Micronutrient homeostasis in plants for more sustainable agriculture and healthier human nutrition

Review: Nutrient-nutrient interactions governing underground plant adaptation strategies in a heterogeneous environment

Plant growth relies on the mineral nutrients present in the rhizosphere. The distribution of nutrients in soils varies depending on their mobility and capacity to bind with soil particles. Consequently, plants often encounter either low or high levels of nutrients in the rhizosphere. Plant roots are the essential organs that sense changes in soil mineral content, leading to the activation of signaling pathways associated with the adjustment of plant architecture and metabolic responses. During differential availability of minerals in the rhizosphere, plants trigger adaptation strategies such as cellular remobilization of minerals, secretion of organic molecules, and the attenuation or enhancement of root growth to balance nutrient uptake. The interdependency, availability, and uptake of minerals, such as phosphorus (P), iron (Fe), zinc (Zn), potassium (K), nitrogen (N) forms, nitrate (NO3-), and ammonium (NH4+), modulate the root architecture and metabolic functioning of plants. Here, we summarized the interactions of major nutrients (N, P, K, Fe, Zn) in shaping root architecture, physiological responses, genetic components involved, and address the current challenges associated with nutrient-nutrient interactions. Furthermore, we discuss the major gaps and opportunities in the field for developing plants with improved nutrient uptake and use efficiency for sustainable agriculture.

Nutritional status of Zombi pea (Vigna vexillata) as influenced by plant density and deblossoming

Feeding billions, a healthy and nutritious diet in the era of climate change is a major challenge before plant breeders, geneticists and agronomist. In this context, the continuous search for adaptive and nutritious crops could be a better alternative to combat the problems of hunger and malnutrition. The zombi pea, a nutritious and underutilized leguminous vegetable, is one of such better alternatives to feed billions a nutritious food besides being a potential gene source for breeding abiotic stress resistant varieties. To evaluate its potential as a wonder crop in the tropical and subtropical regions of India, the nutritional status of tubers, pods and pericarp were investigated under different treatments of plant spacings and deblossoming. The experiment was conducted in split plot design with three replications and eight treatments during 2021-2022 in the coastal regions of India. The nutrient profiling in tubers and pericarp of pods in zombi pea revealed higher accumulation of nutrients viz. potassium (K), magnesium (Mg), iron (Fe), manganese (Mn) and zinc (Zn) with blossom retention. The zombi pea tubers reflected significantly high protein accumulation with the increase in plant spacing. The results pertaining to nutrient profiling in the pods of zombi pea indicated that the plant spacing has no significant effect on the accumulation of majority of nutrients under study. The above-mentioned findings are conspicuously novel and valuable. The present study would pave the way for understanding nutritional importance and breeding potential of this orphan crop. The blossom retention renders higher nutrient accumulation in tubers, pods and pericarp of zombi pea. Deblossoming has no significant influence on nutritional profile of this wonder crop but, wider spacing is effective in producing tubers with high protein content.

Application of ZnO Nanoparticles Encapsulated in Mesoporous Silica on the Abaxial Side of a Solanum lycopersicum Leaf Enhances Zn Uptake and Translocation via the Phloem

Foliar application of nutrient nanoparticles (NPs) is a promising strategy for improving fertilization efficiency in agriculture. Phloem translocation of NPs from leaves is required for efficient fertilization but is currently considered to be feasible only for NPs smaller than a cell wall pore size exclusion limit of <20 nm. Using mass spectrometry imaging, we provide here the first direct evidence for phloem localization and translocation of a larger (∼70 nm) fertilizer NP comprised of ZnO encapsulated in mesoporous SiO2 (ZnO@MSN) following foliar deposition. The Si content in the phloem tissue of the petiole connected to the dosed leaf was ∼10 times higher than in the xylem tissue, and ∼100 times higher than the phloem tissue of an untreated tomato plant petiole. Direct evidence of NPs in individual phloem cells has only previously been shown for smaller NPs introduced invasively in the plant. Furthermore, we show that uptake and translocation of the NPs can be enhanced by their application on the abaxial (lower) side of the leaf. Applying ZnO@MSN to the abaxial side of a single leaf resulted in a 56% higher uptake of Zn as well as higher translocation to the younger (upper) leaves and to the roots, than dosing the adaxial (top) side of a leaf. The higher abaxial uptake of NPs is in alignment with the higher stomatal density and lower density of mesophyll tissues on that side and has not been demonstrated before.

