Recent advances and applications of laser-based imaging techniques in food crops and products: a critical review

To meet the growing demand for food quality and safety, there is a pressing need for fast and visible techniques to monitor the food crop and product production processing, and to understand the chemical changes that occur during these processes. Herein, the fundamental principles, instruments, and characteristics of three major laser-based imaging techniques (LBITs), namely, laser-induced breakdown spectroscopy, Raman spectroscopy, and laser ablation-inductively coupled plasma-mass spectrometry, are introduced. Additionally, the advances, challenges, and prospects for the application of LBITs in food crops and products are discussed. In recent years, LBITs have played a crucial role in mapping primary metabolites, secondary metabolites, nanoparticles, toxic metals, and mineral elements in food crops, as well as visualizing food adulteration, composition changes, pesticide residue, microbial contamination, and elements in food products. However, LBITs are still facing challenges in achieving accurate and sensitive quantification of compositions due to the complex sample matrix and minimal laser sampling quantity. Thus, further research is required to develop comprehensive data processing strategies and signal enhancement methods. With the continued development of imaging methods and equipment, LBITs have the potential to further explore chemical distribution mechanisms and ensure the safety and quality of food crops and products.

Cell type-specific mapping of ion distribution in Arabidopsis thaliana roots

Cell type-specific mapping of element distribution is critical to fully understand how roots partition nutrients and toxic elements with aboveground parts. In this study, we developed a method that combines fluorescence-activated cell sorting (FACS) with inductively coupled plasma mass spectrometry (ICP-MS) to assess the ionome of different cell populations within Arabidopsis thaliana roots. The method reveals that most elements exhibit a radial concentration gradient increasing from the rhizodermis to inner cell layers, and detected previously unknown ionomic changes resulting from perturbed xylem loading processes. With this approach, we also identify a strong accumulation of manganese in trichoblasts of iron-deficient roots. We demonstrate that confining manganese sequestration in trichoblasts but not in endodermal cells efficiently retains manganese in roots, therefore preventing toxicity in shoots. These results indicate the existence of cell type-specific constraints for efficient metal sequestration in roots. Thus, our approach opens an avenue to investigate element compartmentation and transport pathways in plants.

A Multimodal Desorption Electrospray Ionisation Workflow Enabling Visualisation of Lipids and Biologically Relevant Elements in a Single Tissue Section

The colocation of elemental species with host biomolecules such as lipids and metabolites may shed new light on the dysregulation of metabolic pathways and how these affect disease pathogeneses. Alkali metals have been the subject of extensive research, are implicated in various neurodegenerative and infectious diseases and are known to disrupt lipid metabolism. Desorption electrospray ionisation (DESI) is a widely used approach for molecular imaging, but previous work has shown that DESI delocalises ions such as potassium (K) and chlorine (Cl), precluding the subsequent elemental analysis of the same section of tissue. The solvent typically used for the DESI electrospray is a combination of methanol and water. Here we show that a novel solvent system, (50:50 (%v/v) MeOH:EtOH) does not delocalise elemental species and thus enables elemental mapping to be performed on the same tissue section post-DESI. Benchmarking the MeOH:EtOH electrospray solvent against the widely used MeOH:H₂O electrospray solvent revealed that the MeOH:EtOH solvent yielded increased signal-to-noise ratios for selected lipids. The developed multimodal imaging workflow was applied to a lung tissue section containing a tuberculosis granuloma, showcasing its applicability to elementally rich samples displaying defined structural information.

The key micronutrient copper orchestrates broad-spectrum virus resistance in rice

Copper is a critical regulator of plant growth and development. However, the mechanisms by which copper responds to virus invasion are unclear. We previously showed that SPL9-mediated transcriptional activation of miR528 adds a previously unidentified regulatory layer to the established ARGONAUTE (AGO18)-miR528-L-ascorbate oxidase (AO) antiviral defense. Here, we report that rice promotes copper accumulation in shoots by inducing copper transporter genes, including HMA5 and COPT, to counteract viral infection. Copper suppresses the transcriptional activation of miR528 by inhibiting the protein level of SPL9, thus alleviating miR528-mediated cleavage of AO transcripts to strengthen the antiviral response. Loss-of-function mutations in HMA5, COPT1, and COPT5 caused a significant reduction in copper accumulation and plant viral resistance because of the increased SPL9-mediated miR528 transcription. Gain in viral susceptibility was mitigated when SPL9 was mutated in the hma5 mutant background. Our study elucidates the molecular mechanisms and regulatory networks of copper homeostasis and the SPL9-miR528-AO antiviral pathway.

Cryogenic Laser Ablation in a Rapid Cooling Chamber Ensures Excellent Elemental Imaging in Fresh Biological Tissues

Laser ablation inductively coupled plasma mass spectrometry imaging of biologically significant targets largely relies on maintaining the original structures of samples. The temperature regulation capability of the ablation cell is crucial. Herein, a rapid cooling cryogenic sample cell (RCCSC) was developed. In the RCCSC chamber, the temperature reduces to -20 °C in 4 min with a minimum 10 h variation of ±0.1 °C at -26 °C. Improvements on the precision were achieved for the elements of interest in NIST 612 and spiked agarose gel under cryogenic conditions. The limits of detection improved by up to 1.57, 1.70, 3.26, and 1.33 fold for ⁶³Cu, ⁶⁶Zn, ⁵⁷Fe, and ¹⁴⁰Ce in agarose gel, respectively, were obtained under cryogenic conditions compared with those at room temperature. In a time period of testing (10 h), the cryogenic ablation maintains the native state of biological tissues with a high water content to ensure better elemental imaging by reducing thermal effects in ablation and suppressing evaporation of water. The rapid cooling cryogenic ablation significantly improves elemental imaging, as demonstrated by the imaging of various elements in coriander leaves. The present study may provide further insights into elemental distributions in fresh biological samples.

