Phosphorus dynamics: from soil to plant

Major Crop Species Show Differential Balance between Root Morphological and Physiological Responses to Variable Phosphorus Supply

The relationship between root morphological and physiological responses to variable P supply in different plant species is poorly understood. We compared root morphological and physiological responses to P supply in seven crop species (Zea mays, Triticum aestivum, Brassica napus, Lupinus albus, Glycine max, Vicia faba, Cicer arietinum) treated with or without 100 mg P kg-1 in two soils (acidic and calcareous). Phosphorus deficiency decreased root length more in fibrous root species (Zea mays, Triticum aestivum, Brassica napus) than legumes. Zea mays and Triticum aestivum had higher root/shoot biomass ratio and Brassica napus had higher specific root length compared to legumes, whereas legumes (except soybean) had higher carboxylate exudation than fibrous root species. Lupinus albus exhibited the highest P-acquisition efficiency due to high exudation of carboxylates and acid phosphatases. Lupinus albus and Cicer arietinum depended mostly on root exudation (i.e., physiological response) to enhance P acquisition, whereas Zea mays, Triticum aestivum and Brassica napus had higher root morphology dependence, with Glycine max and Vicia faba in between. Principal component analysis using six morphological and six physiological responses identified root size and diameter as the most important morphological traits, whereas important physiological responses included carboxylate exudation, and P-acquisition and P-utilization efficiency followed by rhizosphere soil pH and acid phosphatase activity. In conclusion, plant species can be grouped on the basis of their response to soil P being primarily via root architectural or exudation plasticity, suggesting a potential benefit of crop-specific root-trait-based management to cope with variable soil P supply in sustainable grain production.

Soil properties and species composition under different grazing intensity in an alpine meadow on the eastern Tibetan Plateau, China

As the main form of land use and human disturbance of grassland, livestock grazing has great influences on the soil resources and plant communities. This study observed the variation of soil properties and community characteristics of four treatments of different grazing intensity (no grazing, UG; light grazing, LG; moderate grazing, MG; and heavy grazing, HG) in an alpine meadow of Sichuan Province on the northeastern margin of the Tibetan Plateau. The results showed that grazing increased the pH, soil bulk density (BD), and contents of total carbon (TC) and total nitrogen (TN), and the BD increased while the others decreased with the grazing intensity. At the community level, with the increase of the grazing intensity, the vegetation coverage (R ² = 0.61, P < 0.001), mean height of community (R ² = 0.37, P < 0.001), aboveground biomass (R ² = 0.54, P < 0.001), litter biomass (R ² = 0.84, P < 0.001), and percentage of aboveground biomass of palatable grasses to total biomass (R ² = 0.74, P < 0.001) significantly decreased, while the belowground biomass (R ² = 0.72, P < 0.001) and the root/shoot (R/S) ratio (R ² = 0.65, P < 0.001) increased. The species richness was the greatest at LG and the total biomass at UG. With grazing, the dominant species of the plant community shifted from palatable grasses (Gramineae and Cyperaceae) to unpalatable grasses (Compositae and Ranunculaceae). Based on the results, LG may be the optimal grassland management mode to be used in the long time in the alpine meadow of the Tibetan Plateau.

Phosphate availability regulates ethylene biosynthesis gene expression and protein accumulation in white clover (Trifolium repens L.) roots

The expression and accumulation of members of the 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) and ACC oxidase (ACO) gene families was examined in white clover roots grown in either Pi (phosphate) sufficient or Pi-deprived defined media. The accumulation of one ACO isoform, TR-ACO1, was positively influenced after only 1 h of exposure to low Pi, and this was maintained over a 7-day time-course. Up-regulation of TR-ACS1, TR-ACS2 and TR-ACS3 transcript abundance was also observed within 1 h of exposure to low Pi in different tissue regions of the roots, followed by a second increase in abundance of TR-ACS2 after 5-7 days of exposure. An increase in transcript abundance of TR-ACO1 and TR-ACO3, but not TR-ACO2, was observed after 1 h of exposure to low Pi, with a second increase in TR-ACO1 transcripts occurring after 2-5 days. These initial increases of the TR-ACS and TR-ACO transcript abundance occurred before the induction of Trifolium repens PHOSPHATE TRANSPORTER 1 (TR-PT1), and the addition of sodium phosphite did not up-regulate TR-ACS1 expression over 24 h. In situ hybridization revealed some overlap of TR-ACO mRNA accumulation, with TR-ACO1 and TR-ACO2 in the root tip regions, and TR-ACO1 and TR-ACO3 mRNA predominantly in the lateral root primordia. TR-ACO1p-driven GFP expression showed that activation of the TR-ACO1 promoter was initiated within 24 h of exposure to low Pi (as determined by GFP protein accumulation). These results suggest that the regulation of ethylene biosynthesis in white clover roots is biphasic in response to low Pi supply.

Genotypic Differences in Phosphorus Efficiency and the Performance of Physiological Characteristics in Response to Low Phosphorus Stress of Soybean in Southwest of China

Southwest of China is one of the major soybean (Glycine max L.) production regions in China with low availability of soil phosphorus (P). Whereas little information is available on P-efficient soybean genotypes in this region, even though using P-efficient soybean genotypes is a sustainable P management strategy for enhancing yield and P use efficiency. To assess the genetic variation on P use efficiency, 274 soybean genotypes were employed to compare the yields and P acquisition potentials in the field. Additionally, 10 representational genotypes (5 P-efficient genotypes and 5 P-inefficient genotypes) were grown in hydroponic media containing low P treatment (0.05 mM L-1) and high P treatment (0.25 mM L-1) to further investigate P assimilation characteristics and the related mechanisms of P-efficient soybean genotypes. In the field trial, the models described the relationships between yield and seed P concentration (R2 = 0.85), shoot P accumulation (R2 = 0.84), HI (R2 = 0.82) well. The yield, seed P concentration and shoot P accumulation ranged from 5.5 to 36.0 g plant-1, from 0.045 to 0.93% and from 0.065 to 0.278 mg plant-1, respectively. In the hydroponic trial, P-efficient genotypes under low P treatment showed significantly better plant growth, P accumulation and root: shoot ratio than P-inefficient genotypes. Simultaneously, total root length, specific root length, root surface area and root volume of P-efficient were significantly greater than P-inefficient under low P treatment. Higher rate of organic acid exudation and acid phosphatase activities was observed in the P-efficient soybean genotypes under low P condition when compared to the P-inefficient soybean genotypes. It indicated that significant genetic variation for P use efficiency existed in this region, and the P-efficient soybean genotypes, especially E311 and E141, demonstrated great tolerance to P deficiency, which could be potential materials using in improving production and P use efficiency in low availability of soil P region.

Increase phosphorus availability from the use of alfalfa (Medicago sativa L) green manure in rice (Oryza sativa L.) agroecosystem

Alfalfa is a good green manure source, but its effect on rice growth has not been fully elucidated. Two green manure species, alfalfa and broad bean (Vicia faba L.), and two N fertilizer levels, alone or combination, were applied to a rice field. The results indicated that alfalfa had more pronounced effects on increasing soil labile phosphorus (P) fractions (including NaHCO₃-Pi, NaOH-Pi), P uptake and soil enzyme activities (dehydrogenase, urease, acid phosphatase and β-glucosidase) than broad bean and N fertilizer. The transformation of NaHCO₃-Po to labile P regulated by alfalfa played a significant direct and indirect effect on grain yield. Although a much lower N input from alfalfa addition, a similar grain yield with N fertilizer treatment was achieved, and the integration of alfalfa with N fertilizer produced the highest grain yield and P availability, which was associated with the highest urease, acid phosphatase and β-glucosidase activity in soil. These results indicate that alfalfa green manure had a great ability of increasing grain yield through enhancing P availability in rice paddy, which could give us a way to reduce N fertilizer application by enhancing P availability.

