Channel-like characteristics of the low-affinity barley phosphate transporter PHT1;6 when expressed in Xenopus oocytes

Effects of Soybean Phosphate Transporter Gene GmPHT2 on Pi Transport and Plant Growth under Limited Pi Supply Condition

Phosphorus is an essential macronutrient for plant growth and development, but phosphate resources are limited and rapidly depleting due to massive global agricultural demand. This study identified two genes in the phosphate transporter 2 (PHT2) family of soybean by bioinformatics. The expression patterns of two genes by qRT-PCR at leaves and all were induced by low-phosphate stress. After low-phosphate stress, GmPHT2;2 expression was significantly higher than GmPHT2;1, and the same trend was observed throughout the reproductive period. The result of heterologous expression of GmPHT2 in Arabidopsis knockout mutants of atpht2;1 shows that chloroplasts and whole-plant phosphorus content were significantly higher in plants complementation of GmPHT2;2 than in plants complementation of GmPHT2;1. This suggests that GmPHT2;2 may play a more important role in plant phosphorus metabolic homeostasis during low-phosphate stress than GmPHT2;1. In the yeast backfill assay, both genes were able to backfill the ability of the defective yeast to utilize phosphorus. GmPHT2 expression was up-regulated by a low-temperature treatment at 4 °C, implying that GmPHT2;1 may play a role in soybean response to low-temperature stress, in addition to being involved in phosphorus transport processes. GmPHT2;1 and GmPHT2;2 exhibit a cyclic pattern of circadian variation in response to light, with the same pattern of gene expression changes under red, blue, and white light conditions. GmPHT2 protein was found in the chloroplast, according to subcellular localization analysis. We conclude that GmPHT2 is a typical phosphate transporter gene that can improve plant acquisition efficiency.

Cloning, subcellular localization and expression of phosphate transporter gene HvPT6 of hulless barley

Deficiency of phosphate (Pi) is one of the main growth-limiting factors for crops. Generally, phosphate transporters play a key role in the uptake of P in the crops. However, current knowledge regarding the molecular mechanism underlying Pi transport is still limited. In this study, a phosphate transporter (PT) gene, designated HvPT6, was isolated from a cDNA library constructed from hulless barley "Kunlun 14." The promoter of HvPT6 showed a large number of elements related to plant hormones. The expression pattern also indicated that HvPT6 was highly induced by low phosphorus, drought, abscisic acid, methyl jasmonate and gibberellin. Phylogenetic tree analysis revealed that HvPT6 belongs to the same subfamily of the major facilitator superfamily as OsPT6 from Oryza sativa. Subcellular localization of HvPT6:GFP using transient expression of Agrobacterium tumefaciens showed the green fluorescent protein signal in the membrane and nucleus of the Nicotiana benthamiana leaves. Overexpressing HvPT6 led to a longer and higher lateral root length and dry matter yield in the transgenic Arabidopsis lines under low Pi conditions, indicating that HvPT6 improves plant tolerance under Pi-deficient conditions. This study will lay a molecular basis for phosphate absorption mechanism in barley and breeding barley with high-efficient phosphate uptake.

Integrated multi-omics reveals the molecular mechanisms underlying efficient phosphorus use under phosphate deficiency in elephant grass (Pennisetum purpureum)

Phosphorus (P) is an essential macronutrient element for plant growth, and deficiency of inorganic phosphate (Pi) limits plant growth and yield. Elephant grass (Pennisetum purpureum) is an important fodder crop cultivated widely in tropical and subtropical areas throughout the world. However, the mechanisms underlying efficient P use in elephant grass under Pi deficiency remain poorly understood. In this study, the physiological and molecular responses of elephant grass leaves and roots to Pi deficiency were investigated. The results showed that dry weight, total P concentration, and P content decreased in Pi-deprived plants, but that acid phosphatase activity and P utilization efficiency (PUE) were higher than in Pi-sufficient plants. Regarding Pi starvation-responsive (PSR) genes, transcriptomics showed that 59 unigenes involved in Pi acquisition and transport (especially 18 purple acid phosphatase and 27 phosphate transporter 1 unigenes) and 51 phospholipase unigenes involved in phospholipids degradation or Pi-free lipids biosynthesis, as well as 47 core unigenes involved in the synthesis of phenylpropanoids and flavonoids, were significantly up-regulated by Pi deprivation in leaves or roots. Furthermore, 43 unigenes related to Pi-independent- or inorganic pyrophosphate (PPi)-dependent bypass reactions were markedly up-regulated in Pi-deficient leaves, especially five UDP-glucose pyrophosphorylase and 15 phosphoenolpyruvate carboxylase unigenes. Consistent with PSR unigene expression changes, metabolomics revealed that Pi deficiency significantly increased metabolites of Pi-free lipids, phenylpropanoids, and flavonoids in leaves and roots, but decreased phospholipid metabolites. This study reveals the mechanisms underlying the responses to Pi starvation in elephant grass leaves and roots, which provides candidate unigenes involved in efficient P use and theoretical references for the development of P-efficient elephant grass varieties.

Plant Plasma Membrane Proton Pump: One Protein with Multiple Functions

In plants, the plasma membrane proton pump (PM H⁺-ATPase) regulates numerous transport-dependent processes such as growth, development, basic physiology, and adaptation to environmental conditions. This review explores the multifunctionality of this enzyme in plant cells. The abundance of several PM H⁺-ATPase isogenes and their pivotal role in energizing transport in plants have been connected to the phenomena of pleiotropy. The multifunctionality of PM H⁺-ATPase is a focal point of numerous studies unraveling the molecular mechanisms of plant adaptation to adverse environmental conditions. Furthermore, PM H⁺-ATPase is a key element in plant defense mechanisms against pathogen attack; however, it also functions as a target for pathogens that enable plant tissue invasion. Here, we provide an extensive review of the PM H⁺-ATPase as a multitasking protein in plants. We focus on the results of recent studies concerning PM H⁺-ATPase and its role in plant growth, physiology, and pathogenesis.

The efficiency of arbuscular mycorrhiza in increasing tolerance of Triticum aestivum L. to alkaline stress

BACKGROUND

Evaluation of native soil microbes is a realistic way to develop bio-agents for ecological restoration. Soil alkalinity, which has a high pH, is one of the most common concerns in dry and semi-arid climates. Alkaline soils face problems due to poor physical properties, which affect plant growth and crop production. A pot experiment was carried out to investigate the impact of native mycorrhizal fungi (AMF) on the wheat plant (Triticum aestivum L.) under two levels of alkalinity stress -T1 (37 mM NaHCO₃), T2 (74 mM NaHCO₃) - at two developmental stages (the vegetative and productive stages).

RESULTS

Alkalinity stress significantly inhibited the germination percentage, plant biomass, photosynthetic pigments, and some nutrients (K, N, and P). Mycorrhizal inoculation improved growth parameters and productivity of wheat-stressed plants. However, lipid peroxidation was significantly lowered in mycorrhizal-inoculated plants compared to non-inoculated plants. Catalase and peroxidase were inhibited in wheat leaves and roots by alkalinity, while mycorrhiza promoted the activity of these enzymes.

CONCLUSION

The results of this study demonstrated that alkalinity stress had highly negative effects on some growth parameters of the wheat plant, while AMF inoculation attenuated these detrimental effects of alkalinity stress at two stages by reducing the pH and Na concentration and increasing the availability of P and the productivity of wheat in particular crop yield parameters.

Picrotoxin Delineates Different Transport Configurations for Malate and γ Aminobutyric Acid through TaALMT1

Plant-derived pharmacological agents have been used extensively to dissect the structure–function relationships of mammalian GABA receptors and ion channels. Picrotoxin is a non-competitive antagonist of mammalian GABAA receptors. Here, we report that picrotoxin inhibits the anion (malate) efflux mediated by wheat (Triticum aestivum) ALMT1 but has no effect on GABA transport. The EC50 for inhibition was 0.14 nM and 0.18 nM when the ALMTs were expressed in tobacco BY2 cells and in Xenopus oocytes, respectively. Patch clamping of the oocyte plasma membrane expressing wheat ALMT1 showed that picrotoxin inhibited malate currents from both sides of the membrane. These results demonstrate that picrotoxin inhibits anion efflux effectively and can be used as a new inhibitor to study the ion fluxes mediated by ALMT proteins that allow either GABA or anion transport.

Comparative Transcriptome Profiling Reveals Potential Candidate Genes, Transcription Factors, and Biosynthetic Pathways for Phosphite Response in Potato (Solanum tuberosum L.)

