSL15, a key gene for Fe(III) uptake, is equally applicable for Cr(III) uptake in rice. Firstly, the antagonistic effect of Cr(III) and Fe(III) in the uptake process was found. Rice could accumulate more Cr(III) under Fe-deficient conditions. And the Fe(III) content in the protoplasts of rice root cells gradually decreased with the increase exposure of Cr(III). Knockdown of OsYSL15 in rice significantly reduced the Cr(III) uptake rate. Compared with wild type rice, the accumulation of Cr(III) in OsYSL15 mutant was decreased by 40.7%- 70.6% after gene editing. These results indicate that OsYSL15 is a key gene responsible for Cr(III) uptake in rice, which can guide the screening or genetic modification for low-Cr-accumulation rice varieties.

Linking the key physiological functions of essential micronutrients to their deficiency symptoms in plants

In this review, we untangle the physiological key functions of the essential micronutrients and link them to the deficiency responses in plants. Knowledge of these responses at the mechanistic level, and the resulting deficiency symptoms, have improved over the last decade and it appears timely to review recent insights for each of them. A proper understanding of the links between function and symptom is indispensable for an accurate and timely identification of nutritional disorders, thereby informing the design and development of sustainable fertilization strategies. Similarly, improved knowledge of the molecular and physiological functions of micronutrients will be important for breeding programmes aiming to develop new crop genotypes with improved nutrient-use efficiency and resilience in the face of changing soil and climate conditions.

Identification and characterization of yellow stripe-like genes in maize suggest their roles in the uptake and transport of zinc and iron

BACKGROUND

Yellow Stripe-Like (YSL) proteins are involved in the uptake and transport of metal ions. They play important roles in maintaining the zinc and iron homeostasis in Arabidopsis, rice (Oryza sativa), and barley (Hordeum vulgare). However, proteins in this family have not been fully identified and comprehensively analyzed in maize (Zea mays L.).

RESULTS

In this study, we identified 19 ZmYSLs in the maize genome and analyzed their structural features. The results of a phylogenetic analysis showed that ZmYSLs are homologous to YSLs of Arabidopsis and rice, and these proteins are divided into four independent branches. Although their exons and introns have structural differences, the motif structure is relatively conserved. Analysis of the cis-regulatory elements in the promoters indicated that ZmYSLs might play a role in response to hypoxia and light. The results of RNA sequencing and quantitative real-time PCR analysis revealed that ZmYSLs are expressed in various tissues and respond differently to zinc and iron deficiency. The subcellular localization of ZmYSLs in the protoplast of maize mesophyll cells showed that they may function in the membrane system.

CONCLUSIONS

This study provided important information for the further functional analysis of ZmYSL, especially in the spatio-temporal expression and adaptation to nutrient deficiency stress. Our findings provided important genes resources for the maize biofortification.

Molecular bases for the stronger plastic response to high nitrate in the invasive plant Xanthium strumarium compared with its native congener

MAIN CONCLUSION

High expressions of nitrate use and photosynthesis-related transcripts contribute to the stronger plasticity to high nitrate for the invader relative to its native congener, which may be driven by hormones. Strong phenotypic plasticity is often considered as one of the main mechanisms underlying exotic plant invasions. However, few studies have been conducted to investigate the related molecular mechanisms. Here, we determined the differences in the plastic responses to high nitrate between the invasive plant X. strumarium and its native congener, and the molecular bases by transcriptome analysis and quantitative real-time PCR validation. Our results showed that the invader had higher plasticity of growth, nitrogen accumulation and photosynthesis in responses to high nitrate than its native congener. Compared with its congener, more N utilization-related transcripts, including nitrate transporter 1/peptide transporter family 6.2 and nitrate reductase 1, were induced by high nitrate in the root of X. strumarium, improving its N utilization ability. More transcripts coding for photosynthetic antenna proteins were also induced by high nitrate in the shoot of X. strumarium, enhancing its photosynthesis. Hormones may be involved in the regulation of the plastic responses to high nitrate in the two species. Our study contributes to understanding the molecular mechanisms underlying the stronger plasticity of the invader in responses to high nitrate, and the potential function of plant hormones in these processes, providing bases for precise control of invasive plants using modern molecular techniques.

