The rice "fruit-weight 2.2-like" gene family member OsFWL4 is involved in the translocation of cadmium from roots to shoots

Cadmium Minimization in Crops: A Trade-Off With Mineral Nutrients in Safe Breeding

Cadmium (Cd) contamination poses a threat to global crop safety. To address this issue, researchers mainly focused on the Cd, explored mechanism of accumulation to low-Cd breeding technologies and created several low-Cd varieties over the past decades. However, new challenges have emerged, particularly the yield reduction due to disturbances in mineral nutrient balance. The goals of breeding have been transferred from a primary focus on 'low-Cd crops' to 'low-Cd/nutrient-balanced' crops, which means limiting Cd content while maintaining other nutrient elements like iron (Fe), manganese (Mn) and zinc (Zn) at a proper content, thus to meet the future agricultural demands. Here, on a multielement perspective, we reviewed the mechanisms of Cd and mineral nutrient transport system in crops and summarized the research advances in Cd minimization through artificial mutations, natural variations and genetic engineering. Furthermore, the challenge of disruption of mineral nutrients in low-Cd crops was discussed and two potential approaches designing Cd-mineral nutrient-optimized artificial transporters and pyramiding Cd-mineral nutrient-optimized variations were proposed. Aiming at addressing these challenges, these approaches represent promising advancements in the field and offer potential pathways for future research and development in the creation of safe and high-quality crops.

Rice and heavy metals: A review of cadmium impact and potential remediation techniques

In recent decades, the menace of heavy metals to food security and human health has become a serious concern. Given its status as the primary provider of food globally, significant research has been done to ensure the safe cultivation of rice, particularly concerning the mitigation of heavy metal contamination. Therefore, this article focuses on the effects and poisoning mechanism of heavy metals, primarily cadmium, on rice. Here, we have discussed the absorption, translocation, and toxicity mechanism of cadmium in rice and the external factors, such as soil pH, organic matter, microorganisms, and climate change, associated with this pollution. It also discusses in detail the sources of heavy metal pollution and the countermeasures against their effects on rice, such as the use of nanoparticles, biochar, plant growth regulators, nutrient management, molecular approaches, tolerant genotypes, and associated genes/proteins. Lastly, a number of significant research prospects concerning heavy metals in rice fields were suggested for future investigation. This review serves as a crucial reference for addressing the issue of heavy metal contamination in paddy fields, ensuring the safe cultivation of rice, promoting environmentally friendly fish farming practices, and safeguarding future food security and human health.

The Uptake, Transfer, and Detoxification of Cadmium in Plants and Its Exogenous Effects

Cadmium (Cd) exerts a toxic influence on numerous crucial growth and development processes in plants, notably affecting seed germination rate, transpiration rate, chlorophyll content, and biomass. While considerable advances in Cd uptake and detoxification of plants have been made, the mechanisms by which plants adapt to and tolerate Cd toxicity remain elusive. This review focuses on the relationship between Cd and plants and the prospects for phytoremediation of Cd pollution. We highlight the following issues: (1) the present state of Cd pollution and its associated hazards, encompassing the sources and distribution of Cd and the risks posed to human health; (2) the mechanisms underlying the uptake and transport of Cd, including the physiological processes associated with the uptake, translocation, and detoxification of Cd, as well as the pertinent gene families implicated in these processes; (3) the detrimental effects of Cd on plants and the mechanisms of detoxification, such as the activation of resistance genes, root chelation, vacuolar compartmentalization, the activation of antioxidant systems and the generation of non-enzymatic antioxidants; (4) the practical application of phytoremediation and the impact of incorporating exogenous substances on the Cd tolerance of plants.