The study was conducted with C31 and C80 genotypes of the potato (Solanum tuberosum L.), which are tolerant and susceptible to phosphite (Phi, H₂PO₃), respectively. To decipher the molecular mechanisms underlying tolerance and susceptibility to Phi in the potato, RNA sequencing was used to study the global transcriptional patterns of the two genotypes. Media were prepared with 0.25 and 0.50 mM Phi, No-phosphorus (P), and 1.25 mM (phosphate, Pi as control). The values of fragments per kilobase of exon per million mapped fragments of the samples were also subjected to a principal component analysis, grouping the biological replicates of each sample. Using stringent criteria, a minimum of 819 differential (DEGs) were detected in both C80-Phi-0.25_vs_C80-Phi-0.50 (comprising 517 upregulated and 302 downregulated) and C80-Phi-0.50_vs_C80-Phi-0.25 (comprising 302 upregulated and 517 downregulated) and a maximum of 5214 DEGs in both C31-Con_vs_C31-Phi-0.25 (comprising 1947 upregulated and 3267 downregulated) and C31-Phi-0.25_vs_C31-Con (comprising 3267 upregulated and 1947 downregulated). DEGs related to the ribosome, plant hormone signal transduction, photosynthesis, and plant–pathogen interaction performed important functions under Phi stress, as shown by the Kyoto Encyclopedia of Genes and Genomes annotation. The expressions of transcription factors increased significantly in C31 compared with C80. For example, the expressions of Soltu.DM.01G047240, Soltu.DM.08G015900, Soltu.DM.06G012130, and Soltu.DM.08G012710 increased under P deficiency conditions (Phi-0.25, Phi-0.50, and No-P) relative to the control (P sufficiency) in C31. This study adds to the growing body of transcriptome data on Phi stress and provides important clues to the Phi tolerance response of the C31 genotype.

Insights to improve the plant nutrient transport by CRISPR/Cas system

We need to improve food production to feed the ever growing world population especially in a changing climate. Nutrient deficiency in soils is one of the primary bottlenecks affecting the crop production both in developed and developing countries. Farmers are forced to apply synthetic fertilizers to improve the crop production to meet the demand. Understanding the mechanism of nutrient transport is helpful to improve the nutrient-use efficiency of crops and promote the sustainable agriculture. Many transporters involved in the acquisition, export and redistribution of nutrients in plants are characterized. In these studies, heterologous systems like yeast and Xenopus were most frequently used to study the transport function of plant nutrient transporters. CRIPSR/Cas system introduced recently has taken central stage for efficient genome editing in diverse organisms including plants. In this review, we discuss the key nutrient transporters involved in the acquisition and redistribution of nutrients from soil. We draw insights on the possible application CRISPR/Cas system for improving the nutrient transport in plants by engineering key residues of nutrient transporters, transcriptional regulation of nutrient transport signals, engineering motifs in promoters and transcription factors. CRISPR-based engineering of plant nutrient transport not only helps to study the process in native plants with conserved regulatory system but also aid to develop non-transgenic crops with better nutrient use-efficiency. This will reduce the application of synthetic fertilizers and promote the sustainable agriculture strengthening the food and nutrient security.

The rice phosphate transporter OsPHT1;7 plays a dual role in phosphorus redistribution and anther development

Inorganic phosphate (Pi) is the predominant form of phosphorus (P) readily accessible to plants, and Pi Transporter 1 (PHT1) genes are the major contributors to root Pi uptake. However, the mechanisms underlying the transport and recycling of Pi within plants, which are vital for optimizing P use efficiency, remain elusive. Here, we characterized a functionally unknown rice (Oryza sativa) PHT1 member barely expressed in roots, OsPHT1;7. Yeast complementation and Xenopus laevis oocyte assay demonstrated that OsPHT1;7 could mediate Pi transport. Reverse-transcription quantitative polymerase chain reaction and histochemical analyses showed that OsPHT1;7 was preferentially expressed in source leaves and nodes. A further fine-localization analysis by immunostaining showed that OsPHT1;7 expression was restricted in the vascular bundle (VB) sheath and phloem of source leaves as well as in the phloem of regular/diffuse- and enlarged-VBs of nodes. In accordance with this expression pattern, mutation of OsPHT1;7 led to increased and decreased P distribution in source (old leaves) and sink organs (new leaves/panicles), respectively, indicating that OsPHT1;7 is involved in P redistribution. Furthermore, OsPHT1;7 showed an overwhelmingly higher transcript abundance in anthers than other PHT1 members, and ospht1;7 mutants were impaired in P accumulation in anthers but not in pistils or husks. Moreover, the germination of pollen grains was significantly inhibited upon OsPHT1;7 mutation, leading to a >80% decrease in seed-setting rate and grain yield. Taken together, our results provide evidence that OsPHT1;7 is a crucial Pi transporter for Pi transport and recycling within rice plants, stimulating both vegetative and reproductive growth.

Understanding the Adaptive Mechanisms of Plants to Enhance Phosphorus Use Efficiency on Podzolic Soils in Boreal Agroecosystems

Being a macronutrient, phosphorus (P) is the backbone to complete the growth cycle of plants. However, because of low mobility and high fixation, P becomes the least available nutrient in podzolic soils; hence, enhancing phosphorus use efficiency (PUE) can play an important role in different cropping systems/crop production practices to meet ever-increasing demands in food, fiber, and fuel. Additionally, the rapidly decreasing mineral phosphate rocks/stocks forced to explore alternative resources and methods to enhance PUE either through improved seed P reserves and their remobilization, P acquisition efficiency (PAE), or plant's internal P utilization efficiency (IPUE) or both for sustainable P management strategies. The objective of this review article is to explore and document important domains to enhance PUE in crop plants grown on Podzol in a boreal agroecosystem. We have discussed P availabilities in podzolic soils, root architecture and morphology, root exudates, phosphate transporters and their role in P uptake, different contributors to enhance PAE and IPUE, and strategies to improve plant PUE in crops grown on podzolic soils deficient in P and acidic in nature.

Uptake patterns of critical metals in alpine plant species growing in an unimpaired natural site

The range of metals used for industrial purposes - electrical engineering, solar panels, batteries - has increased substantially over the last twenty years. Some of these emerging metals are the subject of geopolitical conflict and are considered critical as their unique properties make them irreplaceable. Many of these elements are poorly studied and their biogeochemical cycles still raise many questions. Aim of this study is to analyse the soil-to-plant transfer of some of these chemical elements and to shed light on their uptake pathways. For this purpose, the geological site of Jas Roux (France) was chosen as this alpine site is naturally rich in critical and potentially toxic elements such as As, Sb, Ba and Tl, but nevertheless is host to a high diversity of plants. Elemental concentrations were analysed in the topsoil and in 12 selected alpine plant species sampled in situ. Statistical tools were used to detect species dependent characteristics in elemental uptake. Our analyses revealed accumulation of rare earth elements by Saxifraga paniculata, selective oxyanion absorption by Hippocrepis comosa, accumulation of Tl by Biscutella laevigata and Galium corrudifolium and an exclusion strategy in Juniperus communis. These findings advance our understanding of the environmental behaviour of critical metals and metalloids such as V, As, Y, Sb, Ce, Ba and Tl and might bare valuable information for phytoremediation applications.

Regulation of Cytosolic pH: The Contributions of Plant Plasma Membrane H+-ATPases and Multiple Transporters

Cytosolic pH homeostasis is a precondition for the normal growth and stress responses in plants, and H+ flux across the plasma membrane is essential for cytoplasmic pH control. Hence, this review focuses on seven types of proteins that possess direct H+ transport activity, namely, H+-ATPase, NHX, CHX, AMT, NRT, PHT, and KT/HAK/KUP, to summarize their plasma-membrane-located family members, the effect of corresponding gene knockout and/or overexpression on cytosolic pH, the H+ transport pathway, and their functional regulation by the extracellular/cytosolic pH. In general, H+-ATPases mediate H+ extrusion, whereas most members of other six proteins mediate H+ influx, thus contributing to cytosolic pH homeostasis by directly modulating H+ flux across the plasma membrane. The fact that some AMTs/NRTs mediate H+-coupled substrate influx, whereas other intra-family members facilitate H+-uncoupled substrate transport, demonstrates that not all plasma membrane transporters possess H+-coupled substrate transport mechanisms, and using the transport mechanism of a protein to represent the case of the entire family is not suitable. The transport activity of these proteins is regulated by extracellular and/or cytosolic pH, with different structural bases for H+ transfer among these seven types of proteins. Notably, intra-family members possess distinct pH regulatory characterization and underlying residues for H+ transfer. This review is anticipated to facilitate the understanding of the molecular basis for cytosolic pH homeostasis. Despite this progress, the strategy of their cooperation for cytosolic pH homeostasis needs further investigation.