Nicotianamine: A Key Player in Metal Homeostasis and Hyperaccumulation in Plants

Nicotianamine (NA) is a low-molecular-weight N-containing metal-binding ligand, whose accumulation in plant organs changes under metal deficiency or excess. Although NA biosynthesis can be induced in vivo by various metals, this non-proteinogenic amino acid is mainly involved in the detoxification and transport of iron, zinc, nickel, copper and manganese. This review summarizes the current knowledge on NA biosynthesis and its regulation, considers the mechanisms of NA secretion by plant roots, as well as the mechanisms of intracellular transport of NA and its complexes with metals, and its role in radial and long-distance metal transport. Its role in metal tolerance is also discussed. The NA contents in excluders, storing metals primarily in roots, and in hyperaccumulators, accumulating metals mainly in shoots, are compared. The available data suggest that NA plays an important role in maintaining metal homeostasis and hyperaccumulation mechanisms. The study of metal-binding compounds is of interdisciplinary significance, not only regarding their effects on metal toxicity in plants, but also in connection with the development of biofortification approaches to increase the metal contents, primarily of iron and zinc, in agricultural plants, since the deficiency of these elements in food crops seriously affects human health.

Deciphering the functional roles of transporter proteins in subcellular metal transportation of plants

MAIN CONCLUSION

The role of transporters in subcellular metal transport is of great significance for plants in coping with heavy metal stress and maintaining their proper growth and development. Heavy metal toxicity is a serious long-term threat to plant growth and agricultural production, becoming a global environmental concern. Excessive heavy metal accumulation not only damages the biochemical and physiological functions of plants but also causes chronic health hazard to human beings through the food chain. To deal with heavy metal stress, plants have evolved a series of elaborate mechanisms, especially a variety of spatially distributed transporters, to strictly regulate heavy metal uptake and distribution. Deciphering the subcellular role of transporter proteins in controlling metal absorption, transport and separation is of great significance for understanding how plants cope with heavy metal stress and improving their adaptability to environmental changes. Hence, we herein introduce the detrimental effects of excessive common essential and non-essential heavy metals on plant growth, and describe the structural and functional characteristics of transporter family members, with a particular emphasis on their roles in maintaining heavy metal homeostasis in various organelles. Besides, we discuss the potential of controlling transporter gene expression by transgenic approaches in response to heavy metal stress. This review will be valuable to researchers and breeders for enhancing plant tolerance to heavy metal contamination.

Cross talk between Cu excess and Fe deficiency in the roots of rice

Copper (Cu) and iron (Fe) share similar characteristics and participate as coenzymes in several physiological processes. Both Cu excess and Fe deficiency result in chlorosis, however, the crosstalk between the two is not clear in rice. In this study, we performed transcriptome analysis for Cu excess and Fe deficiency in rice. Some WRKY family members (such as WRKY26) and some bHLH family members (such as late flowering) were selected as novel potential transcription factors involved in the regulation of Cu detoxification and Fe utilization, respectively. These genes were induced under corresponding stress conditions. Many Fe uptake-related genes were induced by Cu excess, while Cu detoxification-related genes were not induced by Fe deficiency. Meanwhile, some genes, such as metallothionein 3a, gibberellin 3beta-dioxygenase 2 and WRKY11, were induced by Cu excess but repressed by Fe deficiency. Concisely, our results highlight the crosstalk between Cu excess and Fe deficiency in rice. Cu excess caused Fe deficiency response, while Fe deficiency did not lead to Cu toxicity response. Metallothionein 3a might be responsible for Cu toxicity-induced chlorosis in rice. The crosstalk between Cu excess and Fe deficiency might be regulated by gibberellic acid.

Physiological and Molecular Mechanisms of Plant Responses to Copper Stress

Copper (Cu) is an essential micronutrient for humans, animals, and plants, and it participates in various morphological, physiological, and biochemical processes. Cu is a cofactor for a variety of enzymes, and it plays an important role in photosynthesis, respiration, the antioxidant system, and signal transduction. Many studies have demonstrated the adverse effects of excess Cu on crop germination, growth, photosynthesis, and antioxidant activity. This review summarizes the biological functions of Cu, the toxicity of excess Cu to plant growth and development, the roles of Cu transport proteins and chaperone proteins, and the transport process of Cu in plants, as well as the mechanisms of detoxification and tolerance of Cu in plants. Future research directions are proposed, which provide guidelines for related research.