Cell number regulator 8 from Salix linearistipularis enhances cadmium tolerance in poplar by reducing cadmium uptake and accumulation

Trace metals have relatively high density and high toxicity at low concentrations. Willow (Salix genus) is an excellent phytoremediation species for soil contaminated by trace metal ions. This study identified a cell number regulator (CNR) gene family member in Salix linearistipularis exhibiting strong metal ion resistance: SlCNR8. SlCNR8 expression was affected by various metal ions, including cadmium (Cd), zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn). SlCNR8 overexpression enhanced Cd, Zn, Cu, and Fe resistance in transgenic poplar seedlings (84K) compared with the wild-type (WT). Moreover, transgenic poplar seedlings showed lower root Cd uptake and less Cd accumulation than WT under Cd stress. SlCNR8 was primarily localized to the nucleus and the plasma membrane-like cell periphery. Furthermore, SlCNR8 had transcriptional activation activity in yeast. The transcript levels of multiple metal ion transporters were altered in the roots of transgenic poplar seedlings compared to WT roots under Cd stress. These results suggest that SlCNR8 may enhance Cd resistance in transgenic poplar by reducing Cd uptake and accumulation. This may be related to altered transcription levels of other transporters or to itself. Our study suggests that SlCNR8 can be used as a candidate gene for genetic improvement of phytostabilisation of trace metals by genetic engineering.

Combined application of biochar and sulfur alleviates cadmium toxicity in rice by affecting root gene expression and iron plaque accumulation

Biochar and sulfur are considered useful amendments for soil cadmium (Cd) contamination remediation. However, there is still a gap in the understanding of how combined biochar and sulfur application affects Cd resistance in rice, and the role of the accumulation of iron plaque and the expression of Cd efflux transporter-related genes are still unclear in this type of treatment. In this study, we screened an effective combination of biochar and sulfur (0.75 % biochar, 60 mg/kg sulfur) that significantly reduced the Cd content of rice roots (32.9 %) and shoots (12.3 %); significantly reduced the accumulation of amino acids and their derivatives, organic acids and their derivatives and flavonoids in rice roots; and altered secondary metabolite production and release. This combined biochar and sulfur application alleviated the toxicity of Cd to rice, in which the enhancement of iron plaque (24.8 %) formation and upregulated expression of heavy metal effector genes (NRAMP3, MTP3, ZIP1) were important factors. These findings show that iron plaque and heavy metal transport genes are involved in the detoxification of rice under the combined application of biochar and sulfur, which provides useful information for the combined treatment of soil Cd pollution.

Genome-Wide Analyses of SlFWL Family Genes and Their Expression Profiles under Cold, Heat, Salt and Drought Stress in Tomato

PLAC8 is a cysteine-rich protein that serves as a central mediator of tumor evolution in mammals. PLAC8 motif-containing proteins widely distribute in fungi, algae, higher plants and animals that have been described to be implicated in fruit size, cell number and the transport of heavy metals such as cadmium or zinc. In tomatoes, FW2.2 is a PLAC8 motif-containing gene that negatively controls fruit size by regulating cell division and expansion in the carpel ovary during fruit development. However, despite FW2.2, other FWL (FW2.2-Like) genes in tomatoes have not been investigated. In this study, we identified the 21 SlFWL genes, including FW2.2, examined their expression profiles under various abiotic adversity-related conditions. The SlFWL gene structures and motif compositions are conserved, indicating that tomato SlFWL genes may have similar roles. Cis-acting element analysis revealed that the SlFWL genes may participate in light and abiotic stress responses, and they also interacted with a variety of phytohormone-responsive proteins and plant development elements. Phylogenetic analyses were performed on five additional plant species, including Arabidopsis, pepper, soybean, rice and maize, these genes were classified into five subfamilies. Based on the results of collinearity analyses, the SlFWL genes have a tighter homologous evolutionary relationship with soybean, and these orthologous FWL gene pairs might have the common ancestor. Expression profiling of SlFWL genes show that they were all responsive to abiotic stresses, each subgroup of genes exhibited a different expression trend. Our findings provide a strong foundation for investigating the function and abiotic stress responses of the SlFWL family genes.