Phosphorus speciation by (31)P NMR spectroscopy in bracken (Pteridium aquilinum (L.) Kuhn) and bluebell (Hyacinthoides non-scripta (L.) Chouard ex Rothm.) dominated semi-natural upland soil

Access to P species is a driver for plant community composition based on nutrient acquisition. Here we investigated the distribution and accumulation of soil inorganic P (Pi) and organic P (Po) forms in a bracken and bluebell dominated upland soil for the period between bluebell above ground dominance until biomass is formed from half bluebells and half bracken. Chemical characterisation and (31)P Nuclear Magnetic Resonance spectroscopy was used to determine the organic and inorganic P species. Total P concentration in soils was 0.87gkg(-1), while in plants (above- and below-ground parts) total P ranged between 0.84-4.0gkg(-1) and 0.14-2.0gkg(-1) for bluebell and bracken, respectively. The P speciation in the plant samples was reflected in the surrounding soil. The main forms of inorganic P detected in the NaOH-EDTA soil extracts were orthophosphate (20.0-31.5%), pyrophosphate (0.6-2.5%) and polyphosphate (0.4-7.0%). Phytate (myo-IP6) was the most dominant organic P form (23.6-40.0%). Other major peaks were scyllo-IP6 and α- and β- glycerophosphate (glyP). In bluebells and bracken the main P form detected was orthophosphate ranging from (21.7-80.4%) and 68.5-81.1%, in above-ground and below-ground biomass, respectively. Other detected forms include α-glyP (4.5-14.4%) and β-glyP (0.9-7.7%) in bluebell, while in bracken they were detected only in stripe and blade in ranges of 2.5-5.5% and 4.4-9.6%, respectively. Pyrophosphate, polyphosphate, scyllo-IP6, phosphonates, found in soil samples, were not detected in any plant parts. In particular, the high abundance of phytate in the soil and in bluebell bulbs, may be related to a mechanism through which bluebells create a recalcitrant phosphorus store which form a key part of their adaptation to nutrient poor conditions.

Comparative genomic, proteomic and exoproteomic analyses of three Pseudomonas strains reveals novel insights into the phosphorus scavenging capabilities of soil bacteria

Bacteria that inhabit the rhizosphere of agricultural crops can have a beneficial effect on crop growth. One such mechanism is the microbial-driven solubilization and remineralization of complex forms of phosphorus (P). It is known that bacteria secrete various phosphatases in response to low P conditions. However, our understanding of their global proteomic response to P stress is limited. Here, exoproteomic analysis of Pseudomonas putida BIRD-1 (BIRD-1), Pseudomonas fluorescens SBW25 and Pseudomonas stutzeri DSM4166 was performed in unison with whole-cell proteomic analysis of BIRD-1 grown under phosphate (Pi) replete and Pi deplete conditions. Comparative exoproteomics revealed marked heterogeneity in the exoproteomes of each Pseudomonas strain in response to Pi depletion. In addition to well-characterized members of the PHO regulon such as alkaline phosphatases, several proteins, previously not associated with the response to Pi depletion, were also identified. These included putative nucleases, phosphotriesterases, putative phosphonate transporters and outer membrane proteins. Moreover, in BIRD-1, mutagenesis of the master regulator, phoBR, led us to confirm the addition of several novel PHO-dependent proteins. Our data expands knowledge of the Pseudomonas PHO regulon, including species that are frequently used as bioinoculants, opening up the potential for more efficient and complete use of soil complexed P.

Overcoming Phosphorus Deficiency in West African Pearl Millet and Sorghum Production Systems: Promising Options for Crop Improvement

West Africa (WA) is among the most food insecure regions. Rapid human population growth and stagnating crop yields greatly contribute to this fact. Poor soil fertility, especially low plant available phosphorus (P) is constraining food production in the region. P-fertilizer use in WA is among the lowest in the world due to inaccessibility and high prices, often unaffordable to resource-poor subsistence farmers. This article provides an overview of soil P-deficiency in WA and opportunities to overcome it by exploiting sorghum and pearl millet genetic diversity. The topic is examined from the perspectives of plant breeding, soil science, plant physiology, plant nutrition, and agronomy, thereby referring to recent results obtained in a joint interdisciplinary research project, and reported literature. Specific objectives are to summarize: (1) The global problem of P scarcity and how it will affect WA farmers; (2) Soil P dynamics in WA soils; (3) Plant responses to P deficiency; (4) Opportunities to breed for improved crop adaptation to P-limited conditions; (5) Challenges and trade-offs for improving sorghum and pearl millet adaptation to low-P conditions in WA; and (6) Systems approaches to address soil P-deficiency in WA. Sorghum and pearl millet in WA exhibit highly significant genetic variation for P-uptake efficiency, P-utilization efficiency, and grain yield under P-limited conditions indicating the possibility of breeding P-efficient varieties. Direct selection under P-limited conditions was more efficient than indirect selection under high-P conditions. Combining P-uptake and P-utilization efficiency is recommendable for WA to avoid further soil mining. Genomic regions responsible for P-uptake, P-utilization efficiency, and grain yield under low-P have been identified in WA sorghum and pearl millet, and marker-assisted selection could be possible once these genomic regions are validated. Developing P-efficient genotypes may not, however, be a sustainable solution in itself in the long-term without replenishing the P removed from the system in harvested produce. We therefore propose the use of integrated soil fertility management and systems-oriented management such as enhanced crop-tree-livestock integration in combination with P-use-efficiency-improved varieties. Recycling P from animal bones, human excreta and urine are also possible approaches toward a partially closed and efficient P cycle in WA.

Grain production versus resource and environmental costs: towards increasing sustainability of nutrient use in China

Over the past five decades, Chinese grain production has increased 4-fold, from 110 Mt in 1961 to 557 Mt in 2014, with less than 9% of the world's arable land feeding 22% of the world's population, indicating a substantial contribution to global food security. However, compared with developed economies, such as the USA and the European Union, more than half of the increased crop production in China can be attributed to a rapid increase in the consumption of chemicals, particularly fertilizers. Excessive fertilization has caused low nutrient use efficiency and high environmental costs in grain production. We analysed the key requirements underpinning increased sustainability of crop production in China, as follows: (i) enhance nutrient use efficiency and reduce nutrient losses by fertilizing roots not soil to maximize root/rhizosphere efficiency with innovative root zone nutrient management; (ii) improve crop productivity and resource use efficiency by matching the best agronomic management practices with crop improvement; and (iii) promote technology transfer of the root zone nutrient management to achieve the target of high yields and high efficiency with low environmental risks on a broad scale. Coordinating grain production and environmental protection by increasing the sustainability of nutrient use will be a key step in achieving sustainable crop production in Chinese agriculture.

Responses of Reclamation Plants to High Root Zone pH: Effects of Phosphorus and Calcium Availability

Low phosphorus (P) availability and high pH inhibit plant growth in calcareous soils and some oil sands reclamation sites in northeastern Alberta, Canada. In this study, we used a split-root hydroponic setup to test the effects of supplemental P with different calcium (Ca) concentrations and root-zone pH conditions on the growth and physiological response of trees commonly found in the region: paper birch ( Marsh.), trembling aspen ( Michx.), green alder [ (Chaix) DC.], and black spruce [ (Mill.) Britton, Sterns & Poggenb.] seedlings. Plant roots were divided and treated with different combinations of P (0.5 and 15 mmol L), Ca (2 and 50 mmol L), and pH (5.0 and 9.0) for 6 wk. After that time, we measured seedling height, net photosynthesis and transpiration rates, and the concentration of chlorophyll and different elements in the leaves. Plant responses varied between species; black spruce was most resistant to high pH and high Ca concentrations. We did not find any strong beneficial effects of adding P to plants subjected to high root zone pH and high Ca concentration. However, exposure of part of the root system to low pH alleviated the effects of high pH, likely through the improved supply of micronutrients. Because pH conditions are often not uniform in disturbed sites and reclamation soils, our findings may help improve potential reclamation and phytoremediation strategies for the oil sands, bauxite, and coal-combustion residue utilization industries.

Regulation of phosphorus uptake and utilization: transitioning from current knowledge to practical strategies

Phosphorus is a poorly bioavailable macronutrient that is essential for crop growth and yield. Overuse of phosphorus fertilizers results in low phosphorus use efficiency (PUE), has serious environmental consequences and accelerates the depletion of phosphorus mineral reserves. It has become extremely challenging to improve PUE while preserving global food supplies and maintaining environmental sustainability. Molecular and genetic analyses have revealed the primary mechanisms of phosphorus uptake and utilization and their relationships to phosphorus transporters, regulators, root architecture, metabolic adaptations, quantitative trait loci, hormonal signaling and microRNA. The ability to improve PUE requires a transition from this knowledge of molecular mechanisms and plant architecture to practical strategies. These could include: i) the use of arbuscular mycorrhizal fungal symbioses for efficient phosphorus mining and uptake; ii) intercropping with suitable crop species to achieve phosphorus activation and mobilization in the soil; and iii) tissue-specific overexpression of homologous genes with advantageous agronomic properties for higher PUE along with breeding for phosphorus-efficient varieties and introgression of key quantitative trait loci. More effort is required to further dissect the mechanisms controlling phosphorus uptake and utilization within plants and provide new insight into the means to efficiently improve PUE.

