Phytate (Inositol Hexakisphosphate) in Soil and Phosphate Acquisition from Inositol Phosphates by Higher Plants. A Review

Control of inorganic and organic phosphorus molecules on microbial activity, and the stoichiometry of nutrient cycling in soils in an arid, agricultural ecosystem

Background

The dynamics of carbon (C), nitrogen (N), and phosphorus (P) in soils determine their fertility and crop growth in agroecosystems. These dynamics depend on microbial metabolism, which in turn depends on nutrient availability. Farmers typically apply either mineral or organic fertilizers to increase the availability of nutrients in soils. Phosphorus, which usually limits plant growth, is one of the most applied nutrients. Our knowledge is limited regarding how different forms of P impact the ability of microbes in soils to produce the enzymes required to release nutrients, such as C, N and P from different substrates.

Methods

In this study, we used the arable layer of a calcareous soil obtained from an alfalfa cropland in Cuatro Cienegas, México, to perform an incubation experiment, where five different phosphate molecules were added as treatments substrates: three organic molecules (RNA, adenine monophosphate (AMP) and phytate) and two inorganic molecules (calcium phosphate and ammonium phosphate). Controls did not receive added phosphorus. We measured nutrient dynamics and soil microbial activity after 19 days of incubation.

Results

Different P molecules affected potential microbial C mineralization (CO2-C) and enzyme activities, specifically in the organic treatments. P remained immobilized in the microbial biomass (Pmic) regardless of the source of P, suggesting that soil microorganisms were limited by phosphorus. Higher mineralization rates in soil amended with organic P compounds depleted dissolved organic carbon and increased nitrification. The C:N:P stoichiometry of the microbial biomass implied a change in the microbial community which affected the carbon use efficiency (CUE), threshold elemental ratio (TER), and homeostasis.

Conclusion

Different organic and inorganic sources of P affect soil microbial community structure and metabolism. This modifies the dynamics of soil C, N and P. These results highlight the importance of considering the composition of organic matter and phosphate compounds used in agriculture since their impact on the microbial activity of the soil can also affect plant productivity.

The role of inositol in the environmental organic phosphate cycle

Cellular synthesis of phytic acid sequesters phosphates in the sugar inositol. Phytic acid in soil represents the most abundant form of organic phosphates. The supplementation of phytase or phytase-producing organisms has been considered as a strategy to improve usable soil phosphates. However, the impacts on the environmental flow of inositol, which is generated along with phosphate by phytase, have not been examined. In this review, we discuss the origin and nature of inositol produced in soil and the several possible destinations of inositol released by phytase activities. We emphasise how an improved understanding of soil inositol flow could help to provide new solutions to the phosphate shortage problem in agriculture.

The Phosphorus-Iron Nexus: Decoding the Nutrients Interaction in Soil and Plant

Phosphorus (P) and iron (Fe) are two essential mineral nutrients in plant growth. It is widely observed that interactions of P and Fe could influence their availability in soils and affect their homeostasis in plants, which has received significant attention in recent years. This review presents a summary of latest advances in the activation of insoluble Fe-P complexes by soil properties, microorganisms, and plants. Furthermore, we elucidate the physiological and molecular mechanisms underlying how plants adapt to Fe-P interactions. This review also discusses the current limitations and presents potential avenues for promoting sustainable agriculture through the optimization of P and Fe utilization efficiency in crops.

Exploring the biotechnological potential of novel soil-derived Klebsiella sp. and Chryseobacterium sp. strains using phytate as sole carbon source

Phosphorus (P) is essential for biological systems, playing a pivotal role in energy metabolism and forming crucial structural components of DNA and RNA. Yet its bioavailable forms are scarce. Phytate, a major form of stored phosphorus in cereals and soils, is poorly bioavailable due to its complex structure. Phytases, enzymes that hydrolyze phytate to release useable phosphorus, are vital in overcoming this limitation and have significant biotechnological applications. This study employed novel method to isolate and characterize bacterial strains capable of metabolizing phytate as the sole carbon and phosphorus source from the Andes mountains soils. Ten strains from the genera Klebsiella and Chryseobacterium were isolated, with Chryseobacterium sp. CP-77 and Klebsiella pneumoniae CP-84 showing specific activities of 3.5 ± 0.4 nkat/mg and 40.8 ± 5 nkat/mg, respectively. Genomic sequencing revealed significant genetic diversity, suggesting CP-77 may represent a novel Chryseobacterium species. A fosmid library screening identified several phytase genes, including a 3-phytase in CP-77 and a glucose 1-phosphatase and 3-phytase in CP-84. Phylogenetic analysis confirmed the novelty of these enzymes. These findings highlight the potential of phytase-producing bacteria in sustainable agriculture by enhancing phosphorus bioavailability, reducing reliance on synthetic fertilizers, and contributing to environmental management. This study expands our biotechnological toolkit for microbial phosphorus management and underscores the importance of exploring poorly characterized environments for novel microbial functions. The integration of direct cultivation with metagenomic screening offers robust approaches for discovering microbial biocatalysts, promoting sustainable agricultural practices, and advancing environmental conservation.

Phosphorus speciation in manure and fertilizer impacted Mid-Atlantic coastal plain soils

Historical applications of manures and fertilizers at rates exceeding crop P removal in the Mid-Atlantic region (United States) have resulted in decades of increased water quality degradation from P losses in agricultural runoff. As such, many growers in this region face restrictions on future P applications. An improved understanding of the fate, transformations, and availability of P is needed to manage P-enriched soils. We paired chemical extractions (i.e., Mehlich-3, water extractable P, and chemical fractionation) with nondestructive methods (i.e., x-ray absorption near edge structure [XANES] spectroscopy and x-ray fluorescence [XRF]) to investigate P dynamics in eight P-enriched Mid-Atlantic soils with various management histories. Chemical fractionation and XRF data were used to support XANES linear combination fits, allowing for identification of various Al, Ca, and Fe phosphates and P sorbed phases in soils amended with fertilizer, poultry litter, or dairy manure. Management history and P speciation were used to make qualitative comparisons between the eight legacy P soils; we also speculate about how P speciation may affect future management of these soils with and without additional P applications. With continued P applications, we expect an increase in semicrystalline Al and Fe-P, P sorbed to Al (hydro)oxides, and insoluble Ca-P species in these soils for all P sources. Under drawdown scenarios, we expect plant P uptake first from semicrystalline Al and Fe phosphates followed by P sorbed phases. Our results can help guide management decisions on coastal plain soils with a history of P application.

Structural dynamics of the CROPs domain control stability and toxicity of Paeniclostridium sordellii lethal toxin

Paeniclostridium sordellii lethal toxin (TcsL) is a potent exotoxin that causes lethal toxic shock syndrome associated with fulminant bacterial infections. TcsL belongs to the large clostridial toxin (LCT) family. Here, we report that TcsL with varied lengths of combined repetitive oligopeptides (CROPs) deleted show increased autoproteolysis as well as higher cytotoxicity. We next present cryo-EM structures of full-length TcsL, at neutral (pH 7.4) and acidic (pH 5.0) conditions. The TcsL at neutral pH exhibits in the open conformation, which resembles reported TcdB structures. Low pH induces the conformational change of partial TcsL to the closed form. Two intracellular interfaces are observed in the closed conformation, which possibly locks the cysteine protease domain and hinders the binding of the host receptor. Our findings provide insights into the structure and function of TcsL and reveal mechanisms for CROPs-mediated modulation of autoproteolysis and cytotoxicity, which could be common across the LCT family.

