Introduction

Over the past few decades, an upswing in anthropogenic activities, such as growing industrial production and intensified agricultural practices, has resulted in a rise in environmental heavy metal contamination, posing a formidable threat to various life forms (Eapen and D’Souza 2005; Kavamura and Esposito 2010; Martinez et al. 2011). Notably, in 2021, the Food and Agriculture Organization (FAO) and the United Nations Environment Program (UNEP) collaboratively issued the "Global Soil Pollution Assessment.” The central message in it was that the increasing demand for agricultural production required to meet growing global population demands coupled with extensive rate of industrialization, has led to a severe issue of soil pollution. Due to its potential for long-term environmental impact, such pollution poses a serious threat for agricultural food systems and human health (UNEP and FAO 2021). Taking China as an example, 82.4% of the government recognized contaminated land includes Cd contamination (Chen et al. 2017). There are also many natural soils with high Cd content and high Cd:Zn ratio which imposes severe limitations of growth of all crops (Duan et al. 2020). Finally, many soils in China’s Southern provinces are acidic and are at risk of aluminum toxicity that significantly impacts plant growth and stands as a primary factor contributing to reduced crop yields in the region (Guo et al. 2023). Elevated concentrations of these metal(loid)s not only induce stress and toxicity reactions in crops but, upon accumulation in the human body through the food chain, can also contribute to the occurrence of diseases.

Among those contaminants, Al phytotoxicity poses a widespread challenge, impeding the growth and yield of numerous crops in acidic soils globally. Roughly 50% of the world’s cultivable lands are classified as acidic (Jaskowiak et al. 2019). Acidic soils are particularly prevalent in subtropical and tropical regions, accounting for approximately 60% of such soils, where natural acidification processes take place (Kochian et al. 2015). The majority of these acidic soils are concentrated in developing nations, encompassing areas in South America, Central Africa, the Indian subcontinent, and portions of Southeast Asia (Bian et al. 2013). Notably, China, and specifically Guangdong province, faces a significant challenge with soil acidity. This issue is exacerbated by substantial sulfur dioxide emissions from coal burning in Guangdong and its surrounding areas. Furthermore, the marked increase in nitrogen oxide from motor vehicles and the rapid expansion of extensive township industries contribute significantly to acid rain in this region (Roy and Bhadra 2014).

Acidic soils expose plants to numerous physical and chemical constraints, leading to poor growth primarily due to proton and metal ion toxicities as well as nutrient deficiencies, such as for phosphorus, magnesium, and calcium (Ryan and Delhaize 2017). While proton stress is less damaging, the soluble aluminum and manganese present in acid soils pose more significant threats to plant health. The solubility of aluminum significantly rises with a decrease in a soil pH below 5 (Kochian 1995). An initial sign of aluminum toxicity in plants is the suppression of root growth, detectable within the first hour of exposure to micromolar concentrations of aluminum (Llugany et al. 1995; Delhaize and Ryan 1995; Rengel 2004). Prolonged exposure to aluminum toxicity results in reduced leaf size, darkening of leaves, chlorotic patches, and marginal necrosis and, ultimately, crop yield. Consequently, crops cultivated in acidic soils suffer substantial yield losses (Sarwar et al. 2010; World Health Organization et al. 2011; Zhu et al. 2020). The estimated economic impact of soil acidity on agricultural crop production in only one Australian state (Western Australia) exceeds 1.6 billion AUD every year (Weng et al. 2021).

Amongst other contaminants, inorganic arsenic (As) and cadmium (Cd) have been categorized as group-1 carcinogens due to their intrinsic toxicity, widespread occurrence, and potential for human exposure (Sarwar et al. 2010; World Health Organization et al. 2011). Essential food staples such as barley (Hordeum vulgare), rice (Oryza sativa), wheat (Triticum aestivum), and maize (Zea mays) emerge as the primary sources of exposure to these harmful trace elements. Arsenic, cadmium, nickel, lead and zinc are commonly encountered heavy metals in contaminated soils and wastewater, posing risks to the environment and human health (Lambert et al.2000).

The metalloid arsenic (As) is ubiquitous in the environment, originating from both geological processes and human activities. Many plants and, especially, rice absorb arsenic from the soil, storing it in consumable parts, thus entering the food chain for humans and other organisms. Considering the severe health risks associated with arsenic ingestion, including the development of cancers ( Bundschuh et al. 2012; Martinez et al. 2011), millions of individuals, particularly in South and Southeast Asia, are exposed to the threat of As-contaminated food and drinking water (Verbruggen et al. 2009).