Molecular Evolution and Association of Natural Variation in ZmARF31 with Low Phosphorus Tolerance in Maize

Low-phosphorus (P) stress is one of the major factors constraining plant growth and yield. Improving plant tolerance to P starvation through molecular breeding is an efficient alternative to increase grain production. In the study, 331 diverse maize inbreds were used to detect nucleotide diversity and favorable alleles of ZmARF31, which plays a key role in low P responses and root architecture regulation. Significant phenotypic variation was found in each of 11 tested traits under both P and no-P treatments, and 30 single nucleotide polymorphisms (SNPs) and 14 insertion-deletions (InDels) were detected in ZmARF31 among the 331 maize inbreds. The 5'-untranslated region (UTR) of ZmARF31 showed a small linkage disequilibrium (LD) block under significant purifying selection, whereas the 3'-UTR showed the most abundant diversity and a larger LD block. Thirty, fourteen, and nine natural variations were identified in ZmARF31 that were associated with P-deficiency-tolerance traits (P ≤ 0.01) by using the general linear model (GLM), GLM incorporated with population structure, and mixed linear model, respectively. Four SNPs were significantly associated with the total dry weight (TDW) in the three models, of which SNPs S410 and S462 were located in a complete LD block. A further verification conducted in a recombinant inbred line population revealed that favorable allele G/G of non-synonymous mutation S410 and favorable allele with a 38 bp insertion of InDel S1442 exhibited positive genetic effects on the TDW and total root tips, respectively. Expression analysis further confirmed that ZmARF31 was highly expressed in the roots of low-P-tolerant inbred 178. The protein encoded by ZmARF31 was located both in the nucleus and cytoplasm. Haplotypes carrying more favorable alleles showed a greater effect on phenotypic variation than single loci. Such haplotypes should be helpful to develop valuable genetic markers and perform maize molecular breeding.

Characterization of purple acid phosphatases involved in extracellular dNTP utilization in Stylosanthes

Stylo (Stylosanthes spp.) is a pasture legume predominant in tropical and subtropical areas, where low phosphorus (P) availability is a major constraint for plant growth. Therefore, stylo might exhibit superior utilization of the P pool on acid soils, particularly organic P. However, little is known about mechanisms of inorganic phosphate (Pi) acquisition employed by stylo. In this study, the utilization of extracellular deoxy-ribonucleotide triphosphate (dNTP) and the underlying physiological and molecular mechanisms were examined for two stylo genotypes with contrasting P efficiency. Results showed that the P-efficient genotype, TPRC2001-1, was superior to the P-inefficient genotype, Fine-stem, when using dNTP as the sole P source. This was reflected by a higher dry weight and total P content for TPRC2001-1 than for Fine-stem, which was correlated with higher root-associated acid phosphatase (APase) activities in TPRC2001-1 under low P conditions. Subsequently, three PAP members were cloned from TPRC2001-1: SgPAP7, SgPAP10, and SgPAP26 Expression levels of these three SgPAPs were up-regulated by Pi starvation in stylo roots. Furthermore, there was a higher abundance of transcripts of SgPAP7 and SgPAP10 in TPRC2001-1 than in Fine-stem. Subcellular localization analysis demonstrated that these three SgPAPs were localized on the plasma membrane. Overexpression of these three SgPAPs could result in significantly increased root-associated APase activities, and thus extracellular dNTP utilization in bean hairy roots. Taken together, the results herein suggest that SgPAP7, SgPAP10, and SgPAP26 may differentially contribute to root-associated APase activities, and thus control extracellular dNTP utilization in stylo.

Preliminary validation of a sequential fractionation method to study phosphorus chemistry in a calcareous soil

A sequential fractionation method proposed by Jiang and Gu (1989) distinguished three types of calcium phosphates (Ca-P) according to their different plant availabilities. Three extractants, NaHCO3, NH4Ac, and H2SO4 were used to extract Ca2-P, Ca8-P, and Ca10-P types, respectively, from soil. This sequential fractionation method was tested and modified for analyzing the P chemistry of a calcareous soil. The solubility test and the model diagrams of the stability of the major Ca-P minerals showed that NaHCO3 was able to extract brushite (Ca2-P type), and NH4Ac extracted brushite and β-tricalcium P (Ca8-P type) as well as hydroxyapatite (Ca10-P type). Therefore the P forms targeted by extraction with NH4Ac should include both Ca8-and Ca10-P types. The sum of the P extracted by all extractants in the sequential fractionation method in the calcareous soil was in agreement with the total P measured by the perchloric acid digestion method. A proportion of organic P measured by the sequential fractionation method was in agreement with the result from solution (31)P NMR spectroscopy. This study showed that the modified sequential fractionation method and its target P forms would be useful for quantifying and characterizing inorganic and organic P in a calcareous soil, even though it should be used in combination with other techniques, such as solution (31)P NMR spectroscopy.

Genome-wide identification of microRNAs responding to early stages of phosphate deficiency in maize

Phosphorus (P) is an essential element involved in numerous biochemical reactions. In plants, stress responses, such as the expression of microRNAs (miRNAs), are induced to help them adapt to low phosphate (Pi) concentrations. In this study, deep sequencing was performed using the roots and leaves of maize seedlings grown under low Pi concentrations to identify miRNAs that are differentially expressed during the early stages of Pi deficiency. Eight small RNA libraries were constructed, and 159 known miRNAs representing 32 miRNA families and 10 novel miRNAs. Members of the miR396 family were extremely abundant. Further, 28 Pi-responsive miRNAs were identified (27 known and 1 novel) of which 8 and 7 were significantly expressed exclusively in leaf and root tissues, respectively. The analysis of Pi-responsive miRNAs target genes suggested that most target genes functioning as transcription factors were involved in root and leaf development. The expression profiles of selected Pi-responsive miRNAs and target genes were confirmed by quantitative real-time polymerase chain reaction (qRT-PCR). Moreover, we discuss the significance of the differences in expression patterns of these miRNAs during the early and later stages of Pi starvation. This study provides useful information concerning the role of miRNAs in response to Pi starvation and will further our understanding of the mechanisms governing Pi homeostasis in maize.

NPKS uptake, sensing, and signaling and miRNAs in plant nutrient stress

Sessile nature of higher plants consequently makes it highly adaptable for nutrient absorption and acquisition from soil. Plants require 17 essential elements for their growth and development which include 14 minerals (macronutrients: N, P, K, Mg, Ca, S; micronutrients: Cl, Fe, B, Mn, Zn, Cu, Ni, Mo) and 3 non-mineral (C, H, O) elements. The roots of higher plants must acquire these macronutrients and micronutrients from rhizosphere and further allocate to other plant parts for completing their life cycle. Plants evolved an intricate series of signaling and sensing cascades to maintain nutrient homeostasis and to cope with nutrient stress/availability. The specific receptors for nutrients in root, root system architecture, and internal signaling pathways help to develop plasticity in response to the nutrient starvation. Nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) are essential for various metabolic processes, and their deficiency negatively effects the plant growth and yield. Genes coding for transporters and receptors for nutrients as well as some small non-coding RNAs have been implicated in nutrient uptake and signaling. This review summarizes the N, P, K, and S uptake, sensing and signaling events in nutrient stress condition especially in model plant Arabidopsis thaliana and involvement of microRNAs in nutrient deficiency. This article also provides a framework of uptake, sensing, signaling and to highlight the microRNA as an emerging major players in nutrient stress condition. Nutrient-plant-miRNA cross talk may help plant to cope up nutrient stress, and understanding their precise mechanism(s) will be necessary to develop high yielding smart crop with low nutrient input.

Comparative expression profiling reveals a role of the root apoplast in local phosphate response

BACKGROUND

Plant adaptation to limited phosphate availability comprises a wide range of responses to conserve and remobilize internal phosphate sources and to enhance phosphate acquisition. Vigorous restructuring of root system architecture provides a developmental strategy for topsoil exploration and phosphate scavenging. Changes in external phosphate availability are locally sensed at root tips and adjust root growth by modulating cell expansion and cell division. The functionally interacting Arabidopsis genes, LOW PHOSPHATE RESPONSE 1 and 2 (LPR1/LPR2) and PHOSPHATE DEFICIENCY RESPONSE 2 (PDR2), are key components of root phosphate sensing. We recently demonstrated that the LOW PHOSPHATE RESPONSE 1 - PHOSPHATE DEFICIENCY RESPONSE 2 (LPR1-PDR2) module mediates apoplastic deposition of ferric iron (Fe(3+)) in the growing root tip during phosphate limitation. Iron deposition coincides with sites of reactive oxygen species generation and triggers cell wall thickening and callose accumulation, which interfere with cell-to-cell communication and inhibit root growth.