Synergistic Interaction between Copper and Nitrogen-Uptake, Translocation, and Distribution in Rice Plant

Interactions among nutrients have been widely recognized in plants and play important roles in crop growth and yield formation. However, the interplay of Cu and N in rice plants is not yet clear. In this study, rice plants were grown with different combinations of Cu and N supply. The effects of Cu-N interaction on the growth, yield production, Cu and N transport, and gene expression levels were analyzed. The results showed that the effect of N supply on rice growth and yield formation was more pronounced than that of Cu supply. The Cu supply significantly improved the uptake of N (by 9.52-30.64%), while the N supply significantly promoted the root-to-shoot translocation of Cu (by 27.28-38.45%) and distributed more Cu (1.85-19.16%) into the shoots and leaves. The results of qRT-PCR showed that +Cu significantly up-regulated the expression levels of both NO₃^- and NH₄⁺ transporter genes OsNRTs and OsAMTs, including OsNRT1.1B, OsNRT2.1, OsNRT2.3a, OsNRT2.4, OsAMT1.2, OsAMT1.3, and OsAMT3.1. Meanwhile, +N significantly up-regulated the expression levels of Cu transporter genes OsHMA5 and OsYSL16. In addition, the supply of Cu up-regulated the expression levels of OsGS1;2, OsGS2, and OsNADH-GOGAT to 12.61-, 6.48-, and 6.05-fold, respectively. In conclusion, our study demonstrates a synergistic effect between Cu and N in rice plants. It is expected that our results would be helpful to optimize the application of N and Cu fertilizers in agriculture.

Genome-Wide Identification and Transcript Analysis Reveal Potential Roles of Oligopeptide Transporter Genes in Iron Deficiency Induced Cadmium Accumulation in Peanut

The oligopeptide transporter (OPT) family is a group of proton-coupled symporters that play diverse roles, including metal homeostasis. However, little is known about this family of peanuts. To reveal the potential roles of AhOPT genes in Fe/Cd interactions, peanut AhOPT genes were genome-widely identified, and the relationships between gene expression and Cd accumulation were detected in two contrasting peanut cultivars (Fenghua 1 and Silihong) under Fe-sufficient or Fe-deficient conditions. A total of 40 AhOPT genes were identified in peanuts, which were divided into two subfamilies (PT and YS). Most AhOPT genes underwent gene duplication events predominated by whole-genome duplication. Clustered members generally have similar protein structures. However, gene structural divergences occurred in most of the duplicated genes. Transcription analysis revealed that AhOPT3.2/3.4 and AhYSL3.1/3.2 might be responsible for Fe deficiency tolerance, while AhOPT3.1/3.4, AhOPT7.1/7.2, and AhYSL1.1 be involved in Fe/Cd interactions. These genes might be regulated by transcription factors, including ATHB-12, ATHB-6, DIVARICATA, MYB30, NAC02, DOF3.4, IDD7, and LUX. Reduced expressions of AhYSL3.1/3.2 and higher expressions of AhOPT3.4 might contribute to higher Fe-deficiency tolerance in Silihong. Higher expression of AhOPT7.3 and AhOPT6.1 might be responsible for low Cd accumulation in Fenghua 1. Our results confirmed that AhOPT3/6/7 and AhYSL1/3 might be involved in the transport of Fe and/or Cd in peanuts and provided new clues to understanding potential mechanisms of Fe/Cd interactions.