A phytoexclusion strategy for reducing contamination risk of rice based on low-Cd natural variations pyramid of root transporters

Cadmium pollution in rice is a threat to human health. Phytoexclusion is an effective strategy to reduce the Cd accumulation. Soil-to-root is the first step of Cd entering rice and plays a crucial role in Cd accumulation, so targeting root transporters could be an effective approach for phytoexclusion. This study utilized single-gene & multi-gene joint haplotype analysis to reveal the law of natural variations. The result showed that natural variations of rice root transporters assembled regularly following a certain pattern, rather than randomly. A total of 3 dominant nature variation combinations with 2 high-Cd combinations and 1 low-Cd combination were identified. In addition, indica-japonica differentiation was observed, with indica germplasms harboring high-Cd combinations while japonica germplasms harboring. In Chinese rice landraces, most of the collected indica landraces contained high-Cd combinations, indicating a high Cd contamination risk in indica landraces in terms of both phenotype and genotype. To address this issue, multiple superior low-Cd natural variations were pyramided to create two new low-Cd germplasms. In both pond and farmland trials, the ameliorated rice grain Cd did not exceed safety standards. This research provided a framework for future phytoexclusion, thus to reduce Cd-contamination risk in soil-rice system.

A Coculture of Enterobacter and Comamonas Species Reduces Cadmium Accumulation in Rice

The accumulation of cadmium (Cd) in plants is strongly impacted by soil microbes, but its mechanism remains poorly understood. Here, we report the mechanism of reduced Cd accumulation in rice by coculture of Enterobacter and Comamonas species. In pot experiments, inoculation with the coculture decreased Cd content in rice grain and increased the amount of nonbioavailable Cd in Cd-spiked soils. Fluorescence in situ hybridization and scanning electron microscopy detection showed that the coculture colonized in the rhizosphere and rice root vascular tissue and intercellular space. Soil metagenomics data showed that the coculture increased the abundance of sulfate reduction and biofilm formation genes and related bacterial species. Moreover, the coculture increased the content of organic matter, available nitrogen, and potassium and increased the activities of arylsulfatase, β-galactosidase, phenoloxidase, arylamidase, urease, dehydrogenase, and peroxidase in soils. In subsequent rice transcriptomics assays, we found that the inoculation with coculture activated a hypersensitive response, defense-related induction, and mitogen-activated protein kinase signaling pathway in rice. Heterologous protein expression in yeast confirmed the function of four Cd-binding proteins (HIP28-1, HIP28-4, BCP2, and CID8), a Cd efflux protein (BCP1), and three Cd uptake proteins (COPT4, NRAM5, and HKT6) in rice. Succinic acid and phenylalanine were subsequently proved to inhibit rice divalent Cd [Cd(II)] uptake and activate Cd(II) efflux in rice roots. Thus, we propose a model that the coculture protects rice against Cd stress via Cd immobilization in soils and reducing Cd uptake in rice. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Genetic Regulation Mechanism of Cadmium Accumulation and Its Utilization in Rice Breeding

Cadmium (Cd) is a heavy metal whose pollution in rice fields leads to varying degrees of Cd accumulation in rice. Furthermore, the long-term consumption of Cd-contaminated rice is harmful to human health. Therefore, it is of great theoretical significance and application value to clarify the genetic regulation mechanism of Cd accumulation in rice and cultivate rice varieties with low Cd accumulation for the safe use of Cd-contaminated soils. This review summarizes the effects of Cd on rice growth, yield, and quality; the physiological and molecular mechanisms of Cd absorption in the roots, loading, and transport of Cd in the xylem, the distribution of Cd in nodes, redistribution of Cd in leaves, and accumulation of Cd in the grains; the regulation mechanism of the Cd stress response; and the breeding of rice with low Cd accumulation. Future directions on the genetic regulation of Cd in rice and application are also discussed. This review provides a theoretical basis for studies exploring the genetic regulation of Cd stress in rice. It also offers a basis for formulating effective strategies to reduce the Cd content in rice.