RESULTS

We took advantage of the opposite phosphate-conditional root phenotype of the phosphate deficiency response 2 mutant (hypersensitive) and low phosphate response 1 and 2 double mutant (insensitive) to investigate the phosphate dependent regulation of gene and protein expression in roots using genome-wide transcriptome and proteome analysis. We observed an overrepresentation of genes and proteins that are involved in the regulation of iron homeostasis, cell wall remodeling and reactive oxygen species formation, and we highlight a number of candidate genes with a potential function in root adaptation to limited phosphate availability. Our experiments reveal that FERRIC REDUCTASE DEFECTIVE 3 mediated, apoplastic iron redistribution, but not intracellular iron uptake and iron storage, triggers phosphate-dependent root growth modulation. We further highlight expressional changes of several cell wall-modifying enzymes and provide evidence for adjustment of the pectin network at sites of iron accumulation in the root.

CONCLUSION

Our study reveals new aspects of the elaborate interplay between phosphate starvation responses and changes in iron homeostasis. The results emphasize the importance of apoplastic iron redistribution to mediate phosphate-dependent root growth adjustment and suggest an important role for citrate in phosphate-dependent apoplastic iron transport. We further demonstrate that root growth modulation correlates with an altered expression of cell wall modifying enzymes and changes in the pectin network of the phosphate-deprived root tip, supporting the hypothesis that pectins are involved in iron binding and/or phosphate mobilization.

Enhancing the Mobilization of Native Phosphorus in the Mung Bean Rhizosphere Using ZnO Nanoparticles Synthesized by Soil Fungi

Phosphorus (P) is a limiting factor to plant growth and productivity in almost half of the world's arable soil, and its uptake in plants is often constrained because of its low solubility in the soil. To avoid repeated and large quantity application of rock phosphate as a P fertilizer and enhance the availability of native P acquisition by the plant root surface, in this study a biosynthesized ZnO nanoparticle was used. Zn acts as a cofactor for P-solubilizing enzymes such as phosphatase and phytase, and nano ZnO increased their activity between 84 and 108%. The level of resultant P uptake in mung bean increased by 10.8%. In addition, biosynthesized ZnO also improves plant phenology such as stem height, root volume, and biochemical indicators such as leaf protein and chlorophyll contents. In the rhizosphere, increased chlorophyll content and root volume attract microbial populations that maintain soil biological health. ICP-MS results showed ZnO nanoparticles were distributed in all plant parts, including seeds. However, the concentration of Zn was within the limit of the dietary recommendation. To the best of our knowledge, this is the first holistic study focusing on native P mobilization using ZnO nanoparticles in the life cycle of mung bean plants.

Dynamics of soil available phosphorus and its impact factors under simulated climate change in typical farmland of Taihu Lake region, China

Global climate change affects the availability of soil nutrients, thereby influencing crop productivity. This research was conducted to investigate the effects of elevated CO2, elevated temperature, and the interaction of the elevated CO2 and temperature on the soil available phosphorus (P) of a paddy-wheat rotation in the Taihu Lake region, China. Winter wheat (Triticum aestivum L.) was cultivated during the study period from 2011 to 2014 at two CO2 levels (350 μL•L(-1) ambient and 500 μL•L(-1) elevated by 150 μL•L(-1)) and two temperatures (ambient and 2 °C above the ambient). Soil available P content increased at the first season and decreased at the last season during the three wheat growing seasons. Soil available P content showed seasonal variation, whereas dynamic changes were not significant within each growing season. Soil available P content had no obvious trends under different treatments. But for the elevated temperature, CO2, and their combination treatments, soil available P content decreased in a long time period. During the period of wheat ripening stage, significant positive correlations were found between soil available P content and saturated hydraulic conductivity (Ks) and organic matter, but significant negative correlations with soil clay content and pH value; the correlation coefficients were 0.9400 (p < 0.01), 0.9942 (p < 0.01), -0.9383 (p < 0.01), and -0.6403 (p < 0.05), respectively. Therefore, Ks, organic matter, soil clay, and pH were the major impact factors on soil available P content. These results can provide a basis for predicting the trend of soil available P variation, as well as guidance for managing the soil nutrients and best fertilization practices in the future climate change scenario.

Legacy phosphorus and no tillage agriculture in tropical oxisols of the Brazilian savanna

Crop production in the Brazilian Cerrado is limited by soil phosphorus (P) supply without large inputs of inorganic P fertilizer, which may become more costly and scarce in the future. Reducing dependency on fertilizer P requires a greater understanding of soil P supply in the highly weathered soils in this important agricultural region. We investigated the impact of no tillage (NT) and conventional tillage (CT) agriculture on accumulated (legacy) soil P and P forms in four long-term sites. Compared to the native savanna soils, tilled soils receiving regular annual P fertilizer inputs (30-50 kg P ha(-1)) increased all forms of inorganic and organic P, except highly recalcitrant P associated with the background lithology. However, 70-85% of the net added P was bound in moderately labile and non-labile forms associated with Fe/Al oxyhydroxides rather than in plant available forms. Under NT agriculture, organic P forms and labile and non-labile inorganic P forms were all significantly (P<0.05) increased in the surface soil, except for one site with maize residues where labile inorganic P was increased more under CT agriculture. The contribution of organic P cycling in these tropical soils increased after conversion to agriculture and was proportionally greater under NT. The results highlight the large amounts of unutilized legacy P present in Brazil's Cerrado soils that could be better exploited to reduce dependency on imports of finite phosphate rock. No tillage agriculture confers a positive albeit relatively small benefit for soil P availability and overall soil function.

Bacterial Mobilization of Nutrients From Biochar-Amended Soils

Soil amendments with biochar to improve soil fertility and increase soil carbon stocks have received some high-level attention. Physical and chemical analyses of amended soils and biochars from various feedstocks are reported, alongside some evaluations of plant growth promotion capabilities. Fewer studies investigated the soil microbiota and their potential to increase cycling and mobilization of nutrients in biochar-amended soils. This review is discussing the latest findings in the bacterial contribution to cycling and mobilizing nitrogen, phosphorus, and sulfur in biochar-amended soils and potential contributions to plant growth promotion. Depending on feedstock, pyrolysis, soil type, and plant cover, changes in the bacterial community structure were observed for a majority of the studies using amplicon sequencing or genetic fingerprinting methods. Prokaryotic nitrification largely depends on the availability of ammonium and can vary considerably under soil biochar amendment. However, denitrification to di-nitrogen and in particular, nitrous oxide reductase activity is commonly enhanced, resulting in reduced nitrous oxide emissions. Likewise, bacterial fixation of di-nitrogen appears to be regularly enhanced. A paucity of studies suggests that bacterial mobilization of phosphorus and sulfur is enhanced as well. However, most studies only tested for extracellular sulfatase and phosphatase activity. Further research is needed to reveal details of the bacterial nutrient mobilizing capabilities and this is in particular the case for the mobilization of phosphorus and sulfur.

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.

Increased soil phosphorus availability induced by faba bean root exudation stimulates root growth and phosphorus uptake in neighbouring maize

Root growth is influenced by soil nutrients and neighbouring plants, but how these two drivers affect root interactions and regulate plant growth dynamics is poorly understood. Here, interactions between the roots of maize (Zea mays) and faba bean (Vicia faba) are characterized. Maize was grown alone (maize) or with maize (maize/maize) or faba bean (maize/faba bean) as competitors under five levels of phosphorus (P) supply, and with homogeneous or heterogeneous P distribution. Maize had longer root length and greater shoot biomass and P content when grown with faba bean than with maize. At each P supply rate, faba bean had a smaller root system than maize but greater exudation of citrate and acid phosphatase, suggesting a greater capacity to mobilize P in the rhizosphere. Heterogeneous P availability enhanced the root-length density of maize but not faba bean. Maize root proliferation in the P-rich patches was associated with increased shoot P uptake. Increased P availability by localized P application or by the presence of faba bean exudation stimulated root morphological plasticity and increased shoot growth in maize in the maize/faba bean mixture, suggesting that root interactions of neighbouring plants can be modified by increased P availability.

