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Abstract. The cellular compartmentation of elements was analysed in the Zn hyperaccumulator Arabidopsis halleri (L.) O'Kane & Al-Shehbaz (=Cardaminopsis halleri) using energy-dispersive X-ray microanalysis of frozen-hydrated tissues. Quantitative data were obtained using oxygen as an internal standard in the analyses of vacuoles, whereas a peak/background ratio method was used for quanti®cation of elements in pollen and dehydrated trichomes. Arabidopsis halleri was found to hyperaccumulate not only Zn but also Cd in the shoot biomass. While large concentrations of Zn and Cd were found in the leaves and roots,¯owers contained very little. In roots grown hydroponically, Zn and Cd accumulated in the cell wall of the rhizodermis (root epidermis), mainly due to precipitation of Zn/Cd phosphates. In leaves, the trichomes had by far the largest concentrations of Zn and Cd. Inside the trichomes there was a striking sub-cellular compartmentation, with almost all the Zn and Cd being accumulated in a narrow ring in the trichome base. This distribution pattern was very dierent from that for Ca and P. The epidermal cells other than trichomes were very small and contained lower concentrations of Zn and Cd than mesophyll cells. In particular, the concentrations of Cd and Zn in the mesophyll cells increased markedly in response to increasing Zn and Cd concentrations in the nutrient solution. This indicates that the mesophyll cells in the leaves of A. halleri are the major storage site for Zn and Cd, and play an important role in their hyperaccumulation.

Nanoenabled foliar-applied agrochemicals can potentially be safer and more efficient than conventional products. However, limited understanding about how nanoparticle properties influence their interactions with plant leaves, uptake, translocation through the mesophyll to the vasculature, and transport to the rest of the plant prevents rational design. This study used a combination of Au quantification and spatial analysis to investigate how size (3, 10, or 50 nm) and coating chemistry (PVP versus citrate) of gold nanoparticles (AuNPs) influence these processes. Following wheat foliar exposure to AuNPs suspensions (∼280 ng per plant), adhesion on the leaf surface was increased for smaller sizes, and PVP-AuNPs compared to citrate-AuNPs. After 2 weeks, there was incomplete uptake of citrate-AuNPs with some AuNPs remaining on the outside of the cuticle layer. However, the fraction of citrate-AuNPs that had entered the leaf was translocated efficiently to the plant vasculature. In contrast, for similar sizes, virtually all of the PVP-AuNPs crossed the cuticle layer after 2 weeks, but its transport through the mesophyll cells was lower. As a consequence of PVP-AuNP accumulation in the leaf mesophyll, wheat photosynthesis was impaired. Regardless of their coating and sizes, the majority of the transported AuNPs accumulated in younger shoots (10−30%) and in roots (10−25%), and 5−15% of the NPs <50 nm were exuded into the rhizosphere soil. A greater fraction of larger sizes AuNPs (presenting lower ζ potentials) was transported to the roots. The key hypotheses about the NPs physical−chemical and plant physiology parameters that may matter to predict leaf-to-rhizosphere transport are also discussed.

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Nickel uptake and cellular compartmentation were investigated in three Ni hyperaccumulators: Alyssum bertolonii (Desv), Alyssum lesbiacum (Candargy) and Thlaspi goesingense (Hálácsy). The three species showed similar hyperaccumulation of Ni, but T. goesingense was less tolerant to Ni than the two Alyssum species. An addition of 500 mg Ni kg(-1) to a nutrient-rich growth medium significantly increased shoot biomass of all three species, suggesting that the Ni hyperaccumulators have a higher requirement for Ni than normal plants. Energy-dispersive X-ray microanalysis (EDXA) was performed on frozen-hydrated tissues of leaves (all species) and stems (Alyssum only). In all species analysed, Ni was distributed preferentially in the epidermal cells, most likely in the vacuoles, of the leaves and stems. In stems, there was a second peak of Ni in the boundary cells between the cortical parenchyma and the vascular cylinder. The non-glandular trichomes on the leaf surfaces of the two Alyssum species were highly enriched with Ca, but contained little Ni except in the base. In the leaves of T. goesingense, the large elongated epidermal cells contained more Ni than the cells of the stomatal complexes. The role of cellular compartmentation in Ni hyperaccumulation is discussed.

BackgroundRice is the primary source of food for billions of people in developing countries, yet the commonly consumed polished grain contains insufficient levels of the key micronutrients iron (Fe), zinc (Zn) and Vitamin A to meet daily dietary requirements. Experts estimate that a rice-based diet should contain 14.5 µg g−1 Fe in endosperm, the main constituent of polished grain, but breeding programs have failed to achieve even half of that value. Transgenic efforts to increase the Fe concentration of rice endosperm include expression of ferritin genes, nicotianamine synthase genes (NAS) or ferritin in conjunction with NAS genes, with results ranging from two-fold increases via single-gene approaches to six-fold increases via multi-gene approaches, yet no approach has reported 14.5 µg g−1 Fe in endosperm.Methodology/Principal FindingsThree populations of rice were generated to constitutively overexpress OsNAS1, OsNAS2 or OsNAS3, respectively. Nicotianamine, Fe and Zn concentrations were significantly increased in unpolished grain of all three of the overexpression populations, relative to controls, with the highest concentrations in the OsNAS2 and OsNAS3 overexpression populations. Selected lines from each population had at least 10 µg g−1 Fe in polished grain and two OsNAS2 overexpression lines had 14 and 19 µg g−1 Fe in polished grain, representing up to four-fold increases in Fe concentration. Two-fold increases of Zn concentration were also observed in the OsNAS2 population. Synchrotron X-ray fluorescence spectroscopy demonstrated that OsNAS2 overexpression leads to significant enrichment of Fe and Zn in phosphorus-free regions of rice endosperm.ConclusionsThe OsNAS genes, particularly OsNAS2, show enormous potential for Fe and Zn biofortification of rice endosperm. The results demonstrate that rice cultivars overexpressing single rice OsNAS genes could provide a sustainable and genetically simple solution to Fe and Zn deficiency disorders affecting billions of people throughout the world.

The capacity to accumulate cadmium (Cd) and zinc (Zn) was compared in Thlaspi goesingense and four populations of Thlaspi caerulescens. Two populations of T. caerulescens were grown in hydroponics at five concentrations of Cd. In addition, plants were grown in pots containing compost in which three different concentrations of Cd and two concentrations of Zn were added. A field trial was conducted to compare Zn and Cd uptake by three populations of T. caerulescens on nine selected plots of the Woburn Market Garden Experiment (UK) which had been contaminated to different degrees with heavy metals owing to past applications of sewage sludge. Results show that the four populations of T. caerulescens had the same ability to hyperaccumulate Zn but were significantly different in terms of Cd accumulation. Two populations of T. caerulescens from Southern France accumulated much more Cd than the populations from Prayon (Belgium) and Whitesike (UK). Generally, uptake of Cd was not decreased by increased concentrations of Zn in the substrate. These results indicate that the mechanisms of Cd and Zn hyperaccumulation are not identical in this species. This is the first report of hyperaccumulation of Cd by T. goesingense, but the growth of this species was markedly reduced by the large concentrations of Zn in the substrate. Future work should focus on the differences between Cd and Zn uptake in hyperaccumulator plants at the species and population level.

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