Overexpression of METAL TOLERANCE PROTEIN8 reveals new aspects of metal transport in Arabidopsis thaliana seeds

MTP family analysis and association study reveal the role of ZmMTP11 in lead (Pb) accumulation

The metal tolerance protein (MTP) gene family plays an essential role in the transport of heavy metals, however the function of the MTP family in transporting lead (Pb) was still unclear in plants. In this study, we identified and characterized 12 ZmMTPs in the whole genome of maize. These ZmMTP genes were divided into three subfamilies in evolution, namely Zn-CDF, Zn/Fe-CDF, Mn-CDF subfamilies, which showed diverse expression patterns in different tissues of maize. Using gene-based association analyses, we identified a Pb accumulation-related MTP member in maize, ZmMTP11, which was located in plasma membrane and had the potential of transporting Pb ion. Under the Pb treatment, ZmMTP11 showed a generally decreased expression relative to the normal conditions. Heterologous expressions of ZmMTP11 in yeast, Arabidopsis, and rice demonstrated that ZmMTP11 enhanced Pb accumulation in the cells without affecting yeast and plant growth under Pb stress. Remarkably, the increased Pb concentration in the plant roots did not cause changes in Pb content in the shoots. Our study provides new insights into the genetic improvement of heavy metal tolerance in plants and contributes to bioremediation of Pb-contaminant soils.

Using an embryo specific promoter to modify iron distribution pattern in Arabidopsis

Iron is an essential micronutrient for life. During the development of the seed, iron accumulates during embryo maturation. In Arabidopsis thaliana, iron mainly accumulates in the vacuoles of only one cell type, the cell layer that surrounds provasculature in hypocotyl and cotyledons. Iron accumulation pattern in Arabidopsis is an exception in plant phylogeny, most part of the dicot embryos accumulate iron in several cell layers including cortex and, in some cases, even in protodermis. It remains unknown how does iron reach the internal cell layers of the embryo, and in particular, the molecular mechanisms responsible of this process. Here, we use transgenic approaches to modify the iron accumulation pattern in an Arabidopsis model. Using the SDH2-3 embryo-specific promoter, we were able to express VIT1 ectopically in both a wild type background and a mutant vit1 background lacking expression of this vacuolar iron transporter. These manipulations modify the iron distribution pattern in Arabidopsis from one cell layer to several cell layers, including protodermis, cortex cells, and the endodermis. Interestingly, total seed iron content was not modified compared with the wild type, suggesting that iron distribution in embryos is not involved in the control of the total iron amount accumulated in seeds. This experimental model can be used to study the processes involved in iron distribution patterning during embryo maturation and its evolution in dicot plants.

The Malus domestica metal tolerance protein MdMTP11.1 was involved in the detoxification of excess manganese in Arabidopsis thaliana

Ion homeostasis is maintained in plant cells by specialized transporters. However, functional studies on Mn transporters in apple trees have not been reported. MdMTP11.1, which encodes a putative Mn-MTP transporter in Malus domestica, was expressed highly in leaves and induced by Mn stress. Subcellular localization analysis of the MdMTP11.1-GFP fusion protein indicated that MdMTP11.1 was targeted to the Golgi. Meanwhile, overexpression of MdMTP11.1 in Arabidopsis thaliana conferred increased resistance to plants under toxic Mn levels, as evidenced by increased biomass of whole plant and length of primary root. Analysis of Mn bioaccumulation indicated that overexpression of MdMTP11.1 effectively reduced the content of Mn in every subcellular component and chemical forms when the plants were subjected with Mn stress. The majority of Mn of action were bound to cell wall and combined with un-dissolved phosphate. Besides, contents of malondialdehyde (MDA), proline and hydrogen peroxide (H2O2) were significantly lower, while content of chlorophyll and activities of CAT, SOD, POD and APX were significantly higher in MdMTP11.1-over-expressing plants compared with that in wild type plants under Mn stress. Taken together, these results suggest that MdMTP11.1 is a Mn specific transporter localized to the Golgi can maintain the phenotype, reduce the Mn accumulation and alleviate damage of oxidative stress, conferring the positive role of Mn tolerance.