The auxin-inducible phosphate transporter AsPT5 mediates phosphate transport and is indispensable for arbuscule formation in Chinese milk vetch at moderately high phosphate supply

Phosphorus is a macronutrient that is essential for plant survival. Most land plants have evolved the ability to form a mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, which enhances phosphate (Pi) acquisition. Modulation of Pi transporter systems is the master strategy used by mycorrhizal plants to adapt to ambient Pi concentrations. However, the specific functions of PHOSPHATE TRANSPORTER 1 (PHT1) genes, which are Pi transporters that are responsive to high Pi availability, are largely unknown. Here, we report that AsPT5, an Astragalus sinicus (Chinese milk vetch) member of the PHT1 gene family, is conserved across dicotyledons and is constitutively expressed in a broad range of tissues independently of Pi supply, but is remarkably induced by indole-3-acetic acid (auxin) treatment under moderately high Pi conditions. Subcellular localization experiments indicated that AsPT5 localizes to the plasma membrane of plant cells. Using reverse genetics, we showed that AsPT5 not only mediates Pi transport and remodels root system architecture but is also essential for arbuscule formation in A. sinicus under moderately high Pi concentrations. Overall, our study provides insight into the function of AsPT5 in Pi transport, AM development and the cross-talk between Pi nutrition and auxin signalling in mycorrhizal plants.

Functional Analysis of the Phosphate Transporter Gene MtPT6 From Medicago truncatula

Phosphorus is one of the essential macronutrients required by plant growth and development, but phosphate resources are finite and diminishing rapidly because of the huge need in global agriculture. In this study, 11 genes were found in the Phosphate Transporter 1 (PHT1) family of Medicago truncatula. Seven genes of the PHT1 family were available by qRT-PCR. Most of them were expressed in roots, and almost all genes were induced by low-phosphate stress in the nodule. The expression of MtPT6 was relatively high in nodules and induced by low-phosphate stress. The fusion expression of MtPT6 promoter-GUS gene in M. truncatula suggested that the expression of MtPT6 was induced in roots and nodules by phosphate starvation. In roots, MtPT6 was mainly expressed in vascular tissue and tips, and it was also expressed in cortex under low-phosphate stress; in nodules, it was mainly expressed in vascular bundles, cortical cells, and fixation zone cells. MtPT6 had a close relationship with other PHT1 family members according to amino acid alignment and phylogenetic analysis. Subcellular localization analysis in tobacco revealed that MtPT6 protein was localized to the plasma membrane. The heterologous expression of MtPT6 in Arabidopsis knockout mutants of pht1.1 and pht1.4 made seedlings more susceptible to arsenate treatment, and the phosphate concentrations in pht1.1 were higher in high phosphate condition by expressing MtPT6. We conclude that MtPT6 is a typical phosphate transporter gene and can promote phosphate acquisition efficiency of plants.

Comparative transcriptome analysis reveals unique genetic adaptations conferring salt tolerance in a xerohalophyte

Most studies on salt tolerance in plants have been conducted using glycophytes like Arabidopsis thaliana (L.) Heynh., with limited resistance to salinity. The xerohalophyte Zygophyllum xanthoxylum (Bunge) Engl. is a salt-accumulating desert plant that efficiently transports Na+ into vacuoles to manage salt and exhibits increased growth under salinity conditions, suggesting a unique transcriptional response compared with glycophytes. We used transcriptome profiling by RNA-seq to compare gene expression in roots of Z. xanthoxylum and A. thaliana under 50 mM NaCl treatments. Gene Ontology (GO) functional annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathway analysis suggested that 50 mM NaCl was perceived as a stimulus for Z. xanthoxylum whereas a stress for A. thaliana. Exposure to 50 mM NaCl caused metabolic shifts towards gluconeogenesis to stimulate growth of Z. xanthoxylum, but triggered defensive systems in A. thaliana. Compared with A. thaliana, a vast array of ion transporter genes was induced in Z. xanthoxylum, revealing an active strategy to uptake Na+ and nutrients from the environment. An ascorbate-glutathione scavenging system for reactive oxygen species was also crucial in Z. xanthoxylum, based on high expression of key enzyme genes. Finally, key regulatory genes for the biosynthesis pathways of abscisic acid and gibberellin showed distinct expression patterns between the two species and auxin response genes were more active in Z. xanthoxylum compared with A. thaliana. Our results provide an important framework for understanding unique patterns of gene expression conferring salt resistance in Z. xanthoxylum.

OsPHT1;3 Mediates Uptake, Translocation, and Remobilization of Phosphate under Extremely Low Phosphate Regimes

Plant roots rely on inorganic orthophosphate (Pi) transporters to acquire soluble Pi from soil solutions that exists at micromolar levels in natural ecosystems. Here, we functionally characterized a rice (Oryza sativa) Pi transporter, Os Phosphate Transporter-1;3 (OsPHT1;3), that mediates Pi uptake, translocation, and remobilization. OsPHT1;3 was directly regulated by Os Phosphate Starvation Response-2 and, in response to Pi starvation, showed enhanced expression in young leaf blades and shoot basal regions and even more so in roots and old leaf blades. OsPHT1;3 was able to complement a yeast mutant strain defective in five Pi transporters and mediate Pi influx in Xenopus laevis oocytes. Overexpression of OsPHT1;3 led to increased Pi concentration both in roots and shoots. However, unlike that reported for other known OsPHT1 members that facilitate Pi uptake at relatively higher Pi levels, mutation of OsPHT1;3 impaired Pi uptake and root-to-shoot Pi translocation only when external Pi concentration was below 5 μm Moreover, in basal nodes, the expression of OsPHT1;3 was restricted to the phloem of regular vascular bundles and enlarged vascular bundles. An isotope labeling experiment with ³²P showed that ospht1;3 mutant lines were impaired in remobilization of Pi from source to sink leaves. Furthermore, overexpression and mutation of OsPHT1;3 led to reciprocal alteration in the expression of OsPHT1;2 and several other OsPHT1 genes. Yeast-two-hybrid, bimolecular fluorescence complementation, and coimmunoprecipitation assays all demonstrated a physical interaction between OsPHT1;3 and OsPHT1;2. Taken together, our results indicate that OsPHT1;3 acts as a crucial factor for Pi acquisition, root-to-shoot Pi translocation, and redistribution of phosphorus in plants growing in environments with extremely low Pi levels.

The Hebeloma cylindrosporum HcPT2 Pi transporter plays a key role in ectomycorrhizal symbiosis

Through a mutualistic relationship with woody plant roots, ectomycorrhizal fungi provide growth-limiting nutrients, including inorganic phosphate (Pi), to their host. Reciprocal trades occur at the Hartig net, which is the symbiotic interface of ectomycorrhizas where the two partners are symplasmically isolated. Fungal Pi must be exported to the symbiotic interface, but the proteins facilitating this transfer are unknown. In the present study, we combined transcriptomic, microscopy, whole plant physiology, X-ray fluorescence mapping, ³² P labeling and fungal genetic approaches to unravel the role of HcPT2, a fungal Pi transporter, during the Hebeloma cylindrosporum-Pinus pinaster ectomycorrhizal association. We localized HcPT2 in the extra-radical hyphae and the Hartig net and demonstrated its determinant role for both the establishment of ectomycorrhizas and Pi allocation towards P. pinaster. We showed that the host plant induces HcPT2 expression and that the artificial overexpression of HcPT2 is sufficient to significantly enhance Pi export towards the central cylinder. Together, our results reveal that HcPT2 plays an important role in ectomycorrhizal symbiosis, affecting both Pi influx in the mycelium and efflux towards roots under the control of P. pinaster.

Reduction of geomagnetic field (GMF) to near null magnetic field (NNMF) affects Arabidopsis thaliana root mineral nutrition

The Earth magnetic field (or geomagnetic field, GMF) is a natural component of our planet and variations of the GMF are perceived by plants with a still uncharacterized magnetoreceptor. The purpose of this work was to assess the effect of near null magnetic field (NNMF, ∼40 nT) on Arabidopsis thaliana Col0 root ion modulation. A time-course (from 10 min to 96 h) exposure of Arabidopsis to NNMF was compared to GMF and the content of some cations (NH₄⁺, K⁺, Ca²⁺ and Mg²⁺) and anions (Cl^-, SO₄⁼, NO₃^- and PO₄⁼) was evaluated by capillary electrophoresis. The expression of several cation and anion channel- and transporter-related genes was assessed by gene microarray. A few minutes after exposure to NNMF, Arabidopsis roots responded with a significant change in the content and gene expression of all nutrient ions under study, indicating the presence of a plant magnetoreceptor that responds immediately to MF variations by modulating channels, transporters and genes involved in mineral nutrition. The response of Arabidopsis to reduced MF was a general reduction of plant ion uptake and transport. Our data suggest the importance to understand the nature and function of the plant magnetoreceptor for future space programs involving plant growth in environments with a reduced MF.

