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Background Hyperaccumulation of trace elements is a rare trait among plants which is being investigated to advance our understanding of the regulation of metal accumulation and applications in phytotechnologies. Noccaea caerulescens (Brassicaceae) is an intensively studied hyperaccumulator model plant capable of attaining extremely high tissue concentrations of zinc and nickel with substantial genetic variation at the population-level. Micro-X-ray Fluorescence spectroscopy (µXRF) mapping is a sensitive high-resolution technique to obtain information of the spatial distribution of the plant metallome in hydrated samples. We used laboratory-based µXRF to characterize a collection of 86 genetically diverse Noccaea caerulescens accessions from across Europe. We developed an image-processing method to segment different plant substructures in the µXRF images. We introduced the concentration quotient (CQ) to quantify spatial patterns of metal accumulation and linked that to genetic variation. Results Image processing resulted in automated segmentation of µXRF plant images into petiole, leaf margin, leaf interveinal and leaf vasculature substructures. The harmonic means of recall and precision (F1 score) were 0.79, 0.80, 0.67, and 0.68, respectively. Spatial metal accumulation as determined by CQ is highly heritable in Noccaea caerulescens for all substructures, with broad-sense heritability (H2) ranging from 76 to 92%, and correlates only weakly with other heritable traits. Insertion of noise into the image segmentation algorithm barely decreases heritability scores of CQ for the segmented substructures, illustrating the robustness of the trait and the quantification method. Very low heritability was found for CQ if randomly generated substructures were compared, validating the approach. Conclusions A strategy for segmenting µXRF images of Noccaea caerulescens is proposed and the concentration quotient is developed to provide a quantitative measure of metal accumulation pattern, which can be used to determine genetic variation for such pattern. The metric is robust to segmentation error and provides reliable H2 estimates. This strategy provides an avenue for quantifying XRF data for analysis of the genetics of metal distribution patterns in plants and the subsequent discovery of new genes that regulate metal homeostasis and sequestration in plants.

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Purpose Thallium (Tl) is one of the most toxic elements known and its contamination is an emerging environmental issue associated with base metal (zinc-lead) mining wastes. This study investigated the nature of Tl tolerance and accumulation in Silene latifolia, which has so far only been reported from field-collected samples. Methods Silene latifolia was grown in hydroponics at different Tl concentrations (0, 2.5, 5, 30 and 60 μM Tl). Elemental analysis with Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and laboratory-based micro-X-ray fluorescence spectroscopy (μ-XRF) were used to determine Tl accumulation and distribution in hydrated organs and tissues. Results This study revealed unusually high Tl concentrations in the shoots of S. latifolia, reaching up to 35,700 μg Tl g−1 in young leaves. The species proved to have exceptionally high levels of Tl tolerance and had a positive growth response when exposed to Tl dose rates of up to 5 μM. Laboratory-based μXRF analysis revealed that Tl is localized mainly at the base of the midrib and in the veins of leaves. This distribution differs greatly from that in other known Tl hyperaccumulators. Conclusions Our findings show that S. latifolia is among the strongest known Tl hyperaccumulators in the world. The species has ostensibly evolved mechanisms to survive excessive concentrations of Tl accumulated in its leaves, whilst maintaining lower Tl concentrations in the roots. This trait is of fundamental importance for developing future phytoextraction technologies using this species to remediate Tl-contaminated mine wastes.

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