Iron Transport across Symbiotic Membranes of Nitrogen-Fixing Legumes

Iron sensing, signalling and acquisition by microbes and plants under environmental stress: Use of iron-solubilizing bacteria in crop biofortification for sustainable agriculture

Iron is very crucial micronutrient prerequisite for growth of all cellular organisms including plants, microbes, animals and humans. Though iron (Fe) is present in abundance in earth's crust, but most of its forms present in soil are biologically unavailable, thus putting a constraint to utilize it. Plants and microorganisms maintain iron homeostasis to balance the supply of enough Fe for metabolism from their surrounding environments and to avoid excessive toxic levels. Microorganisms and plants employ different strategies for sensing, signaling, transportation and uptake of Fe under different types of stressed environments. Microbial communities present in soil and vicinity of roots contribute in biogeochemical cycling and uptake of different nutrients including Fe resulting into improved soil fertility and plant health. In this review, the regulation of iron uptake and transport under different kinds of biotic and abiotic stresses is described. In addition, the insights have been provided for enhancing bioavailability of Fe in sustainable agriculture practices. The inoculation of different crop plants with iron solubilizing microbes improved bioavailablilty of Fe in soil and increased plant growth and crop yield. Insights were provided about possible role of recent bioengineering techniques to improve Fe availability and uptake by plants. However, well-planned and large-scale field trials are required before recommending particular iron solubilizing microbes as biofertilizers for increasing Fe availability, improving plant development and crop yields in sustainable agriculture.

Nano-Selenium Elevating Leaf Quality and Growth Via Microbial-Regulating Nitrogen Availability Under Ammonium and Nitrate Spraying in Tea Plants

Nano-selenium fertilizers can promote plant growth and nitrogen availability. However, little information is available on the effects of nano-selenium on tea leaf quality, soil nutrient availability and associated microbe-driven mechanisms. This study examined the effects of nano-selenium on the tea leaf quality and soil nitrogen cycling in 20-year-old tea plantations when the leaves were sprayed with ammonium or nitrate. Leaf selenium and amino acid contents increased ninefold and 9%, respectively, with nano-selenium in "Zhongcha108" and "Longjing43." Rhizosphere bacterial and fungal community compositions were more sensitive to selenium and nitrogen applications in "Longjing43" than in "Zhongcha108." "Zhongcha108" enriched more taxa related to microbial growth, while more taxa related to cellular maintenance and nutrient acquisition enriched in "Longjing43." Nano-selenium application decreased the copy number of AOA and AOB genes, and nosZ and nirK genes by 59%, 53%, 37% and 46% under ammonium, and by 77%, 43%, 38% and 65%, respectively, under nitrate spraying, in "Longjing43." However, the expression of these genes increased by nano-selenium in "Zhongcha108" with ammonium spraying. It is concluded that a nano-selenium application increases tea leaf quality, and this effect on nitrogen cycling and ecological functioning largely depends on the tea cultivar-specific bacterial and fungal composition and function.

Barriers and carriers for transition metal homeostasis in plants

Transition metals are types of metals with high chemical activity. They play critical roles in plant growth, development, reproduction, and environmental adaptation, as well as in human health. However, the acquisition, transport, and storage of these metals pose specific challenges due to their high reactivity and poor solubility. In addition, distinct yet interconnected apoplastic and symplastic diffusion barriers impede their movement throughout plants. To overcome these obstacles, plants have evolved sophisticated carrier systems to facilitate metal transport, relying on the tight coordination of vesicles, enzymes, metallochaperones, low-molecular-weight metal ligands, and membrane transporters for metals, ligands, and metal-ligand complexes. This review highlights recent advances in the homeostasis of transition metals in plants, focusing on the barriers to transition metal transport and the carriers that facilitate their passage through these barriers.