Rice, a major staple for over 50% of the global population, is specifically prone to accumulate of unacceptable levels of As and Cd in grain under a range of conditions in some soils. Extensive areas of arable land in major rice-producing regions, especially in South and Southeast Asia, as well as China, confront significant contamination with As and/or Cd (Mandal and Suzuki 2002; Zhao et al. 2010, 2015; Zhu et al. 2008). Assessments based on rice consumption and the inorganic As content in polished grains indicate heightened cancer risks from As poisoning in populations of Bangladesh and China compared to Italian American and Japanese populations (Meharg et al. 2009). Moreover, infants and toddlers are notably exposed to As through rice-based infant food (European Food Safety Authority and European Centre for Disease Prevention and Control 2018). Rice significantly contributes to food-related Cd exposure, constituting approximately 40% of the Japanese diet intake (Watanabe et al. 2000). In China, grains and cereals contribute about 32% of the total Cd intake (World Health Organization et al. 2011), also as a result of rice being a major staple food. Cereals and grains also emerge as primary sources of dietary Cd exposure for European adults (Olsson et al. 2002; World Health Organization et al. 2011).

Facing the above-mentioned challenges, alleviating aluminium toxicity and metal(loid) pollution requires researching soil amendments, promoting sustainable agriculture, and cultivating resistant crop varieties. This review focuses on some issues and possible solutions to this problem that may be critical for developing agronomical and genetic approaches to tackle the problem.

Zooming in on membrane transporters

Cellular membranes constitute the interface between living organisms and environment and thus represent the most critical component of sensing and responding to environmental constraints. Biologic membranes are indispensable components of any living organism, from bacteria to plants and humans. By separating the cytosol from the external medium, they operate as “gatekeepers”, allowing selective uptake of desirable nutrients and excluding those toxic to the organism. Plant productivity is determined by uptake and partitioning of resources, such as water, inorganic and organic C, nitrogen (N), phosphorus (P), silicon (Si), and important metals (e.g., Fe and Zn). Transport proteins (transporters) in plant-cell membranes control the movement of these resources and toxic substances into, from and around plants. This is also true for toxic metal(loid)s. Thus, understanding the molecular basis and modes of regulation for membrane transporters for As, Cd and Al is critical to both ensure global food safety and minimize detrimental effect of these toxins on human health.

Although several methodologies are available to study ion flux through membrane proteins, two methods are most suitable for understanding operation of membrane transporters. The first one is a patch-clamp technique that remains the gold standard for quantitative analysis of electrogenic ion exchange across membranes. The method is based on measuring currents through membrane proteins (ion channels) by forming a high-resistance seal on the cell membrane. This (Nobel Prize winning) technique allows studies on extremely low (picoamperes) currents through a single transporter protein (in a single-channel mode) and represent an excellent tool for characterization of the properties (such as selectivity and/or gating mode) of specific ion transporters. However, it is an invasive technique with high execution demands, low throughput, and the potential to disrupt the cell membrane. It is only applicable to specific cell types and is primarily used in “blue-sky” studies but not suitable for phenotyping. Yet, broader application of this approach may be instrumental for discovery of new traits related to fundamental mechanisms conferring plants tolerance to toxic metals. For example, patch-clamp studies on vacuolar membranes in wheat have uncovered notable distinctions in the properties of slow vacuolar channel between near isogenic wheat cultivars, the ET8 (Al-tolerant) and ES8 (Al-sensitive) cultivars. The findings suggested that the Alt1 locus might influence more than just the plasma membrane malate channel in wheat (Wherrett et al. 2005), but also contain some genes conferring ability of plants to deal with Al by its efficient sequestration in the vacuole. Earlier, Zhang et al (2008) used patch-clamp techniques to investigate TaALMT1 and Al3+-induced malate efflux in wheat root apices. The results revealed that protoplasts exhibited an inward current with malate2− as the permeant anion, sensitive to niflumate. External AlCl3 increased the inward current, showing high selectivity to malate. This investigation provided direct evidence supporting TaALMT1 as an Al3+-activated malate2− channel, allowing its targeting in the breeding programs.

Another major tool that is highly suitable for understanding operation of key membrane transporters is a MIFE technique for non-invasive microelectrode ion flux measurements (Shabala et al. 1997; Wherrett et al. 2005). Its principle is based on measuring electrochemical gradients for the ion of interest in the proximity of the root tissue. The technique possesses high spatial (several micrometers) and temporal (5 s) resolution and is non-invasive. Combined with the fact that MIFE method measures fluxes from intact tissues (where ion transporters operate in the natural environment) it is an ideal tool for cell-based phenotyping and has already been widely used for screening various germplasm for some toxic metals such as Na+ (Park et al. 2007; Shahzad et al. 2022; Wu et al. 2019) and Cd3+ (Wu et al. 2020) as well as to understand the mode of regulation of relevant transporters by various ligands and signalling molecules (Pedersen et al. 2020).