Mineral and Carbon Metabolic Adjustments in Nodules of Symbiotically Grown Faba Bean (Vicia faba L.) Varieties in Response to Organic Phosphorus Supplementation

Phosphorus (P) is a major limiting factor for legume and symbiotic nitrogen fixation (SNF). Although overall adaptations of legumes to P supplementation have been extensively studied in connection with inorganic P, little information is currently available regarding nodulation or SNF responses to organic P (Po) in hydroponics. We investigated the mineral and carbon metabolism of Po-induced nodules of two contrasting faba bean varieties grown hydroponically under inorganic P (Pi), viz., in P-deficient (2 µM KH2PO4, -Pi), sufficient-P (200 µM KH2PO4, +Pi), and phytic acid (200 µM, Po) conditions, and were inoculated with Rhizobium leguminosarum bv. viciae 3841 and grown for 30 days. The results consistently reveal similar growth and biomass partitioning patterns between +Pi and Po, with both varying substantially from -Pi. In comparison, +Pi and Po observed equivalent accumulations of overall elemental P concentrations, with both increasing by 114 and 119%, respectively, relative to -Pi. A principal component analysis on metabolites showed a clear separation of the -Pi treatment from the others, with +Pi and Po correlating closely together, highlighting the nonsignificant differences between them. Additionally, the δ15N abundance of shoots, roots, and nodules was not significantly different between treatments and varieties and exhibited negative δ15N signatures for all tissues. Our study provides a novel perspective on mineral and carbon metabolism and their regulation of the growth, functioning, and reprogramming of nodules upon phytate supply.

Recent Advances in Microbial-Assisted Remediation of Cadmium-Contaminated Soil

Soil contamination with cadmium (Cd) is a severe concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Industries such as mining, manufacturing, building, etc., rapidly produce a substantial amount of Cd, posing environmental risks. Cd toxicity in crop plants decreases nutrient and water uptake and translocation, increases oxidative damage, interferes with plant metabolism and inhibits plant morphology and physiology. However, various conventional physicochemical approaches are available to remove Cd from the soil, including chemical reduction, immobilization, stabilization and electro-remediation. Nevertheless, these processes are costly and unfriendly to the environment because they require much energy, skilled labor and hazardous chemicals. In contrasting, contaminated soils can be restored by using bioremediation techniques, which use plants alone and in association with different beneficial microbes as cutting-edge approaches. This review covers the bioremediation of soils contaminated with Cd in various new ways. The bioremediation capability of bacteria and fungi alone and in combination with plants are studied and analyzed. Microbes, including bacteria, fungi and algae, are reported to have a high tolerance for metals, having a 98% bioremediation capability. The internal structure of microorganisms, their cell surface characteristics and the surrounding environmental circumstances are all discussed concerning how microbes detoxify metals. Moreover, issues affecting the effectiveness of bioremediation are explored, along with potential difficulties, solutions and prospects.

Arbuscular mycorrhizal fungi enhance plant phosphorus uptake through stimulating hyphosphere soil microbiome functional profiles for phosphorus turnover

The extraradical hyphae of arbuscular mycorrhizal (AM) fungi are colonized by different bacteria in natural and agricultural systems, but the mechanisms by which AM fungi interact with the hyphosphere soil microbiome and influence soil organic phosphorus (P) mobilization remain unclear. We grew Medicago in two-compartment microcosms, inoculated with Rhizophagus irregularis, or not, in the root compartment and set up P treatments (without P, with P addition as KH₂ PO₄ or nonsoluble phytate) in the hyphal compartment. We studied the processes of soil P turnover and characterized the microbiome functional profiles for P turnover in the hyphosphere soil by metagenomic sequencing. Compared with the bulk soil, the hyphosphere soil of R. irregularis was inhabited by a specific bacterial community and their functional profiles for P turnover was stimulated. At the species level, the shift in hyphosphere soil microbiome was characterized by the recruitment of the genome bin2.39 harbouring both gcd and phoD genes and genome bin2.97 harbouring the phoD gene, which synergistically drove nonsoluble phytate mobilization in the hyphosphere soil. Our results suggest that AM fungi recruits a specific hyphosphere soil microbiome and stimulated their functional profiles for P turnover to enhance utilization of phytate.

In-depth characterization of phytase-producing plant growth promotion bacteria isolated in alpine grassland of Qinghai-Tibetan Plateau

The use of plant growth promoting bacteria (PGPB) express phytase (myo-inositol hexakisphosphate phosphohydrolase) capable of hydrolyzing inositol phosphate in soil was a sustainable approach to supply available phosphorus (P) to plants. A total of 73 bacterial isolates with extracellular phytase activity were selected from seven dominant grass species rhizosphere in alpine grassland of Qinghai-Tibetan Plateau. Then, the plant growth promoting (PGP) traits of candidate bacteria were screened by qualitative and quantitative methods, including organic/inorganic Phosphorus solubilization (P. solubilization), plant hormones (PHs) production, nitrogen fixation, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity and antimicrobial activity. Further experiment were conducted to test their growth promoting effect on Lolium perenne L. under P-limitation. Our results indicated that these bacteria as members of phyla Proteobacteria (90.41%) and Actinobacteria (9.59%) were related to 16 different genera. The isolates of Pseudomonas species showed the highest isolates number (36) and average values of phytase activity (0.267 ± 0.012 U mL^-1), and showed a multiple of PGP traits, which was a great candidate for PGPBs. In addition, six strains were positive in phytase gene (β-propeller phytase, bpp) amplification, which significantly increased the shoot length, shoot/root fresh weight, root average diameter and root system phytase activity of Lolium perenne L. under P-limitation, and the expression of phytase gene (bppP) in root system were verified by qPCR. Finally, the PHY101 gene encoding phytase from Pseudomonas mandelii GS10-1 was cloned, sequenced, and recombinantly expressed in Escherichia coli. Biochemical characterization demonstrated that the recombinant phytase PHY101 revealed the highest activity at pH 6 and 40°C temperature. In particular, more than 60% of activity was retained at a low temperature of 15°C. This study demonstrates the opportunity for commercialization of the phytase-producing PGPB to developing localized microbial inoculants and engineering rhizobacteria for sustainable use in alpine grasslands.