Mapping and functional analysis of high-copper accumulation mutant oshc1 in rice

Copper (Cu) is an essential but potentially toxic element in rice. Little is known about the mechanism of rice grain Cu accumulation. In this study, we identified a high copper accumulation in grain 1 (oshc1) mutant from the wild type indica rice cultivar 9311 (WT) mutant bank. Compared with those in WT, more Cu was shown to accumulate in the shoots of seedlings and the above-ground tissues except nodes although less total Cu content in oshc1. Further analysis showed that the mutant had an accelerated Cu transport ratio from roots to shoots and higher Cu concentration in xylem sap than WT. This phenomenon in oshc1 was controlled by a single recessive gene, which was identified as BGIOSGA007732, and named OsHMA4. The eight base frame-shift from 1021 to 1028 bp in the coding sequence of OsHMA4 led to a modification after the 341st amino acid and resulted in premature translation termination of OsHMA4 at the 377th amino acid. This may change the function of OsHMA4. Furthermore, the up-regulated OsCOPT7 and OsATX1 and down-regulated OsHMA4 probably decrease Cu compartmentalization in roots of oshc1. In summary, the frame-shift in OsHMA4 changes the function of OsHMA4 and the expression of genes relative to Cu transport in the mutant, which leads to more Cu transport upward and higher Cu accumulation in the rice grains. Moreover, oshc1 was more tolerance to Cu-shortage than WT, while more sensitive to Cu excess exposure than WT. However, RNA-Seq analysis shown that changes in transcription levels of genes in oshc1 involving in molecular function of ions binding and biological processes of cell wall organization and defense response to bio-stress. Which indicates that oshc1 is advantage to Cu limited condition than WT. This work reveals the mechanism of high Cu accumulation in the grains of oshc1 and provides a material to breed new cultivars with optimum levels of Cu in brown rice by crossing with other dominant varieties, which can be planted in different soils to ensure the yield and quality of rice.

Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation

Heavy metal pollution in soil is a global problem with serious impacts on human health and ecological security. Phytoextraction in phytoremediation, in which plants uptake and transport heavy metals (HMs) to the tissues of aerial parts, is the most environmentally friendly method to reduce the total amount of HMs in soil and has wide application prospects. However, the molecular mechanism of phytoextraction is still under investigation. The uptake, translocation, and retention of HMs in plants are mainly mediated by a variety of transporter proteins. A better understanding of the accumulation strategy of HMs via transporters in plants is a prerequisite for the improvement of phytoextraction. In this review, the biochemical structure and functions of HM transporter families in plants are systematically summarized, with emphasis on their roles in phytoremediation. The accumulation mechanism and regulatory pathways related to hormones, regulators, and reactive oxygen species (ROS) of HMs concerning these transporters are described in detail. Scientific efforts and practices for phytoremediation carried out in recent years suggest that creation of hyperaccumulators by transgenic or gene editing techniques targeted to these transporters and their regulators is the ultimate powerful path for the phytoremediation of HM contaminated soils.

Association mapping uncovers maize ZmbZIP107 regulating root system architecture and lead absorption under lead stress

Lead (Pb) is a highly toxic contaminant to living organisms and the environment. Excessive Pb in soils affects crop yield and quality, thus threatening human health via the food chain. Herein, we investigated Pb tolerance among a maize association panel using root bushiness (BSH) under Pb treatment as an indicator. Through a genome-wide association study of relative BSH, we identified four single nucleotide polymorphisms (SNPs) and 30 candidate genes associated with Pb tolerance in maize seedlings. Transcriptome analysis showed that four of the 30 genes were differentially responsive to Pb treatment between two maize lines with contrasting Pb tolerance. Among these, the ZmbZIP107 transcription factor was confirmed as the key gene controlling maize tolerance to Pb by using gene-based association studies. Two 5' UTR_variants in ZmbZIP107 affected its expression level and Pb tolerance among different maize lines. ZmbZIP107 protein was specifically targeted to the nucleus and ZmbZIP107 mRNA showed the highest expression in maize seedling roots among different tissues. Heterologous expression of ZmbZIP107 enhanced rice tolerance to Pb stress and decreased Pb absorption in the roots. Our study provided the basis for revelation of the molecular mechanism underlying Pb tolerance and contributed to cultivation of Pb-tolerant varieties in maize.