Nucleotide polymorphisms of the maize ZmFWL7 gene and their association with ear-related traits

Plant fw2.2-like (FWL) genes, encoding proteins harboring a placenta-specific eight domain, have been suggested to control fruit and grain size through regulating cell division, differentiation, and expansion. Here, we re-sequenced the nucleotide sequences of the maize ZmFWL7 gene, a member of the FWL family, in 256 elite maize inbred lines, and the associations of nucleotide polymorphisms in this locus with 11 ear-related traits were further detected. A total of 175 variants, including 159 SNPs and 16 InDels, were identified in the ZmFWL7 locus. Although the promoter and downstream regions showed higher nucleotide polymorphism, the coding region also possessed 61 SNPs and 6 InDels. Eleven polymorphic sites in the ZmFWL7 locus were found to be significantly associated with eight ear-related traits. Among them, two nonsynonymous SNPs (SNP2370 and SNP2898) showed significant association with hundred kernel weight (HKW), and contributed to 7.11% and 8.62% of the phenotypic variations, respectively. In addition, the SNP2898 was associated with kernel width (KW), and contributed to 7.57% of the phenotypic variations. Notably, the elite allele T of SNP2370 was absent in teosintes and landraces, while its frequency in inbred lines was increased to 12.89%. By contrast, the frequency of the elite allele A of SNP2898 was 3.12% in teosintes, and it was raised to 12.68% and 19.92% in landraces and inbred lines, respectively. Neutral tests show that this locus wasn’t artificially chosen during the process of domestication and genetic improvement. Our results revealed that the elite allelic variants in ZmFWL7 might possess potential for the genetic improvement of maize ear-related traits.

Cadmium in Cereal Crops: Uptake and Transport Mechanisms and Minimizing Strategies

Cadmium (Cd) contamination in soils and accumulation in cereal grains have posed food security risks and serious health concerns worldwide. Understanding the Cd transport process and its management for minimizing Cd accumulation in cereals may help to improve crop growth and grain quality. In this review, we summarize Cd uptake, translocation, and accumulation mechanisms in cereal crops and discuss efficient measures to reduce Cd uptake as well as potential remediation strategies, including the applications of plant growth regulators, microbes, nanoparticles, and cropping systems and developing low-Cd grain cultivars by CRISPR/Cas9. In addition, miRNAs modulate Cd translocation, and accumulation in crops through the regulation of their target genes was revealed. Combined use of multiple remediation methods may successfully decrease Cd concentrations in cereals. The findings in this review provide some insights into innovative and applicable approaches for reducing Cd accumulation in cereal grains and sustainable management of Cd-contaminated paddy fields.

Mutation in OsFWL7 Affects Cadmium and Micronutrient Metal Accumulation in Rice

Micronutrient metals, such as Mn, Cu, Fe, and Zn, are essential heavy metals for plant growth and development, while Cd is a nonessential heavy metal that is highly toxic to both plants and humans. Our understanding of the molecular mechanisms underlying Cd and micronutrient metal accumulation in plants remains incomplete. Here, we show that OsFWL7, an FW2.2-like (FWL) family gene in Oryza sativa, is preferentially expressed in the root and encodes a protein localized to the cell membrane. The osfwl7 mutation reduces both the uptake and the root-to-shoot translocation of Cd in rice plants. Additionally, the accumulation of micronutrient metals, including Mn, Cu, and Fe, was lower in osfwl7 mutants than in the wildtype plants under normal growth conditions. Moreover, the osfwl7 mutation affects the expression of several heavy metal transporter genes. Protein interaction analyses reveal that rice FWL proteins interact with themselves and one another, and with several membrane microdomain marker proteins. Our results suggest that OsFWL7 is involved in Cd and micronutrient metal accumulation in rice. Additionally, rice FWL proteins may form oligomers and some of them may be located in membrane microdomains.