The Mechanisms of Molybdate Distribution and Homeostasis with Special Focus on the Model Plant Arabidopsis thaliana

This review article deals with the pathways of cellular and global molybdate distribution in plants, especially with a full overview for the model plant Arabidopsis thaliana. In its oxidized state as bioavailable molybdate, molybdenum can be absorbed from the environment. Especially in higher plants, molybdenum is indispensable as part of the molybdenum cofactor (Moco), which is responsible for functionality as a prosthetic group in a variety of essential enzymes like nitrate reductase and sulfite oxidase. Therefore, plants need mechanisms for molybdate import and transport within the organism, which are accomplished via high-affinity molybdate transporter (MOT) localized in different cells and membranes. Two different MOT families were identified. Legumes like Glycine max or Medicago truncatula have an especially increased number of MOT1 family members for supplying their symbionts with molybdate for nitrogenase activity. In Arabidopsis thaliana especially, the complete pathway followed by molybdate through the plant is traceable. Not only the uptake from soil by MOT1.1 and its distribution to leaves, flowers, and seeds by MOT2-family members was identified, but also that inside the cell. the transport trough the cytoplasm and the vacuolar storage mechanisms depending on glutathione were described. Finally, supplying the Moco biosynthesis complex by MOT1.2 and MOT2.1 was demonstrated.

The power of magnesium: unlocking the potential for increased yield, quality, and stress tolerance of horticultural crops

Magnesium (Mg2+) is pivotal for the vitality, yield, and quality of horticultural crops. Central to plant physiology, Mg2+ powers photosynthesis as an integral component of chlorophyll, bolstering growth and biomass accumulation. Beyond basic growth, it critically affects crop quality factors, from chlorophyll synthesis to taste, texture, and shelf life. However, Mg2 + deficiency can cripple yields and impede plant development. Magnesium Transporters (MGTs) orchestrate Mg2+ dynamics, with notable variations observed in horticultural species such as Cucumis sativus, Citrullus lanatus, and Citrus sinensis. Furthermore, Mg2+ is key in fortifying plants against environmental stressors and diseases by reinforcing cell walls and spurring the synthesis of defense substances. A burgeoning area of research is the application of magnesium oxide nanoparticles (MgO-NPs), which, owing to their nanoscale size and high reactivity, optimize nutrient uptake, and enhance plant growth and stress resilience. Concurrently, modern breeding techniques provide insights into Mg2+ dynamics to develop crops with improved Mg2+ efficiency and resilience to deficiency. Effective Mg2+ management through soil tests, balanced fertilization, and pH adjustments holds promise for maximizing crop health, productivity, and sustainability. This review unravels the nuanced intricacies of Mg2+ in plant physiology and genetics, and its interplay with external factors, serving as a cornerstone for those keen on harnessing its potential for horticultural excellence.

Development and Evaluation of Zinc and Iron Nanoparticles Functionalized with Plant Growth-Promoting Rhizobacteria (PGPR) and Microalgae for Their Application as Bio-Nanofertilizers

Micronutrient deficiencies are widespread and growing global concerns. Nanoscale nutrients present higher absorption rates and improved nutrient availability and nutrient use efficiency. Co-application of nanofertilizers (NFs) with biological agents or organic compounds increases NF biocompatibility, stability, and efficacy. This study aimed to develop and evaluate zinc and iron bio-nanofertilizers formulated with plant growth-promoting rhizobacteria (PGPR) and microalgae. Nanoparticles (NPs) were synthesized with the co-precipitation method and functionalized with Pseudomonas species and Spirulina platensis preparation. NPs were characterized and evaluated on seed germination, soil microbial growth, and early plant response under seedbed conditions. NPs corresponded to zinc oxide (ZnO; 77 nm) and maghemite (γ-Fe2O3; 68 nm). Functionalized nanoparticles showed larger sizes, around 145-233 nm. The seedling vigor index of tomato and maize was significantly increased (32.9-46.1%) by bacteria-functionalized ZnO- and γ-Fe2O3-NPs at 75 ppm. NFs at 250 and 75 ppm significantly increased bacterial growth. NFs also improved early plant growth by increasing plant height (14-44%), leaf diameter (22-47%), and fresh weight (46-119%) in broccoli and radish, which were mainly influenced by bacteria capped ZnO- and γ-Fe2O3-NPs at 250 ppm. Beneficial effects on plant growth can be attributed to the synergistic interaction of the biological components and the zinc and iron NPs in the bio-nanofertilizers.