Phosphorus Acquisition Efficiency Related to Root Traits: Is Mycorrhizal Symbiosis a Key Factor to Wheat and Barley Cropping?

Wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) are major crops cultivated around the world, thus playing a crucial role on human diet. Remarkably, the growing human population requires a significant increase in agricultural production in order to feed everybody. In this context, phosphorus (P) management is a key factor as it is component of organic molecules such as nucleic acids, ATP and phospholipids, and it is the most abundant macronutrient in biomass after nitrogen (N), although being one of the scarcest elements in the lithosphere. In general, P fertilization has low efficiency, as only a fraction of the applied P is acquired by roots, leaving a substantial amount to be accumulated in soil as not readily available P. Breeding for P-efficient cultivars is a relatively low cost alternative and can be done through two mechanisms: i) improving P use efficiency (PUE), and/or ii) P acquisition efficiency (PAE). PUE is related to the internal allocation/mobilization of P, and is usually represented by the amount of P accumulated per biomass. PAE relies on roots ability to acquire P from the soil, and is commonly expressed as the relative difference of P acquired under low and high P availability conditions. In this review, plant adaptations related to improved PAE are described, with emphasis on arbuscular mycorrhizal (AM) symbiosis, which is generally accepted to enhance plant P acquisition. A state of the art (1980-2018) of AM growth responses and P uptake in wheat and barley is made to discuss about the commonly accepted growth promoting effect and P increased uptake by AM fungi and the contrasting evidence about the generally accepted lack of positive responses in both plant species. Finally, the mechanisms by which AM symbiosis can affect wheat and barley PAE are discussed, highlighting the importance of considering AM functional diversity on future studies and the necessity to improve PAE definition by considering the carbon trading between all the directly related PAE traits and its return to the host plant.

Aluminum-Activated Malate Transporters Can Facilitate GABA Transport

Plant aluminum-activated malate transporters (ALMTs) are currently classified as anion channels; they are also known to be regulated by diverse signals, leading to a range of physiological responses. Gamma-aminobutyric acid (GABA) regulation of anion flux through ALMT proteins requires a specific amino acid motif in ALMTs that shares similarity with a GABA binding site in mammalian GABA_A receptors. Here, we explore why TaALMT1 activation leads to a negative correlation between malate efflux and endogenous GABA concentrations ([GABA]_i) in both wheat (Triticum aestivum) root tips and in heterologous expression systems. We show that TaALMT1 activation reduces [GABA]_i because TaALMT1 facilitates GABA efflux but GABA does not complex Al³⁺ TaALMT1 also leads to GABA transport into cells, demonstrated by a yeast complementation assay and via ¹⁴C-GABA uptake into TaALMT1-expressing Xenopus laevis oocytes; this was found to be a general feature of all ALMTs we examined. Mutation of the GABA motif (TaALMT1^F213C) prevented both GABA influx and efflux, and resulted in no correlation between malate efflux and [GABA]_i We conclude that ALMTs are likely to act as both GABA and anion transporters in planta. GABA and malate appear to interact with ALMTs in a complex manner to regulate each other's transport, suggestive of a role for ALMTs in communicating metabolic status.

Molecular Mechanisms of Acclimatization to Phosphorus Starvation and Recovery Underlying Full-Length Transcriptome Profiling in Barley (Hordeum vulgare L.)

A lack of phosphorus (P) in plants can severely constrain growth and development. Barley, one of the earliest domesticated crops, is extensively planted in poor soil around the world. To date, the molecular mechanisms of enduring low phosphorus, at the transcriptional level, in barley are still unclear. In the present study, two different barley genotypes (GN121 and GN42)-with contrasting phosphorus efficiency-were used to reveal adaptations to low phosphorus stress, at three time points, at the morphological, physiological, biochemical, and transcriptome level. GN121 growth was less affected by phosphorus starvation and recovery than that of GN42. The biomass and inorganic phosphorus concentration of GN121 and GN42 declined under the low phosphorus-induced stress and increased after recovery with normal phosphorus. However, the range of these parameters was higher in GN42 than in GN121. Subsequently, a more complete genome annotation was obtained by correcting with the data sequenced on Illumina HiSeq X 10 and PacBio RSII SMRT platform. A total of 6,182 and 5,270 differentially expressed genes (DEGs) were identified in GN121 and GN42, respectively. The majority of these DEGs were involved in phosphorus metabolism such as phospholipid degradation, hydrolysis of phosphoric enzymes, sucrose synthesis, phosphorylation/dephosphorylation and post-transcriptional regulation; expression of these genes was significantly different between GN121 and GN42. Specifically, six and seven DEGs were annotated as phosphorus transporters in roots and leaves, respectively. Furthermore, a putative model was constructed relying on key metabolic pathways related to phosphorus to illustrate the higher phosphorus efficiency of GN121 compared to GN42 under low phosphorus conditions. Results from this study provide a multi-transcriptome database and candidate genes for further study on phosphorus use efficiency (PUE).

Transport and homeostasis of potassium and phosphate: limiting factors for sustainable crop production

Potassium (K) and phosphate (Pi) are both macronutrients essential for plant growth and crop production, but the unrenewable resources of phosphorus rock and potash have become limiting factors for food security. One critical measure to help solve this problem is to improve nutrient use efficiency (NUE) in plants by understanding and engineering genetic networks for ion uptake, translocation, and storage. Plants have evolved multiple systems to adapt to various nutrient conditions for growth and production. Within the NUE networks, transport proteins and their regulators are the primary players for maintaining nutrient homeostasis and could be utilized to engineer high NUE traits in crop plants. A large number of publications have detailed K+ and Pi transport proteins in plants over the past three decades. Meanwhile, the discovery and validation of their regulatory mechanisms are fast-track topics for research. Here, we provide an overview of K+ and Pi transport proteins and their regulatory mechanisms, which participate in the uptake, translocation, storage, and recycling of these nutrients in plants.

Roles, Regulation, and Agricultural Application of Plant Phosphate Transporters

Phosphorus (P) is an essential mineral nutrient for plant growth and development. Low availability of inorganic phosphate (orthophosphate; Pi) in soil seriously restricts the crop production, while excessive fertilization has caused environmental pollution. Pi acquisition and homeostasis depend on transport processes controlled Pi transporters, which are grouped into five families so far: PHT1, PHT2, PHT3, PHT4, and PHT5. This review summarizes the current understanding on plant PHT families, including phylogenetic analysis, function, and regulation. The potential application of Pi transporters and the related regulatory factors for developing genetically modified crops with high phosphorus use efficiency (PUE) are also discussed in this review. At last, we provide some potential strategies for developing high PUE crops under salt or drought stress conditions, which can be valuable for improving crop yields challenged by global scarcity of water resources and increasing soil salinization.

Genome-wide Identification, Characterization, and Expression Analysis of PHT1 Phosphate Transporters in Wheat

The PHT1 family of phosphate (Pi) transporters mediates phosphorus (P) uptake and re-mobilization in plants. A genome-wide sequence analysis of PHT1 genes in wheat (Triticum aestivum) was conducted, and their expression locations and responses to P availability were further investigated. We cloned 21 TaPHT1 genes from the homologous alleles at TaPHT1.1 to 1.10 through screening a BAC library and amplifying genomic sequences. The TaPHT1 transporters were clustered into five branches in the phylogenetic tree of PHT1 proteins, and the TaPHT1 genes from a given branch shared high similarities in sequences, expression locations, and responses to P availability. The seven tested PHT1 genes all showed Pi-transport activity in yeast (Saccharomyces cerevisiae) cells grown under both low Pi and high Pi conditions. The expression of TaPHT1.1/1.9, 1.2, and 1.10 were root specific. The expression of these TaPHT1 genes at flowering positively correlated with P uptake after stem elongation across three P application rates and two wheat varieties in a field experiment. Therefore, modification of PHT1 expression may improve P use efficiency in a broad regime of P availability.

Phosphate Uptake and Allocation - A Closer Look at Arabidopsis thaliana L. and Oryza sativa L

This year marks the 20th anniversary of the discovery and characterization of the two Arabidopsis PHT1 genes encoding the phosphate transporter in Arabidopsis thaliana. So far, multiple inorganic phosphate (Pi) transporters have been described, and the molecular basis of Pi acquisition by plants has been well-characterized. These genes are involved in Pi acquisition, allocation, and/or signal transduction. This review summarizes how Pi is taken up by the roots and further distributed within two plants: A. thaliana and Oryza sativa L. by plasma membrane phosphate transporters PHT1 and PHO1 as well as by intracellular transporters: PHO1, PHT2, PHT3, PHT4, PHT5 (VPT1), SPX-MFS and phosphate translocators family. We also describe the role of the PHT1 transporters in mycorrhizal roots of rice as an adaptive strategy to cope with limited phosphate availability in soil.