Membrane transporters mediating root ion uptake in hostile or heavy metal-polluted soils

Analyzing the arsenic (As) speciation in plants is crucial for comprehending As metabolism within plants and evaluating the toxicity of plant-derived As to consumers at higher trophic levels. Studies on various plant species collected from As-contaminated environments reveal that both terrestrial and freshwater aquatic plants primarily contain arsenite (Geiszinger et al. 2002; Koch et al. 2000; Kuehnelt et al. 2000). Although both arsenate and arsenite are present, their relative proportions vary across different plant species. Additionally, many plant species contain trace amounts of methylated As compounds, including MMA, DMA, TMAO, and occasionally tetramethylarsonium ion. Methylated As species were found to be the predominant As compounds in red clover, with MMA and DMA constituting significant portions of the extracted As (Geiszinger et al. 2002). MMA(III) can form complexes with thiol compounds, such as MMA(III)-PC2 identified in sunflower roots exposed to inorganic As (Raab et al. 2005). While pentavalent As does not directly interact with thiol groups, pentavalent As in DMA can bind to GSH in sulfur-rich plant species like Brassica oleracea, forming the DMAS-GS complex when activated by sulfide (Raab et al. 2007).

In aerobic soils, As(V) emerges as the primary arsenic species, entering plant roots through phosphate (Pi) transporters. In Arabidopsis thaliana, two Pht1 isoforms, namely AtPht1;1 (Pi transporter 1;1) and AtPht1;4, play a crucial role in facilitating the acquisition of both Pi and As(V) from soils with varying phosphorus levels (Shin et al. 2004). It is noteworthy that the absence of AtPht1;5 has been demonstrated to mitigate As(V) toxicity in plants (Khan et al. 2021). Conversely, in rice, the overexpression of OsPht1;8 significantly boosts As(V) uptake and translocation, with the transcription factor OsPHR2 (Pi starvation response 2) regulating the expression of OsPht1;8 (Wu et al. 2011). In anaerobic environments such as submerged soils, As(III) becomes the dominant form of arsenic. Within plants, members of the nodulin 26-like intrinsic protein (NIP; TC 1.A.8, the MIP family) family of plant aquaporins play a pivotal role in facilitating As(III) uptake by roots (Xu et al. 2015). Complementing the action of NIPs, aquaporins from the rice plasma membrane intrinsic protein (PIP; TC 1.A.8, MIP family) family, including OsPIP2;4, OsPIP2;6, and OsPIP2;7, also exhibit permeability to As(III), contributing to overall As(III) tolerance in plants (Mosa et al. 2012) (Fig. 1). Given such diversity of transporters, the next step would be to identify which of them may be the main target(s) for plant breeders. This should be keeping in mind the following issues:

  • Given the above specificity, it is essential to gain a thorough understanding of the transport mechanisms of arsenic into, within, and out of plants, for each particular crop, as extrapolating this knowledge from model Arabidopsis or rice species may be not appropriate in all cases.

  • Reduction of As uptake by roots needs to occur without compromising plant’s ability to take inorganic phosphate or water. This objective is rooted in the hypothesis that, akin to transporters like HKT1;5 in Arabidopsis, the transporter’s selectivity is determined by specific amino acid sequences in its selectivity filter (Platten et al. 2006). Thus, using a combined physiological and genetic approach, it may be possible to identify crucial domains in OsPHT1 that differentiate between arsenic and phosphorus uptake through Genome-Wide Association Study (GWAS). This involves analyzing OsPht1;1 transcription in selected rice accessions with SNPs in the OsPht1;1 promoter and conducting western blots with anti-OsPHT1;1 in rice accessions with nonsynonymous SNPs in OsABCC1 to assess OsPHT1;1 protein levels.

  • To identify important domains in OsPHT1;1 potentially differentiating between As and Pi uptake, one may need to re-sequence OsPHT1;1 in selected rice accessions, clone the OsPHT1;1 gene for expression in yeast, and conduct uptake studies. Important domains for As and Pi transport in OsPHT1;1 may be also determined using site-directed mutagenesis, followed by analysis of As and Pi uptake and accumulation in transgenic plants expressing OsPHT1;1 with SNPs or crossed rice lines (BC3F3) with cultivation line and OsPHT1;1 SNPs rice accession.

  • It is unlikely that the crop ability to exclude toxic As occurs at transcriptional but most likely involves post-translational regulation (Basharat and Yasmin. 2016). Thus, a better understanding of gating modes and signalling mechanisms enabling operation of such transporters is required. Electrophysiological techniques may be ideal for such studies.

