Arabidopsis thaliana NIP7;1 is involved in tissue arsenic distribution and tolerance in response to arsenate

Targeting OsNIP3;1 via CRISPR/Cas9: A strategy for minimizing arsenic accumulation and boosting rice resilience

Arsenic (As) contamination in rice poses a significant threat to human health due to its toxicity and widespread consumption. Identifying and manipulating key genes governing As accumulation in rice is crucial for reducing this threat. The large NIP gene family of aquaporins in rice presents a promising target due to functional redundancy, potentially allowing for gene manipulation without compromising plant growth. This study aimed to utilize genome editing to generate knock-out (KO) lines of genes of NIP family (OsLsi1, OsNIP3;1) and an anion transporter family (OsLsi2), in order to assess their impact on As accumulation and stress tolerance in rice. KO lines were created using CRISPR/Cas9 technology, and the As accumulation patterns, physiological performance, and grain yield were compared against wild-type (WT) under As-treated conditions. KO lines exhibited significantly reduced As accumulation in grain compared to WT. Notably, Osnip3;1 KO line displayed reduced As in xylem sap (71-74%) and grain (32-46%) upon treatment. Additionally, these lines demonstrated improved silicon (23%) uptake, photosynthetic pigment concentrations (Chl a: 77%; Chl b: 79%, Total Chl: 79% & Carotenoid: 49%) overall physiological and agronomical performance under As stress compared to WT. This study successfully utilized genome editing for the first time to identify OsNIP3;1 as a potential target for manipulating As accumulation in rice without compromising grain yield or plant vigor.

Understanding phloem's role in long-distance transport and accumulation of arsenic (As) in rice: toward low-As-accumulating grain development

MAIN CONCLUSION

This review highlights the roles of phloem in the long-distance transport and accumulation of As in rice plants, facilitating the formulation of new strategies to reduce the grain As content. Rice is a staple diet for a significant proportion of the global population. As toxicity is a major issue affecting the rice productivity and quality worldwide. Phloem tissues of rice plants play vital roles in As speciation, long-distance transport, and unloading, thereby controlling the As accumulation in rice grains. Phloem transport accounts for a significant proportion of As transport to grains, ranging from 54 to 100% depending on the species [inorganic arsenate (As(V)), arsenite (As(III)), or organic dimethylarsinic acid (DMA(V)]. However, the specific mechanism of As transport through phloem leading to its accumulation in grains remains unknown. Therefore, understanding the molecular mechanism of phloem-mediated As transport is necessary to determine the roles of phloem in long-distance As transport and subsequently reduce the grain As content via biotechnological interventions. This review discusses the roles of phloem tissues in the long-distance transport and accumulation of As in rice grains. This review also highlights the biotechnological approaches using critical genetic factors involved in nodal accumulation, vacuolar sequestration, and cellular efflux of As in phloem- or phloem-associated tissues. Furthermore, the limitations of existing transgenic techniques are outlined to facilitate the formulation of novel strategies for the development of rice with reduced grain As content.

From genes to ecosystems: Decoding plant tolerance mechanisms to arsenic stress

Arsenic (As), a metalloid of considerable toxicity, has become increasingly bioavailable through anthropogenic activities, raising As contamination levels in groundwater and agricultural soils worldwide. This bioavailability has profound implications for plant biology and farming systems. As can detrimentally affect crop yield and pose risks of bioaccumulation and subsequent entry into the food chain. Upon exposure to As, plants initiate a multifaceted molecular response involving crucial signaling pathways, such as those mediated by calcium, mitogen-activated protein kinases, and various phytohormones (e.g., auxin, methyl jasmonate, cytokinin). These pathways, in turn, activate enzymes within the antioxidant system, which combat the reactive oxygen/nitrogen species (ROS and RNS) generated by As-induced stress. Plants exhibit a sophisticated genomic response to As, involving the upregulation of genes associated with uptake, chelation, and sequestration. Specific gene families, such as those coding for aquaglyceroporins and ABC transporters, are key in mediating As uptake and translocation within plant tissues. Moreover, we explore the gene regulatory networks that orchestrate the synthesis of phytochelatins and metallothioneins, which are crucial for As chelation and detoxification. Transcription factors, particularly those belonging to the MYB, NAC, and WRKY families, emerge as central regulators in activating As-responsive genes. On a post-translational level, we examine how ubiquitination pathways modulate the stability and function of proteins involved in As metabolism. By integrating omics findings, this review provides a comprehensive overview of the complex genomic landscape that defines plant responses to As. Knowledge gained from these genomic and epigenetic insights is pivotal for developing biotechnological strategies to enhance crop As tolerance.

Plasma membrane intrinsic protein OsPIP2;6 is involved in root-to-shoot arsenic translocation in rice (Oryza sativa L.)