Biofortification of Broccoli Microgreens (Brassica oleracea var. italica) with Glucosinolates, Zinc, and Iron through the Combined Application of Bio- and Nanofertilizers

There is a severe need to develop a sustainable, affordable, and nutritious food supply system. Broccoli microgreens have attracted attention due to their rich nutritional content and abundant bioactive compounds, constituting an important opportunity to feed the ever-increasing population and fight global health problems. This study aimed to measure the impact of the combined application of biofertilizers and zinc and iron nanofertilizers on plant growth and the biofortification of glucosinolates (GLSs) and micronutrients in broccoli microgreens. Biofertilizers were based on plant growth-promoting (PGP) bacterial consortia previously isolated and characterized for multiple PGP traits. Nanofertilizers consisted of ZnO (77 nm) and γ-Fe2O3 (68 nm) nanoparticles synthesized with the coprecipitation method and functionalized with a Pseudomonas species preparation. Treatments were evaluated under seedbed conditions. Plant growth parameters of plant height (37.0-59.8%), leaf diameter (57.6-81.1%) and fresh weight (112.1-178.0%), as well as zinc (122.19-363.41%) and iron contents (55.19-161.57%), were mainly increased by nanoparticles subjected to the functionalization process with Pseudomonas species and uncapped NPs applied together with the biofertilizer treatment. Regarding GLSs, eight compounds were detected as being most positively influenced by these treatments. This work demonstrated the synergistic interactions of applying ZnO and γ-Fe2O3 nanofertilizers combined with biofertilizers to enhance plant growth and biofortify micronutrients and glucosinolates in broccoli microgreens.

Iron and zinc homeostasis in plants: a matter of trade-offs

This article comments on:

Stanton C, Rodríguez-Celma J, Krämer U, Sanders D, Balk J. 2023. BRUTUS-LIKE (BTSL) E3 ligase-mediated fine-tuning of Fe regulation negatively affects Zn tolerance of Arabidopsis. Journal of Experimental Botany 74, 5767–5782.

Role of Soil and Foliar-Applied Carbon Dots in Plant Iron Biofortification and Cadmium Mitigation by Triggering Opposite Iron Signaling in Roots

In China, iron (Fe) availability is low in most soils but cadmium (Cd) generally exceeds regulatory soil pollution limits. Thus, biofortification of Fe along with mitigation of Cd in edible plant parts is important for human nutrition and health. Carbon dots (CDs) are considered as potential nanomaterials for agricultural applications. Here, Salvia miltiorrhiza-derived CDs are an efficient modulator of Fe, manganese (Mn), zinc (Zn), and Cd accumulation in plants. CDs irrigation (1 mg mL-1 , performed every week starting at the jointing stage for 12 weeks) increased Fe content by 18% but mitigated Cd accumulation by 20% in wheat grains. This finding was associated with the Fe3+ -mobilizing properties of CDs from the soil and root cell wall, as well as endocytosis-dependent internalization in roots. The resulting excess Fe signaling mitigated Cd uptake via inhibiting TaNRAMP5 expression. Foliar spraying of CDs enhanced Fe (44%), Mn (30%), and Zn (19%) content with an unchanged Cd accumulation in wheat grains. This result is attributed to CDs-enhanced light signaling, which triggered shoot-to-root Fe deficiency response. This study not only reveals the molecular mechanism underlying CDs modulation of Fe signaling in plants but also provides useful strategies for concurrent Fe biofortification and Cd mitigation in plant-based foods.

Heavy metal(loid)s contaminations in soils of Pakistan: a review for the evaluation of human and ecological risks assessment and spatial distribution

Heavy metal(loid)s (HM) contaminations in the soil poses threats to the human and ecological community due to their bioaccumulation, toxicity, and persistent nature in the ecosystem. This review was designed to know about the HM contamination in soils, ecological risk, distribution, and potential health risks. Soil HM concentrations published in the last 30 years were collected from Springer, Science Direct, Willey, Mendeley, ResearchGate, Google Scholar, etc. HM concentrations were used for the geo-accumulation index (Igeo), contamination factor, as well as integrated indices such as spatial distribution of ecological risk index. Similarly, the Igeo pattern was observed in Sindh > Baluchistan > Punjab > Khyber Pakhtunkhwa > Gilgit-Baltistan > Islamabad. Moreover, the high ecological risk mean values ranged (160 < ERI < 320) due to cadmium (Cd) was exhibited in the Punjab and Khyber Pakhtunkhwa provinces and Islamabad. Non-carcinogenic risk like hazard quotient was found higher for children (1.59) of Punjab due to arsenic (As) ingestion, whereas the lower risk was observed due to Zn (2.5E-08) for adults of Punjab province via inhalation pathway. Similarly, the health index (HI) from exposure to As (1.61) in soil was higher than the rest of the HM. Moreover, cancerous risk was determined and found in the tolerable range (10^-4-10^-6). This study recommended that HM contaminants in the soil need to be monitored on regular basis, especially in Baluchistan, Gilgit-Baltistan, and Sindh provinces.