Fig. 1
figure 1

Arsenic transport in plants and its dependence of environmental conditions (using rice as an example). In non-flooded areas, arsenite (As(V)), the primary inorganic form of arsenic, is absorbed through phosphate transporters (PHTs) and transformed into As(III) by cytosolic arsenic reductase. Conversely, in flooded rice paddies, As(III) prevails as the major inorganic form. It enters the plant through aquaporins (PIPS, OsLSi1) located at the plasma membrane of root exodermal and endodermal cells. OsLsi2, situated at the plasma membrane of root endodermal cells, facilitates the translocation of As(III) into the xylem (Ma et al. 2008). Consequently, arsenic is transported to the shoots and accumulates in grains. To mitigate As toxicity in the cytosol, As(III) transporters in the rhizodermis may re-export As(III) to the rhizosphere. Additionally, in the nodes of shoots, ABC transporters facilitate the translocation of phytochelatin-conjugated As(III) [PCs-As(III)] into vacuoles, preventing the movement of As to grains (Song et al. 2014)

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Cadmium (Cd), a highly toxic heavy metal, exhibits relatively high mobility in soil, presenting a significant global environmental challenge (Chaney 2015). Even when plants do not display immediate signs of Cd toxicity, and their growth remains unaffected, the accumulation of Cd in the shoot poses potential hazards to humans through food chain cycles (Rizwan et al. 2016, 2017). Consequently, gaining a deeper understanding of the mechanisms governing Cd uptake and transport in plants, particularly in rice and edible leafy vegetables, and devising practical solutions to mitigate Cd accumulation in above-ground plant parts are crucial steps in ensuring food safety. Given its chemical similarity to other essential divalent cations such as iron (Fe), calcium (Ca), zinc (Zn), and manganese (Mn), Cd can infiltrate plant cells through the transporters and channels responsible for these essential elements. This intricate interplay highlights the importance of unraveling the specific mechanisms involved to develop effective strategies for minimizing Cd accumulation and, consequently, enhancing food safety.

Our understanding of how plants absorb and remove cadmium (Cd) is steadily improving. The essential manganese transporter in rice, OsNramp5, has been demonstrated to play a dual role by also facilitating the transport of cadmium ions from the soil into root cells (Sasaki et al. 2012; Ishikawa et al.2012), with field studies confirming that 98% of rice shoot Cd is a result of having active NRAMP5 (Ishikawa et al. 2012). Additional transporters, such as AtHMA3, AtPCR2, AtMTP1, IRT1 and OsNramp1, are potential contributors to Cd influx, particularly following soil reaeration post-flooding events (Nakanishi et al. 2006; Song et al. 2010; Takahashi et al. 2011) although the role of IRT1 remains to be proven under filed-related conditions. In Arabidopsis, the Fe transporter AtIRT1 and Calcium-dependent protein kinases CPK21 and CPK23 had been identified as significant players in Cd uptake (Rogers et al. 2000; Zhang et al. 2023). This was substantiated by the observation of a considerably lower rate of root Cd uptake in the AtIRT1 knockout mutant (Yoshihara et al. 2006). Exploring beyond traditional model plants, the roots of the halophytic plant Suaeda salsa exhibited Cd2+ influx inhibition when treated with Ca2+ channel blockers. This suggests the potential involvement of Ca2+ permeable channels in transporting Cd2+ into cells (Li et al. 2012) (Fig. 2). These insights underscore the diversity of mechanisms employed by different plant species to manage Cd uptake, emphasizing the need for a nuanced understanding to develop targeted strategies for mitigating Cd accumulation in plants.

Fig. 2
figure 2

Cadmium uptake and translocation in plants. Cd is primarily absorbed by OsNramp5, located at the distal side of both root exodermis and endodermis cells (Sasaki et al. 2012). Additionally, OsIRT1, OsIRT2, and OsNramp1 also contribute to Cd uptake (Nakanishi et al. 2006; Takahashi et al. 2011). OsHMA3, situated at the tonoplast of roots, is responsible for accumulating Cd within vacuoles and inhibiting radial translocation of Cd into the stele. Low Cd-accumulating rice was produced by expressing a functional OsHMA3 transporter under the control of OsHMA2 (Sasaki et al. 2014), a rice root pericycle and nodal phloem-specific promoter (Shao et al. 2018). OsLCT1, expressed in cells surrounding enlarged vascular bundles and diffuse vascular bundles of nodes, particularly during the reproductive stage

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In the light of above, the following “burning questions” need to be resolved in a priority manner:

  • A better understanding of structure–function relations of various transporters isoforms for Cd is needed, to uncouple transport of toxic Cd metals from that for essential micronutrients such as Fe or Zn (Bashir et al. 2019).