KEY MESSAGE

This study demonstrates the crucial role of OsPIP2;6 for translocation of arsenic from roots to shoots, which can decrease arsenic accumulation in rice for improved food safety. Arsenic (As) contamination in food and water, primarily through rice consumption, poses a significant health risk due to its natural tendency to accumulate inorganic arsenic (iAs). Understanding As transport mechanisms is vital for producing As-free rice. This study investigates the role of rice plasma membrane intrinsic protein, OsPIP2;6, for AsIII tolerance and accumulation. RNAi-mediated suppression of OsPIP2;6 expression resulted in a substantial (35-65%) reduction in As accumulation in rice shoots, while root arsenic levels remained largely unaffected. Conversely, OsPIP2;6 overexpression led to 15-76% higher arsenic accumulation in shoots, with no significant change in root As content. In mature plants, RNAi suppression caused (19-26%) decrease in shoot As, with flag leaves and grains showing a 16% reduction. OsPIP2;6 expression was detected in both roots and shoots, with higher transcript levels in shoots. Localization studies revealed its presence in vascular tissues of both roots and shoots. Overall, our findings highlight OsPIP2;6's role in root-to-shoot As translocation, attributed to its specific localization in the vascular tissue of roots and leaves. This knowledge can facilitate the development of breeding programs to mitigate As accumulation in rice and other food crops for improved food safety and increasing productivity on As-contaminated soils.

Genome-wide analysis and expression of the aquaporin gene family in Avena sativa L

Background

Oat (Avena sativa L.) belongs to the early maturity grass subfamily of the Gramineae subfamily oats (Avena) and has excellent characteristics, such as tolerance to barrenness, salt, cold, and drought. Aquaporin (AQP) proteins belong to the major intrinsic protein (MIP) superfamily, are widely involved in plant growth and development, and play an important role in abiotic stress responses. To date, previous studies have not identified or analyzed the AsAQP gene family system, and functional studies of oat AQP genes in response to drought, cold, and salt stress have not been performed.

Methods

In this study, AQP genes (AsAQP) were identified from the oat genome, and various bioinformatics data on the AQP gene family, gene structure, gene replication, promoters and regulatory networks were analyzed. Quantitative real-time PCR technology was used to verify the expression patterns of the AQP gene family in different oat tissues under different abiotic stresses.

Results

In this study, a total of 45 AQP genes (AsAQP) were identified from the oat reference genome. According to a phylogenetic analysis, 45 AsAQP were divided into 4 subfamilies (PIP, SIP, NIP, and TIP). Among the 45 AsAQP, 23 proteins had interactions, and among these, 5AG0000633.1 had the largest number of interacting proteins. The 20 AsAQP genes were expressed in all tissues, and their expression varied greatly among different tissues and organs. All 20 AsAQP genes responded to salt, drought and cold stress. The NIP subfamily 6Ag0000836.1 gene was significantly upregulated under different abiotic stresses and could be further verified as a key candidate gene.

Conclusion

The findings of this study provide a comprehensive list of members and their sequence characteristics of the AsAQP protein family, laying a solid theoretical foundation for further functional analysis of AsAQP in oats. This research also offers valuable reference for the creation of stress-tolerant oat varieties through genetic engineering techniques.

Overexpression of gamma-glutamyl cyclotransferase 2;1 (CsGGCT2;1) reduces arsenic toxicity and accumulation in Camelina sativa (L.)

KEY MESSAGE

Overexpressing CsGGCT2;1 in Camelina enhances arsenic tolerance, reducing arsenic accumulation by 40-60%. Genetically modified Camelina can potentially thrive on contaminated lands and help safeguard food quality and sustainable food and biofuel production. Environmental arsenic contamination is a serious global issue that adversely affects human health and diminishes the quality of harvested produce. Glutathione (GSH) is known to bind and detoxify arsenic and other toxic metals. A steady level of GSH is maintained within cells via the γ-glutamyl cycle. The γ-glutamyl cyclotransferases (GGCTs) have previously been shown to be involved in GSH degradation and increased tolerance to toxic metals in plants. In this study, we characterized the GGCT2;1 homolog from Camelina sativa for its role in arsenic tolerance and accumulation. Overexpression of CsGGCT2;1 in Camelina under CaMV35S constitutive promoter resulted in strong tolerance to arsenite (AsIII). The overexpression (OE) lines had 2.6-3.5-fold higher shoots and sevenfold to tenfold enhanced root biomass on media supplemented with AsIII, relative to wild-type plants. The CsGGCT2;1 OE lines accumulated 40-60% less arsenic in root and shoot tissues compared to wild-type plants. Further, the OE lines had ~ twofold higher chlorophyll content and 35% lesser levels of malondialdehyde (MDA), an indicator of membrane damage via lipid peroxidation. There was a slight but non-significant increase in 5-oxoproline (5-OP), a product of GSH degradation, in OE lines. However, the transcript levels of Oxoprolinase 1 (OXP1) were upregulated, indicating the accelerated conversion of 5-OP to glutamate, which is further utilized for the resynthesis of GSH to maintain GSH homeostasis. Overall, this research suggests that genetically modified Camelina may have the potential for cultivation on contaminated marginal lands to reduce As accumulation; thereby could help in addressing food safety issues as well as future food and biofuel needs.

