Extracellular secretion of Aspergillus phytase from Arabidopsis roots enables plants to obtain phosphorus from phytate

Phosphate environment and phosphate uptake studies: past and future

The present review explains briefly the importance of phosphorus in the biological activities and states that the most phosphorus of living organisms is absorbed by plants from the soil. Next, previous studies on the mechanisms of phosphate uptake by plants are reviewed as H+-dependent or Na+-dependent co-transport systems and the phosphate environment in which plants grow is discussed. The evolution of transporter genes and their regulation mechanisms of expression is discussed in relation to the phosphorus environment.

Insoluble-Phytate Improves Plant Growth and Arsenic Accumulation in As-Hyperaccumulator Pteris vittata: Phytase Activity, Nutrient Uptake, and As-Metabolism

Phytate, the principal P storage in plant seeds, is also an important organic P in soils, but it is unavailable for plant uptake. However, the As-hyperaccumulator Pteris vittata can effectively utilize soluble Na-phytate, while its ability to utilize insoluble Ca/Fe-phytate is unclear. Here, we investigated phytate uptake and the underlying mechanisms based on the phytase activity, nutrient uptake, and expression of genes involved in As metabolisms. P. vittata plants were cultivated hydroponically in 0.2-strength Hoagland nutrient solution containing 50 μM As and 0.2 mM Na/Ca/Fe-phytate, with 0.2 mM soluble-P as the control. As the sole P source, all three phytates supported P. vittata growth, with its biomass being 3.2-4.1 g plant-1 and Ca/Fe-phytate being 19-29% more effective than Na-phytate. Phytate supplied soluble P to P. vittata probably via phytase hydrolysis, which was supported by 0.4-0.7 nmol P min-1 g-1 root fresh weight day-1 phytase activity in its root exudates, with 29-545 μM phytate-P being released into the growth media. Besides, compared to Na-phytate, Ca/Fe-phytate enhanced the As contents by 102-140% to 657-781 mg kg-1 in P. vittata roots and by 43-86% to 1109-1447 mg kg-1 in the fronds, which was accompanied by 21-108% increase in Ca and Fe uptake. The increased plant As is probably attributed to 1.3-2.6 fold upregulation of P transporters PvPht1;3/4 for root As uptake, and 1.8-4.3 fold upregulation of arsenite antiporters PvACR3/3;1/3;3 for As translocation to and As sequestration into the fronds. This is the first report to show that, besides soluble Na-phytate, P. vittata can also effectively utilize insoluble Ca/Fe-phytate as the sole P source, which sheds light onto improving its application in phytoremediation of As-contaminated sites.

Unraveling the potential of bacterial phytases for sustainable management of phosphorous

Phosphorous actively participates in numerous metabolic and regulatory activities of almost all living organisms including animals and humans. Therefore, it is considered as an essential macronutrient required supporting their proper growth. On contrary, phytic acid (PA), an antinutritional substance, is widely known for its strong affinity to chelate essential mineral ions including PO4 3- , Ca2+ , Fe2+ , Mg2+ , and Zn2+ . Being one the major reservoir of PO4 3- ions, PA has great potential to bind PO4 3- ions in diverse range of foods. Once combined with P, PA transforms into an undigested and insoluble complex namely phytate. Produced phytate leads to a notable reduction in the bioavailability of P due to negligible activity of phytases in monogastric animals and humans. This highlights the importance and consequent need of enhancement of phytase level in these life forms. Interestingly, phytases, catalyzing the breakdown of phytate complex and recycling the phosphate into ecosystem to its available form, have naturally been reported in a variety of plants and microorganisms over past few decades. In pursuit of a reliable solution, the focus of this review is to explore the keynote potential of bacterial phytases for sustainable management of phosphorous via efficient utilization of soil phytate. The core of the review covers detailed discussion on bacterial phytases along with their widely reported applications viz. biofertilizers, phosphorus acquisition, and plant growth promotion. Moreover, meticulous description on fermentation-based strategies and future trends on bacterial phytases have also been included.