Complex Regulation of Plant Phosphate Transporters and the Gap between Molecular Mechanisms and Practical Application: What Is Missing?

It has been almost 25 years since the first report of the gene encoding a high-affinity phosphate transporter (PT), PHO84, in yeast. Since then, an increasing number of yeast PHO84 homologs as well as other genes encoding proteins with phosphate (Pi) transport activities have been identified and functionally characterized in diverse plant species. Great progress has been made also in deciphering the molecular mechanism underlying the regulation of the abundance and/or activity of these genes and their products. The regulatory genes affect plant Pi homeostasis commonly through direct or indirect regulation of the abundance of PTs at different levels. However, little has been achieved in the use of PTs for developing genetically modified crops with high phosphorus use efficiency (PUE). This might be a consequence of overemphasizing Pi uptake from the rhizosphere and lack of knowledge about the roles of PTs in Pi transport and recycling within the plant that are required to optimize PUE. Here, we mainly focused on the genes encoding proteins with Pi transport activities and the emerging understanding of their regulation at the transcriptional, post-transcriptional, translational, and post-translational levels. In addition, we propose potential strategies for effective use of PTs in improving plant growth and development.

A member of the Phosphate transporter 1 (Pht1) family from the arsenic-hyperaccumulating fern Pteris vittata is a high-affinity arsenate transporter

Pteris vittata exhibits enhanced arsenic uptake, but the corresponding mechanisms are not well known. The prevalent form of arsenic in most soils is arsenate, which is a phosphate analog and a substrate for Phosphate transporter 1 (Pht1) transporters. Herein we identify and characterize three P. vittata Pht1 transporters. Pteris vittata Pht1 cDNAs were isolated and characterized via heterologous expression in Saccharomyces cerevisiae (yeast) and Nicotiana benthamiana leaves. Expression of the PvPht1 loci in P. vittata gametophytes was also examined in response to phosphate deficiency and arsenate exposure. Expression of each of the PvPht1 cDNAs complemented the phosphate uptake defect of a yeast mutant. Compared with yeast cells expressing Arabidopsis thaliana Pht1;5, cells expressing PvPht1;3 were more sensitive to arsenate, and accumulated more arsenic. Uptake assays with yeast cells and radiolabeled (32)P revealed that PvPht1;3 and AtPht1;5 have similar affinities for phosphate, but the affinity of PvPht1;3 for arsenate is much greater. In P. vittata gametophytes, PvPht1;3 transcript levels increased in response to phosphate (Pi) deficiency and arsenate exposure. PvPht1;3 is induced by Pi deficiency and arsenate, and encodes a phosphate transporter that has a high affinity for arsenate. PvPht1;3 probably contributes to the enhanced arsenate uptake capacity and affinity exhibited by P. vittata.

Heat Stress Affects Pi-related Genes Expression and Inorganic Phosphate Deposition/Accumulation in Barley

Phosphorus (P) in plants is taken from soil as an inorganic phosphate (Pi) and is one of the most important macroelements in growth and development. Plants actively react to Pi starvation by the induced expression of Pi transporters, MIR399, MIR827, and miR399 molecular sponge - IPS1 genes and by the decreased expression of the ubiquitin-conjugating enzyme E2 (PHOSPHATE2 - PHO2) and Pi sensing and transport SPX-MFS genes. The PHO2 protein is involved in the degradation of Pi transporters PHT1;1 (from soil to roots) and PHO1 (from roots to shoots). The decreased expression of PHO2 leads to Pi accumulation in shoots. In contrast, the pho1 mutant shows a decreased level of Pi concentration in shoots. Finally, Pi starvation leads to decreased Pi concentration in all plant tissues. Little is known about plant Pi homeostasis in other abiotic stress conditions. We found that, during the first hour of heat stress, Pi accumulated in barley shoots but not in the roots, and transcriptomic data analysis as well as RT-qPCR led us to propose an explanation for this phenomenon. Pi transport inhibition from soil to roots is balanced by lower Pi efflux from roots to shoots directed by the PHO1 transporter. In shoots, the PHO2 mRNA level is decreased, leading to an increased Pi level. We concluded that Pi homeostasis in barley during heat stress is maintained by dynamic changes in Pi-related genes expression.

Genomic Identification and Expression Analysis of the Phosphate Transporter Gene Family in Poplar

Inorganic phosphate is one of key macronutrients essential for plant growth. The acquisition and distribution of phosphate are mediated by phosphate transporters functioning in various physiological and biochemical processes. In the present study, we comprehensively evaluated the phosphate transporter (PHT) gene family in the latest release of the Populus trichocarpa genome (version 3.0; Phytozome 11.0) and a total of 42 PHT genes were identified which formed five clusters: PHT1, PHT2, PHT3, PHT4, and PHO. Among the 42 PHT genes, 41 were localized to 15 Populus chromosomes. Analysis of these genes led to identification of 5-14 transmembrane segments, most of which were conserved within the same cluster. We identified 234 putative cis elements in the 2-kb upstream regions of the 42 PHT genes, many of which are related to development, stress, or hormone. Tissue-specific expression analysis of the 42 PtPHT genes revealed that 25 were highly expressed in the roots of P. tremula, suggesting that most of them might be involved in Pi uptake. Some PtPHT genes were highly expressed in more than six of the twelve investigated tissues of P. tremula, while the expression of a few of them was very low in all investigated tissues. In addition, the expression of the PtPHT genes was verified by quantitative real-time PCR in four tissues of P. simonii. Transcripts of 7 PtPHT genes were detected in all four tested tissues of P. simonii. Most PtPHT genes were expressed in the roots of P. simonii at high levels. Further, PtPHT1.2 and PtPHO9 expression was increased under drought conditions, irrespective of the phosphate levels. In particular, PtPHT1.2 expression was significantly induced by approximately 90-fold. However, the transcriptional changes of some PtPHT genes under drought stress were highly dependent on the phosphate levels. These results will aid in elucidation of the functions of PtPHT in the growth, development, and stress response of the poplar plant.

Rice SPX-Major Facility Superfamily3, a Vacuolar Phosphate Efflux Transporter, Is Involved in Maintaining Phosphate Homeostasis in Rice

To maintain a stable cytosol phosphate (Pi) concentration, plant cells store Pi in their vacuoles. When the Pi concentration in the cytosol decreases, Pi is exported from the vacuole into the cytosol. This export is mediated by Pi transporters on the tonoplast. In this study, we demonstrate that SYG1, PHO81, and XPR1 (SPX)-Major Facility Superfamily (MFS) proteins have a similar structure with yeast (Saccharomyces cerevisiae) low-affinity Pi transporters Phosphatase87 (PHO87), PHO90, and PHO91. OsSPX-MFS1, OsSPX-MFS2, and OsSPX-MFS3 all localized on the tonoplast of rice (Oryza sativa) protoplasts, even in the absence of the SPX domain. At high external Pi concentration, OsSPX-MFS3 could partially complement the yeast mutant strain EY917 under pH 5.5, which lacks all five Pi transporters present in yeast. In oocytes, OsSPX-MFS3 was shown to facilitate Pi influx or efflux depending on the external pH and Pi concentrations. In contrast to tonoplast localization in plants cells, OsSPX-MFS3 was localized to the plasma membrane when expressed in both yeast and oocytes. Overexpression of OsSPX-MFS3 results in decreased Pi concentration in the vacuole of rice tissues. We conclude that OsSPX-MFS3 is a low-affinity Pi transporter that mediates Pi efflux from the vacuole into cytosol and is coupled to proton movement.

Increased expression of six ZIP family genes by zinc (Zn) deficiency is associated with enhanced uptake and root-to-shoot translocation of Zn in barley (Hordeum vulgare)

Low zinc (Zn) in soils reduces yield and grain Zn content. Regulation of ZRT/IRT-like protein (ZIP) family genes is a major mechanism in plant adaptation to low and fluctuating Zn in soil. Although several Zn deficiency-inducible ZIP genes are identified in cereals, there has been no systematic study on the association of Zn deficiency-induced uptake and root-to-shoot translocation with expression of ZIP family genes. We measured Zn deficiency-induced uptake and root-to-shoot translocation of Zn in barley (Hordeum vulgare) plants by resupplying 0.5 μM Zn, and quantified the transcripts of thirteen HvZIP genes. Subcellular localization and tissue-specific expression were also determined for Zn deficiency-inducible HvZIP genes. Zn deficiency enhanced the capacity of uptake and root-to-shoot translocation of Zn, and sustained the enhanced capacity for 6 d after Zn resupply. Six HvZIP genes were highly induced in roots of Zn-deficient plants, and their proteins were localized in the plasma membrane. Tissue-specific expression in roots supports their roles in uptake and root-to-shoot translocation of Zn under low Zn conditions. Our results provide a comprehensive view on the physiological roles of ZIP genes in plant adaptation to low and fluctuating Zn in soil, and pave the way for development of new strategies to improve Zn-deficiency tolerance and biofortification in cereals.