Identification of VrNIP2-1 aquaporin with novel selective filter regulating the transport of beneficial as well as hazardous metalloids in mungbean (Vigna radiata L.)

Nodulin 26-like intrinsic protein (NIP) subfamily of aquaporins (AQPs) in plants, is known to be involved in the uptake of metalloids including boron, germanium (Ge), arsenic (As), and silicon (Si). In the present study, a thorough evaluation of 55 AQPs found in the mungbean genome, including phylogenetic distribution, sequence homology, expression profiling, and structural characterization, contributed to the identification of VrNIP2-1 as a metalloid transporter. The pore-morphology of VrNIP2-1 was studied using molecular dynamics simulation. Interestingly, VrNIP2-1 was found to harbor an aromatic/arginine (ar/R) selectivity filter formed with ASGR amino acids instead of GSGR systematically reported in metalloid transporters (NIP2s) in higher plants. Evaluation of diverse cultivars showed a high level of Si accumulation in leaves indicating functional Si transport in mungbean. In addition, heterologous expression of VrNIP2-1 in yeast revealed As(III) and GeO2 transport activity. Similarly, VrNIP2-1 expression in Xenopus oocytes confirmed its Si transport ability. The metalloid transport activity with unique structural features will be helpful to better understand the solute specificity of NIP2s in mungbean and related pulses. The information provided here will also serve as a basis to improve Si uptake while restricting hazardous metalloids like As in plants.

VOZ1 and VOZ2 transcription factors regulate arsenic tolerance and distribution in rice and Arabidopsis

Rice is the major source of arsenic (As) intake in humans, as this staple crop readily accumulates As in the grain. Identifying the genes and molecular mechanisms underlying As accumulation and tolerance is a crucial step toward developing rice with reduced As levels. We identified 25 rice genes that improve As tolerance in yeast cells by expressing a complementary DNA (cDNA) library generated from As-treated rice roots. Among them, a zinc finger-type transcription factor VASCULAR PLANT ONE- ZINC FINGER 1 (OsVOZ1) (OsVOZ1) conferred the most pronounced As tolerance. OsVOZ1 inhibits As accumulation in yeast via activation of As efflux transporter Acr3p by post-transcriptional modification in yeast. The Arabidopsis voz1 voz2 double-knockout mutant exhibited As hypersensitivity, altered As concentrations in various tissues, and reduced As transport activity via the phloem. Arabidopsis and rice VOZs were highly expressed in phloem cells in various tissues, which are critical for As distribution in plant tissues. The double-knockdown and single-knockout plants of OsVOZ1 and OsVOZ2 reduced As accumulation in their seeds. These findings suggest that rice and Arabidopsis VOZs regulate the translocation of As into tissues by regulating the phloem loading of this element.

Effect of phosphate on arsenic species uptake in plants under hydroponic conditions

Monothioarsenate (MTA) is a newly discovered arsenic (As) compound that can be formed under reduced sulfur conditions, mainly in paddy soil pore waters. It is structurally similar to arsenate As(V) and inorganic phosphate (Pi), which is taken up through phosphate transporters. Due to the similarity between As(V) and Pi, As(V) enters into plants instead of Pi. The important role played by phytochelatin (PC), glutathione (GSH), and the PC-vacuolar transporters ABCC1 and ABCC2 under As stress in plants is well known. However, the plant uptake and mechanisms surrounding MTA still have not been completely addressed. This investigation was divided in two stages: first, several hydroponic assays were set up to establish the sensibility-tolerance of wild-type Arabidopsis thaliana (accession Columbia-0, Col-0). Then Col-0 was used as a control plant to evaluate the effects of As(V) or MTA in (PC)-deficient mutant (cad1-3), glutathione biosynthesis mutant (cad2), and PC transport (abcc1-2). The inhibitory concentration (IC50) root length was calculated for both As species. According to the results, both arsenic species (As(V) and MTA) exhibited high toxicity for the genotypes evaluated. This could mean that these mechanisms play a constitutive role in MTA detoxification. Second, for the Pi-MTA and As(V)-Pi competition assays, a series of experiments on hydroponic seedlings of A. thaliana were carried out using Col-0 and a pht1;1. The plants were grown under increasing Pi concentrations (10 μM, 0.1 mM, or 1 mM) at 10 μM As(V) or 50 μM MTA. The total As concentration in the roots was significantly lower in plants exposed to MTA, there being less As content in the pht1;1 mutant at the lowest Pi concentrations tested compared with the As(V)/Pi treatments. In addition, a higher rate of As translocation from the roots to the shoots under MTA was observed in comparison to the As(V)-treatments.