Assessment of Biofortification Approaches Used to Improve Micronutrient-Dense Plants That Are a Sustainable Solution to Combat Hidden Hunger

Malnutrition causes diseases, immune system disorders, deterioration in physical growth, mental development, and learning capacity worldwide. Micronutrient deficiency, known as hidden hunger, is a serious global problem. Biofortification is a cost-effective and sustainable agricultural strategy for increasing the concentrations or bioavailability of essential elements in the edible parts of plants, minimizing the risks of toxic metals, and thus reducing malnutrition. It has the advantage of delivering micronutrient-dense food crops to a large part of the global population, especially poor populations. Agronomic biofortification and biofertilization, traditional plant breeding, and optimized fertilizer applications are more globally accepted methods today; however, genetic biofortification based on genetic engineering such as increasing or manipulating (such as CRISPR-Cas9) the expression of genes that affect the regulation of metal homeostasis and carrier proteins that serve to increase the micronutrient content for higher nutrient concentration and greater productivity or that affect bioavailability is also seen as a promising high-potential strategy in solving this micronutrient deficiency problem. Data that micronutrients can help strengthen the immune system against the COVID-19 pandemic and other diseases has highlighted the importance of tackling micronutrient deficiencies. In this study, biofortification approaches such as plant breeding, agronomic techniques, microbial fertilization, and some genetic and nanotechnological methods used in the fight against micronutrient deficiency worldwide were compiled.

Challenges and opportunities to regulate mineral transport in rice

Iron (Fe) is an essential mineral for plants, and its deficiency as well as toxicity severely affects plant growth and development. Although Fe is ubiquitous in mineral soils, its acquisition by plants is difficult to regulate particularly in acidic and alkaline soils. Under alkaline conditions, where lime is abundant, Fe and other mineral elements are sparingly soluble. In contrast, under low pH conditions, especially in paddy fields, Fe toxicity could occur. Fe uptake is complicated and could be integrated with copper (Cu), manganese (Mn), zinc (Zn), and cadmium (Cd) uptake. Plants have developed sophisticated mechanisms to regulate the Fe uptake from soil and its transport to root and above-ground parts. Here, we review recent developments in understanding metal transport and discuss strategies to effectively regulate metal transport in plants with a particular focus on rice.

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

Heavy metal stress (HMS) is one of the most destructive abiotic stresses which seriously affects the growth and development of plants. Recent studies have shown significant progress in understanding the molecular mechanisms underlying plant tolerance to HMS. In general, three core signals are involved in plants' responses to HMS; these are mitogen-activated protein kinase (MAPK), calcium, and hormonal (abscisic acid) signals. In addition to these signal components, other regulatory factors, such as microRNAs and membrane proteins, also play an important role in regulating HMS responses in plants. Membrane proteins interact with the highly complex and heterogeneous lipids in the plant cell environment. The function of membrane proteins is affected by the interactions between lipids and lipid-membrane proteins. Our review findings also indicate the possibility of membrane protein-lipid-metal ion interactions in regulating metal homeostasis in plant cells. In this review, we investigated the role of membrane proteins with specific substrate recognition in regulating cell metal homeostasis. The understanding of the possible interaction networks and upstream and downstream pathways is developed. In addition, possible interactions between membrane proteins, metal ions, and lipids are discussed to provide new ideas for studying metal homeostasis in plant cells.

Recent advances in physiological and molecular mechanisms of heavy metal accumulation in plants

Among abiotic stress, the toxicity of metals impacts negatively on plants' growth and productivity. This toxicity promotes various perturbations in plants at different levels. To withstand stress, plants involve efficient mechanisms through the implication of various signaling pathways. These pathways enhance the expression of many target genes among them gene coding for metal transporters. Various metal transporters which are localized at the plasma membrane and/or at the tonoplast are crucial in metal stress response. Furthermore, metal detoxification is provided by metal-binding proteins like phytochelatins and metallothioneins. The understanding of the molecular basis of metal toxicities signaling pathways and tolerance mechanisms is crucial for genetic engineering to produce transgenic plants that enhance phytoremediation. This review presents an overview of the recent advances in our understanding of metal stress response. Firstly, we described the effect of metal stress on plants. Then, we highlight the mechanisms involved in metal detoxification and the importance of the regulation in the response to heavy metal stress. Finally, we mentioned the importance of genetic engineering for enhancing the phytoremediation technique. In the end, the response to heavy metal stress is complex and implicates various components. Thus, further studies are needed to better understand the mechanisms involved in response to this abiotic stress.