Phosphorus and nitrogen physiology of two contrasting poplar genotypes when exposed to phosphorus and/or nitrogen starvation

Phosphorus (P) and nitrogen (N) are the two essential macronutrients for tree growth and development. To elucidate the P and N physiology of woody plants during acclimation to P and/or N starvation, we exposed saplings of the slow-growing Populus simonii Carr (Ps) and the fast-growing Populus × euramericana Dode (Pe) to complete nutrients or starvation of P, N or both elements (NP). P. × euramericana had lower P and N concentrations and greater P and N amounts due to higher biomass production, thereby resulting in greater phosphorus use efficiency/N use efficiency (PUE/NUE) compared with Ps. Compared with the roots of Ps, the roots of Pe exhibited higher enzymatic activities in terms of acid phosphatases (APs) and malate dehydrogenase (MDH), which are involved in P mobilization, and nitrate reductase (NR), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH), which participate in N assimilation. The responsiveness of the transcriptional regulation of key genes encoding transporters for phosphate, ammonium and nitrate was stronger in Pe than in Ps. These results suggest that Pe possesses a higher capacity for P/N uptake and assimilation, which promote faster growth compared with Ps. In both poplars, P or NP starvation caused significant decreases in the P concentrations and increases in PUE. Phosphorus deprivation induced the activity levels of APs, phosphoenolpyruvate carboxylase and MDH in both genotypes. Nitrogen or NP deficiency resulted in lower N concentrations, amino acid levels, NR and GOGAT activities, and higher NUE in both poplars. Thus, in Ps and Pe, the mRNA levels of PHT1;5, PHT1;9, PHT2;1, AMT2;1 and NR increased in the roots, while PHT1;9, PHO1;H1, PHO2, AMT1;1 and NRT2;1 increased in the leaves during acclimation to P, N or NP deprivation. These results suggest that both poplars suppress P/N uptake, mobilization and assimilation during acclimation to P, N or NP starvation.

A vacuolar phosphate transporter essential for phosphate homeostasis in Arabidopsis

Inorganic phosphate (Pi) is stored in the vacuole, allowing plants to adapt to variable Pi availability in the soil. The transporters that mediate Pi sequestration into vacuole remain unknown, however. Here we report the functional characterization of Vacuolar Phosphate Transporter 1 (VPT1), an SPX domain protein that transports Pi into the vacuole in Arabidopsis. The vpt1 mutant plants were stunted and consistently retained less Pi than wild type plants, especially when grown in medium containing high levels of Pi. In seedlings, VPT1 was expressed primarily in younger tissues under normal conditions, but was strongly induced by high-Pi conditions in older tissues, suggesting that VPT1 functions in Pi storage in young tissues and in detoxification of high Pi in older tissues. As a result, disruption of VPT1 rendered plants hypersensitive to both low-Pi and high-Pi conditions, reducing the adaptability of plants to changing Pi availability. Patch-clamp analysis of isolated vacuoles showed that the Pi influx current was severely reduced in vpt1 compared with wild type plants. When ectopically expressed in Nicotiana benthamiana mesophyll cells, VPT1 mediates vacuolar influx of anions, including Pi, SO4(2-), NO3(-), Cl(-), and malate with Pi as that preferred anion. The VPT1-mediated Pi current amplitude was dependent on cytosolic phosphate concentration. Single-channel analysis showed that the open probability of VPT1 was increased with the increase in transtonoplast potential. We conclude that VPT1 is a transporter responsible for vacuolar Pi storage and is essential for Pi adaptation in Arabidopsis.

Plant Survival in a Changing Environment: The Role of Nitric Oxide in Plant Responses to Abiotic Stress

Nitric oxide in plants may originate endogenously or come from surrounding atmosphere and soil. Interestingly, this gaseous free radical is far from having a constant level and varies greatly among tissues depending on a given plant's ontogeny and environmental fluctuations. Proper plant growth, vegetative development, and reproduction require the integration of plant hormonal activity with the antioxidant network, as well as the maintenance of concentration of reactive oxygen and nitrogen species within a narrow range. Plants are frequently faced with abiotic stress conditions such as low nutrient availability, salinity, drought, high ultraviolet (UV) radiation and extreme temperatures, which can influence developmental processes and lead to growth restriction making adaptive responses the plant's priority. The ability of plants to respond and survive under environmental-stress conditions involves sensing and signaling events where nitric oxide becomes a critical component mediating hormonal actions, interacting with reactive oxygen species, and modulating gene expression and protein activity. This review focuses on the current knowledge of the role of nitric oxide in adaptive plant responses to some specific abiotic stress conditions, particularly low mineral nutrient supply, drought, salinity and high UV-B radiation.

Maize yield response to a phosphorus-solubilizing microbial inoculant in field trials

Findings from multi-year, multi-site field trial experiments measuring maize yield response to inoculation with the phosphorus-solubilizing fungus, Penicillium bilaiae Chalabuda are presented. The main objective was to evaluate representative data on crop response to the inoculant across a broad set of different soil, agronomic management and climate conditions. A statistical analysis of crop yield response and its variability was conducted to guide further implementation of a stratified trial and sampling plan. Field trials, analysed in the present study, were conducted across the major maize producing agricultural cropland of the United States (2005-11) comprising 92 small (with sampling replication) and 369 large (without replication) trials. The multi-plot design enabled both a determination of how sampling area affects the estimation of maize yield and yield variance and an estimation of the ability of inoculation with P. bilaiae to increase maize yield. Inoculation increased maize yield in 66 of the 92 small and 295 of the 369 large field trials (within the small plots, yield increased significantly at the 95% confidence level, by 0·17 ± 0·044 t/ha or 1·8%, while in the larger plots, yield increases were higher and less variable (i.e., 0·33 ± 0·026 t/ha or 3·5%). There was considerable inter-annual variability in maize yield response attributed to inoculation compared to the un-inoculated control, with yield increases varying from 0·7 ± 0·75 up to 3·7 ± 0·73%. No significant correlation between yield response and soil acidity (i.e., pH) was detected, and it appears that pH reduction (through organic acid or proton efflux) was unlikely to be the primary pathway for better phosphorus availability measured as increased yield. Seed treatment and granular or dribble band formulations of the inoculant were found to be equally effective. Inoculation was most effective at increasing maize yield in fields that had low or very low soil phosphorus status for both small and large plots. At higher levels of soil phosphorus, yield in the large plots increased more with inoculation than in the small plots, which could be explained by phosphorus fertilization histories for the different field locations, as well as transient (e.g., rainfall) and topographic effects.

The impact of elevated carbon dioxide on the phosphorus nutrition of plants: a review

BACKGROUND

Increasing attention is being focused on the influence of rapid increases in atmospheric CO2 concentration on nutrient cycling in ecosystems. An understanding of how elevated CO2 affects plant utilization and acquisition of phosphorus (P) will be critical for P management to maintain ecosystem sustainability in P-deficient regions.

SCOPE

This review focuses on the impact of elevated CO2 on plant P demand, utilization in plants and P acquisition from soil. Several knowledge gaps on elevated CO2-P associations are highlighted.

CONCLUSIONS

Significant increases in P demand by plants are likely to happen under elevated CO2 due to the stimulation of photosynthesis, and subsequent growth responses. Elevated CO2 alters P acquisition through changes in root morphology and increases in rooting depth. Moreover, the quantity and composition of root exudates are likely to change under elevated CO2, due to the changes in carbon fluxes along the glycolytic pathway and the tricarboxylic acid cycle. As a consequence, these root exudates may lead to P mobilization by the chelation of P from sparingly soluble P complexes, by the alteration of the biochemical environment and by changes to microbial activity in the rhizosphere. Future research on chemical, molecular, microbiological and physiological aspects is needed to improve understanding of how elevated CO2 might affect the use and acquisition of P by plants.

Genes and networks regulating root anatomy and architecture

The root is an excellent model for studying developmental processes that underlie plant anatomy and architecture. Its modular structure, the lack of cell movement and relative accessibility to microscopic visualization facilitate research in a number of areas of plant biology. In this review, we describe several examples that demonstrate how cell type-specific developmental mechanisms determine cell fate and the formation of defined tissues with unique characteristics. In the last 10 yr, advances in genome-wide technologies have led to the sequencing of thousands of plant genomes, transcriptomes and proteomes. In parallel with the development of these high-throughput technologies, biologists have had to establish computational, statistical and bioinformatic tools that can deal with the wealth of data generated by them. These resources provide a foundation for posing more complex questions about molecular interactions, and have led to the discovery of new mechanisms that control phenotypic differences. Here we review several recent studies that shed new light on developmental processes, which are involved in establishing root anatomy and architecture. We highlight the power of combining large-scale experiments with classical techniques to uncover new pathways in root development.