Imaging of Potassium and Calcium Distribution in Plant Tissues and Cells to Monitor Stress Response and Programmed Cell Death

In plants, the response to stress, such as salinity, pathogen attack, drought, high concentration of metals, hyperthermia, and hypothermia, is usually accompanied by potassium ion (K⁺) leakage from the cytosol to the cell wall, mediated by plasma membrane cation conductivity. Stress-induced electrolyte leakage co-occurs with accumulation of reactive oxygen species (ROS) and calcium ions (Ca²⁺) and often results in programmed cell death (PCD). The development of X-ray and mass spectrometry (MS) based imaging techniques has enabled insight into the spatial tissue and cell-specific redistribution of major and trace elements during the stress response. In this chapter a workflow for sample preparation, imaging, and image analysis by X-ray and MS based techniques is presented.

In situ ESEM using 3-D printed and adapted accessories to observe living plantlets and their interaction with enzyme and fungus

This paper describes an innovative way of using environmental scanning electron microscopy (ESEM) and the development of a suitable accessory to perform in situ observation of living seedlings in the ESEM. We provide details on fabrication of an accessory that proved to be essential for such experiments but inexpensive and easy to build in the laboratory, and present our in situ observations of the tissue and cell surfaces. Sample-specific configurations and optimized tuning of the ESEM were defined to maintain Arabidopsis and flax seedlings viable throughout repetitive exposure to the imaging conditions in the microscope chamber. This method permitted us to identify cells and tissues of the live plantlets and characterize their surface morphology during their early stage of growth and development. We could extend the application of this technique, to visualize the response of living cells and tissues to exogenous enzymatic treatments with polygalacturonase in Arabidopsis, and their interaction with hyphae of the wilt fungus Verticillium dahliae during artificial infection in flax plantlets. Our results provide an incentive to the use of the ESEM for in situ studies in plant science and a guide for researchers to optimize their electron microscopy observation in the relevant fields.

Rare earth elements, aluminium and silicon distribution in the fern Dicranopteris linearis revealed by μPIXE Maia analysis

BACKGROUND

The fern Dicranopteris linearis is a hyperaccumulator of rare earth elements (REEs), aluminium (Al) and silicon (Si). However, the physiological mechanisms of tissue-level tolerance of high concentrations of REE and Al, and possible interactions with Si, are currently incompletely known.

METHODS

A particle-induced X-ray emission (μPIXE) microprobe with the Maia detector, scanning electron microscopy with energy-dispersive spectroscopy and chemical speciation modelling were used to decipher the localization and biochemistry of REEs, Al and Si in D. linearis during uptake, translocation and sequestration processes.

RESULTS

In the roots >80 % of REEs and Al were in apoplastic fractions, among which the REEs were most significantly co-localized with Si and phosphorus (P) in the epidermis. In the xylem sap, REEs were nearly 100 % present as REEH3SiO42+, without significant differences between the REEs, while 24-45 % of Al was present as Al-citrate and only 1.7-16 % Al was present as AlH3SiO42+. In the pinnules, REEs were mainly concentrated in necrotic lesions and in the epidermis, and REEs and Al were possibly co-deposited within phytoliths (SiO2). Different REEs had similar spatial localizations in the epidermis and exodermis of roots, the necrosis, veins and epidermis of pinnae of D. linearis.

CONCLUSIONS

We posit that Si plays a critical role in REE and Al tolerance within the root apoplast, transport within the vascular bundle and sequestration within the blade of D. linearis.

Analytical Methodologies for Agrometallomics: A Critical Review

Agrometallomics, as an independent interdiscipline, is first defined and described in this review. Metallic elements widely exist in agricultural plants, animals and edible fungi, seed, fertilizer, pesticide, feedstuff, as well as the agricultural environment and ecology, and even functional and pathogenic microorganisms. So, the agrometallome plays a vital role in molecular and organismic mechanisms like environmetallomics, metabolomics, proteomics, lipidomics, glycomics, immunomics, genomics, etc. To further reveal the inner and mutual mechanism of the agrometallome, comprehensive and systematic methodologies for the analysis of beneficial and toxic metals are indispensable to investigate elemental existence, concentration, distribution, speciation, and forms in agricultural lives and media. Based on agrometallomics, this review summarizes and discusses the advanced technical progress and future perspectives of metallic analytical approaches, which are categorized into ultrasensitive and high-throughput analysis, elemental speciation and state analysis, and spatial- and microanalysis. Furthermore, the progress of agrometallomic innovativeness greatly depends on the innovative development of modern metallic analysis approaches including, but not limited to, high sensitivity, elemental coverage, and anti-interference; high-resolution isotopic analysis; solid sampling and nondestructive analysis; metal chemical species and metal forms, associated molecular clusters, and macromolecular complexes analysis; and metal-related particles or metal within the microsize and even single cell or subcellular analysis.

Differing effects of inorganic and organic arsenic on uptake and distribution of multi-elements in Rice grain

Arsenic (As) pollution can lead to an element imbalance in rice. A hydroponic study was carried out to examine the influence of inorganic (arsenate) and organic (dimethylarsinic acid (DMA)) arsenic compounds on the concentration and distribution of iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni), carbon (C), nitrogen (N), and sulfur (S) in rice caryopsis at maturity using laser confocal microscopy and synchrotron X-ray fluorescence (SXRF). Results showed that treatments with inorganic (iAs) and organic (DMA) arsenic did not change the distribution characteristics of the above elements in rice grains. Fe, Mn, and iAs were mainly limited to the ventral ovular vascular trace, while Cu, Zn, and DMA extended into the endosperm. This implies that milling processes are likely to remove a majority of Fe, Mn, and iAs, but not Cu, Zn, and DMA. With regard to the average fluorescent intensity of the rice endosperm, iAs exposure caused significant reductions in Mn (53%), Fe (40%), Cu (27%), and Zn (74%) while DMA treatments decreased Mn (49%), Fe (37%), and Zn (21%). Compared with DMA, iAs exerted more influence on the reduction of these elements in rice caryopsis. In addition, the elemental analysis revealed a significant 12.7% increase for N and 8% reduction for S in DMA-treated rice caryopsis while a significant decrease of 24.0% for S in iAs-exposed rice caryopsis. These findings suggest that Cu, Zn, and S are more easily impacted by iAs, while N is mostly affected by DMA.