Nutritional components and protein quality analysis of genetically modified phytase maize

The nutritional components and protein quality of genetically modified maize expressing phytase gene (GM) were analyzed and evaluated in this study. The nutritional components were analyzed by Chinese national standard methods. The ileostomy Bama miniature pigs were utilized to analyze the true digestibility of protein and amino acids. The digestible indispensable amino acid score (DIAAS) was adopted to evaluate the protein quality of GM, its parental maize (PM) and commercial available maize Zhengdan 958 (ZD). Meanwhile, the widely used protein digestibility corrected amino acid score (PDCAAS) was also calculated and compared with DIAAS. The content of protein, fat, vitamins, and minerals of all the strains of maize are in the normal ranges of OECD and/or ILSI. The DIAAS of GM, PM, and ZD were 54.57, 31.75, and 33.91, respectively, and the first limiting amino acid for GM, PM, and ZD was lysine. In conclusion, the introduction of phyA2 gene in GM maize does not disturb the digestion of protein/amino acid, but has the ability to promote the digestion of amino acids.

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.

Combining analyses of metabolite profiles and phosphorus fractions to explore high phosphorus utilization efficiency in maize

Phosphorus (P) limitation is a significant factor restricting crop production in agricultural systems, and enhancing the internal P utilization efficiency (PUE) of crops plays an important role in ensuring sustainable P use in agriculture. To better understand how P is remobilized to affect crop growth, we first screened P-efficient (B73 and GEMS50) and P-inefficient (Liao5114) maize genotypes at the same shoot P content, and then analyzed P pools and performed non-targeted metabolomic analyses to explore changes in cellular P fractions and metabolites in maize genotypes with contrasting PUE. We show that lipid P and nucleic acid P concentrations were significantly lower in lower leaves of P-efficient genotypes, and these P pools were remobilized to a major extent in P-efficient genotypes. Broad metabolic alterations were evident in leaves of P-efficient maize genotypes, particularly affecting products of phospholipid turnover and phosphorylated compounds, and the shikimate biosynthesis pathway. Taken together, our results suggest that P-efficient genotypes have a high capacity to remobilize lipid P and nucleic acid P and promote the shikimate pathway towards efficient P utilization in maize.

Mechanisms for improving phosphorus utilization efficiency in plants

BACKGROUND

Limitation of plant productivity by phosphorus (P) supply is widespread and will probably increase in the future. Relatively large amounts of P fertilizer are applied to sustain crop growth and development and to achieve high yields. However, with increasing P application, plant P efficiency generally declines, which results in greater losses of P to the environment with detrimental consequences for ecosystems.

SCOPE

A strategy for reducing P input and environmental losses while maintaining or increasing plant performance is the development of crops that take up P effectively from the soil (P acquisition efficiency) or promote productivity per unit of P taken up (P utilization efficiency). In this review, we describe current research on P metabolism and transport and its relevance for improving P utilization efficiency.

CONCLUSIONS

Enhanced P utilization efficiency can be achieved by optimal partitioning of cellular P and distributing P effectively between tissues, allowing maximum growth and biomass of harvestable plant parts. Knowledge of the mechanisms involved could help design and breed crops with greater P utilization efficiency.