GABA signalling modulates plant growth by directly regulating the activity of plant-specific anion transporters

The non-protein amino acid, gamma-aminobutyric acid (GABA) rapidly accumulates in plant tissues in response to biotic and abiotic stress, and regulates plant growth. Until now it was not known whether GABA exerts its effects in plants through the regulation of carbon metabolism or via an unidentified signalling pathway. Here, we demonstrate that anion flux through plant aluminium-activated malate transporter (ALMT) proteins is activated by anions and negatively regulated by GABA. Site-directed mutagenesis of selected amino acids within ALMT proteins abolishes GABA efficacy but does not alter other transport properties. GABA modulation of ALMT activity results in altered root growth and altered root tolerance to alkaline pH, acid pH and aluminium ions. We propose that GABA exerts its multiple physiological effects in plants via ALMT, including the regulation of pollen tube and root growth, and that GABA can finally be considered a legitimate signalling molecule in both the plant and animal kingdoms.

The Phosphate Transporter Gene OsPht1;4 Is Involved in Phosphate Homeostasis in Rice

A total of 13 phosphate transporters in rice (Oryza sative) have been identified as belonging to the Pht1 family, which mediates inorganic phosphate (Pi) uptake and transport. We report the biological property and physiological role of OsPht1;4 (OsPT4). Overexpressing OsPT4 resulted in significant higher Pi accumulation in roots, straw and brown rice, and suppression of OsPT4 caused decreased Pi concentration in straw and brown rice. Expression of the β-glucuronidase reporter gene driven by the OsPT4 promoter showed that OsPT4 is expressed in roots, leaves, ligules, stamens, and caryopses under sufficient Pi conditions, consistent with the expression profile showing that OsPT4 has high expression in roots and flag leaves. The transcript level of OsPT4 increased significantly both in shoots and roots with a long time Pi starvation. OsPT4 encoded a plasma membrane-localized protein and was able to complement the function of the Pi transporter gene PHO84 in yeast. We concluded that OsPT4 is a functional Pi-influx transporter involved in Pi absorption in rice that might play a role in Pi translocation. This study will enrich our understanding about the physiological function of rice Pht1 family genes.

Involvement of OsPht1;4 in phosphate acquisition and mobilization facilitates embryo development in rice

Phosphate (Pi) transporters mediate acquisition and transportation of Pi within plants. Here, we investigated the functions of OsPht1;4 (OsPT4), one of the 13 members of the Pht1 family in rice. Quantitative real-time RT-PCR analysis revealed strong expression of OsPT4 in roots and embryos, and OsPT4 promoter analysis using reporter genes confirmed these findings. Analysis using rice protoplasts showed that OsPT4 localized to the plasma membrane. OsPT4 complemented a yeast mutant defective in Pi uptake, and also facilitated increased accumulation of Pi in Xenopus oocytes. Further, OsPT4 genetically modified (GM) rice lines were generated by knockout/knockdown or over-expression of OsPT4. Pi concentrations in roots and shoots were significantly lower and higher in knockout/knockdown and over-expressing plants, respectively, compared to wild-type under various Pi regimes. (33) Pi uptake translocation assays corroborated the altered acquisition and mobilization of Pi in OsPT4 GM plants. We also observed effects of altered expression levels of OsPT4 in GM plants on the concentration of Pi, the size of the embryo, and several attributes related to seed development. Overall, our results suggest that OsPT4 encodes a plasma membrane-localized Pi transporter that facilitates acquisition and mobilization of Pi, and also plays an important role in development of the embryo in rice.

Phosphate transporter OsPht1;8 in rice plays an important role in phosphorus redistribution from source to sink organs and allocation between embryo and endosperm of seeds

Phosphorus (P) redistribution from source to sink organs within plant is required for optimizing growth and development under P deficient condition. In this study, we knocked down expression of a phosphate transporter gene OsPht1;8 (OsPT8) selectively in shoot and/or in seed endosperm by RNA-interference using RISBZ1 and GluB-1 promoter (designate these transgenic lines as SSRi and EnSRi), respectively, to characterize the role of OsPT8 in P redistribution of rice. In comparison to wild type (WT) and EnSRi lines, SSRi lines under P deficient condition accumulated more P in old blades and less P in young blades, corresponding to attenuated and enriched transcripts of P-responsive genes in old and young blades, respectively. The ratio of total P in young blades to that in old blades decreased from 2.6 for WT to 0.9-1.2 for SSRi lines. During the grain-filling stage, relative to WT, SSRi lines showed the substantial decrease of total P content in both endosperm and embryo, while EnSRi lines showed 40-50% decrease of total P content in embryo but similar P content in endosperm. Taken together, our results demonstrate that OsPT8 plays a critical role in redistribution of P from source to sink organs and P homeostasis in seeds of rice.

Phosphate concentration and arbuscular mycorrhizal colonisation influence the growth, yield and expression of twelve PHT1 family phosphate transporters in foxtail millet (Setaria italica)

Phosphorus (P) is an essential element which plays several key roles in all living organisms. Setaria italica (foxtail millet) is a model species for panacoid grasses including several millet species widely grown in arid regions of Asia and Africa, and for the bioenergy crop switchgrass. The growth responses of S. italica to different levels of inorganic phosphate (Pi) and to colonisation with the arbuscular mycorrhizal fungus Funneliformis mosseae (syn. Glomus mosseae) were studied. Phosphate is taken up from the environment by the PHT1 family of plant phosphate transporters, which have been well characterized in several plant species. Bioinformatic analysis identified 12 members of the PHT1 gene family (SiPHT1;1-1;12) in S. italica, and RT and qPCR analysis showed that most of these transporters displayed specific expression patterns with respect to tissue, phosphate status and arbuscular mycorrhizal colonisation. SiPHT1;2 was found to be expressed in all tissues and in all growth conditions tested. In contrast, expression of SiPHT1;4 was induced in roots after 15 days growth in hydroponic medium of low Pi concentration. Expression of SiPHT1;8 and SiPHT1;9 in roots was selectively induced by colonisation with F. mosseae. SiPHT1;3 and SiPHT1;4 were found to be predominantly expressed in leaf and root tissues respectively. Several other transporters were expressed in shoots and leaves during growth in low Pi concentrations. This study will form the basis for the further characterization of these transporters, with the long term goal of improving the phosphate use efficiency of foxtail millet.

Molecular basis of adaptation to high soil boron in wheat landraces and elite cultivars

Environmental constraints severely restrict crop yields in most production environments, and expanding the use of variation will underpin future progress in breeding. In semi-arid environments boron toxicity constrains productivity, and genetic improvement is the only effective strategy for addressing the problem. Wheat breeders have sought and used available genetic diversity from landraces to maintain yield in these environments; however, the identity of the genes at the major tolerance loci was unknown. Here we describe the identification of near-identical, root-specific boron transporter genes underlying the two major-effect quantitative trait loci for boron tolerance in wheat, Bo1 and Bo4 (ref. 2). We show that tolerance to a high concentration of boron is associated with multiple genomic changes including tetraploid introgression, dispersed gene duplication, and variation in gene structure and transcript level. An allelic series was identified from a panel of bread and durum wheat cultivars and landraces originating from diverse agronomic zones. Our results demonstrate that, during selection, breeders have matched functionally different boron tolerance alleles to specific environments. The characterization of boron tolerance in wheat illustrates the power of the new wheat genomic resources to define key adaptive processes that have underpinned crop improvement.

Phosphate transporters OsPHT1;9 and OsPHT1;10 are involved in phosphate uptake in rice

We characterized the function of two rice phosphate (Pi) transporters: OsPHT1;9 (OsPT9) and OsPHT1;10 (OsPT10). OsPT9 and OsPT10 were expressed in the root epidermis, root hairs and lateral roots, with their expression being specifically induced by Pi starvation. In leaves, expression of the two genes was observed in both mesophyll and vasculature. High-affinity Km values for Pi transport of OsPT9 and OsPT10 were determined by yeast experiments and two-electrode voltage clamp analysis of anion transport in Xenopus oocytes expressing OsPT9 and OsPT10. Pi uptake and Pi concentrations in transgenic plants harbouring overexpressed OsPT9 and OsPT10 were determined by Pi concentration analysis and (33) P-labelled Pi uptake rate analysis. Significantly higher Pi uptake rates in transgenic plants compared with wild-type plants were observed under both high-Pi and low-Pi solution culture conditions. Conversely, although no alterations in Pi concentration were found in OsPT9 or OsPT10 knockdown plants, a significant reduction in Pi concentration in both shoots and roots was observed in double-knockdown plants grown under both high- and low-Pi conditions. Taken together, our results suggest that OsPT9 and OsPT10 redundantly function in Pi uptake.