Global Phylogeography and Microdiversity of the Marine Diazotrophic Cyanobacteria Trichodesmium and UCYN-A

Cyanobacterial diazotrophs, specifically the genera Trichodesmium and UCYN-A, play a pivotal role in marine nitrogen cycling through their capacity for nitrogen fixation. Despite their global distribution, the microdiversity and environmental drivers of these diazotrophs remain underexplored. This study provides a comprehensive analysis of the global diversity and distribution of Trichodesmium and UCYN-A using the nitrogenase gene ( nifH ) as a genetic marker. We sequenced 954 samples from the Pacific, Atlantic, and Indian Oceans as part of the Bio-GO-SHIP project. Our results reveal significant phylogenetic and biogeographic differences between and within the two genera. Trichodesmium exhibited greater microdiversity compared to UCYN-A, with clades showing region-specific distribution. Trichodesmium clades were primarily influenced by temperature and nutrient availability, and particularly frequent in regions of phosphorus stress. In contrast UCYN-A was found in regions of iron stress. UCYN-A clades demonstrated a more homogeneous distributions, with a single sequencing variant within the UCYN-A1 clade dominating across varied environments. The biogeographic patterns and environmental correlations of Trichodesmium and UCYN-A highlight the role of microdiversity in their ecological adaptation and reflect their different ecological strategies. This study underscores the importance of characterizing the global patterns of fine-scale genetic diversity to better understand the functional roles and distribution of marine nitrogen-fixing cyanobacteria.

Expression and mutagenesis studies in the Medicago truncatula iron transporter MtVTL8 confirm its role in symbiotic nitrogen fixation and reveal amino acids essential for transport

The model legume Medicago truncatula establishes a symbiosis with soil bacteria (rhizobia) that carry out symbiotic nitrogen fixation (SNF) in plant root nodules. SNF requires the exchange of nutrients between the plant and rhizobia in the nodule that occurs across a plant-derived symbiosome membrane. One iron transporter, belonging to the Vacuolar iron Transporter-Like (VTL) family, MtVTL8, has been identified as essential for bacteria survival and therefore SNF. In this work we investigated the spatial expression of MtVTL8 in nodules and addressed whether it could be functionally interchangeable with a similar nodule-expressed iron transporter, MtVTL4. Using a structural model for MtVTL8 and the previously hypothesized mechanism for iron transport in a phylogenetically-related Vacuolar Iron Transporter (VIT), EgVIT1 with known crystal structure, we identified critical amino acids and obtained their mutants. Mutants were tested in planta for complementation of an SNF defective line and in an iron sensitive mutant yeast strain. An extended phylogenetic assessment of VTLs and VITs showed that amino acids critical for function are conserved differently in VTLs vs. VITs. Our studies showed that some amino acids are essential for iron transport leading us to suggest a model for MtVTL8 function, one that is different for other iron transporters (VITs) studied so far. This study extends the understanding of iron transport mechanisms in VTLs as well as those used in SNF.

Genetic basis of the historical iron-accumulating dgl and brz mutants in pea

The Pisum sativum (pea) mutants degenerate leaves (dgl) and bronze (brz) accumulate large amounts of iron in leaves. First described several decades ago, the two mutants have provided important insights into iron homeostasis in plants but the underlying mutations have remained unknown. Using exome sequencing we identified an in-frame deletion associated with dgl in a BRUTUS homolog. The deletion is absent from wild type and the original parent line. BRUTUS belongs to a small family of E3 ubiquitin ligases acting as negative regulators of iron uptake in plants. The brz mutation was previously mapped to chromosome 4, and superimposing this region to the pea genome sequence uncovered a mutation in OPT3, encoding an oligopeptide transporter with a plant-specific role in metal transport. The causal nature of the mutations was confirmed by additional genetic analyses. Identification of the mutated genes rationalizes many of the previously described phenotypes and provides new insights into shoot-to-root signaling of iron deficiency. Furthermore, the non-lethal mutations in these essential genes suggest new strategies for biofortification of crops with iron.