Regulatory Mechanisms Underlying Arsenic Uptake, Transport, and Detoxification in Rice

Arsenic (As) is a metalloid environmental pollutant ubiquitous in nature that causes chronic and irreversible poisoning to humans through its bioaccumulation in the trophic chain. Rice, the staple food crop for 350 million people worldwide, accumulates As more easily compared to other cereal crops due to its growth characteristics. Therefore, an in-depth understanding of the molecular regulatory mechanisms underlying As uptake, transport, and detoxification in rice is of great significance to solving the issue of As bioaccumulation in rice, improving its quality and safety and protecting human health. This review summarizes recent studies on the molecular mechanisms of As toxicity, uptake, transport, redistribution, regulation, and detoxification in rice. It aims to provide novel insights and approaches for preventing and controlling As bioaccumulation in rice plants, especially reducing As accumulation in rice grains.

Heterologous expression of HsPstS gene reducing arsenic accumulation and improving As-tolerance in transgenic tobaccos by enhancing CAT activity

Arsenic (As) is a toxic metalloid element that affects plant growth and development. Reducing the uptake of arsenic by plants via genetic engineering strategy can effectively improve the tolerance and safety of economic crops in As-contaminated soil. In this paper, the HsPstS gene coded ABC-type periplasmic phosphate-binding protein (PBP) of Halomonas strain GFAJ-1 was introduced into tobacco K326 by Agrobacterium-mediated genetic transformation to create transgenic tobaccos. Under As stress, NBT and DAB staining of tobacco leaves showed significant accumulation of H₂O₂ in wild-type and CK plants, and the further determination showed that the H₂O₂ content in CK plants was higher than that in transgenic plants except for L_35S-2 and L_REL-4 at 3 d after stress. Generally, the activity of antioxidant enzymes (CAT and POD) in tobaccos increased first and then decreased under As stress, and the CAT activity in most transgenic tobacco plants was significantly higher than that in wild-type and CK plants at 5 d after stress. By contrast, POD activity in CK and wild-type plants was significantly higher than that in transgenic tobaccos except for L_35S-2. Additionally, As content determination showed that all transgenic tobacco plants except for CK showed the characteristic of low As-accumulation, especially in transgenic tobaccos L_35S-2 and L_REL-4, which suggested that the introduction of HsPstS could significantly reduce the As absorption in HsPstS-contained transgenic tobaccos, while there was no significant influence on agronomic traits and photosynthetic characteristics of transgenic tobaccos compared with wild-type ones. Interestingly, the introduction of HsPstS gene also reduced the content of nicotine and nornicotine in transgenic tobacco plants, while there was no significant difference on K content between transgenic and non-transgenic tobaccos. These results above provided ideal parental materials for cultivating tobacco germplasm with the characteristic of low As-accumulation.

Aquaporin-mediated transport: Insights into metalloid trafficking

Metalloids in plants have diverse physiological effects. From being essential to beneficial to toxic, they have significant effects on many physiological processes, influencing crop yield and quality. Aquaporins are a group of membrane channels that have several physiological substrates along with water. Metalloids have emerged as one of their important substrates and they are found to have a substantial role in regulating plant metalloid homeostasis. The present review comprehensively details the multiple isoforms of aquaporins having specificity for metalloids and being responsible for their influx, distribution or efflux. In addition, it also highlights the usage of aquaporin-mediated transport as a selection marker in toxic screens and as tracer elements for closely related metalloids. Therefore, aquaporins, with their imperative contribution to the regulation of plant growth, development and physiological processes, need more research to unravel the metalloid trafficking mechanisms and their future applications.