Organ-specific phosphorus-allocation patterns and transcript profiles linked to phosphorus efficiency in two contrasting wheat genotypes

Recent studies have identified genotypic variation in phosphorus (P) efficiency, but rarely have the underlying mechanisms been described at the molecular level. We demonstrate that the highly P-efficient wheat (Triticum aestivum L.) cultivar Chinese 80-55 maintains higher inorganic phosphate (Pi ) concentrations in all organs upon Pi withdrawal in combination with higher Pi acquisition in the presence of Pi when compared with the less-efficient cultivar Machete. These findings correlated with differential organ-specific expression of Pi transporters TaPHT1;2, TaPHT1;5, TaPHT1;8, TaPHT2;1 and H(+) -ATPase TaHa1. Observed transcript level differences between the cultivars suggest that higher de novo phospholipid biosynthetic activities in Pi -limited elongating basal leaf sections are another crucial adaptation in Chinese 80-55 for sustaining growth upon Pi withdrawal. These activities may be supported through enhanced breakdown of starch in Chinese 80-55 stems as suggested by higher TaGPho1 transcript levels. Chinese 80-55 fine roots on the other hand show strong suppression of transcripts involved in glycolysis, transcriptional regulation and ribosomal activities. Our work reveals major differences in the way the two contrasting cultivars allocate Pi and organic P compounds between source and sink tissues and in the acclimation of their metabolism to changes in Pi availability.

Genome-wide investigation and expression analysis suggest diverse roles and genetic redundancy of Pht1 family genes in response to Pi deficiency in tomato

BACKGROUND

Phosphorus (P) deficiency is one of the major nutrient stresses limiting plant growth. The uptake of P by plants is well considered to be mediated by a number of high-affinity phosphate (Pi) transporters belonging to the Pht1 family. Although the Pht1 genes have been extensively identified in several plant species, there is a lack of systematic analysis of the Pht1 gene family in any solanaceous species thus far.

RESULTS

Here, we report the genome-wide analysis, phylogenetic evolution and expression patterns of the Pht1 genes in tomato (Solanum lycopersicum). A total of eight putative Pht1 genes (LePT1 to 8), distributed on three chromosomes (3, 6 and 9), were identified through extensive searches of the released tomato genome sequence database. Chromosomal organization and phylogenetic tree analysis suggested that the six Pht1 paralogues, LePT1/3, LePT2/6 and LePT4/5, which were assigned into three pairs with very close physical distance, were produced from recent tandem duplication events that occurred after Solanaceae splitting with other dicot families. Expression analysis of these Pht1 members revealed that except LePT8, of which the transcript was undetectable in all tissues, the other seven paralogues showed differential but partial-overlapping expression patterns. LePT1 and LePT7 were ubiquitously expressed in all tissues examined, and their transcripts were induced abundantly in response to Pi starvation; LePT2 and LePT6, the two paralogues harboring identical coding sequence, were predominantly expressed in Pi-deficient roots; LePT3, LePT4 and LePT5 were strongly activated in the roots colonized by arbuscular mycorrhizal fungi under low-P, but not high-P condition. Histochemical analysis revealed that a 1250-bp LePT3 promoter fragment and a 471-bp LePT5 promoter fragment containing the two elements, MYCS and P1BS, were sufficient to direct the GUS reporter expression in mycorrhizal roots and were limited to distinct cells harboring AM fungal structures. Additionally, the four paralogues, LePT1, LePT2, LePT6 and LePT7, were very significantly down-regulated in the mycorrhizal roots under low Pi supply condition.

CONCLUSIONS

The results obtained from this study provide new insights into the evolutionary expansion, functional divergence and genetic redundancy of the Pht1 genes in response to Pi deficiency and mycorrhizal symbiosis in tomato.

Functional properties and expression quantitative trait loci for phosphate transporter GmPT1 in soybean

Phosphate (Pi) remobilization within a plant is critical for plant survival under Pi-limiting conditions. In this paper, a soybean Pi transporter gene, GmPT1, was characterized. A marked induction of GmPT1 transcript was observed in young leaves, mature leaves and lateral roots during long-term Pi starvation. Transgenic tobacco plants containing the GmPT1 gene were obtained using an Agrobacterium-mediated transformation system. Compared with wild-type plants, transgenic plants showed significant increases in phosphorus-use efficiency (PUE), photosystem II (PSII) function, total dry weight and seed weight under Pi-deficient conditions. GmPT1 expression levels and PUE were determined in a soybean recombinant inbred line population during a pot experiment that was conducted to measure chlorophyll fluorescence parameters, photosynthetic rate (PN ) and seed yield. Correlation analysis revealed that GmPT1 expression levels had significantly positive correlations with seed yield, PUE, PN and the quantum yield of PSII primary photochemistry (ΦPSII ). Expression quantitative trait loci (eQTL) mapping for GmPT1 revealed two eQTLs, one of which coincided with both the physical location of GmPT1 and a QTL associated with seed yield. These results suggest that GmPT1 plays a role in Pi remobilization, and it may be possible to improve soybean seed yields under Pi-limiting conditions by modulating GmPT1 expression levels.

HvZIP7 mediates zinc accumulation in barley (Hordeum vulgare) at moderately high zinc supply

High expression of zinc (Zn)-regulated, iron-regulated transporter-like protein (ZIP) genes increases root Zn uptake in dicots, leading to high accumulation of Zn in shoots. However, none of the ZIP genes tested previously in monocots could enhance shoot Zn accumulation. In this report, barley (Hordeum vulgare) HvZIP7 was investigated for its functions in Zn transport. The functions of HvZIP7 in planta were studied using in situ hybridization and transient analysis of subcellular localization with a green fluorescent protein (GFP) reporter. Transgenic barley lines overexpressing HvZIP7 were also generated to further understand the functions of HvZIP7 in metal transport. HvZIP7 is strongly induced by Zn deficiency, primarily in vascular tissues of roots and leaves, and its protein was localized in the plasma membrane. These properties are similar to its closely related homologs in dicots. Overexpression of HvZIP7 in barley plants increased Zn uptake when moderately high concentrations of Zn were supplied. Significantly, there was a specific enhancement of shoot Zn accumulation, with no measurable increase in iron (Fe), manganese (Mn), copper (Cu) or cadmium (Cd). HvZIP7 displays characteristics of low-affinity Zn transport. The unique function of HvZIP7 provides new insights into the role of ZIP genes in Zn homeostasis in monocots, and offers opportunities to develop Zn biofortification strategies in cereals.

Isolation and characterization of a novel wheat cysteine-rich receptor-like kinase gene induced by Rhizoctonia cerealis

Cysteine-rich receptor kinases (CRKs) belong to the receptor-like kinase family. Little is known about CRK genes in wheat. We isolated a wheat CRK gene TaCRK1 from Rhizoctonia cerealis-resistant wheat CI12633 based on a differentially expressed sequence identified by RNA-Sequencing (RNA-Seq) analysis. TaCRK1 was more highly expressed in CI12633 than in susceptible Wenmai 6. Transcription of TaCRK1 in wheat was induced in CI12633 after R. cerealis infection and exogenous abscisic acid (ABA) treatment. The deduced TaCRK1 protein contained a signal peptide, two DUF26 domains, a transmembrane domain, and a serine/threonine protein kinase domain. Transient expression of a green fluorescence protein fused with TaCRK1 in wheat and onion indicated that TaCRK1 may localize to plasma membranes. Characterization of TaCRK1 silencing induced by virus-mediated method in CI12633 showed that the downregulation of TaCRK1 transcript did not obviously impair resistance to R. cerealis. This study paves the way to further CRK research in wheat.