Methodology for the Implementation of Internal Standard to Laser-Induced Breakdown Spectroscopy Analysis of Soft Tissues

The improving performance of the laser-induced breakdown spectroscopy (LIBS) triggered its utilization in the challenging topic of soft tissue analysis. Alterations of elemental content within soft tissues are commonly assessed and provide further insights in biological research. However, the laser ablation of soft tissues is a complex issue and demands a priori optimization, which is not straightforward in respect to a typical LIBS experiment. Here, we focus on implementing an internal standard into the LIBS elemental analysis of soft tissue samples. We achieve this by extending routine methodology for optimization of soft tissues analysis with a standard spiking method. This step enables a robust optimization procedure of LIBS experimental settings. Considering the implementation of LIBS analysis to the histological routine, we avoid further alterations of the tissue structure. Therefore, we propose a unique methodology of sample preparation, analysis, and subsequent data treatment, which enables the comparison of signal response from heterogenous matrix for different LIBS parameters. Additionally, a brief step-by-step process of optimization to achieve the highest signal-to-noise ratio (SNR) is described. The quality of laser-tissue interaction is investigated on the basis of the zinc signal response, while selected experimental parameters (e.g., defocus, gate delay, laser energy, and ambient atmosphere) are systematically modified.

Salt accumulation and effects within foliage of Tilia × vulgaris trees from the street greenery of Riga, Latvia

Green infrastructures within sprawling cities provide essential ecosystem services, increasingly undermined by environmental stress. The main objective in this study was to relate the allocation patterns of NaCl contaminants to injury within foliage of lime trees mechanistically and distinguish between the effects of salt and other environmental stressors. Using field material representative of salt contamination levels in the street greenery of Riga, Latvia, the contribution of salt contaminants to structural and ultrastructural injury was analyzed, combining different microscopy techniques. On severely salt-polluted and dystrophic soils, the foliage of street lime trees showed foliar concentrations of Na/Cl up to 13,600/16,750 mg kg^-1 but a still balanced nutrient content. The salt contaminants were allocated to all leaf blade tissues and accumulated in priority within mesophyll vacuoles, changing the vacuolar ionic composition at the expense of especially K and Ca. The size of mesophyll cells and vacuoles was increased as a function of NaCl concentration, suggesting impeded transpiration stream. In parallel, the cytoplasm showed degenerative changes, suggesting indirect stress effects. Hence, the lime trees in Riga showed tolerance to the dystrophic environmental conditions enhanced by salt pollution but their leaf physiology appeared directly impacted by the accumulation of contaminants within foliage.

Wide field imaging energy dispersive X-ray absorption spectroscopy

A new energy dispersive X-ray absorption spectroscopy (EDXAS) method is presented for simultaneous wide-field imaging and transmission X-ray absorption spectroscopy (XAS) to enable rapid imaging and speciation of elements. Based on spectral K-Edge Subtraction imaging (sKES), a bent Laue imaging system diffracting in the vertical plane was developed on a bend magnet beamline for selenium speciation. The high flux and small vertical focus, forming a wide horizontal line beam for projection imaging and computed tomography applications, is achieved by precise matching of lattice plane orientation and crystal surface (asymmetry angle). The condition generating a small vertical focus for imaging also provides good energy dispersion. Details for achieving sufficient energy and spatial resolution are demonstrated for both full field imaging and computed tomography in quantifying selenium chemical species. While this system has lower sensitivity as it uses transmission and may lack the flux and spatial resolution of a dedicated focused beamline system, it has significant potential in rapid screening of heterogeneous biomedical or environmental systems to correlate metal speciation with function.

Metal ligands in micronutrient acquisition and homeostasis

Acquisition and homeostasis of micronutrients such as iron (Fe) and zinc (Zn) pose specific challenges. Poor solubility and high reactivity require controlled synthesis and supply of ligands to complex these metals extracellularly and intracellularly. Cytosolic labile pools represent only a minute fraction of the total cellular content. Several low-molecular-weight ligands are known in plants, including sulfur ligands (cysteine and peptides), nitrogen/oxygen ligands (S-adenosyl-l-methionine-derived molecules and histidine), and oxygen ligands (phenolics and organic acids). Some ligands are secreted into the extracellular space and influence the phytoavailability of metal ions. A second principal function is the intracellular buffering of micronutrients as well as the facilitation of long-distance transport in xylem and phloem. Furthermore, low-molecular-weight ligands are involved in the storage of metals, predominantly in vacuoles. A detailed molecular understanding is hampered by technical limitations, in particular the difficulty to detect and quantify cellular metal-ligand complexes. More, but still too little, is known about ligand synthesis and the transport across membranes, either with or without a complexed metal. Metal ligands have an immediate impact on human well-being. Engineering metal ligand synthesis and distribution in crops has tremendous potential to improve the nutritional quality of food and to tackle major human health risks.

Spatial Heterogeneity of Cadmium Effects on Salvia sclarea Leaves Revealed by Chlorophyll Fluorescence Imaging Analysis and Laser Ablation Inductively Coupled Plasma Mass Spectrometry

In this study, for a first time (according to our knowledge), we couple the methodologies of chlorophyll fluorescence imaging analysis (CF-IA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), in order to investigate the effects of cadmium (Cd) accumulation on photosystem II (PSII) photochemistry. We used as plant material Salvia sclarea that grew hydroponically with or without (control) 100 μM Cd for five days. The spatial heterogeneity of a decreased effective quantum yield of electron transport (ΦPSΙΙ) that was observed after exposure to Cd was linked to the spatial pattern of high Cd accumulation. However, the high increase of non-photochemical quenching (NPQ), at the leaf part with the high Cd accumulation, resulted in the decrease of the quantum yield of non-regulated energy loss (ΦNO) even more than that of control leaves. Thus, S. sclarea leaves exposed to 100 μM Cd exhibited lower reactive oxygen species (ROS) production as singlet oxygen (1O2). In addition, the increased photoprotective heat dissipation (NPQ) in the whole leaf under Cd exposure was sufficient enough to retain the same fraction of open reaction centers (qp) with control leaves. Our results demonstrated that CF-IA and LA-ICP-MS could be successfully combined to monitor heavy metal effects and plant tolerance mechanisms.