Insights into the physiological responses of the facultative halophyte Aeluropus littoralis to the combined effects of salinity and phosphorus availability

In this work, we investigate the physiological responses to P deficiency (5μM KH2PO4=D), salt stress (400mM NaCl=C+S), and their combination (D+S) on the facultative halophyte Aeluropus littoralis to understand how plants adapt to these combined stresses. When individually applied, both P deficiency and salinity significantly restricted whole plant growth, with a more marked effect of the latter stress. However, the effects of the two stresses were not additive in plant biomass production since the response of plants to combined salinity and P deficiency was similar to that of plants grown under salt stress alone. In addition the observed features under salinity alone are kept when plants are simultaneously subjected to the combined effects of salinity and P deficiency such as biomass partitioning; the synthesis of proline and the K(+)/Na(+) selectivity ratio. Thus, increasing P availability under saline conditions has no significant effect on salt tolerance in this species. Plants cultivated under the combined effects of salinity and P deficiency exhibited the lowest leaf water potential. This trend was associated with a high accumulation of Na(+), Cl(-) and proline in shoots of salt treated plants suggesting the involvement of these solutes in osmotic adjustment. Proline could be involved in other physiological processes such as free radical scavenging. Furthermore, salinity has no significant effect on phosphorus acquisition when combined with a low P supply and it significantly decreased this parameter when combined with a sufficient P supply. This fact was probably due to salt's effect on P transporters. In addition, shoot soluble sugars accumulation under both P deficiency treatments with and without salt likely play an important role in the adaptation of A. littoralis plants to P shortage applied alone or combined with salinity. Moreover, there was a strong correlation between shoot and root intracellular acid phosphatase activity and phosphorus use efficiency which strengthens the assumption that intracellular acid phosphatase enzymes are involved in P remobilization in this species. Finally, our results showed that P availability has no significant effect on salt excretion in A. littorlais which suggests that independently of the P status in the plant, excretion remains priority over other functions requiring energy such as growth. This result could also indicate that salt excretion is not energy-dependent in this species.

Phosphate solubilization and promotion of maize growth by Penicillium oxalicum P4 and Aspergillus niger P85 in a calcareous soil

Alternative tactics for improving phosphorus nutrition in crop production are needed in China and elsewhere, as the overapplication of phosphatic fertilizers can adversely impact agricultural sustainability. Penicillium oxalicum P4 and Aspergillus niger P85 were isolated from a calcareous soil in China that had been exposed to excessive application of phosphatic fertilizer for decades. Each isolate excreted a number of organic acids into, acidified, and solubilized phosphorus in a synthetic broth containing insoluble tricalcium phosphate or rock phosphate. Isolate P4, applied as a seed treatment, increased maize fresh mass per plant when rock phosphate was added to the calcareous soil in greenhouse pot studies. Isolate P85 did not increase maize fresh mass per plant but did significantly increase total phosphorus per plant when rock phosphate was added. Significant increases in 7 and 4 organic acids were detected in soil in association with isolates P4 and P85, respectively, relative to the soil-only control. The quantity and (or) number of organic acids produced by these isolates increased when rock phosphate was added to the soil. Both isolates also significantly increased available phosphorus in soil in the presence of added rock phosphate and effectively colonized the maize rhizosphere. Studies reported here indicate that isolate P4 is adapted to and capable of promoting maize growth in a calcareous soil. Plant-growth promotion by this isolate is likely due, at least in part, to increased phosphorus availability resulting from the excretion of organic acids into, and the resulting acidification of, this soil.

Contrasting responses of root morphology and root-exuded organic acids to low phosphorus availability in three important food crops with divergent root traits

Phosphorus (P) is an important element for crop productivity and is widely applied in fertilizers. Most P fertilizers applied to land are sorbed onto soil particles, so research on improving plant uptake of less easily available P is important. In the current study, we investigated the responses in root morphology and root-exuded organic acids (OAs) to low available P (1 μM P) and sufficient P (50 μM P) in barley, canola and micropropagated seedlings of potato-three important food crops with divergent root traits, using a hydroponic plant growth system. We hypothesized that the dicots canola and tuber-producing potato and the monocot barley would respond differently under various P availabilities. WinRHIZO and liquid chromatography triple quadrupole mass spectrometry results suggested that under low P availability, canola developed longer roots and exhibited the fastest root exudation rate for citric acid. Barley showed a reduction in root length and root surface area and an increase in root-exuded malic acid under low-P conditions. Potato exuded relatively small amounts of OAs under low P, while there was a marked increase in root tips. Based on the results, we conclude that different crops show divergent morphological and physiological responses to low P availability, having evolved specific traits of root morphology and root exudation that enhance their P-uptake capacity under low-P conditions. These results could underpin future efforts to improve P uptake of the three crops that are of importance for future sustainable crop production.

Role of microRNAs involved in plant response to nitrogen and phosphorous limiting conditions

Plant microRNAs (miRNAs) are a class of small non-coding RNAs which target and regulate the expression of genes involved in several growth, development, and metabolism processes. Recent researches have shown involvement of miRNAs in the regulation of uptake and utilization of nitrogen (N) and phosphorus (P) and more importantly for plant adaptation to N and P limitation conditions by modifications in plant growth, phenology, and architecture and production of secondary metabolites. Developing strategies that allow for the higher efficiency of using both N and P fertilizers in crop production is important for economic and environmental benefits. Improved crop varieties with better adaptation to N and P limiting conditions could be a key approach to achieve this effectively. Furthermore, understanding on the interactions between N and P uptake and use and their regulation is important for the maintenance of nutrient homeostasis in plants. This review describes the possible functions of different miRNAs and their cross-talk relevant to the plant adaptive responses to N and P limiting conditions. In addition, a comprehensive understanding of these processes at molecular level and importance of biological adaptation for improved N and P use efficiency is discussed.

Changes in Olsen Phosphorus Concentration and Its Response to Phosphorus Balance in Black Soils under Different Long-Term Fertilization Patterns

The Olsen phosphorus (P) concentration of a soil is a key index that can be used to evaluate the P supply capacity of the soil and to estimate the optimal P fertilization rate. A study of the relationship between the soil Olsen P concentration and the P balance (P input minus P output) and their variations among different fertilization patterns will help to provide useful information for proper management of P fertilization. In this paper, the two investigated long-term experiments were established on black soils in the northeast region of China. Six fertilization treatments were selected: (1) unfertilized (CK); (2) nitrogen only (N); (3) nitrogen and potassium (NK); (4) nitrogen and phosphorus (NP); (5) nitrogen, phosphorus, and potassium (NPK); and (6) nitrogen, phosphorus, potassium and manure (NPKM). The results showed that the average Olsen P concentrations in the black soils at Gongzhuling and Harbin (16- and 31-year study periods, respectively), decreased by 0.49 and 0.56 mg kg^-1 a^-1, respectively, without P addition and increased by 3.17 and 1.78 mg kg^-1 a^-1, respectively, with P fertilization. The changes in soil Olsen P concentrations were significantly (P<0.05) correlated with the P balances at both sites except for the NP and NPK treatments at Gongzhuling. Under an average deficit of 100 kg ha^-1 P, the soil Olsen P concentration at both sites decreased by 1.36~3.35 mg kg^-1 in the treatments without P addition and increased by 4.80~16.04 mg kg^-1 in the treatments with 100 kg ha^-1 of P accumulation. In addition, the changes in Olsen P concentrations in the soil with 100 kg ha^-1of P balance were significantly correlated with the P activation coefficient (PAC, percentage of Olsen P to total P, r=0.99, P<0.01) and soil organic matter content (r=0.91, P<0.01). A low pH was related to large changes of Olsen P by 1 kg ha^-1 of P balance. These results indicated that soil organic matter and pH have important effects on the change in soil Olsen P by 1 kg ha^-1 of P balance.