Heavy metal stress in rice: Uptake, transport, signaling, and tolerance mechanisms

Heavy metal contamination of agricultural fields has become a global concern as it causes a direct impact on human health. Rice is the major food crop for almost half of the world population and is grown under diverse environmental conditions, including heavy metal-contaminated soil. In recent years, the impact of heavy metal contamination on rice yield and grain quality has been shown through multiple approaches. In this review article, different aspects of heavy metal stress, that is uptake, transport, signaling and tolerance mechanisms, are comprehensively discussed with special emphasis on rice. For uptake, some of the transporters have specificity to one or two metal ions, whereas many other transporters are able to transport many different ions. After uptake, the intercellular signaling is mediated through different signaling pathways involving the regulation of various hormones, alteration of calcium levels, and the activation of mitogen-activated protein kinases. Heavy metal stress signals from various intermediate molecules activate various transcription factors, which triggers the expression of various antioxidant enzymes. Activated antioxidant enzymes then scavenge various reactive oxygen species, which eventually leads to stress tolerance in plants. Non-enzymatic antioxidants, such as ascorbate, metalloids, and even metal-binding peptides (metallothionein and phytochelatin) can also help to reduce metal toxicity in plants. Genetic engineering has been successfully used in rice and many other crops to increase metal tolerance and reduce heavy metals accumulation. A comprehensive understanding of uptake, transport, signaling, and tolerance mechanisms will help to grow rice plants in agricultural fields with less heavy metal accumulation in grains.

YSL3-mediated copper distribution is required for fertility, seed size and protein accumulation in Brachypodium

Addressing the looming global food security crisis requires the development of high-yielding crops. In agricultural soils, deficiency in the micronutrient copper significantly decreases grain yield in wheat (Triticum aestivum), a globally important crop. In cereals, grain yield is determined by inflorescence architecture, flower fertility, grain size, and weight. Whether copper is involved in these processes, and how it is delivered to the reproductive organs is not well understood. We show that copper deficiency alters not only the grain set but also flower development in both wheat and its recognized model, Brachypodium distachyon. We then show that the Brachypodium yellow stripe-like 3 (YSL3) transporter localizes to the phloem, transports copper in frog (Xenopus laevis) oocytes, and facilitates copper delivery to reproductive organs and grains. Failure to deliver copper, but not iron, zinc, or manganese to these structures in the ysl3 CRISPR-Cas9 mutant results in delayed flowering, altered inflorescence architecture, reduced floret fertility, grain size, weight, and protein accumulation. These defects are rescued by copper supplementation and are complemented by YSL3 cDNA. This knowledge will help to devise sustainable approaches for improving grain yield in regions where soil quality is a major obstacle for crop production. Copper distribution by a phloem-localized transporter is essential for the transition to flowering, inflorescence architecture, floret fertility, size, weight, and protein accumulation in seeds.

Iron Biofortification in Rice: An Update on Quantitative Trait Loci and Candidate Genes

Rice is the most versatile model for cereals and also an economically relevant food crop; as a result, it is the most suitable species for molecular characterization of Fe homeostasis and biofortification. Recently there have been significant efforts to dissect genes and quantitative trait loci (QTL) associated with Fe translocation into rice grains; such information is highly useful for Fe biofortification of cereals but very limited in other species, such as maize (Zea mays) and wheat (Triticum aestivum). Given rice's centrality as a model for Poaceae species, we review the current knowledge on genes playing important roles in Fe transport, accumulation, and distribution in rice grains and QTLs that might explain the variability in Fe concentrations observed in different genotypes. More than 90 Fe QTLs have been identified over the 12 rice chromosomes. From these, 17 were recorded as stable, and 25 harbored Fe-related genes nearby or within the QTL. Among the candidate genes associated with Fe uptake, translocation, and loading into rice grains, we highlight the function of transporters from the YSL and ZIP families; transporters from metal-binding molecules, such as nicotianamine and deoxymugineic acid; vacuolar iron transporters; citrate efflux transporters; and others that were shown to play a role in steps leading to Fe delivery to seeds. Finally, we discuss the application of these QTLs and genes in genomics assisted breeding for fast-tracking Fe biofortification in rice and other cereals in the near future.