Molecular insight into arsenic uptake, transport, phytotoxicity, and defense responses in plants: a critical review

A critical investigation into arsenic uptake and transportation, its phytotoxic effects, and defense strategies including complex signaling cascades and regulatory networks in plants. The metalloid arsenic (As) is a leading pollutant of soil and water. It easily finds its way into the food chain through plants, more precisely crops, a common diet source for humans resulting in serious health risks. Prolonged As exposure causes detrimental effects in plants and is diaphanously observed through numerous physiological, biochemical, and molecular attributes. Different inorganic and organic As species enter into the plant system via a variety of transporters e.g., phosphate transporters, aquaporins, etc. Therefore, plants tend to accumulate elevated levels of As which leads to severe phytotoxic damages including anomalies in biomolecules like protein, lipid, and DNA. To combat this, plants employ quite a few mitigation strategies such as efficient As efflux from the cell, iron plaque formation, regulation of As transporters, and intracellular chelation with an array of thiol-rich molecules such as phytochelatin, glutathione, and metallothionein followed by vacuolar compartmentalization of As through various vacuolar transporters. Moreover, the antioxidant machinery is also implicated to nullify the perilous outcomes of the metalloid. The stress ascribed by the metalloid also marks the commencement of multiple signaling cascades. This whole complicated system is indeed controlled by several transcription factors and microRNAs. This review aims to understand, in general, the plant-soil-arsenic interaction, effects of As in plants, As uptake mechanisms and its dynamics, and multifarious As detoxification mechanisms in plants. A major portion of this article is also devoted to understanding and deciphering the nexus between As stress-responsive mechanisms and its underlying complex interconnected regulatory networks.

The role of roots and rhizosphere in providing tolerance to toxic metals and metalloids

Human activity and natural processes have led to the widespread dissemination of metals and metalloids, many of which are toxic and have a negative impact on plant growth and development. Roots, as the first point of contact, are essential in endowing plants with tolerance to excess metal(loid) in the soil. The most important root processes that contribute to tolerance are: adaptation of transport processes that affect uptake efflux and long-distance transport of metal(loid)s; metal(loid) detoxification within root cells via conjugation to thiol rich compounds and subsequent sequestration in the vacuole; plasticity in root architecture; the presence of bacteria and fungi in the rhizosphere that impact on metal(loid) bioavailability; the role of root exudates. In this review, we provide details on these processes and assess their relevance on the detoxification of arsenic, cadmium, mercury and zinc in crops. Furthermore, we assess which of these strategies have been tested in field conditions and whether they are effective in terms of improving crop metal(loid) tolerance.

Arsenic acquisition, toxicity and tolerance in plants - From physiology to remediation: A review

Globally, environmental contamination by potentially noxious metalloids like arsenic is becoming a critical concern to the living organisms. Arsenic is a non-essential metalloid for plants and can be acclimatised in plants to toxic levels. Arsenic acquisition by plants poses serious health risks in human due to its entry in the food chain. High arsenic regimes disturb plant water relations, promote the generation of reactive oxygen species (ROS) and induce oxidative outburst in plants. This review evidences a conceivable tie-up among arsenic levels, speciation, its availability, uptake, acquisition, transport, phytotoxicity and arsenic detoxification in plants. The role of different antioxidant enzymes to confer plant tolerance towards the enhanced arsenic distress has also been summed up. Additionally, the mechanisms involved in the modulation of different genes coupled with arsenic tolerance have been thoroughly discussed. This review is intended to present an overview to rationalise the contemporary progressions on the recent advances in phytoremediation approaches to overcome ecosystem contamination by arsenic.

Arsenic Remediation through Sustainable Phytoremediation Approaches

Arsenic contamination of the environment is a serious problem threatening the health of millions of people exposed to arsenic (As) via drinking water and crops grown in contaminated areas. The remediation of As-contaminated soil and water bodies needs to be sustainable, low-cost and feasible to apply in the most affected low-to-middle income countries, like India and Bangladesh. Phytoremediation is an aesthetically appreciable and successful approach that can be used for As decontamination with use of the best approach(es) and the most promising plant(s). However, phytoremediation lacks the required speed and sometimes the stress caused by As could diminish plants’ potential for remediation. To tackle these demerits, we need augment plants’ potential with appropriate technological methods including microbial and nanoparticles applications and genetic modification of plants to alleviate the As stress and enhance As accumulation in phytoremediator plants. The present review discusses the As phytoremediation prospects of soil and water bodies and the usefulness of various plant systems in terms of high biomass, high As accumulation, bioenergy potential, and economic utility. The potential and prospects of assisted phytoremediation approaches are also presented.

Ionomic Approaches for Discovery of Novel Stress-Resilient Genes in Plants

Plants, being sessile, face an array of biotic and abiotic stresses in their lifespan that endanger their survival. Hence, optimized uptake of mineral nutrients creates potential new routes for enhancing plant health and stress resilience. Recently, minerals (both essential and non-essential) have been identified as key players in plant stress biology, owing to their multifaceted functions. However, a realistic understanding of the relationship between different ions and stresses is lacking. In this context, ionomics will provide new platforms for not only understanding the function of the plant ionome during stresses but also identifying the genes and regulatory pathways related to mineral accumulation, transportation, and involvement in different molecular mechanisms under normal or stress conditions. This article provides a general overview of ionomics and the integration of high-throughput ionomic approaches with other "omics" tools. Integrated omics analysis is highly suitable for identification of the genes for various traits that confer biotic and abiotic stress tolerance. Moreover, ionomics advances being used to identify loci using qualitative trait loci and genome-wide association analysis of element uptake and transport within plant tissues, as well as genetic variation within species, are discussed. Furthermore, recent developments in ionomics for the discovery of stress-tolerant genes in plants have also been addressed; these can be used to produce more robust crops with a high nutritional value for sustainable agriculture.