Forging a symbiosis: transition metal delivery in symbiotic nitrogen fixation

Symbiotic nitrogen fixation carried out by the interaction between legumes and rhizobia is the main source of nitrogen in natural ecosystems and in sustainable agriculture. For the symbiosis to be viable, nutrient exchange between the partners is essential. Transition metals are among the nutrients delivered to the nitrogen-fixing bacteria within the legume root nodule cells. These elements are used as cofactors for many of the enzymes controlling nodule development and function, including nitrogenase, the only known enzyme able to convert N2 into NH3 . In this review, we discuss the current knowledge on how iron, zinc, copper, and molybdenum reach the nodules, how they are delivered to nodule cells, and how they are transferred to nitrogen-fixing bacteria within.

Iron in the Symbiosis of Plants and Microorganisms

Iron is an essential element for most organisms. Both plants and microorganisms have developed different mechanisms for iron uptake, transport and storage. In the symbiosis systems, such as rhizobia-legume symbiosis and arbuscular mycorrhizal (AM) symbiosis, maintaining iron homeostasis to meet the requirements for the interaction between the host plants and the symbiotic microbes is a new challenge. This intriguing topic has drawn the attention of many botanists and microbiologists, and many discoveries have been achieved so far. In this review, we discuss the current progress on iron uptake and transport in the nodules and iron homeostasis in rhizobia-legume symbiosis. The discoveries with regard to iron uptake in AM fungi, iron uptake regulation in AM plants and interactions between iron and other nutrient elements during AM symbiosis are also summarized. At the end of this review, we propose prospects for future studies in this fascinating research area.

Biofortification of common bean (Phaseolus vulgaris L.) with iron and zinc: Achievements and challenges

Micronutrient deficiencies (hidden hunger), particularly in iron (Fe) and zinc (Zn), remain one of the most serious public health challenges, affecting more than three billion people globally. A number of strategies are used to ameliorate the problem of micronutrient deficiencies and to improve the nutritional profile of food products. These include (i) dietary diversification, (ii) industrial food fortification and supplements, (iii) agronomic approaches including soil mineral fertilisation, bioinoculants and crop rotations, and (iv) biofortification through the implementation of biotechnology including gene editing and plant breeding. These efforts must consider the dietary patterns and culinary preferences of the consumer and stakeholder acceptance of new biofortified varieties. Deficiencies in Zn and Fe are often linked to the poor nutritional status of agricultural soils, resulting in low amounts and/or poor availability of these nutrients in staple food crops such as common bean. This review describes the genes and processes associated with Fe and Zn accumulation in common bean, a significant food source in Africa that plays an important role in nutritional security. We discuss the conventional plant breeding, transgenic and gene editing approaches that are being deployed to improve Fe and Zn accumulation in beans. We also consider the requirements of successful bean biofortification programmes, highlighting gaps in current knowledge, possible solutions and future perspectives.

Posttranslational regulation of transporters important for symbiotic interactions

Coordinated sharing of nutritional resources is a central feature of symbiotic interactions, and, despite the importance of this topic, many questions remain concerning the identification, activity, and regulation of transporter proteins involved. Recent progress in obtaining genome and transcriptome sequences for symbiotic organisms provides a wealth of information on plant, fungal, and bacterial transporters that can be applied to these questions. In this update, we focus on legume-rhizobia and mycorrhizal symbioses and how transporters at the symbiotic interfaces can be regulated at the protein level. We point out areas where more research is needed and ways that an understanding of transporter mechanism and energetics can focus hypotheses. Protein phosphorylation is a predominant mechanism of posttranslational regulation of transporters in general and at the symbiotic interface specifically. Other mechanisms of transporter regulation, such as protein-protein interaction, including transporter multimerization, polar localization, and regulation by pH and membrane potential are also important at the symbiotic interface. Most of the transporters that function in the symbiotic interface are members of transporter families; we bring in relevant information on posttranslational regulation within transporter families to help generate hypotheses for transporter regulation at the symbiotic interface.