Phosphorus nutrition of phosphorus-sensitive Australian native plants: threats to plant communities in a global biodiversity hotspot

South-western Australia harbours a global biodiversity hotspot on the world's most phosphorus (P)-impoverished soils. The greatest biodiversity occurs on the most severely nutrient-impoverished soils, where non-mycorrhizal species are a prominent component of the flora. Mycorrhizal species dominate where soils contain slightly more phosphorus. In addition to habitat loss and dryland salinity, a major threat to plant biodiversity in this region is eutrophication due to enrichment with P. Many plant species in the south-western Australian biodiversity hotspot are extremely sensitive to P, due to a low capability to down-regulate their phosphate-uptake capacity. Species from the most P-impoverished soils are also very poor competitors at higher P availability, giving way to more competitive species when soil P concentrations are increased. Sources of increased soil P concentrations include increased fire frequency, run-off from agricultural land, and urban activities. Another P source is the P-fertilizing effect of spraying natural environments on a landscape scale with phosphite to reduce the impacts of the introduced plant pathogen Phytophthora cinnamomi, which itself is a serious threat to biodiversity. We argue that alternatives to phosphite for P. cinnamomi management are needed urgently, and propose a strategy to work towards such alternatives, based on a sound understanding of the physiological and molecular mechanisms of the action of phosphite in plants that are susceptible to P. cinnamomi. The threats we describe for the south-western Australian biodiversity hotspot are likely to be very similar for other P-impoverished environments, including the fynbos in South Africa and the cerrado in Brazil.

Phosphorus nutrition of phosphorus-sensitive Australian native plants: threats to plant communities in a global biodiversity hotspot

South-western Australia harbours a global biodiversity hotspot on the world's most phosphorus (P)-impoverished soils. The greatest biodiversity occurs on the most severely nutrient-impoverished soils, where non-mycorrhizal species are a prominent component of the flora. Mycorrhizal species dominate where soils contain slightly more phosphorus. In addition to habitat loss and dryland salinity, a major threat to plant biodiversity in this region is eutrophication due to enrichment with P. Many plant species in the south-western Australian biodiversity hotspot are extremely sensitive to P, due to a low capability to down-regulate their phosphate-uptake capacity. Species from the most P-impoverished soils are also very poor competitors at higher P availability, giving way to more competitive species when soil P concentrations are increased. Sources of increased soil P concentrations include increased fire frequency, run-off from agricultural land, and urban activities. Another P source is the P-fertilizing effect of spraying natural environments on a landscape scale with phosphite to reduce the impacts of the introduced plant pathogen Phytophthora cinnamomi, which itself is a serious threat to biodiversity. We argue that alternatives to phosphite for P. cinnamomi management are needed urgently, and propose a strategy to work towards such alternatives, based on a sound understanding of the physiological and molecular mechanisms of the action of phosphite in plants that are susceptible to P. cinnamomi. The threats we describe for the south-western Australian biodiversity hotspot are likely to be very similar for other P-impoverished environments, including the fynbos in South Africa and the cerrado in Brazil.

TaPht1;4, a high-affinity phosphate transporter gene in wheat (Triticum aestivum), plays an important role in plant phosphate acquisition under phosphorus deprivation

An expressed sequence tag (EST) highly similar to BdPT1-4, a phosphate transporter (PT) gene in Brachypodium distachyon, was obtained in a wheat root cDNA subtractive suppression library containing genes that respond to low-phosphate (Pi) stress. The DNA sequence covering this EST (designated as TaPht1;4) was determined based on screening a wheat DNA library. TaPht1;4 consists of two exons and one intron and encodes a 555 amino acid (aa) polypeptide with a molecular weight of 60.85kDa and an isoelectric point of 7.60. TaPht1;4 contains 12 conserved membrane-spanning domains similar to previously reported PTs in diverse plant species. Yeast complement analysis in low-Pi medium confirmed that TaPht1;4 confers the capacity to uptake Pi to MB192, a yeast strain with a defective high-affinity PT; with an apparent Km of 35.3μM. The TaPht1;4 transcripts were specifically detected in the root and were highly induced under Pi deficiency. TaPht1;4 was also expressed following a diurnal pattern, i.e. high levels during daytime and low levels during night-time. TaPht1;4 overexpression and downregulation dramatically altered the plant phenotypic features under low-Pi conditions. Samples that overexpressed TaPht1;4 had significantly improved growth traits and accumulated more Pi than the wild-type plant and those with downregulated TaPht1;4 expression. Therefore, TaPht1;4 is a high-affinity PT gene that plays a critical role in wheat Pi acquisition under Pi deprivation.

Function of wheat phosphate transporter gene TaPHT2;1 in Pi translocation and plant growth regulation under replete and limited Pi supply conditions

Several phosphate transporters (PTs) that belong to the Pht2 family have been released in bioinformatics databases, but only a few members of this family have been functionally characterized. In this study, we found that wheat TaPHT2;1 shared high identity with a subset of Pht2 in diverse plants. Expression analysis revealed that TaPHT2;1 was strongly expressed in the leaves, was up-regulated by low Pi stress, and exhibited a circadian rhythmic expression pattern. TaPHT2;1-green fluorescent protein fusions in the leaves of tobacco and wheat were specifically detected in the chloroplast envelop. TaPHT2;1 complemented the Pi transporter activities in a yeast mutant with a defect in Pi uptake. Knockdown expression of TaPHT2;1 significantly reduced Pi concentration in the chloroplast under sufficient (2 mM Pi) and deficient Pi (100 μM Pi) conditions, suggesting that TaPHT2;1 is crucial in the mediation of Pi translocation from the cytosol to the chloroplast. The down-regulated expression of TaPHT2;1 resulted in reduced photosynthetic capacities, total P contents, and accumulated P amounts in plants under sufficient and deficient Pi conditions, eventually leading to worse plant growth phenotypes. The TaPHT2;1 knockdown plants exhibited pronounced decrease in accumulated phosphorus in sufficient and deficient Pi conditions, suggesting that TaPHT2;1 is an important factor to associate with a distinct P signaling that up-regulates other PT members to control Pi acquisition and translocation within plants. Therefore, TaPHT2;1 is a key member of the Pht2 family involved in Pi translocation, and that it can function in the improvement of phosphorus usage efficiency in wheat.

A constitutive expressed phosphate transporter, OsPht1;1, modulates phosphate uptake and translocation in phosphate-replete rice

A number of phosphate (Pi) starvation- or mycorrhiza-regulated Pi transporters belonging to the Pht1 family have been functionally characterized in several plant species, whereas functions of the Pi transporters that are not regulated by changes in Pi supply are lacking. In this study, we show that rice (Oryza sativa) Pht1;1 (OsPT1), one of the 13 Pht1 Pi transporters in rice, was expressed abundantly and constitutively in various cell types of both roots and shoots. OsPT1 was able to complement the proton-coupled Pi transporter activities in a yeast mutant defective in Pi uptake. Transgenic plants of OsPT1 overexpression lines and RNA interference knockdown lines contained significantly higher and lower phosphorus concentrations, respectively, compared with the wild-type control in Pi-sufficient shoots. These responses of the transgenic plants to Pi supply were further confirmed by the changes in depolarization of root cell membrane potential, root hair occurrence, (33)P uptake rate and transportation, as well as phosphorus accumulation in young leaves at Pi-sufficient levels. Furthermore, OsPT1 expression was strongly enhanced by the mutation of Phosphate Overaccumulator2 (OsPHO2) but not by Phosphate Starvation Response2, indicating that OsPT1 is involved in the OsPHO2-regulated Pi pathway. These results indicate that OsPT1 is a key member of the Pht1 family involved in Pi uptake and translocation in rice under Pi-replete conditions.

Structure and expression profile of the phosphate Pht1 transporter gene family in mycorrhizal Populus trichocarpa

Gene networks involved in inorganic phosphate (Pi) acquisition and homeostasis in woody perennial species able to form mycorrhizal symbioses are poorly known. Here, we describe the features of the 12 genes coding for Pi transporters of the Pht1 family in poplar (Populus trichocarpa). Individual Pht1 transporters play distinct roles in acquiring and translocating Pi in different tissues of mycorrhizal and nonmycorrhizal poplar during different growth conditions and developmental stages. Pi starvation triggered the up-regulation of most members of the Pht1 family, especially PtPT9 and PtPT11. PtPT9 and PtPT12 showed a striking up-regulation in ectomycorrhizas and endomycorrhizas, whereas PtPT1 and PtPT11 were strongly down-regulated. PtPT10 transcripts were highly abundant in arbuscular mycorrhiza (AM) roots only. PtPT8 and PtPT10 are phylogenetically associated to the AM-inducible Pht1 subfamily I. The analysis of promoter sequences revealed conserved motifs similar to other AM-inducible orthologs in PtPT10 only. To gain more insight into gene regulatory mechanisms governing the AM symbiosis in woody plant species, the activation of the poplar PtPT10 promoter was investigated and detected in AM of potato (Solanum tuberosum) roots. These results indicated that the regulation of AM-inducible Pi transporter genes is conserved between perennial woody and herbaceous plant species. Moreover, poplar has developed an alternative Pi uptake pathway distinct from AM plants, allowing ectomycorrhizal poplar to recruit PtPT9 and PtPT12 to cope with limiting Pi concentrations in forest soils.