Cation diffusion facilitator proteins of Beta vulgaris reveal diversity of metal handling in dicotyledons

Manganese (Mn), iron (Fe), and zinc (Zn) are essential for diverse processes in plants, but their availability is often limiting or excessive. Cation diffusion facilitator (CDF) proteins have been implicated in the allocation of those metals in plants, whereby most of our mechanistic understanding has been obtained in Arabidopsis. It is unclear to what extent this can be generalized to other dicots. We characterized all CDFs/metal tolerance proteins of sugar beet (Beta vulgaris spp. vulgaris), which is phylogenetically distant from Arabidopsis. Analysis of subcellular localization, substrate selectivities, and transcriptional regulation upon exposure to metal deficiencies and toxicities revealed unexpected deviations from their Arabidopsis counterparts. Localization and selectivity of some members were modulated by alternative splicing. Notably, unlike in Arabidopsis, Mn- and Zn-sequestrating members were not induced in Fe-deficient roots, pointing to differences in the Fe acquisition machinery. This was supported by low Zn and Mn accumulation under Fe deficiency and a strikingly increased Fe accumulation under Mn and Zn excess, coinciding with an induction of BvIRT1. High Zn load caused a massive upregulation of Zn-BvMTPs. The results suggest that the employment of the CDF toolbox is highly diverse amongst dicots, which questions the general applicability of metal homeostasis models derived from Arabidopsis.

Biofortification of Crops to Fight Anemia: Role of Vacuolar Iron Transporters

Plant-based foods provide all the crucial nutrients for human health. Among these, iron (Fe) is one of the essential micronutrients for plants and humans. A lack of Fe is a major limiting factor affecting crop quality, production, and human health. There are people who suffer from various health problems due to the low intake of Fe in their plant-based foods. Anemia has become a serious public health issue due to Fe deficiency. Enhancing Fe content in the edible part of food crops is a major thrust area for scientists worldwide. Recent progress in nutrient transporters has provided an opportunity to resolve Fe deficiency or nutritional problems in plants and humans. Understanding the structure, function, and regulation of Fe transporters is essential to address Fe deficiency in plants and to improve Fe content in staple food crops. In this review, we summarized the role of Fe transporter family members in the uptake, cellular and intercellular movement, and long-distance transport of Fe in plants. We draw insights into the role of vacuolar membrane transporters in the crop for Fe biofortification. We also provide structural and functional insights into cereal crops' vacuolar iron transporters (VITs). This review will help highlight the importance of VITs for improving the Fe biofortification of crops and alleviating Fe deficiency in humans.

The role of metal transporters in phytoremediation: A closer look at Arabidopsis

Pollution of the environment by heavy metals (HMs) has recently become a global issue, affecting the health of all living organisms. Continuous human activities (industrialization and urbanization) are the major causes of HM release into the environment. Over the years, two methods (physical and chemical) have been widely used to reduce HMs in polluted environment. However, these two methods are inefficient and very expensive to reduce the HMs released into the atmosphere. Alternatively, researchers are trying to remove the HMs by employing hyper-accumulator plants. This method, referred to phytoremediation, is highly efficient, cost-effective, and eco-friendly. Phytoremediation can be divided into five types: phytostabilization, phytodegradation, rhizofiltration, phytoextraction, and phytovolatilization, all of which contribute to HMs removal from the polluted environment. Brassicaceae family members (particularly Arabidopsis thaliana) can accumulate more HMs from the contaminated environment than those of other plants. This comprehensive review focuses on how HMs pollute the environment and discusses the phytoremediation measures required to reduce the impact of HMs on the environment. We discuss the role of metal transporters in phytoremediation with a focus on Arabidopsis. Then draw insights into the role of genome editing tools in enhancing phytoremediation efficiency. This review is expected to initiate further research to improve phytoremediation by biotechnological approaches to conserve the environment from pollution.

The trans-Golgi-localized protein BICAT3 regulates manganese allocation and matrix polysaccharide biosynthesis

Manganese (Mn2+) is essential for a diversity of processes, including photosynthetic water splitting and the transfer of glycosyl moieties. Various Golgi-localized glycosyltransferases that mediate cell wall matrix polysaccharide biosynthesis are Mn2+ dependent, but the supply of these enzymes with Mn2+ is not well understood. Here, we show that the BIVALENT CATION TRANSPORTER 3 (BICAT3) localizes specifically to trans-cisternae of the Golgi. In agreement with a role in Mn2+ and Ca2+ homeostasis, BICAT3 rescued yeast (Saccharomyces cerevisiae) mutants defective in their translocation. Arabidopsis (Arabidopsis thaliana) knockout mutants of BICAT3 were sensitive to low Mn2+ and high Ca2+ availability and showed altered accumulation of these cations. Despite reduced cell expansion and leaf size in Mn2+-deficient bicat3 mutants, their photosynthesis was improved, accompanied by an increased Mn content of chloroplasts. Growth defects of bicat3 corresponded with an impaired glycosidic composition of matrix polysaccharides synthesized in the trans-Golgi. In addition to the vegetative growth defects, pollen tube growth of bicat3 was heterogeneously aberrant. This was associated with a severely reduced and similarly heterogeneous pectin deposition and caused diminished seed set and silique length. Double mutant analyses demonstrated that the physiological relevance of BICAT3 is distinct from that of ER-TYPE CA2+-ATPASE 3, a Golgi-localized Mn2+/Ca2+-ATPase. Collectively, BICAT3 is a principal Mn2+ transporter in the trans-Golgi whose activity is critical for specific glycosylation reactions in this organelle and for the allocation of Mn2+ between Golgi apparatus and chloroplasts.