Bioimaging of multiple elements by high-resolution LA-ICP-MS reveals altered distribution of mineral elements in the nodes of rice mutants

Inter-vascular transfer in rice (Oryza sativa) nodes is required for delivering mineral elements to developing tissues, which is mediated by various transporters in the nodes. However, the effect of these transporters on distribution of mineral elements in the nodes at a cellular level is still unknown. Here, we established a protocol for bioimaging of multiple elements at a cellular level in rice node by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and compared the mineral distribution profile between wild-type (WT) rice and mutants. Both relative comparison of mineral distribution normalized by endogenous ¹³ C and quantitative analysis using spiked standards combined with soft ablation gave valid results. Overall, macro-nutrients such as K and Mg were accumulated more in the phloem region, while micro-nutrients such as Fe and Zn were highly accumulated at the inter-vascular tissues of the node. In mutants of nodal Zn transporter OsHMA2, Zn localization pattern in the node tissues did not differ from that of WT; however, Zn accumulation in the inter-vascular tissues was lower in uppermost node I but higher in the third upper node III compared with the WT. In contrast, Si deposition in the mutants of three nodal Si transporters Lsi2, Lsi3 and Lsi6 showed different patterns, which are consistent with the localization of these transporters. This improved LA-ICP-MS analysis combined with functional characterization of transporters will provide further insight into mineral element distribution mechanisms in rice and other plant species.

Leaf Age-Dependent Effects of Foliar-Sprayed CuZn Nanoparticles on Photosynthetic Efficiency and ROS Generation in Arabidopsis thaliana

Young and mature leaves of Arabidopsis thaliana were exposed by foliar spray to 30 mg L-1 of CuZn nanoparticles (NPs). The NPs were synthesized by a microwave-assisted polyol process and characterized by dynamic light scattering (DLS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). CuZn NPs effects in Arabidopsis leaves were evaluated by chlorophyll fluorescence imaging analysis that revealed spatiotemporal heterogeneity of the quantum efficiency of PSII photochemistry (ΦPSΙΙ) and the redox state of the plastoquinone (PQ) pool (qp), measured 30 min, 90 min, 180 min, and 240 min after spraying. Photosystem II (PSII) function in young leaves was observed to be negatively influenced, especially 30 min after spraying, at which point increased H2O2 generation was correlated to the lower oxidized state of the PQ pool.. Recovery of young leaves photosynthetic efficiency appeared only after 240 min of NPs spray when also the level of ROS accumulation was similar to control leaves. On the contrary, a beneficial effect on PSII function in mature leaves after 30 min of the CuZn NPs spray was observed, with increased ΦPSΙΙ, an increased electron transport rate (ETR), decreased singlet oxygen (1O2) formation, and H2O2 production at the same level of control leaves.An explanation for this differential response is suggested.

Elemental Mapping of Lithium Diffusion in Doped Plant Leaves Using Laser-Induced Breakdown Spectroscopy (LIBS)

Mapping of element distributions and diffusion processes in plant tissue has great significance for understanding the systematic uptake, transport, and accumulation of nutrients and harmful elements in plants, and for studying the interaction between plants and the environment. In this work, we used laser-induced breakdown spectroscopy (LIBS) to study the elemental accumulation of Li and its diffusion in plant leaves. The spatially resolved information that LIBS offers, combined with its high sensitivity to light elements make this technology highly advantageous for the analysis of Li. Laser-induced breakdown spectroscopy mapping of Li-doped leaf samples is used to directly visualize the diffusion of Li in the plant leaf and study its distribution as a function of LiCl solution exposure time. Our findings demonstrate that diffusion of Li in plant leaves occurs though their veins (i.e., bundles of vascular tissue) and that Li concentration decreases as we move away from the LiCl exposure site. These results underline the importance of veins in transportation of toxic elements in plants, and mapping of their distribution can be instrumental in the development of possible remediation approaches for managing Li toxicity.

Variability of trace element distribution in Noccaea spp., Arabidopsis spp., and Thlaspi arvense leaves: the role of plant species and element accumulation ability

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was applied for the determination of Cd and Zn distributions within the leaves of Cd- and Zn-hyperaccumulating plants, Noccaea caerulescens, N. praecox, and Arabidopsis halleri, in contrast to nonaccumulator species, Thlaspi arvense and A. thaliana. The elemental mapping of the selected leaf area was accomplished via line scans with a 110-μm-diameter laser beam at a 37-μm s^-1 scan speed and repetition rate of 10 Hz. The lines were spaced 180 μm apart and ablated at an energy density of 2 J cm^-2. The elemental imaging clearly confirmed that Cd was predominantly distributed within the parenchyma of the T. arvense, whereas in the Noccaea spp. and A. halleri, the highest intensity Cd signal was observed in the veins of the leaves. For Zn, higher intensities were observed in the veins for all the plant species except for A. thaliana. Close relationships between Zn and Ca were identified for the Noccaea spp. leaves. These relationships were not confirmed for A. halleri. Significant correlations were also proved between the Cd and Zn distribution in A. halleri, but not for the Noccaea spp. For both T. arvense and A. thaliana, no relevant significant relationship for the interpretation of the results was observed. Thus, the LA-ICP-MS imaging is proved as a relevant technique for the description and understanding of the elements in hyperaccumulating or highly accumulating plant species, although its sensitivity for the natural element contents in nonaccumulator plant species is still insufficient.