Genomic Analysis Reveals Potential Mechanisms Underlying Promotion of Tomato Plant Growth and Antagonism of Soilborne Pathogens by Bacillus amyloliquefaciens Ba13

Bacillus amyloliquefaciens Ba13 is a plant beneficial bacterium isolated from loessial soil with notable biological activity. This study clarified potential mechanisms underlying the plant growth-promoting and antipathogenic effects of strain Ba13. A pot experiment was used to verify the plant growth-promoting effects of strain Ba13 on tomato, and the antipathogenic activity was tested in petri dishes. The underlying mechanisms were explored based on whole-genome sequencing of strain Ba13 and liquid chromatography-tandem mass spectrometry (LC-MS/MS) detection of plant hormones and biosynthetic intermediates. The results showed that exposure to strain Ba13 promoted tomato plant growth significantly. Compared with control treatment, bacterial treatment increased plant height and fresh weight by 10.98% and 20.15%, respectively, at 28 days after inoculation. Strain Ba13 exhibited antagonistic activity against all eight plant pathogens tested. The 3,861,210-bp genome of strain Ba13 was predicted to encode antibiotics (e.g., surfactin, bacillaene, bacillomycin D, bacilysin, and bacillibactin) and volatile gaseous compounds (e.g., 2,3-butanediol and acetoin). Genes were also predicted to encode extracellular phytase and β-glucanase that are secreted through the secretory (Sec) system. Strain Ba13 could synthesize indole-3-acetic acid through the indole-3-pyruvic acid pathway. The results of this study indicate that B. amyloliquefaciens Ba13 has multiple effects on tomato plants and associated microorganisms, directly or indirectly promoting plant growth and controlling plant diseases. IMPORTANCE Microbial agents are considered the optimal alternative for chemical agents. Exploring the mechanisms underlying the beneficial effects of microbial agents is essential for rational applications in the field. In this study, we report a functional bacterial strain, Bacillus amyloliquefaciens Ba13, which exhibited plant growth-promoting and antipathogenic effects. The whole genome of strain Ba13 was sequenced, and functional genes of interest were predicted. Strain Ba13 could synthesize indole-3-acetic acid through the indole-3-pyruvic acid pathway.

The grazing activity of Acrobeloides sp. drives phytate mineralisation within its trophic relationship with bacteria

The microbial loop has been suggested as an alternative route for better utilization of phytate, a poorly available P source to plants. We hypothesized that bacterial grazer activity might dramatically enhance bacterial migration and proliferation, increasing the probability of phytate hydrolysis by bacterial phytases and, thus, phytate mineralization and release of free phosphate. We tested this hypothesis in a two-compartment system with a solid medium containing phytate or free phosphate as the source of P. Two bacterial species, B. subtilis 168 or Bradyrhizobium sp., with or without bacterial grazing nematodes belonging to Acrobeloides sp. previously fed on each of the bacterial species, were inoculated at a single point in the medium. Whatever the P source, nematode migration within both zones allowed the proliferation of bacteria. However, B. subtilis 168 was more efficient in using phytate than Bradyrhizobium sp. since the highest bacterial cell density and free phosphate concentrations were reached by Acrobeloides sp. fed on B. subtilis 168. The grazer activity seemed to be crucial to enhance phytate mineralization, despite Acrobeloides sp. showing a higher preference to feed on Bradyrhizobium sp. This study provides new insights into the effects of bacterial grazer activity on phytate mineralization.

Bioprocess for Production, Characteristics, and Biotechnological Applications of Fungal Phytases

Phytases are a group of enzymes that hydrolyze the phospho-monoester bonds of phytates. Phytates are one of the major forms of phosphorus found in plant tissues. Fungi are mainly used for phytase production. The production of fungal phytases has been achieved under three different fermentation methods including solid-state, semi-solid-state, and submerged fermentation. Agricultural residues and other waste materials have been used as substrates for the evaluation of enzyme production in the fermentation process. Nutrients, physical conditions such as pH and temperature, and protease resistance are important factors for increasing phytase production. Fungal phytases are considered monomeric proteins and generally possess a molecular weight of between 14 and 353 kDa. Fungal phytases display a broad substrate specificity with optimal pH and temperature ranges between 1.3 and 8.0 and 37-67°C, respectively. The crystal structure of phytase has been studied in Aspergillus. Notably, thermostability engineering has been used to improve relevant enzyme properties. Furthermore, fungal phytases are widely used in food and animal feed additives to improve the efficiency of phosphorus intake and reduce the amount of phosphorus in the environment.