An amiRNA screen uncovers redundant CBF and ERF34/35 transcription factors that differentially regulate arsenite and cadmium responses

Arsenic stress causes rapid transcriptional responses in plants. However, transcriptional regulators of arsenic-induced gene expression in plants remain less well known. To date, forward genetic screens have proven limited for dissecting arsenic response mechanisms. We hypothesized that this may be due to the extensive genetic redundancy present in plant genomes. To overcome this limitation, we pursued a forward genetic screen for arsenite tolerance using a randomized library of plants expressing >2,000 artificial microRNAs (amiRNAs). This library was designed to knock-down diverse combinations of homologous gene family members within sub-clades of transcription factor and transporter gene families. We identified six transformant lines showing an altered response to arsenite in root growth assays. Further characterization of an amiRNA line targeting closely homologous CBF and ERF transcription factors show that the CBF1,2 and 3 transcription factors negatively regulate arsenite sensitivity. Furthermore, the ERF34 and ERF35 transcription factors are required for cadmium resistance. Generation of CRISPR lines, higher-order T-DNA mutants and gene expression analyses, further support our findings. These ERF transcription factors differentially regulate arsenite sensitivity and cadmium tolerance.

Decreases in arsenic accumulation by the plasma membrane intrinsic protein PIP2;2 in Arabidopsis and yeast

Arsenic (As) is a toxic pollutant that mainly enters the human body via plants. Therefore, understanding the strategy for reducing arsenic accumulation in plants is important to human health and the environment. Aquaporins are ubiquitous water channel proteins that bidirectionally transport water across cell membranes and play a role in the transportation of other molecules, such as glycerol, ammonia, boric acid, and arsenic acid. Previously, we observed that Arabidopsis PIP2;2, encoding a plasma membrane intrinsic protein, is highly expressed in NtCyc07-expressing Arabidopsis, which shows a higher tolerance to arsenite (As(III)). In this study, we report that the overexpression of AtPIP2;2 enhanced As(III) tolerance and reduced As(III) levels in yeast. Likewise, AtPIP2;2-overexpressing Arabidopsis exhibited improved As(III) tolerance and lower accumulation of As(III). In contrast, atpip2;2 knockout Arabidopsis showed reduced As(III) tolerance but no significant change in As(III) levels. Interestingly, the AtPIP2;2 transcript and protein levels were increased in roots and shoots of Arabidopsis in response to As(III). Furthermore, As(III) efflux was enhanced and As(III) influx/accumulation was reduced in AtPIP2;2-expressing plants. The expression of AtPIP2;2 rescued the As(III)-sensitive phenotype of acr3 mutant yeast by reducing As levels and slightly reduced the As(III)-tolerant phenotype of fps1 mutant yeast by enhancing As content, suggesting that AtPIP2; 2 functions as a bidirectional channel of As(III), while the As(III) exporter activity is higher than the As(III) importer activity. All these results indicate that AtPIP2;2 expression promotes As(III) tolerance by decreasing As(III) accumulation through enhancing As(III) efflux in Arabidopsis. This finding can be applied to the generation of low arsenic crops for human health.

A curated list of genes that affect the plant ionome

Understanding the mechanisms underlying plants' adaptation to their environment will require knowledge of the genes and alleles underlying elemental composition. Modern genetics is capable of quickly, and cheaply indicating which regions of DNA are associated with particular phenotypes in question, but most genes remain poorly annotated, hindering the identification of candidate genes. To help identify candidate genes underlying elemental accumulations, we have created the known ionome gene (KIG) list: a curated collection of genes experimentally shown to change uptake, accumulation, and distribution of elements. We have also created an automated computational pipeline to generate lists of KIG orthologs in other plant species using the PhytoMine database. The current version of KIG consists of 176 known genes covering 5 species, 23 elements, and their 1588 orthologs in 10 species. Analysis of the known genes demonstrated that most were identified in the model plant Arabidopsis thaliana, and that transporter coding genes and genes altering the accumulation of iron and zinc are overrepresented in the current list.

Plant abiotic stress response and nutrient use efficiency

Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality. Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress- and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.