The guard cell ionome: Understanding the role of ions in guard cell functions

The ionome is critical for plant growth, productivity, defense, and it eventually affects human food quantity and quality. Located on the leaf surface, stomatal guard cells are critical gatekeepers for water, gas, and pathogens. Insights form ionomics (metallomics) is imperative as we enter an omics-driven systems biology era where an understanding of guard cell function and physiology is advanced through efforts in genomics, transcriptomics, proteomics, and metabolomics. While the roles of major cations (K, Ca) and anions (Cl) are well known in guard cell function, the related physiology, movement and regulation of trace elements, metal ions, and heavy metals are poorly understood. The majority of the information on the role of trace elements in guard cells emanates from classical feeding experiments, field or in vitro fortification, micropropagation, and microscopy studies, while novel insights are available from limited metal ion transporter and ion channel studies. Given the rejuvenated and recent interest in the constantly changing ionome in plant mineral balance and eventually in human nutrition and health, we looked into the far from established guard cell ionome in lieu of the modern omics era of high throughput research endeavors. Newer technologies and tools i.e., high resolution mass spectrometry, advanced imaging, and phenomics are now available to delve into the guard cell ionomes. In this review, research efforts on guard cell ionomes were collated and categorized, and we highlight the underlying role of the largely unknown ionome in guard cell function towards a systems physiology understanding of plant health and productivity.

X-ray elemental mapping techniques for elucidating the ecophysiology of hyperaccumulator plants

Contents Summary 432 I. Introduction 433 II. Preparation of plant samples for X-ray micro-analysis 433 III. X-ray elemental mapping techniques 438 IV. X-ray data analysis 442 V. Case studies 443 VI. Conclusions 446 Acknowledgements 449 Author contributions 449 References 449 SUMMARY: Hyperaccumulators are attractive models for studying metal(loid) homeostasis, and probing the spatial distribution and coordination chemistry of metal(loid)s in their tissues is important for advancing our understanding of their ecophysiology. X-ray elemental mapping techniques are unique in providing in situ information, and with appropriate sample preparation offer results true to biological conditions of the living plant. The common platform of these techniques is a reliance on characteristic X-rays of elements present in a sample, excited either by electrons (scanning/transmission electron microscopy), protons (proton-induced X-ray emission) or X-rays (X-ray fluorescence microscopy). Elucidating the cellular and tissue-level distribution of metal(loid)s is inherently challenging and accurate X-ray analysis places strict demands on sample collection, preparation and analytical conditions, to avoid elemental redistribution, chemical modification or ultrastructural alterations. We compare the merits and limitations of the individual techniques, and focus on the optimal field of applications for inferring ecophysiological processes in hyperaccumulator plants. X-ray elemental mapping techniques can play a key role in answering questions at every level of metal(loid) homeostasis in plants, from the rhizosphere interface, to uptake pathways in the roots and shoots. Further improvements in technological capabilities offer exciting perspectives for the study of hyperaccumulator plants into the future.

The coastal environment affects lead and sodium uptake by the moss Hypnum cupressiforme used as an air pollution biomonitor

Several studies suggest that potential competition exists between marine cations and heavy metals for binding sites on the cell wall of mosses. This competition would impact the heavy metal concentration measured in mosses by biomonitoring programs, which may underestimate air pollution by heavy metals in a coastal environment. In the present study, we aim to identify possible mechanisms affecting lead uptake by mosses in a coastal environment, specifically, the competition between lead (Pb²⁺) and sodium (Na⁺) for binding sites in Hypnum cupressiforme (Hc). We also compared the response of continental and coastal Hc populations to Pb²⁺ exposure by immersing the moss samples in artificial solutions that comprised six experimental treatments and subsequently locating and quantifying Pb²⁺ and Na⁺ using the sequential elution technique and X-ray microanalyses with a scanning electron microscope. We demonstrated that high concentrations of Pb²⁺ prevented Na⁺ from binding to the cell wall. We also examined the effect of the salt acclimation of Hc on Pb²⁺ and Na⁺ accumulation. Coastal Hc populations accumulated more Na and less Pb than continental Hc populations in all treatments. Moreover, our results showed treatment effects on the intra/extracellular distribution of Na⁺, as well as site. This feedback on the influence of salt stress tolerance on Pb²⁺ uptake by mosses requires further study and can be investigated for other heavy metals, leading to a better use of mosses as biomonitoring tools.

Quantitative mapping of elements in basil leaves (Ocimum basilicum) based on cesium concentration and growth period using laser ablation ICP-MS

Quantitative elemental mapping of metallic pollutants in sweet basil was studied by laser ablation (LA)-ICP-MS. For this, the sweet basil was cultivated in Hoagland nutrient solution spiked with 100 and 1000 ng mL^-1 of Cs for 10-60 days. Then, the Cs distribution in collected leaves was determined by LA-ICP-MS using lab-synthesized standard pellets based on NIST 1573a tomato leaves. For comparison, S, Ca, and K were also simultaneously determined in this measurement with a¹³C⁺ signal from the leaves as an internal standard. The obtained calibration curves showed linear coefficient of determination (R²) of 0.991 for K and 0.999 for Cs. The concentration of Cs measured in the basil leaves increased with growth period and pollutant concentration, and accumulation followed the order of leaf margin, petiole, midrib, and veins. Although no visible symptom was detected, significant suppression of the growth rate was observed due to the presence of high-concentration Cs. The experimental model demonstrated herein showed potential for studying the influence of radioactive pollutants on plants and other organisms in the food chain.

Analysis of Plant Leaves Using Laser Ablation Inductively Coupled Plasma Optical Emission Spectrometry: Use of Carbon to Compensate for Matrix Effects

Direct solid sampling by laser ablation into an inductively coupled plasma synchronous vertical dual view optical emission spectroscope (LA-SVDV-ICP-OES) was used for the elemental analysis of nutrient elements Ca, B, Mn, Mg, K, and Zn and essential (non-metallic) elements P and S in plant materials. The samples were mixed with paraffin as a binder, an approach that provides better cohesion of the particles in the pellets in addition to supplying carbon to serve as an internal standard (atomic line C I 193.027 nm) as a way to compensate for matrix effects, and/or variations in the ablation process. Precision was in the range of 1-8% relative standard deviation (RSD) with limit of detection in the range of 0.4-1 mg/kg^-1 and 25-640 mg/kg^-1 for metallic and non-metallic elements, respectively.