Multiple interval QTL mapping and searching for PSTOL1 homologs associated with root morphology, biomass accumulation and phosphorus content in maize seedlings under low-P

BACKGROUND

Modifications in root morphology are important strategies to maximize soil exploitation under phosphorus starvation in plants. Here, we used two multiple interval models to map QTLs related to root traits, biomass accumulation and P content in a maize RIL population cultivated in nutrient solution. In addition, we searched for putative maize homologs to PSTOL1, a gene responsible to enhance early root growth, P uptake and grain yield in rice and sorghum.

RESULTS

Based on path analysis, root surface area was the root morphology component that most strongly contributed to total dry weight and to P content in maize seedling under low-P availability. Multiple interval mapping models for single (MIM) and multiple traits (MT-MIM) were combined and revealed 13 genomic regions significantly associated with the target traits in a complementary way. The phenotypic variances explained by all QTLs and their epistatic interactions using MT-MIM (23.4 to 35.5 %) were higher than in previous studies, and presented superior statistical power. Some of these QTLs were coincident with QTLs for root morphology traits and grain yield previously mapped, whereas others harbored ZmPSTOL candidate genes, which shared more than 55 % of amino acid sequence identity and a conserved serine/threonine kinase domain with OsPSTOL1. Additionally, four ZmPSTOL candidate genes co-localized with QTLs for root morphology, biomass accumulation and/or P content were preferentially expressed in roots of the parental lines that contributed the alleles enhancing the respective phenotypes.

CONCLUSIONS

QTL mapping strategies adopted in this study revealed complementary results for single and multiple traits with high accuracy. Some QTLs, mainly the ones that were also associated with yield performance in other studies, can be good targets for marker-assisted selection to improve P-use efficiency in maize. Based on the co-localization with QTLs, the protein domain conservation and the coincidence of gene expression, we selected novel maize genes as putative homologs to PSTOL1 that will require further validation studies.

Soil carbon, nitrogen and phosphorus changes under sugarcane expansion in Brazil

Historical data of land use change (LUC) indicated that the sugarcane expansion has mainly displaced pasture areas in Central-Southern Brazil, globally the largest producer, and that those pastures were prior established over native forests in the Cerrado biome. We sampled 3 chronosequences of land use comprising native vegetation (NV), pasture (PA), and sugarcane crop (SC) in the sugarcane expansion region to assess the effects of LUC on soil carbon, nitrogen, and labile phosphorus pools. Thirty years after conversion of NV to PA, we found significant losses of original soil organic matter (SOM) from NV, while insufficient new organic matter was introduced from tropical grasses into soil to offset the losses, reflecting in a net C emission of 0.4 Mg ha(-1)yr(-1). These findings added to decreases in (15)N signal indicated that labile portions of SOM are preserved under PA. Afterwards, in the firsts five years after LUC from PA to SC, sparse variations were found in SOM levels. After more than 20 years of sugarcane crop, however, there were losses of 40 and 35% of C and N stocks, respectively, resulting in a rate of C emission of 1.3 Mg ha(-1)yr(-1) totally caused by the respiration of SOM from C4-cycle plants. In addition, conversion of pastures to sugarcane mostly increased (15)N signal, indicating an accumulation of more recalcitrant SOM under sugarcane. The microbe- and plant-available P showed site-specific responses to LUC as a function of different P-input managements, with the biological pool mostly accounting for more than 50% of the labile P in both anthropic land uses. With the projections of 6.4 Mha of land required by 2021 for sugarcane expansion in Brazil to achieve ethanol's demand, this explanatory approach to the responses of SOM to LUC will contribute for an accurate assessment of the CO₂ balance of sugarcane ethanol.

Unmasking Novel Loci for Internal Phosphorus Utilization Efficiency in Rice Germplasm through Genome-Wide Association Analysis

Depletion of non-renewable rock phosphate reserves and phosphorus (P) fertilizer price increases has renewed interest in breeding P-efficient varieties. Internal P utilization efficiency (PUE) is of prime interest because there has been no progress to date in breeding for high PUE. We characterized the genotypic variation for PUE present within the rice gene pool by using a hydroponic system that assured equal plant P uptake, followed by mapping of loci controlling PUE via Genome-Wide Association Studies (GWAS). Loci associated with PUE were mapped on chromosomes 1, 4, 11 and 12. The highest PUE was associated with a minor indica-specific haplotype on chromosome 1 and a rare aus-specific haplotype on chromosome 11. Comparative variant and expression analysis for genes contained within the chromosome 1 haplotype identified high priority candidate genes. Differences in coding regions and expression patterns between genotypes of contrasting haplotypes, suggested functional alterations for two predicted nucleic acid-interacting proteins that are likely causative for the observed differences in PUE. The loci reported here are the first identified for PUE in any crop that is not confounded by differential P uptake among genotypes. Importantly, modern rice varieties lacked haplotypes associated with superior PUE, and would thus benefit from targeted introgressions of these loci from traditional donors to improve plant growth in phosphorus-limited cropping systems.

In situ imaging of interfacial precipitation of phosphate on Goethite

Adsorption and subsequent immobilization of orthophosphate on iron oxides is of considerable importance in soil fertility and eutrophication studies. Here, in situ atomic force microscopy (AFM) has been used to probe the interaction of phosphate-bearing solutions with goethite, α-FeOOH, (010) cleavage surfaces. During the dissolution of goethite we observed simultaneous nucleation of nanoparticles (1.0-3.0 nm in height) of iron phosphate (Fe-P) phases at the earliest nucleation stages, subsequent aggregation to form secondary particles (about 6.0 nm in height) and layered precipitates under various pH values and ionic strengths relevant to acid soil solution conditions. The heterogeneous nucleation rates of Fe-P precipitates at phosphate concentrations ranging from 5.0 to 50.0 mM were quantitatively defined. Enhanced goethite dissolution in the presence of high concentration NaCl or AlCl3 leads to a rapid increase in Fe-P nucleation rates, whereas low concentration MgCl2 inhibits goethite dissolution, this in turn influences Fe-P nucleation. Moreover, kinetic data analyses show that low concentrations of citrate caused an increase in the nucleation rate of Fe-P phases. However, at higher concentrations of citrate, nucleation acceleration was reversed with much longer induction times to form Fe-P nuclei. These in situ observations may improve the mechanistic understanding of processes resulting in phosphate immobilization by goethite-rich acid soils in the presence of various inorganic and organic additive molecules.

Housing helpful invaders: the evolutionary and molecular architecture underlying plant root-mutualist microbe interactions

Plant root rhizosphere interactions with mutualistic microbes are diverse and numerous, having evolved over time in response to selective pressures on plants to attain anchorage and nutrients. These relationships can be considered to be formed through a combination of architectural connections: molecular architecture interactions that control root-microbe perception and regulate the balance between host and symbiont and developmental architecture interactions that enable the microbes to be 'housed' in the root and enable the exchange of compounds. Recent findings that help to understand the common architecture that exists between nodulation and mycorrhizal interactions, and how this architecture could be re-tuned to develop new symbioses, are discussed here.

Improving intercropping: a synthesis of research in agronomy, plant physiology and ecology

Intercropping is a farming practice involving two or more crop species, or genotypes, growing together and coexisting for a time. On the fringes of modern intensive agriculture, intercropping is important in many subsistence or low-input/resource-limited agricultural systems. By allowing genuine yield gains without increased inputs, or greater stability of yield with decreased inputs, intercropping could be one route to delivering ‘sustainable intensification’. We discuss how recent knowledge from agronomy, plant physiology and ecology can be combined with the aim of improving intercropping systems. Recent advances in agronomy and plant physiology include better understanding of the mechanisms of interactions between crop genotypes and species – for example, enhanced resource availability through niche complementarity. Ecological advances include better understanding of the context-dependency of interactions, the mechanisms behind disease and pest avoidance, the links between above- and below-ground systems, and the role of microtopographic variation in coexistence. This improved understanding can guide approaches for improving intercropping systems, including breeding crops for intercropping. Although such advances can help to improve intercropping systems, we suggest that other topics also need addressing. These include better assessment of the wider benefits of intercropping in terms of multiple ecosystem services, collaboration with agricultural engineering, and more effective interdisciplinary research.

Past, present, and future use of phosphorus in Chinese agriculture and its influence on phosphorus losses

Large inputs of phosphorus (P) in chemical fertilizers and feed supplements since 1978 have improved soil P status in arable land in China, but have also created challenges by increasing P concentrations in manure and exacerbating water quality degradation. Arable land in China can be divided into five management zones based on soil P chemistry, with 15-92 % of arable land having lower P status than the agronomic optimum and 0.3-7.2 % having severe risks of P leaching losses. A scenario analysis of soil P budget and agronomic P demand during 2011-2030 highlighted the great pressure China faces in sustainable P management and the need for drastic changes in current practices. This includes new policies to reduce P supplementation of feed and improved P use efficiency by livestock and programs to expand the adoption of appropriate fertilization, soil conservation, and drainage management practices to minimize P losses.