Molecular and Functional Characterization of Grapevine NIPs through Heterologous Expression in aqy-Null Saccharomyces cerevisiae

Plant Nodulin 26-like Intrinsic Proteins (NIPs) are multifunctional membrane channels of the Major Intrinsic Protein (MIP) family. Unlike other homologs, they have low intrinsic water permeability. NIPs possess diverse substrate selectivity, ranging from water to glycerol and to other small solutes, depending on the group-specific amino acid composition at aromatic/Arg (ar/R) constriction. We cloned three NIPs (NIP1;1, NIP5;1, and NIP6;1) from grapevine (cv. Touriga Nacional). Their expression in the membrane of aqy-null Saccharomyces cerevisiae enabled their functional characterization for water and glycerol transport through stopped-flow spectroscopy. VvTnNIP1;1 demonstrated high water as well as glycerol permeability, whereas VvTnNIP6;1 was impermeable to water but presented high glycerol permeability. Their transport activities were declined by cytosolic acidification, implying that internal-pH can regulate NIPs gating. Furthermore, an extension of C-terminal in VvTnNIP6;1M homolog, led to improved channel activity, suggesting that NIPs gating is putatively regulated by C-terminal. Yeast growth assays in the presence of diverse substrates suggest that the transmembrane flux of metalloids (As, B, and Se) and the heavy metal (Cd) are facilitated through grapevine NIPs. This is the first molecular and functional characterization of grapevine NIPs, providing crucial insights into understanding their role for uptake and translocation of small solutes, and extrusion of toxic compounds in grapevine.

Integrative response of arsenic uptake, speciation and detoxification by Salix atrocinerea

Despite arsenic (As) being very toxic with deleterious effects on metabolism, it can be tolerated and accumulated by some plants. General genetic mechanisms responsible for As tolerance in plants, including Salix species, have been described in transcriptomic analysis, but further experimental verification of the significance of particular transcripts is needed. In this study, a Salix atrocinerea clone, able to thrive in an As-contaminated brownfield, was grown hydroponically in controlled conditions under an As concentration similar to the bioavailable fraction of the contaminated area (18 mg kg^-1) for 30 days. At different time points, i.e. short-term and long-term exposure, biometric data, As accumulation, phytochelatin synthesis, non-protein thiol production and expression of target genes related to these processes were studied. Results showed that S. atrocinerea presents a great tolerance to As and accumulates up to 2400 mg As kg^-1 dry weight in roots and 25 mg As kg^-1 dry weight in leaves. Roots reduce As V to As III rapidly, with As III being the predominant form of As accumulated in root tissues, whereas in the leaves it is As V. After 1 d of As exposure, roots and leaves show de novo synthesis and an increase in non-protein thiols as compared to the control. Integrating these data on As accumulation in the plant and its speciation, non-protein thiol production and the kinetic gene expression of related target genes, a fundamental role is highlighted for these processes in As accumulation and tolerance in S. atrocinerea. As such, this study offers new insights in the plant tolerance mechanisms to As, which provides important knowledge for future application of high-biomass willow plants in phytoremediation of As-polluted soils.

Safer food through plant science: reducing toxic element accumulation in crops

Natural processes and human activities have caused widespread background contamination with non-essential toxic elements. The uptake and accumulation of cadmium (Cd), arsenic (As), and lead (Pb) by crop plants results in chronic dietary exposure and is associated with various health risks. Current human intake levels are close to what is provisionally regarded as safe. This has recently triggered legislative actions to introduce or lower limits for toxic elements in food. Arguably, the most effective way to reduce the risk of slow poisoning is the breeding of crops with much lower accumulation of contaminants. The past years have seen tremendous progress in elucidating molecular mechanisms of toxic element transport. This was achieved in the model systems Arabidopsis thaliana and, most importantly, rice, the major source of exposure to As and Cd for a large fraction of the global population. Many components of entry and sequestration pathways have been identified. This knowledge can now be applied to engineer crops with reduced toxic element accumulation especially in edible organs. Most obvious in the case of Cd, it appears likely that subtle genetic intervention has the potential to reduce human exposure to non-essential toxic elements almost immediately. This review outlines the risks and discusses our current state of knowledge with emphasis on transgenic and gene editing approaches.