Two-factor ANOVA of SSH and RNA-seq analysis reveal development-associated Pi-starvation genes in oilseed rape

The 5-leaf-stage rape seedlings were more insensitive to Pi starvation than that of the 3-leaf-stage plants, which may be attributed to the higher expression levels of ethylene signaling and sugar-metabolism genes in more mature seedlings. Traditional suppression subtractive hybridization (SSH) and RNA-Seq usually screen out thousands of differentially expressed genes. However, identification of the most important regulators has not been performed to date. Here, we employed two methods, namely, a two-round SSH and two-factor transcriptome analysis derived from the two-factor ANOVA that is commonly used in the statistics, to identify development-associated inorganic phosphate (Pi) starvation-induced genes in Brassica napus. Several of these genes are related to ethylene signaling (such as EIN3, ACO3, ACS8, ERF1A, and ERF2) or sugar metabolism (such as ACC2, GH3, LHCB1.4, XTH4, and SUS2). Although sucrose and ethylene may counteract each other at the biosynthetic level, they may also work synergistically on Pi-starvation-induced gene expression (such as PT1, PT2, RNS1, ACP5, AT4, and IPS1) and root acid phosphatase activation. Furthermore, three new transcription factors that are responsive to Pi starvation were identified: the zinc-finger MYND domain-containing protein 15 (MYND), a Magonashi family protein (MAGO), and a B-box zinc-finger family salt-tolerance protein. This study indicates that the two methods are highly efficient for functional gene screening in non-model organisms.

Current and Future Applications of Phytases in Poultry Industry: A Critical Review

Phytases are enzymes that initiate the removal of phosphate from phytate. This enzyme has been widely utilized in animal feeding especially in the poultry industry to enhance phosphorus intake and minimize environmental pollution. Phytases are widely distributed in microbial, plants and animals. Supplementations of phytase into the diets of poultry have great impact to the improvement of poultry immune systems and increase bird weight. In addition to that, phytase are able to improve both quantity and quality of eggs, egg mass and egg shell quality. This review covers the classifications and distribution of phytases in different biofactoris. In addition, it shed more light on the recent trends of application and beneficial impact in poultry farming.

A root-associated purple acid phosphatase, SgPAP23, mediates extracellular phytate-P utilization in Stylosanthes guianensis

As a major component of soil organic phosphorus (P), phytate-P is unavailable to plants unless hydrolysed by phytase to release inorganic phosphate. However, knowledge on natural variation in root-associated phytase activity and its underlying molecular mechanisms in plants remains fragmentary. In this study, variations in root internal and associated phytase activity were observed among 39 genotypes of Stylosanthes guianensis (Stylo), which is well adapted to acid soils. Furthermore, TPRC2001-1, the genotype with the highest root-associated phytase activity, was more capable of utilizing extracellular phytate-P than Fine-stem, the genotype with the lowest root-associated phytase activity. After protein liquid chromatography-tandem mass spectrometry analysis, a purple acid phosphatase (PAP), SgPAP23, was identified and cloned from TPRC2001-1. SgPAP23 exhibited high activity against phytate-P and was mainly localized on the plasma membrane. Furthermore, SgPAP23 overexpression resulted in significant increases of root-associated phytase activity and thus facilitated extracellular phytate-P utilization in both bean (Phaseolus vulgaris) hairy roots and Arabidopsis thaliana. The results herein support the conclusion that SgPAP23 is a primary contributor to the superior extracellular phytate-P utilization in stylo and thus is used to develop cultivars with efficient extracellular phytate-P utilization.