  • Another promising and currently underexplored avenue is to understand the role of non-selective cation channels in Cd uptake and reveal mechanisms of their regulation (Wu et al. 2020). In Arabidopsis genome, two groups of non-selective cation channels (NSCC), namely “glutamate receptor-like” (GLR) and “cyclic nucleotide-gated channels” (CNGC) are known, with 20 members in each group. Each of them may potentially represent the possible pathway for Cd entry into plant root and should be functionally characterized for their ability to uptake Cd2+ in functional (MIFE or patch-clamp electrophysiology) assays and then validated in genetic studies.

  • NSCC are known to be regulated by reactive oxygen species (ROS) that are considered as critical signalling molecules mediating plant adaptive responses to environment (Pottosin and Dobrovinskaya. 2014). Thus, understanding plant redox balance and post-transcriptional regulation of NSCC activity by ROS under Cd exposure may be critical for developing low-Cd accumulating germplasm.

  • Reverse genetics experiments will be needed to further validate contribution of each specific transporter. This can be achieved by creating double- and triple mutants between GLR/CNGC, IRT, and NRAMP. The sensitivity of candidate channels to ROS should be then studied using MIFE and patch-clamp techniques.

  • Cd uptake from soils may be also influenced by the presence of dissolved chloride. At high pH, the solubility of Cd can be markedly increases by chloride in saline soils leading to its increased accumulation in crops such as potato (McLaughlin et al. 1994) or wheat (Norvell et al. 2000). The exact mechanism of chloride increasing Cd uptake is not clear yet.

The challenge of aluminum (Al) toxicity significantly hampers crop yields in acid soils at the global scale. To combat Al stress, plants employ various resistance mechanisms. One such strategy involves Al exclusion, where plants release organic acids (OAs), such as malate and citrate, from the root apex into the rhizosphere. These OAs chelate toxic Al3+ ions, increase soil pH, reduce the solubility of aluminum, and prevent their entry into root cells. Additionally, plants employ an internal Al tolerance mechanism, sequestering Al in the vacuole of root cells or translocating it to the shoot for storage in leaf cell vacuoles (Wang et al. 2017).

The Al exclusion and tolerance mechanism, extensively investigated, centers around the exudation of Al-activated root organic acids (OAs), a well-characterized process across numerous plant species. Two key organic acids, malate, and citrate, serve as primary OAs and are transported by two distinct families of plasma membrane (PM)-localized transporters: the Al-activated malate transporter (ALMT) family, comprising anion channels, and the multidrug and toxic compound extrusion (MATE) family, consisting of organic acid/H+ antiporters, respectively. Although the functional roles and operational modes of these transporters have been extensively studied, the understanding of plasma membrane transporters responsible for the initial transport of Al3+ across the root plasma membrane remains limited.

Negishi et al. (2012) demonstrated that the plasma membrane aluminum transporter HmPALT1 in Hydrangea macrophylla mediates the aluminum transport process (Negishi et al. 2012). Remarkably, this transporter belongs to the aquaporin (AQP) family. In a more recent investigation, Wang et al. (2017) identified NIP1;2, a member of the Arabidopsis nodulin 26-like intrinsic protein (NIP) subfamily within the aquaporin (AQP) family, localized in the plasma membrane. NIP1;2 was observed to play a crucial role in transporting Al-malate from the root cell wall into the root symplast. This step is vital for subsequent Al xylem loading and root-to-shoot translocation, representing key elements in an internal Al tolerance mechanism in Arabidopsis (Wang et al. 2017). Notably, the study by Wang et al. (2017) emphasized that NIP1;2 specifically facilitates the transport of Al-malate, rather than Al3+ ions. Despite these findings, the broader role of aquaporins as cation channels and their potential involvement as a pathway for Al3+ uptake is not yet fully understood. The complexities of AQP function in the context of aluminum tolerance warrant further investigation to unravel their precise roles in ion transport mechanisms (Fig. 3).