Leaf-derived ABA regulates rice seed development via a transporter-mediated and temperature-sensitive mechanism

Long-distance transport of the phytohormone abscisic acid (ABA) has been studied for ~50 years, yet its mechanistic basis and biological significance remain very poorly understood. Here, we show that leaf-derived ABA controls rice seed development in a temperature-dependent manner and is regulated by defective grain-filling 1 (DG1), a multidrug and toxic compound extrusion transporter that effluxes ABA at nodes and rachilla. Specifically, ABA is biosynthesized in both WT and dg1 leaves, but only WT caryopses accumulate leaf-derived ABA. Our demonstration that leaf-derived ABA activates starch synthesis genes explains the incompletely filled and floury seed phenotypes in dg1 Both the DG1-mediated long-distance ABA transport efficiency and grain-filling phenotypes are temperature sensitive. Moreover, we extended these mechanistic insights to other cereals by observing similar grain-filling defects in a maize DG1 ortholog mutant. Our study demonstrates that rice uses a leaf-to-caryopsis ABA transport-based mechanism to ensure normal seed development in response to variable temperatures.

Copper deficiency alters shoot architecture and reduces fertility of both gynoecium and androecium in Arabidopsis thaliana

Copper deficiency reduces plant growth, male fertility, and seed set. The contribution of copper to female fertility and the underlying molecular aspects of copper deficiency-caused phenotypes are not well known. We show that among copper deficiency-caused defects in Arabidopsis thaliana were also the increased shoot branching, delayed flowering and senescence, and entirely abolished gynoecium fertility. The increased shoot branching of copper-deficient plants was rescued by the exogenous application of auxin or copper. The delayed flowering was associated with the decreased expression of the floral activator, FT. Copper deficiency also decreased the expression of senescence-associated genes, WRKY53 and SAG13, but increased the expression of SAG12. The reduced fertility of copper-deficient plants stemmed from multiple factors including the abnormal stigma papillae development, the abolished gynoecium fertility, and the failure of anthers to dehisce. The latter defect was associated with reduced lignification, the upregulation of copper microRNAs and the downregulation of their targets, laccases, implicated in lignin synthesis. Copper-deficient plants accumulated ROS in pollen and had reduced cytochrome c oxidase activity in both leaves and floral buds. This study opens new avenues for the investigation into the relationship between copper homeostasis, hormone-mediated shoot architecture, gynoecium fertility, and copper deficiency-derived nutritional signals leading to the delay in flowering and senescence.

Maize YSL2 is required for iron distribution and development in kernels

Iron (Fe) is an essential micronutrient and plays an irreplaceable role in plant growth and development. Although its uptake and translocation are important biological processes, little is known about the molecular mechanism of Fe translocation within seed. Here, we characterized a novel small kernel mutant yellow stripe like 2 (ysl2) in maize (Zea mays). ZmYSL2 was predominantly expressed in developing endosperm and was found to encode a plasma membrane-localized metal-nicotianamine (NA) transporter ZmYSL2. Analysis of transporter activity revealed ZmYSL2-mediated Fe transport from endosperm to embryo during kernel development. Dysfunction of ZmYSL2 resulted in the imbalance of Fe homeostasis and abnormality of protein accumulation and starch deposition in the kernel. Significant changes of nitric oxide accumulation, mitochondrial Fe-S cluster content, and mitochondrial morphology indicated that the proper function of mitochondria was also affected in ysl2. Collectively, our study demonstrated that ZmYSL2 had a pivotal role in mediating Fe distribution within the kernel and kernel development in maize.

Improvement of nutrient use efficiency in rice: current toolbox and future perspectives

Modern agriculture relies heavily on chemical fertilizers, especially in terms of cereal production. The excess application of fertilizers not only increases production cost, but also causes severe environmental problems. As one of the major cereal crops, rice (Oryza sativa L.) provides the staple food for nearly half of population worldwide, especially in developing countries. Therefore, improving rice yield is always the priority for rice breeding. Macronutrients, especially nitrogen (N) and phosphorus (P), are two most important players for the grain yield of rice. However, with economic development and improved living standard, improving nutritional quality such as micronutrient contents in grains has become a new goal in order to solve the "hidden hunger." Micronutrients, such as iron (Fe), zinc (Zn), and selenium (Se), are critical nutritional elements for human health. Therefore, breeding the rice varieties with improved nutrient use efficiency (NUE) is thought to be one of the most feasible ways to increase both grain yield and nutritional quality with limited fertilizer input. In this review, we summarized the progresses in molecular dissection of genes for NUE by reverse genetics on macronutrients (N and P) and micronutrients (Fe, Zn, and Se), exploring natural variations for improving NUE in rice; and also, the current genetic toolbox and future perspectives for improving rice NUE are discussed.