Root physiological adaptations involved in enhancing P assimilation in mining and non-mining ecotypes of Polygonum hydropiper grown under organic P media

It is important to seek out plant species, high in phosphorus (P) uptake, for phytoremediation of P-enriched environments with a large amount of organic P (Po). P assimilation characteristics and the related mechanisms of Polygonum hydropiper were investigated in hydroponic media containing various concentrations of Po (1-8 mmol L(-1)) supplied as phytate. The mining ecotype (ME) showed significantly higher biomass in both shoots and roots compared to the non-mining ecotype (NME) at 4, 6, and 8 m mol L(-1). Shoot P content of both ecotypes increased up to 4 mmol L(-1) while root P content increased continually up to 8 mmol L(-1) for the ME and up to 6 mmol L(-1) for the NME. Root P content of the ME exceeded 1% dry weight under 6 and 8 mmol L(-1). The ME had significantly higher P accumulation in both shoots and roots compared to the NME supplied with 6 and 8 mmol L(-1). The ME showed higher total root length, specific root length, root surface area, root volume, and displayed significantly greater root length, root surface area, and root volume of lateral roots compared to the NME grown in all Po treatments. Average diameter of lateral roots was 0.17-19 mm for the ME and 0.18-0.21 mm for the NME. Greater acid phosphatase and phytase activities were observed in the ME grown under different levels of Po relative to the NME. This indicated fine root morphology, enhanced acid phosphatase and phytase activities might be adaptations to high Po media. Results from this study establish that the ME of P. hydropiper is capable of assimilating P from Po media and is a potential material for phytoremediation of polluted area with high Po.

The Medicago truncatula hypermycorrhizal B9 mutant displays an altered response to phosphate and is more susceptible to Aphanomyces euteiches

Inorganic phosphate (Pi) plays a key role in the development of arbuscular mycorrhizal (AM) symbiosis, which is favoured when Pi is limiting in the environment. We have characterized the Medicago truncatula hypermycorrhizal B9 mutant for its response to limiting (P/10) and replete (P2) Pi. On P2, mycorrhization was significantly higher in B9 plants than in wild-type (WT). The B9 mutant displayed hallmarks of Pi-limited plants, including higher levels of anthocyanins and lower concentrations of Pi in shoots than WT plants. Transcriptome analyses of roots of WT and B9 plants cultivated on P2 or on P/10 confirmed the Pi-limited profile of the mutant on P2 and highlighted its altered response to Pi on P/10. Furthermore, the B9 mutant displayed a higher expression of defence/stress-related genes and was more susceptible to infection by the root oomycete pathogen Aphanomyces euteiches than WT plants. We propose that the hypermycorrhizal phenotype of the B9 mutant is linked to its Pi-limited status favouring AM symbiosis in contrast to WT plants in Pi-replete conditions, and discuss the possible links between the altered response of the B9 mutant to Pi, mycorrhization and infection by A. euteiches.

A major root-associated acid phosphatase in Arabidopsis, AtPAP10, is regulated by both local and systemic signals under phosphate starvation

The induction and secretion of acid phosphatases (APases) is a universal response of plants to phosphate (Pi) starvation. AtPAP10 (Arabidopsis purple acid phosphatase 10) is a major Pi starvation-induced APase that is associated with the root surface in Arabidopsis. So far, the roles of local and systemic signalling in regulating root-associated AtPAP10 activity remain largely unknown. In this work, we show that a decrease of local, external Pi availability is sufficient to induce AtPAP10 transcription in roots in the presence of sucrose, a systemic signal from shoots, whereas the magnitude of the induction is affected by the Pi status of the whole plant. Once the AtPAP10 mRNAs are synthesized in roots, subsequent accumulation of AtPAP10 proteins in root cells and increase in AtPAP10 activity on the root surface are mainly controlled by local signalling. Previously, ethylene has been demonstrated to be a positive regulator of AtPAP10 activity. In this study, we provide evidence that under Pi deficiency ethylene mainly modulates enzymatic activity of AtPAP10 on the root surface, but not AtPAP10 transcription and protein accumulation, suggesting that it functions as a local signal. Furthermore, our work indicates that the effect of ethylene on the induction of root-associated AtPAP10 activity depends on sucrose, but that the effect of sucrose does not depend on ethylene. These results reveal new insights into the distinct roles of local and systemic signalling in the regulation of root-associated AtPAP10 activity under Pi starvation.

Hierarchy of factors exerting an impact on nutrient load of the Baltic Sea and sustainable management of its drainage basin

The aim of the paper was to evaluate 23 catchment factors that determine total phosphorus and total nitrogen load to the Baltic Sea. Standard correlation analysis and clustering were used. Both phosphorus and nitrogen loads were found to be positively related to the number of pigs and the human population associated with wastewater treatment plants (WWTPs) per km(2), while the number of cattle and agricultural area were found to influence nitrogen rather than phosphorus load, and the area of forests is negatively related to loads of both nutrients. Clustering indicates an overall north-south pattern in the spatial co-occurrence of socio-ecological factors, with some exceptions discussed in the paper. Positive steps in the Baltic Sea region have already been taken, but much remains to be done. The development of coherent response policies to reduce eutrophication in the Baltic Sea should be based on a comprehensive knowledge base, an appropriate information strategy and learning alliance platform in each drainage river catchments.

Identification of phosphatin, a drug alleviating phosphate starvation responses in Arabidopsis

Inorganic phosphate (Pi) is present in most soils at suboptimal concentrations, strongly limiting plant development. Plants have the ability to sense and adapt to the surrounding ionic environment, and several genes involved in the response to Pi starvation have been identified. However, a global understanding of the regulatory mechanisms involved in this process is still elusive. Here, we have initiated a chemical genetics approach and isolated compounds that inhibit the response to Pi starvation in Arabidopsis (Arabidopsis thaliana). Molecules were screened for their ability to inhibit the expression of a Pi starvation marker gene (the high-affinity Pi transporter PHT1;4). A drug family named Phosphatin (PTN; Pi starvation inhibitor), whose members act as partial suppressors of Pi starvation responses, was thus identified. PTN addition also reduced various traits of Pi starvation, such as phospholipid/glycolipid conversion, and the accumulation of starch and anthocyanins. A transcriptomic assay revealed a broad impact of PTN on the expression of many genes regulated by low Pi availability. Despite the reduced amount of Pi transporters and resulting reduced Pi uptake capacity, no reduction of Pi content was observed. In addition, PTN improved plant growth; this reveals that the developmental restrictions induced by Pi starvation are not a consequence of metabolic limitation but a result of genetic regulation. This highlights the existence of signal transduction pathway(s) that limit plant development under the Pi starvation condition.

RNA-seq transcriptome profiling reveals that Medicago truncatula nodules acclimate N₂ fixation before emerging P deficiency reaches the nodules

Legume nodules are plant tissues with an exceptionally high concentration of phosphorus (P), which, when there is scarcity of P, is preferentially maintained there rather than being allocated to other plant organs. The hypothesis of this study was that nodules are affected before the P concentration in the organ declines during whole-plant P depletion. Nitrogen (N₂) fixation and P concentration in various organs were monitored during a whole-plant P-depletion process in Medicago truncatula. Nodule gene expression was profiled through RNA-seq at day 5 of P depletion. Until that point in time P concentration in leaves reached a lower threshold but was maintained in nodules. N₂-fixation activity per plant diverged from that of fully nourished plants beginning at day 5 of the P-depletion process, primarily because fewer nodules were being formed, while the activity of the existing nodules was maintained for as long as two weeks into P depletion. RNA-seq revealed nodule acclimation on a molecular level with a total of 1140 differentially expressed genes. Numerous genes for P remobilization from organic structures were increasingly expressed. Various genes involved in nodule malate formation were upregulated, while genes involved in fermentation were downregulated. The fact that nodule formation was strongly repressed with the onset of P deficiency is reflected in the differential expression of various genes involved in nodulation. It is concluded that plants follow a strategy to maintain N₂ fixation and viable leaf tissue as long as possible during whole-plant P depletion to maintain their ability to react to emerging new P sources (e.g. through active P acquisition by roots).