The transcription factor gene RDD4 contributes to the control of nutrient ion accumulation in rice

In this study, we investigated the expression and functions of the transcription factor gene RDD4 (rice Dof daily fluctuations 4), which has sequence similarity to RDD1 that controls nutrient ion accumulation in rice. RDD4 protein was highly accumulated in leaf sheaths and localized to vascular bundles. RDD4-overexpressing plants (RDD4-OX) improved the accumulation of various nutrient ions, irrespective of nutrient concentration in a hydroponic solution. K⁺ and Cl^- deficiencies induced the accumulation of other cations and anions, respectively. Interestingly, in RDD4-OX plants K⁺ and Cl^- deficiencies increased PO₄ ^3- and Mg²⁺ contents, respectively, despite opposite electric charges. Furthermore, PO₄ ^3- deficiency induced NO₃ ^- and Mg²⁺ accumulation in RDD4-OX plants. These data show that RDD4 is associated with the control of nutrient ion contents within plants. Also, photosynthetic CO₂ assimilation in RDD4-OX plants was higher than in wild-type (WT) plants, although the sizes of shoots and panicles decreased in RDD4-OX plants. Subsequent microarray analysis indicated that OsFWL7, similar to maize CNR1 that negatively regulates plant size, showed the most significant difference in its expression levels between WT and RDD4-OX plants. Based on these results, it is hypothesized that a prominent increase in the OsFWL7 expression reduces plant size in RDD4-OX plants.

In search of the still unknown function of FW2.2/CELL NUMBER REGULATOR, a major regulator of fruit size in tomato

The FW2.2 gene is associated with the major quantitative trait locus (QTL) governing fruit size in tomato, and acts by negatively controlling cell division during fruit development. FW2.2 belongs to a multigene family named the CELL NUMBER REGULATOR (CNR) family. CNR proteins harbour the uncharacterized PLAC8 motif made of two conserved cysteine-rich domains separated by a variable region that are predicted to be transmembrane segments, and indeed FW2.2 localizes to the plasma membrane. Although FW2.2 was cloned more than two decades ago, the molecular mechanisms of action remain unknown. In particular, how FW2.2 functions to regulate cell cycle and fruit growth, and thus fruit size, is as yet not understood. Here we review current knowledge on PLAC8-containing CNR/FWL proteins in plants, which are described to participate in organogenesis and the regulation of organ size, especially in fruits, and in cadmium resistance, ion homeostasis, and/or Ca2+ signalling. Within the plasma membrane FW2.2 and some CNR/FWLs are localized in microdomains, which is supported by recent data from interactomics studies. Hence FW2.2 and CNR/FWL could be involved in a transport function of signalling molecules across membranes, influencing organ growth via a cell to cell trafficking mechanism.

Reduction of cadmium accumulation in the grains of male sterile rice Chuang-5S carrying Pi48 or Pi49 through marker-assisted selection

To reduce cadmium (Cd) accumulation in the grains of rice Chuang-5S (C5S), a gene OsHMA3 and a QTL qlGCd3 related to low-Cd accumulation were separately introgressed into the recipient parent C5S (male sterile line) by molecular marker-assisted breeding. The recurrent parent C5S was then replaced by NIL (near-isogenic line)-C5S with the blast resistance gene Pi48 or Pi49 to construct the BC2F1 generation. Finally, two groups of improved materials of C5S, which pyramided the gene/QTL associated with low-Cd accumulation and blast resistance gene, were developed. The Cd accumulation, agronomic traits, genetic background and blast resistance of these improved C5S materials were evaluated. The results showed that the average Cd content of improved C5S material carrying OsHMA3 and qlGCd3 was, respectively, reduced by 52.8% and 50.8% compared with that of C5S, indicating that the gene related to low-Cd accumulation was, successfully, transferred to C5S with stable expression. The main agronomic traits of the improved materials were consistent with those of C5S. Besides, the improved C5S lines showed stronger blast resistance than C5S and more than 88% similarity to the genetic background of C5S. These two groups of improved materials may be further utilized for the breeding of advanced male sterile lines or superior hybrid rice.

Programmed cell death (PCD) control in plants: New insights from the Arabidopsis thaliana deathosome

Programmed cell death (PCD) is a genetically controlled process that leads to cell suicide in both eukaryotic and prokaryotic organisms. In plants PCD occurs during development, defence response and when exposed to adverse conditions. PCD acts controlling the number of cells by eliminating damaged, old, or unnecessary cells to maintain cellular homeostasis. Unlike in animals, the knowledge about PCD in plants is limited. The molecular network that controls plant PCD is poorly understood. Here we present a review of the current mechanisms involved with the genetic control of PCD in plants. We also present an updated version of the AtLSD1 deathosome, which was previously proposed as a network controlling HR-mediated cell death in Arabidopsis thaliana. Finally, we discuss the unclear points and open questions related to the AtLSD1 deathosome.