Insight into phytase-producing microorganisms for phytate solubilization and soil sustainability

The increasing demand for food has increased dependence on chemical fertilizers that promote rapid growth and yield as well as produce toxicity and negatively affect nutritional value. Therefore, researchers are focusing on alternatives that are safe for consumption, non-toxic, cost-effective production process, and high yielding, and that require readily available substrates for mass production. The potential industrial applications of microbial enzymes have grown significantly and are still rising in the 21st century to fulfill the needs of a population that is expanding quickly and to deal with the depletion of natural resources. Due to the high demand for such enzymes, phytases have undergone extensive research to lower the amount of phytate in human food and animal feed. They constitute efficient enzymatic groups that can solubilize phytate and thus provide plants with an enriched environment. Phytases can be extracted from a variety of sources such as plants, animals, and microorganisms. Compared to plant and animal-based phytases, microbial phytases have been identified as competent, stable, and promising bioinoculants. Many reports suggest that microbial phytase can undergo mass production procedures with the use of readily available substrates. Phytases neither involve the use of any toxic chemicals during the extraction nor release any such chemicals; thus, they qualify as bioinoculants and support soil sustainability. In addition, phytase genes are now inserted into new plants/crops to enhance transgenic plants reducing the need for supplemental inorganic phosphates and phosphate accumulation in the environment. The current review covers the significance of phytase in the agriculture system, emphasizing its source, action mechanism, and vast applications.

Biogeochemical dynamics of particulate organic phosphorus and its potential environmental implication in a typical "algae-type" eutrophic lake

Organic phosphorus (P_o) plays a very important role in the process of lake eutrophication, but there is still a lack of knowledge about the internal cycle of P_o in suspended particulate matter (SPM) dominated by algal debris. In this study, the characterization of bioavailable P_o by sequential extraction and enzymatic hydrolysis showed that 45% of extracted TP was P_o in SPM of Lake Dianchi, and 43-98% of total P_o in H₂O, NaHCO₃ and NaOH fractions was enzymatically hydrolyzable P_o (EHP, H₂O-EHP: 31-53%). Importantly, labile monoester P was the main organic form (68%) of EHP, and its potential bioavailability was higher than that of diester P and phytate-like P. According to the estimation of P pools in SPM of the whole lake, the total load of P_i plus EHP in the H₂O extract of SPM was 74.9 t and had great potential risk to enhance eutrophication in the lake water environment. Accordingly, reducing the amount of SPM in the water during the algal blooming period is likely to be a necessary measure that can successfully interfere with or block the continuous stress of unhealthy levels of P on the aquatic ecosystem.

X-ray absorption near edge structure spectroscopy reveals phosphate minerals at surface and agronomic sampling depths in agricultural Ultisols saturated with legacy phosphorus

Legacy phosphorus (P) soils have received excessive P inputs from historic manure and fertilizer applications and present unique management challenges for protecting water quality as soil P saturation leads to increased soluble P to waterways. We used P K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify and quantify the dominant P minerals in four representative legacy P soils under conventional till and no-till management in Maryland, USA. Various measures of extractable soil P, including water-extractable P (20.6-54.1 mg kg^-1 at 1:10 soil-to-water ratio; 52.7-132.2 mg kg^-1 at 1:100 soil-to-water ratio), plant available P extracted with Mehlich 3 (692-1139 mg kg^-1), and Mehlich 3P saturation ratio (0.54-1.37), were above the environmental threshold values, suggesting the accumulation of legacy P in soils. The quantification of dominant P minerals may provide insights into the potential of legacy P soils to contribute to P release for crop use and soluble P losses. Linear combination fits of XANES spectra identified the presence of four phosphate mineral groups, consisting of (i) calcium-phosphate minerals (11-59%) in the form of fluorapatite, β-tricalcium phosphate, and brushite, followed by (ii) iron-phosphate minerals (12-49%) in the form of ludlamite, heterosite, P sorbed to ferrihydrite, and amorphous iron phosphates, (iii) aluminum-phosphate minerals (15-33%) in the form of wavellite and P sorbed to aluminum hydroxide, and (iv) other phosphate minerals (5-35%) in the form of copper-phosphate (cornetite, 5-18%) and manganese-phosphate (hureaulite, 25-35%). Organic P consisting of phytic acid was found in most soils (13-24%) and was more pronounced in the surface layer of no-till (21-24%) than in tilled (16%) fields. Of the P forms identified with XANES, we conclude that P sorbed to Fe and Al, and Ca-P in the form of brushite and β-tricalcium phosphate will likely readily contribute to the soil WEP pool as the soil solution P is depleted by crop uptake and lost via runoff and leaching.

Impacts and Industrial Applications of Phytic Acid and Phytase

Phytic acid is an antinutritional compound that chelates P and essential divalent cations such as Fe, Ca, and Zn in cereals and major staple crops such as wheat, maize, rice, and sorghum. As a result, these cations cannot be absorbed by monogastric animals or humans: phytic acid has an inhibitory effect on nutrient uptake and its levels are negatively correlated with protein and starch digestibility. However, phytic acid can be degraded by the action of the enzyme phytase. Phytase plays important roles in the degradation of phytic acid and in increasing the nutritional quality of staple foods. Microbial phytase is a versatile enzyme that is beneficial for humans, animals, the environment, and the industry. In this review, we summarise the interaction of phytic acid with micronutrients, various approaches to enhancing the nutritional profile of staple foods by reducing the phytic acid content, and current knowledge of microbial-based phytase as a potential reducer of phytic acid.

Enhancing Phytate Availability in Soils and Phytate-P Acquisition by Plants: A Review

Phytate (myo-inositol hexakisphosphate salts) can constitute a large fraction of the organic P in soils. As a more recalcitrant form of soil organic P, up to 51 million metric tons of phytate accumulate in soils annually, corresponding to ∼65% of the P fertilizer application. However, the availability of phytate is limited due to its strong binding to soils via its highly-phosphorylated inositol structure, with sorption capacity being ∼4 times that of orthophosphate in soils. Phosphorus (P) is one of the most limiting macronutrients for agricultural productivity. Given that phosphate rock is a finite resource, coupled with the increasing difficulty in its extraction and geopolitical fragility in supply, it is anticipated that both economic and environmental costs of P fertilizer will greatly increase. Therefore, optimizing the use of soil phytate-P can potentially enhance the economic and environmental sustainability of agriculture production. To increase phytate-P availability in the rhizosphere, plants and microbes have developed strategies to improve phytate solubility and mineralization by secreting mobilizing agents including organic acids and hydrolyzing enzymes including various phytases. Though we have some understanding of phytate availability and phytase activity in soils, the limiting steps for phytate-P acquisition by plants proposed two decades ago remain elusive. Besides, the relative contribution of plant- and microbe-derived phytases, including those from mycorrhizas, in improving phytate-P utilization is poorly understood. Hence, it is important to understand the processes that influence phytate-P acquisition by plants, thereby developing effective molecular biotechnologies to enhance the dynamics of phytate in soil. However, from a practical view, phytate-P acquisition by plants competes with soil P fixation, so the ability of plants to access stable phytate must be evaluated from both a plant and soil perspective. Here, we summarize information on phytate availability in soils and phytate-P acquisition by plants. In addition, agronomic approaches and biotechnological strategies to improve soil phytate-P utilization by plants are discussed, and questions that need further investigation are raised. The information helps to better improve phytate-P utilization by plants, thereby reducing P resource inputs and pollution risks to the wider environment.