Functional characterization and molecular fingerprinting of potential phosphate solubilizing bacterial candidates from Shisham rhizosphere

Phosphate solubilizing bacteria (PSB) are important role players in plant growth promotion. In the present study, we aimed to screen the functionally active phosphate solubilizing bacteria (PSB) associated with Dalbergia sissoo Roxb. (Shisham) from different provenances. Screening for phosphate solubilization was done on Pikovskaya's agar, and 18 bacteria positive for the tri-calcium phosphate (Ca₃(PO₄)₂ solubilization showing visible dissolution halo zones were identified. All 18 isolates showed zinc solubilization, indole acetic acid (IAA), siderophore, and hydrogen cyanide (HCN) production. The morphological and biochemical characterization with 16S rDNA gene-based phylogenetic analysis identified bacterial strains as belonging to the genus Pseudomonas, Klebsiella, Streptomyces, Pantoea, Kitasatospora, Micrococcus, and Staphylococcus. Among all the isolates, one of the isolates named L4, from Lacchiwala region was the most efficient P solubilizer with a high P solubilization index (4.75 ± 0.06) and quantitative P solubilization activity (891.38 ± 18.55 μg mL^-1). The validation of phosphate solubilization activity of PSB isolates was done by amplification of the Pyrroloquinoline quinone (PQQ) genes, pqqA and pqqC. Based on this study, we have selected the bacterial strains which are efficient phosphate solubilizers and could be economical and eco-friendly in plant growth promotion, disease suppression, as an antioxidant, and for subsequent enhancement of yield.

Protein metalation in a nutshell

Metalation, the acquisition of metals by proteins, must avoid mis-metalation with tighter binding metals. This is illustrated by four selected proteins that require different metals: all show similar ranked orders of affinity for bioavailable metals, as described in a universal affinity series (the Irving-Williams series). Crucially, cellular protein metalation occurs in competition with other metal binding sites. The strength of this competition defines the intracellular availability of each metal: its magnitude has been estimated by calibrating a cells' set of DNA-binding, metal-sensing, transcriptional regulators. This has established that metal availabilities (as free energies for forming metal complexes) are maintained to the inverse of the universal series. The tightest binding metals are least available. With these availabilities, correct metalation is achieved.

Effects of 12-Year Nitrogen Addition and Mowing on Plant-Soil Micronutrients in a Typical Steppe

Changes in soil micronutrient availability may have adverse consequences on grassland productivity, yet it’s still largely unclear how concurrent human practices, such as fertilization and mowing, affect micronutrient cycling in the plant-soil systems. Here, we measured six essential micronutrient (Fe, Mn, Cu, Zn, Co and Mo) contents in both plant pool (separated as aboveground plant parts, litter, and belowground roots) at the community level and soil pool (0−10 cm depth) after 12-year consecutive nitrogen (N) addition (0, 2, 10, and 50 g N m−2 year−1) and mowing in a typical steppe of the Mongolian Plateau. The results show that (i) medium-N (10 g m−2 year−1) and high-N (50 g m−2 year−1) addition rates significantly increased contents of soil-available Fe (+310.0%, averaging across the two N addition rates), Mn (+149.2%), Co (+123.6%) and Mo (+73.9%) irrespective of mowing treatment, whereas these addition treatments usually decreased contents of soil total Fe (−8.9%), Mn (−21.6%), Cu (−15.9%), Zn (−19.5%), Co (−16.4%) and Mo (−34.7%). (ii) Contents of Fe in aboveground plant parts, litter, and roots significantly decreased, whereas plant Mn increased with N addition. Contents of above ground plant Cu, Zn, Co, and Mo significantly decreased at high-N addition rate, whereas contents of micronutrients in roots and litters, except for Fe, generally increased with N addition. Moreover, the total amount of micronutrients in the plant pool (contents × biomass) significantly increased at the medium-N addition rate but decreased at the high-N addition rate. All N addition rates significantly enlarged the pool of litter micronutrients, and roots could hold more micronutrients under N addition, especially combined with mowing treatment. Importantly, although mowing could regulate the effects of N addition on variables (i) and (ii), the effects were weaker overall than those of N addition. (iii) Changes in root micronutrients, except for Mn, could explain corresponding changes in plant micronutrients (R2: 0.19−0.56, all p < 0.01), and significant linear correlations were also observed between soil-available Fe and Fe in plant and roots. Aboveground plant Mn was significantly correlated with soil-available Mn, while Co and Mo in roots were also significantly correlated with soil-available Co and Mo. These results indicate that soil micronutrient supply capacity may decrease due to a decrease in total micronutrient contents after long-term N addition and mowing. They also suggest that different magnitude responses of soil micronutrients in plants (i.e., litters, roots) and soil should be considered when comprehensively examining nutrient cycling in grassland ecosystems.