Fig. 3
figure 3

Solute transporters mediating plant adaptation to Al3+ toxicity. Several plasma membrane-localized transporters, such as AtNIP1;2 (Wang et al. 2017), HmPALT1 (Negishi et al. 2012), and OsNrat1 ( Li et al. 2014; Xia et al. 2010), facilitate the entry of Al3+ into the cytosol. Simultaneously, OsMGT1, a plasma membrane transporter that is primarily responsible for Mg uptake (Chen et al. 2012), and OsCDT3, a small cysteine-rich peptide anchored to rice root plasma membrane known for its potential to bind Al and impede its root entry (Xia et al. 2013), play roles in managing Al3+ influx. AtNIP1;2, besides its involvement in Al uptake, serves to transport the malate-Al complex to the shoot via the xylem. Transporters such as OsALS1 (Huang et al. 2012), AtALS1 (Larsen et al. 2007), and HmVALT1 (Negishi et al. 2012), located in the tonoplast, contribute to sequestering Al3+ into the vacuole, regulating internal Al3+ levels. The uptake of Al3+ triggers a signaling cascade that activates transporters like ALMT1 ( Magalhaes et al. 2007; Sasaki et al. 2004) and MATE (Furukawa et al. 2007; Yokosho et al. 2011). These transporters promote the exudation of malate and citrate, respectively, which bind and chelate free Al3+ in the soil. AtALS3, found in the root cortex’s epidermal cells and phloem, potentially aids in distributing Al away from the root tip within the plant. This involves transporting Al directly or in association with a ligand (Huang et al. 2021). During the process of excluding Al3+, the ABC transporter STAR1/STAR2 (Huang et al. 2009) releases UDP-Glucose into the cell wall, contributing to Al3+ exclusion mechanisms

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In recent years, evidence has emerged that aquaporins (AQPs) from the plasma membrane intrinsic protein (PIP) family may operate as ion channels, mediating cation transport across the plasma membrane. In this context, it may be interesting to elucidate the role of various AQPs in Al3+ uptake in plant roots and uncouple it from uptake via other possible routes. For doing this, candidate AQP genes from the PIP and nodulin 26-like intrinsic protein (NIP) families may be cloned and expressed in Xenopus oocytes for functional analysis. Subsequent yeast growth assays could then be conducted to explore whether the expression of AQPs causes increased accumulation of Al3+. Once the functional role of a specific AQP is confirmed, its role in planta may be studied using available mutants or gene editing technologies to modify the operation of the candidate AQP gene in model species.

Ion channels and carriers involved in ion sequestration in vacuoles

The vacuole serves a pivotal role in the detoxification and distribution of xenobiotics, including toxic metals, allowing plants to adapt to dynamic environmental conditions (Park et al. 2012; Sharma et al. 2016; Shitan and Yazaki 2013). Various transporters located at the vacuolar membrane are responsible for sequestering a diverse range of organic and inorganic metabolites and xenobiotics. This approach has been proposed as a pivotal strategy to minimize the presence of toxic metal(loid)s in the living cell organs. In Arabidopsis studies, the vacuolar transporters AtABCC1 and AtABCC2, identified as As-phytochelatin (PC) transporters, exhibited enhanced resistance to and accumulation of arsenic when co-expressed with a wheat phytochelatin synthase gene (TaPCS1) in both yeast and Arabidopsis. Conversely, plants with double knockouts of atabcc1 and atabcc2 demonstrated hypersensitivity to arsenic due to a deficiency in vacuolar sequestration of the toxic minerals conjugated to phytochelatin ( Park et al. 2012; Song et al. 2010). In rice, OsMTP1 primarily functions as a tonoplast-localized transporter that stands out as a candidate gene for enhancing zinc levels and reducing cadmium levels in rice grains (Ning et al. 2023). Furthermore, the highest expression levels of OsHMA3, that is involved in the sequestering cadmium into the vacuole, are found in the root (Ueno et al. 2010; Sasaki et al. 2014).

As commented above, most plant species deal with the issue of Al toxicity by releasing organic acid exudates, thereby increasing the pH in the rhizosphere and reducing the concentration of soluble Al (Kochian et al. 2004; Wang et al. 2006). Numerous genes and quantitative trait loci (QTLs) have been identified as responsible for this process, with MATE or ALMT gene families playing key roles in citric acid or malic acid exudation, respectively [67].

However, some plant species employ an alternative strategy, bypassing the release of organic acids and instead sequestering cytotoxic Al in root vacuoles. The molecular mechanisms governing this sequestration process remain elusive. we utilized two near-isogenic lines of wheat, ET8 and ES8, with a 99% homology, differing in Al tolerance at a single genetic locus, Alt1, responsible for malate extrusion (Wherrett et al. 2005). However, in addition to varying rates of malate extrusion, ET8 and ES8 lines exhibited dramatic differences in the density of tonoplast slow vacuolar (SV) channels and their modes of regulation by vacuolar Al3+. These findings challenged the conventional belief that the Alt1 locus exclusively regulates the plasma membrane malate channel’s expression, suggesting the potential involvement of the slow vacuolar channel in the aluminum tolerance mechanism.