How Plants Handle Trivalent (+3) Elements

Plant development and fitness largely depend on the adequate availability of mineral elements in the soil. Most essential nutrients are available and can be membrane transported either as mono or divalent cations or as mono- or divalent anions. Trivalent cations are highly toxic to membranes, and plants have evolved different mechanisms to handle +3 elements in a safe way. The essential functional role of a few metal ions, with the possibility to gain a trivalent state, mainly resides in the ion's redox activity; examples are iron (Fe) and manganese. Among the required nutrients, the only element with +3 as a unique oxidation state is the non-metal, boron. However, plants also can take up non-essential trivalent elements that occur in biologically relevant concentrations in soils. Examples are, among others, aluminum (Al), chromium (Cr), arsenic (As), and antimony (Sb). Plants have evolved different mechanisms to take up and tolerate these potentially toxic elements. This review considers recent studies describing the transporters, and specific and unspecific channels in different cell compartments and tissues, thereby providing a global vision of trivalent element homeostasis in plants.

Developmental pathways for shaping spike inflorescence architecture in barley and wheat

Grass species display a wide array of inflorescences ranging from highly branched compound/panicle inflorescences to unbranched spike inflorescences. The unbranched spike is a characteristic feature of the species of tribe Triticeae, including economically important crops, such as wheat and barley. In this review, we describe two important developmental genetic mechanisms regulating spike inflorescence architecture in barley and wheat. These include genetic regulation of (i) row-type pathway specific to Hordeum species and (ii) unbranched spike development in barley and wheat. For a comparative understanding, we describe the branched inflorescence phenotypes of rice and maize along with unbranched Triticeae inflorescences. In the end, we propose a simplified model describing a probable mechanism leading to unbranched spike formation in Triticeae species.

The Yellow Stripe-Like (YSL) Gene Functions in Internal Copper Transport in Peanut

Copper (Cu) is involved in fundamental biological processes for plant growth and development. However, Cu excess is harmful to plants. Thus, Cu in plant tissues must be tightly regulated. In this study, we found that the peanut Yellow Stripe-Like family gene AhYSL3.1 is involved in Cu transport. Among five AhYSL genes, AhYSL3.1 and AhYSL3.2 were upregulated by Cu deficiency in peanut roots and expressed mainly in young leaves. A yeast complementation assay suggested that the plasma membrane-localized AhYSL3.1 was a Cu-nicotianamine complex transporter. High expression of AhYSL3.1 in tobacco and rice plants with excess Cu resulted in a low concentration of Cu in young leaves. These transgenic plants were resistant to excess Cu. The above results suggest that AhYSL3.1 is responsible for the internal transport of Cu in peanut.

OsYSL13 Is Involved in Iron Distribution in Rice

The uptake and transport of iron (Fe) in plants are both important for plant growth and human health. However, little is known about the mechanism of Fe transport in plants, especially for crops. In the present study, the function of yellow stripe-like 13 (YSL13) in rice was analyzed. OsYSL13 was highly expressed in leaves, especially in leaf blades, whereas its expression was induced by Fe deficiency both in roots and shoots. Furthermore, the expression level of OsYSL13 was higher in older leaves than that in younger leaves. OsYSL13 was located in the plasma membrane. Metal measurement revealed that Fe concentrations were lower in the youngest leaf and higher in the older leaves of the osysl13 mutant under both Fe sufficiency and deficiency conditions, compared with the wild type and two complementation lines. Moreover, the Fe concentrations in the brown rice and seeds of the osysl13 mutant were also reduced. Opposite results were found in OsYSL13 overexpression lines. These results suggest that OsYSL13 is involved in Fe distribution in rice.