The F-bZIP-regulated Zn deficiency response in land plants

This review describes zinc sensing and transcriptional regulation of the zinc deficiency response in Arabidopsis, and discusses how their evolutionary conservation in land plants facilitates translational approaches for improving the Zn nutritional value of crop species. Zinc is an essential micronutrient for all living organisms due to its presence in a large number of proteins, as a structural or catalytic cofactor. In plants, zinc homeostasis mechanisms comprise uptake from soil, transport and distribution throughout the plant to provide adequate cellular zinc availability. Here, I discuss the transcriptional regulation of the response to zinc deficiency and the zinc sensing mechanisms in Arabidopsis, and their evolutionary conservation in land plants. The Arabidopsis F-group basic region leucine-zipper (F-bZIP) transcription factors bZIP19 and bZIP23 function simultaneously as sensors of intracellular zinc status, by direct binding of zinc ions, and as the central regulators of the zinc deficiency response, with their target genes including zinc transporters from the ZRT/IRT-like Protein (ZIP) family and nicotianamine synthase enzymes that produce the zinc ligand nicotianamine. I note that this relatively simple mechanism of zinc sensing and regulation, together with the evolutionary conservation of F-bZIP transcription factors across land plants, offer important research opportunities. One of them is to use the F-bZIP-regulated zinc deficiency response as a tractable module for evolutionary and comparative functional studies. Another research opportunity is translational research in crop plants, modulating F-bZIP activity as a molecular switch to enhance zinc accumulation. This should become a useful plant-based solution to alleviate effects of zinc deficiency in soils, which impact crop production and crop zinc content, with consequences for human nutrition globally.

Age Dependent Partitioning Patterns of Essential Nutrients Induced by Copper Feeding Status in Leaves and Stems of Poplar

Copper (Cu) is an essential micronutrient, and its deficiency can cause plants to undergo metabolic changes at several levels of organization. It has been shown that leaf age can play a role in nutrient partitioning along the shoot axis of poplar. In this study, we investigated the effect of Cu deficiency on the altered partitioning of essential macro and micronutrients in leaves and stems of different age. Cu deficiency was associated with higher concentrations of calcium, magnesium, sulfur, iron, zinc, manganese, and molybdenum in leaves and relatively higher concentrations of calcium, phosphorus, iron, and zinc in stems. Leaf and stem age had significant effects on nutrient partitioning. Principal component analyses revealed patterns that point to inverse influences in leaves and stems on nutrient partitioning. Specifically, these analyses revealed that nutrient partitioning in leaves was influenced by Cu feeding status more than developmental stage, whereas nutrient partitioning in stems was influenced by developmental stage more than Cu feeding status. These results suggest that Cu deficiency and developmental stage can significantly influence the partitioning and homeostasis of macro and micronutrients in poplar organs.

Impact of polyploidy on plant tolerance to abiotic and biotic stresses

Polyploidy, defined as the coexistence of three or more complete sets of chromosomes in an organism's cells, is considered as a pivotal moving force in the evolutionary history of vascular plants and has played a major role in the domestication of several crops. In the last decades, improved cultivars of economically important species have been developed artificially by inducing autopolyploidy with chemical agents. Studies on diverse species have shown that the anatomical and physiological changes generated by either natural or artificial polyploidization can increase tolerance to abiotic and biotic stresses as well as disease resistance, which may positively impact on plant growth and net production. The aim of this work is to review the current literature regarding the link between plant ploidy level and tolerance to abiotic and biotic stressors, with an emphasis on the physiological and molecular mechanisms responsible for these effects, as well as their impact on the growth and development of both natural and artificially generated polyploids, during exposure to adverse environmental conditions. We focused on the analysis of those types of stressors in which more progress has been made in the knowledge of the putative morpho-physiological and/or molecular mechanisms involved, revealing both the factors in common, as well as those that need to be addressed in future research.