Elemental bio-imaging of PEGylated NaYF4:Yb/Tm/Gd upconversion nanoparticles in mice by laser ablation inductively coupled plasma mass spectrometry to study toxic side effects on the spleen, liver and kidneys

The distribution of PEGylated NaYF₄:Yb/Tm/Gd (PEG-UCNPs) and imaging in mice spleen, liver and kidney were examined by laser ablation inductively coupled plasma mass spectrometry.

In-Field, In Situ, and In Vivo 3-Dimensional Elemental Mapping for Plant Tissue and Soil Analysis Using Laser-Induced Breakdown Spectroscopy

Sensing and mapping element distributions in plant tissues and its growth environment has great significance for understanding the uptake, transport, and accumulation of nutrients and harmful elements in plants, as well as for understanding interactions between plants and the environment. In this study, we developed a 3-dimensional elemental mapping system based on laser-induced breakdown spectroscopy that can be deployed in- field to directly measure the distribution of multiple elements in living plants as well as in the soil. Mapping is performed by a fast scanning laser, which ablates a micro volume of a sample to form a plasma. The presence and concentration of specific elements are calculated using the atomic, ionic, and molecular spectral characteristics of the plasma emission spectra. Furthermore, we mapped the pesticide residues in maize leaves after spraying to demonstrate the capacity of this method for trace elemental mapping. We also used the system to quantitatively detect the element concentrations in soil, which can be used to further understand the element transport between plants and soil. We demonstrate that this method has great potential for elemental mapping in plant tissues and soil with the advantages of 3-dimensional and multi-elemental mapping, in situ and in vivo measurement, flexible use, and low cost.

Multi-Elemental Profiling of Tibial and Maxillary Trabecular Bone in Ovariectomised Rats

Atomic minerals are the smallest components of bone and the content of Ca, being the most abundant mineral in bone, correlates strongly with the risk of osteoporosis. Postmenopausal women have a far greater risk of suffering from OP due to low Ca concentrations in their bones and this is associated with low bone mass and higher bone fracture rates. However, bone strength is determined not only by Ca level, but also a number of metallic and non-metallic elements in bone. Thus, in this study, the difference of metallic and non-metallic elements in ovariectomy-induced osteoporosis tibial and maxillary trabecular bone was investigated in comparison with sham operated normal bone by laser ablation inductively-coupled plasma mass spectrometry using a rat model. The results demonstrated that the average concentrations of ²⁵Mg, ²⁸Si, ³⁹K, ⁴⁷Ti, ⁵⁶Fe, ⁵⁹Co, ⁷⁷Se, ⁸⁸Sr, ¹³⁷Ba, and ²⁰⁸Pb were generally higher in tibia than those in maxilla. Compared with the sham group, Ovariectomy induced more significant changes of these elements in tibia than maxilla, indicating tibial trabecular bones are more sensitive to changes of circulating estrogen. In addition, the concentrations of ²⁸Si, ⁷⁷Se, ²⁰⁸Pb, and Ca/P ratios were higher in tibia and maxilla in ovariectomised rats than those in normal bone at all time-points. The present study indicates that ovariectomy could significantly impact the element distribution and concentrations between tibia and maxilla.

Spatiotemporal distribution of essential elements through Populus leaf ontogeny

We examined the spatiotemporal distribution and accumulation of calcium (Ca), potassium (K), and zinc (Zn) during the growth and maturation of grey poplar (Populus tremula × alba) leaves covering plastochrons 01 through 10. This period spans the sugar sink-to-source transition and requires coordinated changes of multiple core metabolic processes that likely involve alterations in essential and non-essential element distributions as tissues mature and effect a reversal in phloem flow direction. Whole-leaf elemental maps were obtained from dried specimens using micro X-ray fluorescence spectroscopy. Additional cross-sections of fresh leaves were scanned to check for tissue specificity in element accumulation. The anatomical distribution of Zn and K remains relatively consistent throughout leaf development; Ca accumulation varied across leaf developmental stages. The basipetal allocation of Ca to the leaf mesophyll matched spatially and temporally the sequence of phloem maturation, positive carbon balance, and sugar export from leaves. The accumulation of Ca likely reflects the maturation of xylem in minor veins and the enhancement of the transpiration stream. Our results independently confirm that xylem and phloem maturation are spatially and temporally coordinated with the onset of sugar export in leaves.

Bioanalytics in Quantitive (Bio)imaging/Mapping of Metallic Elements in Biological Samples

The aim of this article is to describe selected analytical techniques and their applications in the quantitative mapping/(bio)imaging of metals in biological samples. This work presents the advantages and disadvantages as well as the appropriate methods of scope for research. Distribution of metals in biological samples is currently one of the most important issues in physiology, toxicology, pharmacology, and other disciplines where functional information about the distribution of metals is essential. This issue is a subject of research in (bio)imaging/mapping studies, which use a variety of analytical techniques for the identification and determination of metallic elements. Increased interest in analytical techniques enabling the (bio)imaging of metals in a variety of biological material has been observed more recently. Measuring the distribution of trace metals in tissues after a drug dose or ingestion of poison-containing metals allows for the studying of pathomechanisms and the pathophysiology of various diseases and disorders related to the management of metals in human and animal systems.

Recent advances in quantitative LA-ICP-MS analysis: challenges and solutions in the life sciences and environmental chemistry

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely accepted method for direct sampling of solid materials for trace elemental analysis. The number of reported applications is high and the application range is broad; besides geochemistry, LA-ICP-MS is mostly used in environmental chemistry and the life sciences. This review focuses on the application of LA-ICP-MS for quantification of trace elements in environmental, biological, and medical samples. The fundamental problems of LA-ICP-MS, such as sample-dependent ablation behavior and elemental fractionation, can be even more pronounced in environmental and life science applications as a result of the large variety of sample types and conditions. Besides variations in composition, the range of available sample states is highly diverse, including powders (e.g., soil samples, fly ash), hard tissues (e.g., bones, teeth), soft tissues (e.g., plants, tissue thin-cuts), or liquid samples (e.g., whole blood). Within this article, quantification approaches that have been proposed in the past are critically discussed and compared regarding the results obtained in the applications described. Although a large variety of sample types is discussed within this article, the quantification approaches used are similar for many analytical questions and have only been adapted to the specific questions. Nevertheless, none of them has proven to be a universally applicable method.