Cadmium-zinc cross-talk delineates toxicity tolerance in rice via differential genes expression and physiological / ultrastructural adjustments

Understanding the physiological and molecular response of crop genotypes could be useful in eco-toxicological evaluation with cadmium (Cd) and could be a strategy to solve heavy metal contamination in agriculture. This study corroborates unique patterns of Cd accumulation and molecular mechanisms adopted by plants to acquire Cd tolerance and counteractive effects of zinc (Zn) against Cd toxicity. Two rice (Oryza sativa) genotypes (Heizhan 43 and Yinni 801) differing in cadmium tolerance and its accumulation in plant tissues were investigated hydroponically using two Cd levels [Cd₁₀ (10 μM L^-1) and Cd₁₅ (15 μM L^-1)] and two Zn levels [Zn₂₅ (25 μM L^-2) and Zn₅₀ (50 μM L^-1)] and their combinations. Cadmium toxicity rendered substantial reduction in plant height, biomass, chlorophyll contents and photosynthesis as compared to the control plants after 15 days of treatment. Supplementation of Zn evidently ameliorated Cd toxicity by minimizing the reduction in plant growth, chlorophyll contents and photosynthetic attributes (Pn, gs, Ci, and Tr). Comparatively, lower accumulation of Cd in Yinni 801 under combined treatments revealed a preferential uptake of Zn in this genotype. A cross-talk among Cd, Zn, Fe, Ca and K correlated with fluctuating gs, Ci and Tr. Both genotypes also differed in morphological alterations of cell membrane, chloroplasts and appearance of enlarged plastoglobuli along with distorted mitochondria. An increased ascorbate peroxidase activity in roots of Yinni 801 presented a defensive strategy. Relative expression of Cd and Zn ion transporter genes also confirmed the genotypic background of phenotypic divergence. The OsLCT1 and OsHMA2 expression was significant in Heizhan 43, indicating possible translocation of Cd from shoot to grains contrary to Yinni 801, which accumulated Cd in shoot and showed stunted growth. Zn supplementation promises tolerance to Cd in Yinni 801 by differential expression of putative genes for Cd translocation with minimum ultrastructural modifications by maintaining physiological functions in contrast to Heizhan 43.

Targeted Mutagenesis of the Rice FW 2.2-Like Gene Family Using the CRISPR/Cas9 System Reveals OsFWL4 as a Regulator of Tiller Number and Plant Yield in Rice

The FW2.2-like (FWL) genes encode cysteine-rich proteins with a placenta-specific 8 domain. They play roles in cell division and organ size control, response to rhizobium infection, and metal ion homeostasis in plants. Here, we target eight rice FWL genes using the CRISPR/Cas9 system delivered by Agrobacterium-mediated transformation. We successfully generate transgenic T₀ lines for 15 of the 16 targets. The targeted mutations are detected in the T₀ lines of all 15 targets and the average mutation rate is found to be 81.6%. Transfer DNA (T-DNA) truncation is a major reason for the failure of mutagenesis in T₀ plants. T-DNA segregation analysis reveals that the T-DNA inserts in transgenic plants can be easily eliminated in the T₁ generation. Of the 30 putative off-target sites examined, unintended mutations are detected in 13 sites. Phenotypic analysis reveals that tiller number and plant yield of OsFWL4 gene mutants are significantly greater than those of the wild type. Flag leaves of OsFWL4 gene mutants are wider than those of the wild type. The increase in leaf width of the mutants is caused by an increase in cell number. Additionally, grain length of OsFWL1 gene mutants is higher than that of the wild type. Our results suggest that transgene-free rice plants with targeted mutations can be produced in the T₁ generation using the Agrobacterium-mediated CRISPR/Cas9 system and that the OsFWL4 gene is a negative regulator of tiller number and plant yield.