Micro-analytical and molecular approaches for understanding the distribution, biochemistry, and molecular biology of selenium in (hyperaccumulator) plants.

MAIN CONCLUSION: Micro-analytical techniques to untangle Se distribution and chemical speciation in plants coupled with molecular biology analysis enable the deciphering of metabolic pathways responsible for Se tolerance and accumulation. Selenium (Se) is not essential for plants and is toxic at high concentrations. However, Se hyperaccumulator plants have evolved strategies to both tolerate and accumulate > 1000 µg Se g-1 DW in their living above-ground tissues. Given the complexity of the biochemistry of Se, various approaches have been adopted to study Se metabolism in plants. These include X-ray-based techniques for assessing distribution and chemical speciation of Se, and molecular biology techniques to identify genes implicated in Se uptake, transport, and assimilation. This review presents these techniques, synthesises the current state of knowledge on Se metabolism in plants, and highlights future directions for research into Se (hyper)accumulation and tolerance. We conclude that powerful insights may be gained from coupling information on the distribution and chemical speciation of Se to genome-scale studies to identify gene functions and molecular mechanisms that underpin Se tolerance and accumulation in these ecologically and biotechnologically important plants species. The study of Se metabolism is challenging and is a useful testbed for developing novel analytical approaches that are potentially more widely applicable to the study of the regulation of a wide range of metal(loid)s in hyperaccumulator plants.

Farming for battery metals.

Globally, there is a major shift to electric vehicles to combat climate change and these vehicles are currently powered by lithium-ion batteries that contain nickel cobalt manganese oxide materials. This technological change from internal combustion engines means that demand for battery minerals will need to increase by factors of >20 for the critical metals required for batteries in the next three decades. If this scenario plays out, it will require a dramatic increase in the worldwide capacity to produce nickel, manganese, cobalt, and lithium raw materials of sufficient purity. This demand could partly be met by agromining technology, which is a 'green technology' that extracts valuable products, including high-purity metal salts useful for the battery industry, from selected plants known as 'metal crops'. Farming for nickel, cobalt, and manganese is currently within reach, whereas lithium agromining has not yet been developed but has potential. SYNOPSIS: Agromining offers a sustainable approach to economically produce battery-grade raw materials from unconventional sources, thus, producing 'green technologies' from 'green sources'.

Treasure from trash: Mining critical metals from waste and unconventional sources.

To meet future technological demands of our growing global community new sources of industry critical metals need to be identified. To meet these demands, extracting minerals from larger, lower grade deposits across most commodities is required, which in turn generates ever increasing amounts of mine wastes. We propose that agromining could be used to enables access to unconventional resources not viable using existing minerals processing techniques. This innovative technique relies on so-called hyperaccumulator plants to bio-concentrate high levels of metals into living biomass which can then be extracted from the harvested bio-ore. Producing critical metals, such as nickel, cobalt and thallium, efficiently and sustainably using agromining appears to be well within reach, but this technology needs industrial champions to develop demonstration sites that are scaled appropiately in areas where it is feasible.

Distribution and chemical form of selenium in Neptunia amplexicaulis from Central Queensland, Australia.

Selenium (Se), a trace element essential for human and animal biological processes, is deficient in many agricultural soils. Some extremely rare plants can naturally accumulate extraordinarily high concentrations of Se. The native legume Neptunia amplexicaulis, endemic to a small area near Richmond and Hughenden in Central Queensland, Australia, is one of the strongest Se hyperaccumulators known on Earth, with foliar concentrations in excess of 4000 µg Se g-1 previously recorded. Here, we report on the Se distribution at a whole plant level using laboratory micro X-ray Fluorescence Microscopy (µXRF) and scanning electron microscopy (SEM-EDS), as well as on chemical forms of Se in various tissues using liquid chromatography-mass spectrometry (LC-MS) and synchrotron X-ray absorption spectroscopy (XAS). The results show that Se occurs in the forms of methyl-selenocysteine and seleno-methionine in the foliar tissues, with up to 13 600 µg Se g-1 total in young leaves. Selenium was found to accumulate primarily in the young leaves, flowers, pods and taproot, with lower concentrations present in the fine-roots and stem and the lowest present in the oldest leaves. Trichomes were not found to accumulate Se. We postulate that Se is (re)distributed in this plant via the phloem from older leaves to newer leaves, using the taproot as the main storage organ. High concentrations of Se in the nodes (pulvini) indicate this structure may play an important a role in Se (re)distribution. The overall pattern of Se distribution was similar in a non-Se tolerant closely related species (Neptunia gracilis), although the prevailing Se concentrations were substantially lower than in N. amplexicaulis.