Vacuolar sequestration plays a crucial role in mitigating the harmful effects of cadmium on cell metabolism. As cadmium has a strong affinity for thiol-containing molecules such as cysteine, glutathione, and phytochelatins (PCs), the dominant mechanism by which Cd is sequestered in the vacuole is by using ATP-binding cassette (ABC) transporters that mediate influx of PC-metal (loid) complexes (Wei et al. 2021). Cadmium may also be sequestered in the ionic form by P1B-type ATPases from HMA family. OsHMA3, a vacuolar P1B-type ATPase was found to contribute 85.6% of the variance in Cd content between low-and high-cadmium accumulation varieties of rice (Ueno et al. 2010). Some transporters like CAXs and NRAMPs, which are tonoplast-localized, also play a role in Cd tolerance (Oomen et al. 2009).

Long-distant transport of pollutant ions

Metalloids absorbed by the roots are subsequently translocated to shoots and grains through xylem and phloem transport pathways. In the specific case of rice, the translocation of arsenic (As) from roots to shoots is a critical factor influencing the accumulation of toxic metals in rice grains (Ma et al. 2008). In anaerobic paddy soil, the predominant chemical species of As is arsenite (As(III)). Two silicon transporters, OsLsi1 and OsLsi2, play pivotal roles in the accumulation of As(III) and its translocation from roots to shoots in rice. These transporters are localized at the plasma membrane of root exodermal and endodermal cells. OsLsi1 facilitates the entry of As(III) into these cell layers, while OsLsi2, positioned nearby, is instrumental in the efflux of As(III) toward the stele (Ma et al. 2008). The dual functionality of these silicon transporters underscores their vital role in regulating the movement of As(III) within rice plants, significantly contributing to the overall dynamics of metalloid accumulation in rice grains.

Transgenic rice plants developed by heterologous expression of OsNip1;1 and OsNip3;3 in Xenopus oocytes demonstrated their high permeability to arsenite (As(III)). Transgenic rice plants overexpressing these transporters showed a reduction in root-to-shoot As translocation and diminished As accumulation in the grain. This outcome was attributed to a decrease in xylem loading of As, highlighting the role of OsNip1;1 and OsNip3;3 in modulating As dynamics in rice (Sun et al. 2018). In contrast, in wheat and barley, arsenate (As(V)) is the primary inorganic form of As absorbed by roots. The uptake process is facilitated by phosphate transporters, and subsequently, As(V) is reduced to As(III) by As(V) reductases within root cells (Chao et al. 2014; Shi et al. 2016; Xu et al. 2015). Additionally, transporters responsible for inositol uptake (INT) in the phloem of Arabidopsis may also play a role in arsenic transport (Duan et al. 2015). The disruption of AtINT2 or AtINT4 in these plants resulted in a significant reduction in arsenic concentration in the phloem, silique, and seed, suggesting the involvement of these transporters in the phloem-mediated transport of arsenic. In the light of this, future studies should be focused on quantification of the role of INT and NIP transporters in long-distance arsenic transport and identification of their control modes by root-born signaling molecules. Such experiments may include collection of xylem and phloem sap to measure changes in their chemical and molecular profiles and linking them with stress-induced changes in hormonal (auxins, cytokinins, strigolactones, jasmonate, ABA) and molecular (miRNA, peptides) profiles in studied plant species. Regulation of the loading of Cd into the transpiration stream and its subsequent delivery to the shoot is vital to avoid Cd entry into the food chain, particularly in leafy vegetables. Equally crucial is the avoidance of Cd accumulation in cereal grains, especially rice, which is accomplished through a retranslocation mechanism based on the phloem. The translocation of Cd through the xylem stands out as a pivotal factor influencing the variability in grain Cd accumulation.

Early studies revealed that variations in the rates of Cd translocation from roots to shoots among cultivars of Oryza sativa ssp. indica (high Cd-accumulating) and Oryza sativa ssp. japonica (low Cd-accumulating) rice contribute to the observed genotypic differences in Cd accumulation between these two subspecies (Clemens et al. 2013). This translocation process is primarily influenced by retention in roots and the activity of xylem loading. Proteins implicated in the xylem loading process include other HMA P1B-type ATPases. OsHMA2, predominantly expressed in the vascular bundles of roots, plays a role in the translocation of Cd and Zn. The loss of function in OsHMA2 insertion mutants leads to reduced Cd concentrations in leaves and grains (Takahashi et al. 2012) but also resulted in reduced Zn concentration in the grain. Cd translocation to developing grains involves phloem transport, either through remobilization from leaves or directly after root uptake, xylem loading, and rapid accumulation at the shoot base (Rodda et al. 2011). In rice, the OsLCT1 Cd transporter is believed to mediate this process, expressed in cells surrounding enlarged vascular bundles and diffuse vascular bundles of nodes, particularly during the reproductive stage.