Investigation of the AQP Family in Soybean and the Promoter Activity of TIP2;6 in Heat Stress and Hormone Responses

Aquaporins (AQPs) are one diverse family of membrane channel proteins that play crucial regulatory roles in plant stress physiology. However, the heat stress responsiveness of AQP genes in soybean remains poorly understood. In this study, 75 non-redundant AQP encoding genes were identified in soybean. Multiple sequence alignments showed that all GmAQP proteins possessed the conserved regions, which contained 6 trans-membrane domains (TM1 to TM6). Different GmAQP members consisted of distinct Asn-Pro-Ala (NPA) motifs, aromatic/arginine (ar/R) selectivity filters and Froger's positions (FPs). Phylogenetic analyses distinguished five sub-families within these GmAQPs: 24 GmPIPs, 24 GmTIPs, 17 GmNIPs, 8 GmSIPs, and 2 GmXIPs. Promoter cis-acting elements analyses revealed that distinct number and composition of heat stress and hormone responsive elements existed in different promoter regions of GmAQPs. QRT-PCR assays demonstrated that 12 candidate GmAQPs with relatively extensive expression in various tissues or high expression levels in root or leaf exhibited different expression changes under heat stress and hormone cues (abscisic acid (ABA), l-aminocyclopropane-l-carboxylic acid (ACC), salicylic acid (SA) and methyl jasmonate (MeJA)). Furthermore, the promoter activity of one previously functionally unknown AQP gene-GmTIP2;6 was investigated in transgenic Arabidopsis plants. The beta-glucuronidase (GUS) activity driven by the promoter of GmTIP2;6 was strongly induced in the heat- and ACC-treated transgenic plants and tended to be accumulated in the hypocotyls, vascular bundles, and leaf trichomes. These results will contribute to uncovering the potential functions and molecular mechanisms of soybean GmAQPs in mediating heat stress and hormone signal responses.

Monothioarsenate Uptake, Transformation, and Translocation in Rice Plants

Thioarsenates form under sulfur-reducing conditions in paddy soil pore waters. Sulfur fertilization, recently promoted for decreasing total arsenic (As) grain concentrations, could enhance their formation. Yet, to date, thioarsenate toxicity, uptake, transformation, and translocation in rice are unknown. Our growth inhibition experiments showed that the toxicity of monothioarsenate (MTA) was similar to that of arsenate but lower than that of arsenite. Higher toxicity of MTA with lower phosphate availability might imply uptake through phosphate transporters similar to arsenate. To demonstrate direct uptake of MTA by rice plants, a species-preserving extraction method for plant samples was developed. When plants were exposed to 10 μM MTA for 72 h, up to 19% and 4% of total As accumulated in roots and shoots, respectively, was MTA. Monothioarsenate was detected in xylem sap and root exudates, and its reduction to arsenite in rice roots and shoots was shown. Total As uptake was lower upon exposure to MTA compared to arsenate, but root to shoot translocation was higher, resulting in comparable As shoot concentrations. Thus, before promoting sulfur fertilization, uptake and detoxifying mechanisms of thioarsenates as well as potential contribution to grain As accumulation need to be better understood.

Dissecting the components controlling root-to-shoot arsenic translocation in Arabidopsis thaliana

Arsenic (As) is an important environmental and food-chain toxin. We investigated the key components controlling As accumulation and tolerance in Arabidopsis thaliana. We tested the effects of different combinations of gene knockout, including arsenate reductase (HAC1), γ-glutamyl-cysteine synthetase (γ-ECS), phytochelatin synthase (PCS1) and phosphate effluxer (PHO1), and the heterologous expression of the As-hyperaccumulator Pteris vittata arsenite efflux (PvACR3), on As tolerance, accumulation, translocation and speciation in A. thaliana. Heterologous expression of PvACR3 markedly increased As tolerance and root-to-shoot As translocation in A. thaliana, with PvACR3 being localized to the plasma membrane. Combining PvACR3 expression with HAC1 mutation led to As hyperaccumulation in the shoots, whereas combining HAC1 and PHO1 mutation decreased As accumulation. Mutants of γ-ECS and PCS1 were hypersensitive to As and had higher root-to-shoot As translocation. Combining γ-ECS or PCS1 with HAC1 mutation did not alter As tolerance or accumulation beyond the levels observed in the single mutants. PvACR3 and HAC1 have large effects on root-to-shoot As translocation. Arsenic hyperaccumulation can be engineered in A. thaliana by knocking out the HAC1 gene and expressing PvACR3. PvACR3 and HAC1 also affect As tolerance, but not to the extent of γ-ECS and PCS1.

New Molecular Mechanisms to Reduce Arsenic in Crops

Arsenic is toxic to all life forms and is a potent carcinogen. Its accumulation in crop plants and subsequent consumption poses a serious threat to public health worldwide. Recent developments have enhanced our understanding of the molecular mechanisms governing arsenic uptake, detoxification, and accumulation in plants. In particular, the identification of plant arsenate reductase enzymes and emerging details of the processes underlying arsenic distribution and deposition in the seed will prove invaluable in the development of new strategies to mitigate this threat. Here we provide an outline of these recent developments and suggest new molecular mechanisms that could be employed to reduce arsenic in crops.