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

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

P uptake characteristics and root morphological responses in the mining ecotype of Polygonum hydropiper under high organic P media

Understanding plant phosphorus (P) assimilation and its root morphological responses is important to acquire an ideal material for remediation of P-enriched environments. Pot experiments were conducted to explore P accumulation and root morphological traits in a mining ecotype (ME) and non-mining ecotype (NME) of Polygonum hydropiper under different organic P (Po) sources (G1P, AMP, ATP, IHP) and inorganic P (Pi) source (KH₂PO₄), and also their responses to a high level of IHP for different growth periods. Both ecotypes showed higher biomass in Pi and IHP treatments than other Po sources. P accumulation in seedlings were in the order of Pi > IHP > other Po media. Extending the growth period increased biomass and P accumulation in both ecotypes. The ME demonstrated 1.11-1.46 times higher P accumulation than the NME. Seedlings fed with IHP demonstrated significantly greater morphological parameters of fine, medium, and thick roots compared to other Po sources. Total root length, surface area, and volume of both ecotypes significantly increased with the prolonged growth period. The ME has a higher ability to develop root system and exhibits better distribution of fine roots to enhance P accumulation from high P media, and thus it is a worthy material for P-phytoextraction.

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.

Functional Assessment of an Overexpressed Arabidopsis Purple Acid Phosphatase Gene (AtPAP26) in Tobacco Plants

BACKGROUND

Overexpression of known genes encoding key phosphate (Pi)-metabolizing enzymes, such as acid phosphatases (APases), is presumed to help plants with Pi availability and absorption as they are mostly exposed to suboptimal environmental conditions for this vital element.

OBJECTIVES

In this study, the overexpression effect of AtPAP26, one of the main contributors in retrieving Pi from intracellular and extracellular compounds, was evaluated from various viewes in tobacco plant.

MATERIALS AND METHODS

As a heterologous expression system, the encoding cDNA sequence of AtPAP26 was transferred into tobacco plants.

RESULTS

A high growth rate of the transgenic lines was observed which could be due to an increased APase activity, leading to the high total phosphorus as well as the free Pi content of the transgenic plants. Interestingly, a significant increased activity of the other APases was also noticed, indicating a networking among them. These were accompanied by less branched and short primary roots and a decreased lateral root numbers grown in Pi-starvation condition compared to the wild type seedlings. Besides, a delayed germination and dwarf phenotype indicates the possible reduction in gibberellic acid biosynthesis in the transgenic lines.

CONCLUSIONS

Such transgenic plants are of interest not only for increased yield but also for the reduced need for chemical fertilizers and removal of excessive Pi accumulation in soils as a consequence of fertilizers' or poultry wastes' over-usage.

Phosphorus acquisition by citrate- and phytase-exuding Nicotiana tabacum plant mixtures depends on soil phosphorus availability and root intermingling

Citrate and phytase root exudates contribute to improved phosphorus (P) acquisition efficiency in Nicotiana tabacum (tobacco) when both exudates are produced in a P deficient soil. To test the importance of root intermingling in the interaction of citrate and phytase exudates, Nicotiana tabacum plant-lines with constitutive expression of heterologous citrate (Cit) or fungal phytase (Phy) exudation traits were grown under two root treatments (roots separated or intermingled) and in two soils with contrasting soil P availability. Complementarity of plant mixtures varying in citrate efflux rate and mobility of the expressed phytase in soil was determined based on plant biomass and P accumulation. Soil P composition was evaluated using solution 31 P NMR spectroscopy. In the soil with limited available P, positive complementarity occurred in Cit+Phy mixtures with roots intermingled. Root separation eliminated positive interactions in mixtures expressing the less mobile phytase (Aspergillus niger PhyA) whereas positive complementarity persisted in mixtures that expressed the more mobile phytase (Peniophora lycii PhyA). Soils from Cit+Phy mixtures contained less inorganic P and more organic P compared to monocultures. Exudate-specific strategies for the acquisition of soil P were most effective in P-limited soil and depended on citrate efflux rate and the relative mobility of the expressed phytase in soil. Plant growth and soil P utilization in plant systems with complementary exudation strategies are expected to be greatest where exudates persist in soil and are expressed synchronously in space and time.