Additionally, transporter proteins are applied not only in agricultural crops but also in horticultural crops. Negishi et al. (2012) demonstrated the mediation of the plasma membrane aluminum transporter HmPALT1 in the process in Hydrangea macrophylla. Notably, this transporter belongs to the aquaporin (AQP) family (Negishi et al. 2012). More recently, Wang et al. (2017) identified NIP1;2, a plasma membrane-localized member of the Arabidopsis nodulin 26-like intrinsic protein (NIP) subfamily within the AQP family. NIP1;2 was found to facilitate the transport of Al-malate from the root cell wall into the root symplasm. This process is crucial for subsequent Al xylem loading and root-to-shoot translocation, representing essential steps in an internal Al tolerance mechanism in Arabidopsis. At the same time, it was suggested that NIP1;2 facilitates the transport of (chelated) Al-malate rather than Al3- ions questioning the role of AQPs as cation channels, and their potential involvement as a pathway for Al3+ uptake(Wang et al. 2017). These findings underscore the complexity of the mechanisms governing Al tolerance and suggest the need for further investigation to unravel the specific roles of AQPs in the context of Al3+ transport.

Conclusions and future prospects

In summary, the existing comprehension of metal(loid) absorption, conveyance, and buildup in plants underscores the complex interaction among diverse membrane transporters and channels. To optimize metalloid dynamics in crops and enhance food safety, thorough investigations into the genetic basis and regulatory mechanisms of key transporters (OsLsi1, OsLsi2, OsNip1;1, OsNip3;3, OsHMA2, OsLCT1, NIP1;2; HMA3; NRAMP5) are essential. Genetic modification, including overexpression or targeted breeding, should be explored to enhance the efficiency of these transporters in sequestering arsenic (As) and cadmium (Cd), as well as controlling xylem loading.

Based on the current standing in a field, several promising research directions are envisaged. First, the identification of crucial domains in transporters such as OsPHT1, responsible for differentiating between As(V) and phosphorus uptake, offers the potential for targeted genetic modifications to mitigate arsenic contamination in crops. Similarly, the exploration of non-selective cation channels (NSCC) in Cd uptake provides a basis for understanding and manipulating the specific pathways involved, contributing to food safety. Further research is needed to fully understand the roles of AQPs as cation channels and their involvement in Al3+ uptake. This knowledge could lead to the development of strategies to enhance crop tolerance to aluminum toxicity, particularly in regions facing significant soil acidity issues. More in depth studies into operation of vacuolar transporters, such as AtABCC1 and OsMTP1, may open avenues for genetic modification to enhance metal sequestration, reducing health risks associated with metal(loid) accumulation in edible plant parts. Unraveling the molecular mechanisms of cytotoxic aluminum sequestration in root vacuoles and exploring the role of MATE or ALMT genes might provide alternative strategies for crop improvement. Lastly, the understanding of long-distance transport of metalloids, especially in the context of rice, highlights the importance of transporters like OsLsi1 and OsLsi2. Future research could focus on manipulating these transporters to regulate the translocation of toxic metals from roots to shoots, influencing metal(loid) accumulation in grains.

The knowledge of operation of membrane transporters and their regulation may be also instrumental for developing practical strategies to mitigate impact of toxic metals on agricultural production. For example, high levels of silicon (Si) reduces As uptake in plants, (Bogdan and Schenk 2008; Ma et al. 2008), and some greenhouse studies showed that Si fertilizer reduces As accumulation in rice shoots and grain (Li et al. 2009). Another example is a possibility of using foliar sprays with plant growth regulators. For example, gamma-aminobutyric acid (GABA) is known to be able to regulate ALMT transporters and modulate the extent of malate extrusion (Ramesh et al. 2015) hence modulating biological availability of Al and an extent of its accumulation in plants. This regulation of anion flux through ALMT proteins requires a specific amino acid motif in ALMTs that shares similarity with a GABA binding site in mammalian GABAA receptors (Ramesh et al. 2018). Thus, understanding the gating modes of specific transporters may lead to design of optimal formulations to mitigate the extent of toxic metal stress on crop performance.

In conclusion, the outlined experiments will be instrumental not only for contributing to the fundamental understanding of ion transport in plants but may also have significant implications for sustainable agriculture, environmental health, and food safety. The knowledge gained from these endeavors will likely inform future research and practical applications aimed at addressing the challenges posed by heavy metal contamination in the environment.