The vacuolar transporter LaMTP8.1 detoxifies manganese in leaves of Lupinus albus

Manganese (Mn) is an essential microelement, but overaccumulation is harmful to many plant species. Most plants have similar minimal Mn requirements, but the tolerance to elevated Mn varies considerably. Mobilization of phosphate (P) by plant roots leads to increased Mn uptake, and shoot Mn levels have been reported to serve as an indicator for P mobilization efficiency in the presence of P deficiency. White lupin (Lupinus albus L.) mobilizes P and Mn with outstanding efficiency due to the formation of determinate cluster roots that release carboxylates. The high Mn tolerance of L. albus goes along with shoot Mn accumulation, but the molecular basis of this detoxification mechanism has been unknown. In this study, we identify LaMTP8.1 as the transporter mediating vacuolar sequestration of Mn in the shoot of white lupin. The function of Mn transport was demonstrated by yeast complementation analysis, in which LaMTP8.1 detoxified Mn in pmr1∆ mutant cells upon elevated Mn supply. In addition, LaMTP8.1 also functioned as an iron (Fe) transporter in yeast assays. The expression of LaMTP8.1 was particularly high in old leaves under high Mn stress. However, low P availability per se did not result in transcriptional upregulation of LaMTP8.1. Moreover, LaMTP8.1 expression was strongly upregulated under Fe deficiency, where it was accompanied by Mn accumulation, indicating a role in the interaction of these micronutrients in L. albus. In conclusion, the tonoplast-localized Mn transporter LaMTP8.1 mediates Mn detoxification in leaf vacuoles, providing a mechanistic explanation for the high Mn accumulation and Mn tolerance in this species.

Size- and Ligand-Dependent Transport of Nanoparticles in Matricaria chamomilla as Demonstrated by Mass Spectroscopy and X-ray Fluorescence Imaging

Matricaria chamomilla flowers were incubated with gold nanoparticles of different sizes ranging from 1.4 to 94 nm. After different incubation times of 6, 12, 24, and 48 h, the gold distribution in the flowers was destructively measured by inductively coupled plasma mass spectrometry (ICP-MS) and non-destructively measured by X-ray fluorescence imaging (XFI) with high lateral resolution. As a control, the biodistribution of iodine ions or iodine-containing organic molecules (iohexol) was determined, in order to demonstrate the feasibility of mapping the distribution of several elements in parallel. The results show a clear size-dependent transport of the nanoparticles. In addition, the surface chemistry also plays a decisive role in disposition. Only the 1.6 nm nanoparticles coated with acetylcysteine could be efficiently transported through the stem of the flowers into the petals. In this case, almost 80% of the nanoparticles which were found within each flower were located in the petals. The study also highlights the potential of XFI for in situ recording of in vivo analyte biodistribution.

B3 Transcription Factors Determine Iron Distribution and FERRITIN Gene Expression in Embryo but Do Not Control Total Seed Iron Content

Iron is an essential micronutrient for humans and other organisms. Its deficiency is one of the leading causes of anemia worldwide. The world health organization has proposed that an alternative to increasing iron content in food is through crop biofortification. One of the most consumed part of crops is the seed, however, little is known about how iron accumulation in seed occurs and how it is regulated. B3 transcription factors play a critical role in the accumulation of storage compounds such as proteins and lipids. Their role in seed maturation has been well characterized. However, their relevance in accumulation and distribution of micronutrients like iron remains unknown. In Arabidopsis thaliana and other plant models, three master regulators belonging to the B3 transcription factors family have been identified: FUSCA3 (FUS3), LEAFY COTYLEDON2 (LEC2), and ABSCISIC ACID INSENSITIVE 3 (ABI3). In this work, we studied how seed iron homeostasis is affected in B3 transcription factors mutants using histological and molecular approaches. We determined that iron distribution is modified in abi3, lec2, and fus3 embryo mutants. For abi3-6 and fus3-3 mutant embryos, iron was less accumulated in vacuoles of cells surrounding provasculature compared with wild type embryos. lec2-1 embryos showed no difference in the pattern of iron distribution in hypocotyl, but a dramatic decrease of iron was observed in cotyledons. Interestingly, for the three mutant genotypes, total iron content in dry mutant seeds showed no difference compared to wild type. At the molecular level, we showed that genes encoding the iron storage ferritins proteins are misregulated in mutant seeds. Altogether our results support a role of the B3 transcription factors ABI3, LEC2, and FUS3 in maintaining iron homeostasis in Arabidopsis embryos.