Plant phosphorus-use and -acquisition strategies in Amazonia

In the tropical rainforest of Amazonia, phosphorus (P) is one of the main nutrients controlling forest dynamics, but its effects on the future of the forest biomass carbon (C) storage under elevated atmospheric CO₂ concentrations remain uncertain. Soils in vast areas of Amazonia are P-impoverished, and little is known about the variation or plasticity in plant P-use and -acquisition strategies across space and time, hampering the accuracy of projections in vegetation models. Here, we synthesize current knowledge of leaf P resorption, fine-root P foraging, arbuscular mycorrhizal symbioses, and root acid phosphatase and organic acid exudation and discuss how these strategies vary with soil P concentrations and in response to elevated atmospheric CO₂ . We identify knowledge gaps and suggest ways forward to fill those gaps. Additionally, we propose a conceptual framework for the variations in plant P-use and -acquisition strategies along soil P gradients of Amazonia. We suggest that in soils with intermediate to high P concentrations, at the plant community level, investments are primarily directed to P foraging strategies via roots and arbuscular mycorrhizas, whereas in soils with intermediate to low P concentrations, investments shift to prioritize leaf P resorption and mining strategies via phosphatases and organic acids.

Biosolids increase phosphate adsorption of semi-arid Mediterranean soils

The impact of biosolid compost on the adsorption of orthophosphate (IP) to Mediterranean-type soils was studied. Eight soils were amended with a stable biosolid compost (ADSC) at 9:1 and 97:3 ratios (w/w). Four soils were amended with the dissolved organic matter (DOM) fraction of the ADSC at the amount added at the 9:1 mixture (810 mg C kg^-1). Soils and their 9:1 soil‒ADSC mixtures were incubated for seven years. The maximum ADSC IP-adsorption capacity (S_MAX, Langmuir model) at native pH (≈7.5) was 850 mg P kg^-1. Mixing the ADSC with the soils increased their S_MAX values by ca. 150 and 190 mg P kg^-1 in the 9:1 and 97:3 mixtures, which exceeded additivity by 50% and 575%. The addition of DOM similarly increased the S_MAX of three out of the four soils. Following the incubation, the soils' organic-C decreased by 34% and the ADSC-derived OC decreased by 60%. Still, the corresponding soil's and mixtures' average levels of labile IP either increased (by 60%) or remained steady (at ̴30% of total-P). Incubation increased the S_MAX of three soils and five soil‒ADSC mixtures and decreased their binding affinity (k), trends which were also reflected in the quantity/intensity parameters. This study showed that amending semi-arid Mediterranean soils with stable biosolids, and their long-term oxidative co-stabilization is conducive to increase their IP binding capacity and bioavailability. Finally, the often similar effects of the compost and its DOM on IP adsorption merits further research regarding the role of cation (Ca⁺²) bridging in IP‒DOM‒solid phase interactions.

Reuse of Wasted Bread as Soil Amendment: Bioprocessing, Effects on Alkaline Soil and Escarole (Cichorium endivia) Production

In an era characterized by land degradation, climate change, and a growing population, ensuring high-yield productions with limited resources is of utmost importance. In this context, the use of novel soil amendments and the exploitation of plant growth-promoting microorganisms potential are considered promising tools for developing a more sustainable primary production. This study aimed at investigating the potential of bread, which represents a large portion of the global food waste, to be used as an organic soil amendment. A bioprocessed wasted bread, obtained by an enzymatic treatment coupled with fermentation, together with unprocessed wasted bread were used as amendments in a pot trial. An integrated analytical plan aimed at assessing (i) the modification of the physicochemical properties of a typical Mediterranean alkaline agricultural soil, and (ii) the plant growth-promoting effect on escarole (Cichorium endivia var. Cuartana), used as indicator crop, was carried out. Compared to the unamended soils, the use of biomasses raised the soil organic carbon content (up to 37%) and total nitrogen content (up to 40%). Moreover, the lower pH and the higher organic acid content, especially in bioprocessed wasted bread, determined a major availability of Mn, Fe, and Cu in amended soils. The escaroles from pots amended with raw and bioprocessed bread had a number of leaves, 1.7- and 1.4-fold higher than plants cultivated on unamended pots, respectively, showing no apparent phytotoxicity and thus confirming the possible re-utilization of such residual biomasses as agriculture amendments.

Role of metal complexation on the solubility and enzymatic hydrolysis of phytate

Phytate is a dominant form of organic phosphorus (P) in the environment. Complexation and precipitation with polyvalent metal ions can stabilize phytate, thereby significantly hinder the hydrolysis by enzymes. Here, we studied the stability and hydrolyzability of environmentally relevant metal phytate complexes (Na, Ca, Mg, Cu, Zn, Al, Fe, Al/Fe, Mn, and Cd) under different pHs, presence of metal chelators, and thermal conditions. Our results show that the order of solubility of metal phytate complexes is as follows: i) for metal species: Na, Ca, Mg > Cu, Zn, Mn, Cd > Al, Fe, ii) under different pHs: pH 5.0 > pH 7.5), and iii) in the presence of chelators: EDTA> citric acid. Phytate-metal complexes are mostly resistant towards acid hydrolysis (except Al-phytate), and dry complexes are generally stable at high pressure and temperature under autoclave conditions (except Ca phytate). Inhibition of metal complex towards enzymatic hydrolysis by Aspergillus niger phytase was variable but found to be highest in Fe phytate complex. Strong chelating agents such as EDTA are insufficient for releasing metals from the complexes unless the reduction of metals (such as Fe) occurs first. The insights gained from this research are expected to contribute to the current understanding of the fate of phytate in the presence of various metals that are commonly present in agricultural soils.

Phosphate bacterial solubilization: A key rhizosphere driving force enabling higher P use efficiency and crop productivity

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Phosphate bacteria bio-solubilization significantly increase crop P acquisition and productivity.

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Phosphate solubilizing bacteria increase RP agronomic efficiency as well as P fertilizers efficiency.

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This process can be optimized through a rational bacterial screening to assure efficient PSB are selected.

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Appropriate formulation of PSB is a sustainable approach to enhance P-fertilizers efficiency.

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Development of innovative PSB-Phosphate formulations is likely to sustain crop production.

Background

Increasing crop production to feed a growing population has driven the use of mineral fertilizers to ensure nutrients availability and fertility of agricultural soils. After nitrogen, phosphorus (P) is the second most important nutrient for plant growth and productivity. However, P availability in most agricultural soils is often limited because P strongly binds to soil particles and divalent cations forming insoluble P-complexes. Therefore, there is a constant need to sustainably improve soil P availability. This may include, among other strategies, the application of microbial resources specialized in P cycling, such as phosphate solubilizing bacteria (PSB). This P-mediating bacterial component can improve soil biological fertility and crop production, and should be integrated in well-established formulations to enhance availability and efficiency in use of P. This is of importance to P fertilization, including both organic and mineral P such as rock phosphate (RP) aiming to improve its agronomic efficiency within an integrated crop nutrition system where agronomic profitability of P and PSB can synergistically occur.

Aim of Review

The purpose of this review is to discuss critically the important contribution of PSB to crop P nutrition in concert with P fertilizers, with a specific focus on RP. We also highlight the need for PSB bioformulations being a sustainable approach to enhance P fertilizer use efficiency and crop production.

Key Scientific Concepts of Review

We first recognize the important contribution of PSB to sustain crop production, which requires a rational approach for both screening and evaluation of PSB enabling an accurate assessment of the bacterial effects both alone and in intertwined interaction with plant roots. Furthermore, we propose new research ideas about the development of microbial bioformulations based on PSB with a particular focus on strains exhibiting synergetic effects with RP.