Cation transporters/channels in plants: Tools for nutrient biofortification

Cation transporters/channels are key players in a wide range of physiological functions in plants, including cell signaling, osmoregulation, plant nutrition and metal tolerance. The recent identification of genes encoding some of these transport systems has allowed new studies toward further understanding of their integrated roles in plant. This review summarizes recent discoveries regarding the function and regulation of the multiple systems involved in cation transport in plant cells. The role of membrane transport in the uptake, distribution and accumulation of cations in plant tissues, cell types and subcellular compartments is described. We also discuss how the knowledge of inter- and intra-species variation in cation uptake, transport and accumulation as well as the molecular mechanisms responsible for these processes can be used to increase nutrient phytoavailability and nutrients accumulation in the edible tissues of plants. The main trends for future research in the field of biofortification are proposed.

Diagnosis of nephrogenic systemic fibrosis by means of elemental bioimaging and speciation analysis

The combined use of elemental bioimaging and speciation analysis is presented as a novel means for the diagnosis of nephrogenic systemic fibrosis (NSF), a rare disease occurring after administration of gadolinium-based contrast agents (GBCA) for magnetic resonance imaging (MRI), in skin samples of patients suffering from renal insufficiency. As the pathogenesis of NSF is still largely unknown particularly with regard to the distribution and potential retention of gadolinium in the human organism, a skin biopsy sample from a suspected NSF patient was investigated. The combination of inductively coupled plasma mass spectrometry (ICP-MS), laser ablation (LA) ICP-MS for quantitative elemental bioimaging, and hydrophilic interaction liquid chromatography (HILIC) ICP-MS for speciation analysis allowed one to unambiguously diagnose the patient as a case of NSF. By means of ICP-MS, a total gadolinium concentration from 3.02 to 4.58 mg/kg was determined in the biopsy sample, indicating a considerable deposition of gadolinium in the patient's skin. LA-ICP-MS revealed a distinctly inhomogeneous distribution of gadolinium as well as concentrations of up to 400 mg/kg in individual sections of the skin biopsy. Furthermore, the correlation between the distributions of phosphorus and gadolinium suggests the presence of GdPO4 deposits in the tissue section. Speciation analysis by means of HILIC-ICP-MS showed the presence of the intact GBCA Gd-HP-DO3A eight years after the administration to the patient. The concentration of the contrast agent in the aqueous extract of the skin biopsy was found to be 1.76 nmol/L. Moreover, evidence for the presence of further highly polar gadolinium species in low concentrations was found.

Analytical approaches to support current understanding of exposure, uptake and distributions of engineered nanoparticles by aquatic and terrestrial organisms

Initiatives to support the sustainable development of the nanotechnology sector have led to rapid growth in research on the environmental fate, hazards and risk of engineered nanoparticles (ENP). As the field has matured over the last 10 years, a detailed picture of the best methods to track potential forms of exposure, their uptake routes and best methods to identify and track internal fate and distributions following assimilation into organisms has begun to emerge. Here we summarise the current state of the field, focussing particularly on metal and metal oxide ENPs. Studies to date have shown that ENPs undergo a range of physical and chemical transformations in the environment to the extent that exposures to pristine well dispersed materials will occur only rarely in nature. Methods to track assimilation and internal distributions must, therefore, be capable of detecting these modified forms. The uptake mechanisms involved in ENP assimilation may include a range of trans-cellular trafficking and distribution pathways, which can be followed by passage to intracellular compartments. To trace toxicokinetics and distributions, analytical and imaging approaches are available to determine rates, states and forms. When used hierarchically, these tools can map ENP distributions to specific target organs, cell types and organelles, such as endosomes, caveolae and lysosomes and assess speciation states. The first decade of ENP ecotoxicology research, thus, points to an emerging paradigm where exposure is to transformed materials transported into tissues and cells via passive and active pathways within which they can be assimilated and therein identified using a tiered analytical and imaging approach.

Histological and ultrastructural evidence for zinc sequestration in Solanum nigrum L

The accumulation of contaminants in the environment due to anthropogenic activities is a matter of global concern. Solanum nigrum L. plants, able to accumulate zinc and hyperaccumulate cadmium, were challenged with 0.025 g Zn L(-1) during 35 days. The localization of Zn in roots, stems and leaves of S. nigrum plants was evaluated by autometallography (AMG) in order to determine the specific tissue, cell and subcellular compartments of Zn sequestration. This Zn concentration resulted in stunted plant growth but no other symptoms of Zn toxicity. Zinc concentration in the plants was highest in the roots, 666 mg Zn kg(-1) fresh weight (f.w.), and lower in the stems, 318 mg Zn kg(-1) f.w., and leaves, 101 mg Zn kg(-1) f.w. Roots of Zn-treated plants showed an underdeveloped structure but additional layers of proliferating cortical parenchyma cells. AMG of S. nigrum roots, stems and leaves revealed a generalized Zn distribution associated with the cell walls in all tissues. In the vasculature (xylem and phloem), Zn was observed at the plasma membrane-cell wall complex of vascular parenchyma cells and conducting elements. Conspicuous Zn deposits were detected in the vacuoles of cortical parenchyma and starch sheath, as well as in the tonoplast of the mesophyll cells. Our results suggest that Zn flux through the plant occurs via the xylem and phloem and associated parenchyma until it is conducted to permanent storage sites, namely, the apoplast and vacuoles of cortical parenchyma cells of the root, stem and the leaf mesophyll.

"Afterlife experiment": use of MALDI-MS and SIMS imaging for the study of the nitrogen cycle within plants

As part of a project to demonstrate the science of decay, a series of mass spectrometry imaging experiments were performed. The aim was to demonstrate that decay and decomposition are only part of the story and to show pictorially that atoms and molecules from dead plants and animals are incorporated into new life. Radish plants (Raphanus sativus) were grown hydroponically using a nutrient system containing (15)N KNO3 (98% labeled) as the only source of nitrogen. Plants were cropped and left to ferment in water for 2 weeks to create a radish "tea", which was used as a source of nitrogen for radish grown in a second hydroponics experiment. After 5 weeks of growth, the radish plants were harvested and cryosectioned, and sections were imaged by positive-ion MALDI and SIMS mass spectrometry imaging. The presence of labeled species in the plants grown using (15)N KNO3 as nutrient and those grown from the radish "tea" was readily discernible. The uptake of (15)N into a number of identifiable metabolites has been studied by MALDI-MS and SIMS imaging.

A microautoradiographic method for fresh-frozen sections to reveal the distribution of radionuclides at the cellular level in plants

Microautoradiography (MAR) is a conventional imaging method based on the daguerreotype. The technique is used to visualize the distribution of radionuclide-labeled compounds within a tissue section. However, application of the classical MAR method to plant tissue sections is associated with several difficulties. In this study, we report an MAR method applicable to fresh-frozen plant sections. Our method had two features: (i) the sample was kept frozen from plant tissue collection to radioisotope detection, making it possible to fix solutes without solvent exchange; and (ii) 1.2 µm thick polyphenylene sulfide film was inserted between the fresh-frozen plant section and the photosensitive nuclear emulsion to separate the section from the emulsion before autoradiography was conducted, which significantly improved the quality of the section until microscopic detection, the quality of the MAR image and the success rate. Then, the passage of cadmium (Cd) through vegetative rice stem tissue after 24 h of (109)Cd absorption was described for the first time using the MAR method. MAR clearly revealed the distribution of (109)Cd at the tissue level with high resolution. The (109)Cd concentration in phloem cells was found to be particularly high, whereas the xylem cells contained only small amounts of (109)Cd. The MAR method was also applicable for detecting (109)Cd and [(33)P]phosphate in roots. The MAR method developed here is expected to provide distribution images for a variety of compounds and ions in plant tissue.

Imaging element distribution and speciation in plant cells

To maintain cellular homeostasis, concentrations, chemical speciation, and localization of mineral nutrients and toxic trace elements need to be regulated. Imaging the cellular and subcellular localization of elements and measuring their in situ chemical speciation are challenging tasks that can be undertaken using synchrotron-based techniques, such as X-ray fluorescence and X-ray absorption spectrometry, and mass spectrometry-based techniques, such as secondary ion mass spectrometry and laser-ablation inductively coupled plasma mass spectrometry. We review the advantages and limitations of these techniques, and discuss examples of their applications, which have revealed highly heterogeneous distribution patterns of elements in different cell types, often varying in chemical speciation. Combining these techniques with molecular genetic approaches can unravel functions of genes involved in element homeostasis.

Biochemical and biophysical methods for studying mitochondrial iron metabolism

Iron is a heavily utilized element in organisms and numerous mechanisms accordingly regulate the trafficking, metabolism, and storage of iron. Despite the high regulation of iron homeostasis, several diseases and mutations can lead to the misregulation and often accumulation of iron in the cytosol or mitochondria of tissues. To understand the genesis of iron overload, it is necessary to employ various techniques to quantify iron in organisms and mitochondria. This chapter discusses techniques for determining the total iron content of tissue samples, ranging from colorimetric determination of iron concentrations, atomic absorption spectroscopy, inductively coupled plasma-optical emission spectroscopy, and inductively coupled plasma-mass spectrometry. In addition, we discuss in situ techniques for analyzing iron including electron microscopic nonheme iron histochemistry, electron energy loss spectroscopy, synchrotron X-ray fluorescence imaging, and confocal Raman microscopy. Finally, we discuss biophysical methods for studying iron in isolated mitochondria, including ultraviolet-visible, electron paramagnetic resonance, X-ray absorbance, and Mössbauer spectroscopies. This chapter should aid researchers to select and interpret mitochondrial iron quantifications.

Ironing out the issues: integrated approaches to understanding iron homeostasis in plants

Plants initialize responses to environmental changes at all levels, from signaling to translation and beyond. Such is the case for fluctuations in the availability of iron (Fe), one of the most critical micronutrients for plants. The results of these responses are physiological and morphological changes that lead to increased iron uptake from the rhizosphere, and recycling and reallocation of Fe, which must be properly localized within specific cells and cellular compartment for use. The use of reductionist approaches, in combination with in vivo and in situ Fe localization tools, has been able to shed light on critical signaling molecules, transcriptional regulators, transporters and other proteins involved in Fe homeostasis. Recent advances in elemental distribution and speciation analysis now enable detection and measurement of Fe and other elements at resolutions never seen before. Moreover, increasing use of systems biology approaches provide a substantially broader perspective of how Fe availability affects processes at many levels. This review highlights the latest in vivo and in situ iron localization approaches and some of the recent advances in understanding mechanisms that control Fe translocation. A broad perspective of how Fe localization data might one day be integrated with large-scale data to create models for Fe homeostasis is presented.

Spatially resolved (semi)quantitative determination of iron (Fe) in plants by means of synchrotron micro X-ray fluorescence

Iron (Fe) is an essential element for plant growth and development; hence determining Fe distribution and concentration inside plant organs at the microscopic level is of great relevance to better understand its metabolism and bioavailability through the food chain. Among the available microanalytical techniques, synchrotron μ-XRF methods can provide a powerful and versatile array of analytical tools to study Fe distribution within plant samples. In the last years, the implementation of new algorithms and detection technologies has opened the way to more accurate (semi)quantitative analyses of complex matrices like plant materials. In this paper, for the first time the distribution of Fe within tomato roots has been imaged and quantified by means of confocal μ-XRF and exploiting a recently developed fundamental parameter-based algorithm. With this approach, Fe concentrations ranging from few hundreds of ppb to several hundreds of ppm can be determined at the microscopic level without cutting sections. Furthermore, Fe (semi)quantitative distribution maps were obtained for the first time by using two opposing detectors to collect simultaneously the XRF radiation emerging from both sides of an intact cucumber leaf.