Microbial Phosphorus Mobilization Strategies Across a Natural Nutrient Limitation Gradient and Evidence for Linkage With Iron Solubilization Traits

Microorganisms have evolved several mechanisms to mobilize and mineralize occluded and insoluble phosphorus (P), thereby promoting plant growth in terrestrial ecosystems. However, the linkages between microbial P-solubilization traits and the preponderance of insoluble P in natural ecosystems are not well known. We tested the P solubilization traits of hundreds of culturable bacteria representative of the rhizosphere from a natural gradient where P concentration and bioavailability decline as soil becomes progressively more weathered. Aluminum, iron phosphate and organic P (phytate) were expected to dominate in more weathered soils. A defined cultivation medium with these chemical forms of P was used for isolation. A combination of soil chemical, spectroscopic analyses and 16S rRNA gene sequencing were used to understand the in situ ability for solubilization of these predominant forms of P. Locations with more occluded and organic P harbored the greatest abundance of P-mobilizing microorganisms, especially Burkholderiaceae (Caballeronia and Paraburkholderia spp.). Nearly all bacteria utilized aluminum phosphate, however fewer could subsist on iron phosphate (FePO₄) or phytate. Microorganisms isolated from phytic acid were also most effective at solubilizing FePO₄, suggesting that phytate solubilization may be linked to the ability to solubilize Fe. Significantly, we observed Fe to be co-located with P in organic patches in soil. Siderophore addition in lab experiments reinstated phytase mediated P-solubilization from Fe-phytate complexes. Taken together, these results indicate that metal-organic-P complex formation may limit enzymatic P solubilization from phytate in soil. Additionally, the linked traits of phytase and siderophore production were mostly restricted to specific clades within the Burkholderiaceae. We propose that Fe complexation of organic P (e.g., phytate) represents a major constraint on P turnover and availability in acidic soils, as only a limited subset of bacteria appear to possess the traits required to access this persistent pool of soil P.

The biotransformation of soil phosphorus in the water level fluctuation zone could increase eutrophication in reservoirs

The massive amounts of phosphorus (P) entering into rivers and reservoirs may induce eutrophication. However, the link between the transport and transformation of soil P and the dynamics of P availability in reservoir regions are not well demonstrated. The present study selected the Pengxi River suffering the anti-seasonal water level fluctuation of the Three Gorgers Reservoir as the study area. Soil nutrients along the longitudinal and lateral gradients of the Pengxi River were investigated to illustrate the spatial distribution patterns, analyzed by the Hedley extraction schemes. The effects of biotic and abiotic factors on soil P transformation and the dynamics of bioavailable P were evaluated via determinations of enzymatic hydrolysis phosphorus (EHP) with and without ultraviolet (UV) irradiation. The results indicated that soil nutrients varied significantly between the water level fluctuation zone (WLFZ) and upland along the river longitudinal gradients, where the trends of the extracted OP were the same in H₂O, NaHCO₃ and NaOH extractions. The EHP accounted for 33.67 ± 15.87% of the total extracted OP, of which Monoester P, Phytate-like P and NHOP were determined at all extracts but Diester P was mainly found at H₂O and NaOH extracts. UV irradiation significantly increased P bioavailability up to 24.44%. These results could demonstrate the mechanism of soil P transformation via UV irradiation and enzymatic hydrolysis. Therefore, the bioavailable P enters the water body during the submergence period may lead to eutrophication in the Pengxi River, which could pose a risk to the reservoir ecosystem.

Abiotic Stress in Crop Species: Improving Tolerance by Applying Plant Metabolites

Reductions in crop yields brought about by abiotic stress are expected to increase as climate change, and other factors, generate harsher environmental conditions in regions traditionally used for cultivation. Although breeding and genetically modified and edited organisms have generated many varieties with greater abiotic stress tolerance, their practical use depends on lengthy processes, such as biological cycles and legal aspects. On the other hand, a non-genetic approach to improve crop yield in stress conditions involves the exogenous application of natural compounds, including plant metabolites. In this review, we examine the recent literature related to the application of different natural primary (proline, l-tryptophan, glutathione, and citric acid) and secondary (polyols, ascorbic acid, lipoic acid, glycine betaine, α-tocopherol, and melatonin) plant metabolites in improving tolerance to abiotic stress. We focus on drought, saline, heavy metal, and temperature as environmental parameters that are forecast to become more extreme or frequent as the climate continues to alter. The benefits of such applications are often evaluated by measuring their effects on metabolic, biochemical, and morphological parameters in a variety of crop plants, which usually result in improved yields when applied in greenhouse conditions or in the field. As this strategy has proven to be an effective way to raise plant tolerance to abiotic stress, we also discuss the prospect of its widespread implementation in the short term.

Ab Initio Molecular Dynamics Simulations of the Interaction between Organic Phosphates and Goethite

Today's fertilizers rely heavily on mining phosphorus (P) rocks. These rocks are known to become exhausted in near future, and therefore effective P use is crucial to avoid food shortage. A substantial amount of P from fertilizers gets adsorbed onto soil minerals to become unavailable to plants. Understanding P interaction with these minerals would help efforts that improve P efficiency. To this end, we performed a molecular level analysis of the interaction of common organic P compounds (glycerolphosphate (GP) and inositol hexaphosphate (IHP)) with the abundant soil mineral (goethite) in presence of water. Molecular dynamics simulations are performed for goethite-IHP/GP-water complexes using the multiscale quantum mechanics/molecular mechanics method. Results show that GP forms monodentate (M) and bidentate mononuclear (B) motifs with B being more stable than M. IHP interacts through multiple phosphate groups with the 3M motif being most stable. The order of goethite-IHP/GP interaction energies is GP M < GP B < IHP M < IHP 3M. Water is important in these interactions as multiple proton transfers occur and hydrogen bonds are formed between goethite-IHP/GP complexes and water. We also present theoretically calculated infrared spectra which match reasonably well with frequencies reported in literature.

Accessing Legacy Phosphorus in Soils

Repeated applications of phosphorus (P) fertilizers result in the buildup of P in soil (commonly known as legacy P), a large fraction of which is not immediately available for plant use. Long-term applications and accumulations of soil P is an inefficient use of dwindling P supplies and can result in nutrient runoff, often leading to eutrophication of water bodies. Although soil legacy P is problematic in some regards, it conversely may serve as a source of P for crop use and could potentially decrease dependence on external P fertilizer inputs. This paper reviews the (1) current knowledge on the occurrence and bioaccessibility of different chemical forms of P in soil, (2) legacy P transformations with mineral and organic fertilizer applications in relation to their potential bioaccessibility, and (3) approaches and associated challenges for accessing native soil P that could be used to harness soil legacy P for crop production. We highlight how the occurrence and potential bioaccessibility of different forms of soil inorganic and organic P vary depending on soil properties, such as soil pH and organic matter content. We also found that accumulation of inorganic legacy P forms changes more than organic P species with fertilizer applications and cessations. We also discuss progress and challenges with current approaches for accessing native soil P that could be used for accessing legacy P, including natural and genetically modified plant-based strategies, the use of P-solubilizing microorganisms, and immobilized organic P-hydrolyzing enzymes. It is foreseeable that accessing legacy P will require multidisciplinary approaches to address these limitations.

The impact of biogas digestate typology on nutrient recovery for plant growth: Accessibility indicators for first fertilization prediction

In recent years, anaerobic digestion of organic waste (OW) is rapidly appearing as a winning waste management strategy by producing energy and anaerobic digestates that can be used as fertilizers in agricultural soils. In this context, the management of the OW treatment process to maximize agro-system sustainability satisfying the crop nutrient demands represents the main goal. To investigate these traits, two protocols to assess the plant availability of digestate nitrogen (N) and phosphorus (P) were evaluated. With this aim, the N and P availability was determined on 8 digestates and 2 types of digestate-based compost from different OW via sequential chemical extractions (SCE). In addition, the digestates were tested in soil incubations and in plant pot tests with Italian ryegrass and compared with chemical fertilizer and a non-amended control soil. The N extracted from digestates via SCE was related to soil N mineralization and plant N recovery. The C: N ratio had negative impact on mineralized N and its recovery in shoots (Shoots_N = -0.0085.(C/N)+0.172, r² = 0.67), whereas water extractable mineral N was positevely related to the root N apparent recovery fraction (N-ARF) with (Roots_N = 5E^-5.N_solublemin+0.0138, r² = 0.53). The shoot P-ARF was positively correlated with the inorganic water extractable fraction of P (Shoots_P =0.1153.H₂O-P_i-0.2777.H₂O-P_o+0.0249, r² = 0.71) whereas the root P-ARF was positively correlated with the less accessible fractions (Roots_P = (b)   0.0955.NaHCO₃-P_o+0.0955.NaOH-P_o-0.0584NaHCO₃-P_i+0.0128, r² = 0.8641). Feedstock digestate typology impacted the N and P recovery results leading to a better description of the typology properties and a first nutrients ARF prediction.

Exudation of organic acid anions by tropical grasses in response to low phosphorus availability

It has been suggested that some tropical grasses can acquire phosphorus (P) from hematite and gypsite by exuding organic acid anions (OAs). However, it remains to be determined exactly which OAs could be involved in each case. The objective of this study was to verify the exudation OAs by ruzigrass (Urochloa ruziziensis), palisade grass (U. brizantha), and Guinea grass (Megathyrsus maximus) as a response to P deficiency. The grasses were grown in leachate columns with adequate and deficient P nutrient solutions. The concentration of OAs in the leacheate and root surface, as well as shoot and root dry matter, and P uptake were determined. Citrate, isocitrate, and malate concentration in leachates and root surfaces increased with P starvation, mainly for the Urochloa grasses. Oxalate exudation was similar for the grasses under adequate P supply, but was lower in Guinea grass under P starvation. Palisade grass showed a higher concentration of total OAs in the root surface than the other species due to a great production of oxalate and isocitrate. Palisade grass showed greater dry matter yields regardless of P deficiency, and Guinea grass always had the higher shoot:root ratio. Urochloa grasses have a higher capacity to cope with low P availability by exuding OAs along with a lower shoot:root ratio than Guinea grass.

Bioavailability of organic phosphorus in the water level fluctuation zone soil and the effects of ultraviolet irradiation on it in the Three Gorges Reservoir, China

Ultraviolet (UV) irradiation is an abiotic pathway for the transformation of complex phosphorus (P) components into inorganic P in ecosystems. To explore the effect of UV irradiation on organic P (OP) bioavailability in the water level fluctuation zone (WLFZ) soil, we collected representative soil samples from WLFZ of the Pengxi River, a tributary of the TGR, China. We determined the contents of different forms of OP in the WLFZ soil through sequential extraction. The bioavailability of different forms of OP and the effect of UV light were characterised using a combination of enzymatic hydrolysis and UV irradiation. The OP contents of the different extracts (P_o) were ranked as NaOH-P_o > NaHCO₃-P_o > H₂O-P_o, whereas those of enzymatically hydrolysable organic P (EHP) were ranked as NaOH-EHP > NaHCO₃-EHP > H₂O-EHP. UV irradiation was found to improve OP bioavailability, as demonstrated by increased levels of UV-sensitive P (UV-P) and EHP in the extracts after irradiation. The total contents of bioavailable P_o in extracts were 5.6-35.3% higher after UV irradiation than before irradiation. Thus, the effect of UV irradiation on the OP bioavailability and release activity cannot be neglected in TGR WLFZ soil.

Speciation and sorption of phosphorus in agricultural soil profiles of redoximorphic character

Controlled drainage is considered as a soil management tool to improve water supply to crops and reduce nutrient losses from fields; however, its closure may affect phosphorus (P) mobilization in soil. To assess the P mobilization potential, three soil profiles with redoximorphic features were selected along a slight hill in Northern Germany. Soil samples from three depths of each profile were characterized for basic properties, total element content, oxalate- and dithionite-extractable pedogenic Al, Fe and Mn (hydr)oxides, P pools (sequential extraction), P species [P K-edge X-ray absorption near-edge structure (XANES) spectroscopy] and P sorption behavior. In topsoil (~ 10 cm depth), labile P (H₂O-P + resin-P + NaHCO₃-P) accounted for 26-32% of total P (P_t). Phosphorus K-edge XANES revealed that up to 49% of P_t was bound to Al and/or Fe (hydr)oxides, but sequential fractionation indicated that > 30% of this P was occluded within sesquioxide aggregates. A low binding capacity for P was demonstrated by P sorption capacity and low K_f coefficients (20-33 [Formula: see text]) of the Freundlich equation. In the subsoil layers (~ 30 and ~ 65 cm depth), higher proportions of Al- and Fe-bound P along with other characteristics suggested that all profiles might be prone to P mobilization/leaching risk under reducing conditions even if the degree of P saturation (DPS) of a profile under oxic conditions was < 25%. The results suggest that a closure of the controlled drainage may pose a risk of increased P mobilization, but this needs to be compared with the risk of uncontrolled drainage and P losses to avoid P leaching into the aquatic ecosystem.

QM/MM simulations of organic phosphorus adsorption at the diaspore-water interface

Phosphorus (P) immobilization and thus its availability for plants are mainly affected by the strong interaction of phosphates with soil components especially soil mineral surfaces. The related reactions have been studied extensively via sorption experiments especially by carrying out adsorption of ortho-phosphates onto Fe-oxide surfaces. But a molecular-level understanding of the P-binding mechanisms at the mineral-water interface is still lacking, especially for forest eco-systems. Therefore, the current contribution provides an investigation of the molecular binding mechanisms for two abundant phosphates in forest soils, inositol hexaphosphate (IHP) and glycerolphosphate (GP), at the diaspore mineral surface. Here a hybrid electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) based molecular dynamics simulation has been applied to explore the diaspore-IHP/GP-water interactions. The results provide evidence for the formation of different P-diaspore binding motifs involving monodentate (M) and bidentate (B) for GP and two (2M) as well as three (3M) monodentates for IHP. The interaction energy results indicated the abundance of the GP B motif compared to the M one. The IHP 3M motif has a higher total interaction energy compared to its 2M motif, but exhibits a lower interaction energy per bond. Compared to GP, IHP exhibited stronger interaction with the surface as well as with water. Water was found to play an important role in controlling these diaspore-IHP/GP-water interactions. The interfacial water molecules form moderately strong H-bonds (HBs) with GP and IHP as well as with the diaspore surface. For all the diaspore-IHP/GP-water complexes, the interaction of water with the diaspore exceeds that with the studied phosphates. Furthermore, some water molecules form covalent bonds with diaspore Al atoms while others dissociate at the surface to protons and hydroxyl groups leading to proton transfer processes. Finally, the current results confirm the previous experimental conclusions indicating the importance of the number of phosphate groups, HBs, and proton transfers in controlling the P-binding at soil mineral surfaces.

Abiotic and biotic controls on dynamics of labile phosphorus fractions in calcareous soils under agricultural cultivation

Soil labile inorganic and organic phosphorus (L-Pi and L-Po) extracted by NaHCO₃ are potentially important sources of plant-available P. Their availability is strongly affected by soil physiochemical and biological properties. Here we conducted a field study in an arid region of northwestern China to investigate how L-Pi and L-Po dynamics are affected by changes in physiochemical and biological variables caused by agricultural cultivation of calcareous soils. Topsoils were sampled at multiple sites from natural calcareous grasslands and cultivated farmlands of different ages (32-40 vs. 90-100 years) that had been converted from natural grasslands. We measured L-Pi and L-Po concentrations and a set of key physiochemical (soil pH, concentrations of base cations (K⁺ + Na⁺ + Ca²⁺ + Mg²⁺), acid anions (Cl^- + NO₃^- + SO₄^2-), nitrate and ammonium nitrogen (N), organic carbon, and total P) and biological (soil microbial biomass carbon (C_mic), alkaline phosphatase activity (A_alp), and abundances of soil macroinvertebrates (A_macro) and mesoinvertebrates (A_meso)) variables. The concentration of L-Pi and L-Po was 484% and 128% higher and their proportion in the total P content was 354% and 78% higher in young farmland soils, whereas L-Pi and L-Po concentrations were 583% and 423% higher and their proportions were 353% and 240% higher in old farmland soils compared to grassland soils. Increases in L-Pi and L-Po attributable to soil P release may be driven by crucial processes of soil acidification-induced phosphate dissolution and soil biota-driven Po mineralisation. Path analyses revealed that L-Pi and L-Po dynamics were shaped by the complex interactions among five key controlling factors, soil pH, nitrate-N, A_meso, C_mic, and A_alp, involved in these crucial processes. We conclude that cultivation of calcareous soils significantly increases the availability of L-Pi and L-Po, emphasizing the importance of this land-use change as a regulator of P cycling in calcareous soils.

Characteristics of the First Protein Tyrosine Phosphatase with Phytase Activity from a Soil Metagenome

Protein tyrosine phosphatases (PTPs) fulfil multiple key regulatory functions. Within the group of PTPs, the atypical lipid phosphatases (ALPs) are known for their role as virulence factors associated with human pathogens. Another group of PTPs, which is capable of using inositol-hexakisphosphate (InsP₆) as substrate, are known as phytases. Phytases play major roles in the environmental phosphorus cycle, biotechnology, and pathogenesis. So far, all functionally characterized PTPs, including ALPs and PTP-phytases, have been derived exclusively from isolated microorganisms. In this study, screening of a soil-derived metagenomic library resulted in identification of a gene (pho16B), encoding a PTP, which shares structural characteristics with the ALPs. In addition, the characterization of the gene product (Pho16B) revealed the capability of the protein to use InsP₆ as substrate, and the potential of soil as a source of phytases with so far unknown characteristics. Thus, Pho16B represents the first functional environmentally derived PTP-phytase. The enzyme has a molecular mass of 38 kDa. The enzyme is promiscuous, showing highest activity and affinity toward naphthyl phosphate (K_m 0.966 mM). Pho16B contains the HCXXGKDR[TA]G submotif of PTP-ALPs, and it is structurally related to PtpB of Mycobacterium tuberculosis. This study demonstrates the presence and functionality of an environmental gene codifying a PTP-phytase homologous to enzymes closely associated to bacterial pathogenicity.

Sensitivity of Three Phosphate Extraction Methods to the Application of Phosphate Species Differing in Immediate Plant Availability

Extractive tests for determining the plant-availability of soil phosphorus (P) give varying results due to the inherently different characteristics of the extraction solution. Generally, classical soil P tests such as the Olsen or calcium acetate/lactate (CAL) method do not give an indication on the total amount of plant available P, but merely give an indication of the equilibrium between soil and extraction solution. It is also not entirely clear which fractions of P are directly determined through the various methods of extraction, i.e., determined P must not be immediately plant available, as is the case for rock phosphate. It is therefore possible that extraction methods either over or under estimate the amount of P available for plant consumption. In this research, we compared three methods of soil P determination (CAL, Olsen and diffusive gradients in thin films (DGT)) with regards to their ability to determine P species (Ca(H2PO4)2, CaHPO4, Ca3(PO4)2 and Inositol-6-hexakisphosphate) added to soils of high sorption capacity, immediately after as well as two weeks after application. For each of the methods, it could be shown that sorption processes in the soil immediately (0 days incubation) fix P to a point where it is not extractable through any of the described methods. These sorption processes continue over time, leading to a further decrease of determined P. The acidic CAL extraction method gives higher results of extractable P compared to the Olsen method. Due to the extraction of Ca3(PO4)2, the CAL method may overestimate immediately plant-available P. The most suitable methods for the determination of immediately plant available P may therefore be the Olsen and DGT methods. Organic IP6 is not determined by any of the extraction methods. At low concentrations of soil P, the DGT method may fail to give results.

Characterization of the Catalytic Structure of Plant Phytase, Protein Tyrosine Phosphatase-Like Phytase, and Histidine Acid Phytases and Their Biotechnological Applications

Phytase plays a prominent role in monogastric animal nutrition due to its ability to improve phytic acid digestion in the gastrointestinal tract, releasing phosphorus and other micronutrients that are important for animal development. Moreover, phytase decreases the amounts of phytic acid and phosphate excreted in feces. Bioinformatics approaches can contribute to the understanding of the catalytic structure of phytase. Analysis of the catalytic structure can reveal enzymatic stability and the polarization and hydrophobicity of amino acids. One important aspect of this type of analysis is the estimation of the number of β-sheets and α-helices in the enzymatic structure. Fermentative processes or genetic engineering methods are employed for phytase production in transgenic plants or microorganisms. To this end, phytase genes are inserted in transgenic crops to improve the bioavailability of phosphorus. This promising technology aims to improve agricultural efficiency and productivity. Thus, the aim of this review is to present the characterization of the catalytic structure of plant and microbial phytases, phytase genes used in transgenic plants and microorganisms, and their biotechnological applications in animal nutrition, which do not impact negatively on environmental degradation.

Extracellular Secretion of Phytase from Transgenic Wheat Roots Allows Utilization of Phytate for Enhanced Phosphorus Uptake

A significant portion of organic phosphorus comprises of phytates which are not available to wheat for uptake. Hence for enabling wheat to utilize organic phosphorus in form of phytate, transgenic wheat expressing phytase from Aspergillus japonicus under barley root-specific promoter was developed. Transgenic events were initially screened via selection media containing BASTA, followed by PCR and BASTA leaf paint assay after hardening. Out of 138 successfully regenerated T_o events, only 12 had complete constructs and thus further analyzed. Positive T1 transgenic plants, grown in sand, exhibited 0.08-1.77, 0.02-0.67 and 0.44-2.14 fold increase in phytase activity in root extracts, intact roots and external root solution, respectively, after 4 weeks of phosphorus stress. Based on these results, T2 generation of four best transgenic events was further analyzed which showed up to 1.32, 56.89, and 15.40 fold increase in phytase activity in root extracts, intact roots and external root solution, respectively, while in case of real-time PCR, maximum fold increase of 19.8 in gene expression was observed. Transgenic lines showed 0.01-1.18 fold increase in phosphorus efficiency along with higher phosphorus content when supplied phytate or inorganic phosphorus than control plants. Thus, this transgenic wheat may aid in reducing fertilizer utilization and enhancing wheat yield.

Mechanisms of oat (Avena sativa L.) acclimation to phosphate deficiency

BACKGROUND

Deficiency of available forms of phosphorus is common in most soils and causes reduction of crop plants growth and yield. Recently, model plants responses to phosphate (Pi) deficiency have been intensively studied. However, acclimation mechanisms of cereals like oat (Avena sativa L.), to low Pi stress remains not fully understood. Oat plants have been usually cultured on poor soils, with a low nutrient content, but their responses to such conditions are not well known, therefore the main goal of the study was to investigate the mechanisms that enable oat plants to grow under low Pi conditions.

METHODS

Four oat cultivars (A. sativa, cv. Arab, Krezus, Rajtar and Szakal) were grown for three weeks in a nutrient media with various P sources: inorganic-KH2PO4 (control), organic-phytate (PA) and with no phosphate (-P). The effects of Pi deficiency on the level of P, oat growth parameters, intensity of photosynthesis, plant productivity, root exudation ability, localization, activity and isoforms of acid phosphatases, enzymes involved in Pi mobilization, were estimated. In addition, the effect of mycorrhization on plant growth was also observed.

RESULTS

All studied oat cultivars grown on Pi-deficient media had significantly decreased Pi content in the tissues. Pi deficiency caused inhibition of shoot growth, but generally it did not affect root elongation; root diameter was decreased, root/shoot ratios increased, whereas PA plants showed a similar growth to control. Photosynthesis rate and productivity parameters decreased under low Pi nutrition, however, sugar content generally increased. Studied oat cultivars did not respond to low Pi via increased exudation of carboxylates from the roots, as pH changes in the growth media were not observed. Pi starvation significantly increased the activity of extracellular and intracellular acid phosphatases (APases) in comparison to the control plants. Three major APase isoforms were detected in oat tissues and the isoform pattern was similar in all studied conditions, usually with a higher level of one of the isoforms under Pi starvation. Generally no significant effects of mycorrhizal colonization on growth of oat cultivars were observed.

DISCUSSION

We postulated that acid phosphatases played the most important role in oat cultivars acclimation to Pi deficiency, especially extracellular enzymes involved in Pi acquisition from soil organic P esters. These APases are mainly located in the epidermis of young roots, and may be released to the rhizosphere. On the other hand, intracellular APases could be involved in fast Pi remobilization from internal sources. Our study showed that oat, in contrast to other plants, can use phytates as the sole source of P. The studied oat cultivars demonstrated similar acclimation mechanisms to Pi deficiency, however, depending on stress level, they can use different pools of acid phosphatases.

Colloidal-Bound Polyphosphates and Organic Phosphates Are Bioavailable: A Nutrient Solution Study

Colloidal forms of Fe(III) minerals can be stabilized in solution by coatings of organic or poly-phosphate (P), which reduce the zeta-potential. This opens up a route toward the development of nanoforms of P fertilizers. However, it is unclear if such P forms are bioavailable. To address this question, spinach (Spinacia oleracea) was grown in nutrient solutions, at equal total P, using three different forms of P (orthophosphate = P_i; hexametaphosphate = HMP; myo-inositol hexaphosphate = IHP), free or bound to goethite/ferrihydrite colloids. After 10 days, P uptake was determined with a dose-response curve using colloid-free P_i as a reference treatment. The P_i concentration generating equal P uptake as in colloidal P treatments was used to calculate the relative bioavailability of colloidal P (RBA_colloid). The RBA_colloid was about 60% for P_i-loaded goethite, stabilized with natural organic matter. For HMP/IHP-P_i-loaded colloids, RBA_colloid ranged between 10 and 50%, in line with their higher sorption strength. In conclusion, colloidal organic P or poly-P can stabilize Fe(III) colloids in solution and can contribute to plant-available P. Soil experiments are required to assess their potential as nanofertilizers.

Phosphate Uptake from Phytate Due to Hyphae-Mediated Phytase Activity by Arbuscular Mycorrhizal Maize

Phytate is the most abundant form of soil organic phosphorus (P). Increased P nutrition of arbuscular mycorrhizal plants derived from phytate has been repeatedly reported. Earlier studies assessed acid phosphatase rather than phytase as an indication of mycorrhizal fungi-mediated phytate use. We investigated the effect of mycorrhizal hyphae-mediated phytase activity on P uptake by maize. Two maize (Zea mays L.) cultivars, non-inoculated or inoculated with the arbuscular mycorrhizal fungi Funneliformis mosseae or Claroideoglomus etunicatum, were grown for 45 days in two-compartment rhizoboxes, containing a root compartment and a hyphal compartment. The soil in the hyphal compartment was supplemented with 20, 100, and 200 mg P kg-1 soil as calcium phytate. We measured activity of phytase and acid phosphatase in the hyphal compartment, hyphal length density, P uptake, and plant biomass. Our results showed: (1) phytate addition increased phytase and acid phosphatase activity, and resulted in larger P uptake and plant biomass; (2) increases in P uptake and biomass were correlated with phytase activity but not with acid phosphatase activity; (3) lower phytate addition rate increased, but higher addition rate decreased hyphal length density. We conclude that P from phytate can be taken up by arbuscular mycorrhizal plants and that phytase plays a more important role in mineralizing phytate than acid phosphatase.

Seed Biofortification and Phytic Acid Reduction: A Conflict of Interest for the Plant?

Most of the phosphorus in seeds is accumulated in the form of phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate, InsP₆). This molecule is a strong chelator of cations important for nutrition, such as iron, zinc, magnesium, and calcium. For this reason, InsP₆ is considered an antinutritional factor. In recent years, efforts to biofortify seeds through the generation of low phytic acid (lpa) mutants have been noteworthy. Moreover, genes involved in the biosynthesis and accumulation of this molecule have been isolated and characterized in different species. Beyond its role in phosphorus storage, phytic acid is a very important signaling molecule involved in different regulatory processes during plant development and responses to different stimuli. Consequently, many lpa mutants show different negative pleitotropic effects. The strength of these pleiotropic effects depends on the specific mutated gene, possible functional redundancy, the nature of the mutation, and the spatio-temporal expression of the gene. Breeding programs or transgenic approaches aimed at development of new lpa mutants must take into consideration these different aspects in order to maximize the utility of these mutants.