Heterologous Expression of Secreted Bacterial BPP and HAP Phytases in Plants Stimulates Arabidopsis thaliana Growth on Phytate

Phytases are specialized phosphatases capable of releasing inorganic phosphate from myo-inositol hexakisphosphate (phytate), which is highly abundant in many soils. As inorganic phosphorus reserves decrease over time in many agricultural soils, genetic manipulation of plants to enable secretion of potent phytases into the rhizosphere has been proposed as a promising approach to improve plant phosphorus nutrition. Several families of biotechnologically important phytases have been discovered and characterized, but little data are available on which phytase families can offer the most benefits toward improving plant phosphorus intake. We have developed transgenic Arabidopsis thaliana plants expressing bacterial phytases PaPhyC (HAP family of phytases) and 168phyA (BPP family) under the control of root-specific inducible promoter Pht1;2. The effects of each phytase expression on growth, morphology and inorganic phosphorus accumulation in plants grown on phytate hydroponically or in perlite as the only source of phosphorus were investigated. The most enzymatic activity for both phytases was detected in cell wall-bound fractions of roots, indicating that these enzymes were efficiently secreted. Expression of both bacterial phytases in roots improved plant growth on phytate and resulted in larger rosette leaf area and diameter, higher phosphorus content and increased shoot dry weight, implying that these plants were indeed capable of utilizing phytate as the source of phosphorus for growth and development. When grown on phytate the HAP-type phytase outperformed its BPP-type counterpart for plant biomass production, though this effect was only observed in hydroponic conditions and not in perlite. Furthermore, we found no evidence of adverse side effects of microbial phytase expression in A. thaliana on plant physiology and seed germination. Our data highlight important functional differences between these members of bacterial phytase families and indicate that future crop biotechnologies involving such enzymes will require a very careful evaluation of phytase source and activity. Overall, our data suggest feasibility of using bacterial phytases to improve plant growth in conditions of phosphorus deficiency and demonstrate that inducible expression of recombinant enzymes should be investigated further as a viable approach to plant biotechnology.

Inositol Hexakis Phosphate is the Seasonal Phosphorus Reservoir in the Deciduous Woody Plant Populus alba L

Seasonal recycling of nutrients is an important strategy for deciduous perennials. Deciduous perennials maintain and expand their nutrient pools by the autumn nutrient remobilization and the subsequent winter storage throughout their long life. Phosphorus (P), one of the most important elements in living organisms, is remobilized from senescing leaves during autumn in deciduous trees. However, it remains unknown how phosphate is stored over winter. Here we show that in poplar trees (Populus alba L.), organic phosphates are accumulated in twigs from late summer to winter, and that IP6 (myo-inositol-1,2,3,4,5,6-hexakis phosphate: phytic acid) is the primary storage form. IP6 was found in high concentrations in twigs during winter and quickly decreased in early spring. In parenchyma cells of winter twigs, P was associated with electron-dense structures, similar to globoids found in seeds of higher plants. Various other deciduous trees were also found to accumulate IP6 in twigs during winter. We conclude that IP6 is the primary storage form of P in poplar trees during winter, and that it may be a common strategy for seasonal P storage in deciduous woody plants.

Genetically modified phytase crops role in sustainable plant and animal nutrition and ecological development: a review

Globally, plant-derivatives especially cereals and legumes are the major staple food sources for animals. The seeds of these crops comprise of phytic acid, the major repository form of the phosphorus, which is not digestible by simple-stomached animals. However, it is the most important factor responsible for impeding the absorption of minerals by plants that eventually results in less use of fertilizers that ultimately cause eutrophication in water bodies. Although abundant phosphorus (P) exists in the soils, plants cannot absorb most of the P due to its conversion to unavailable forms. Hence, additional P supplementation is indispensable to the soil to promote crop yields which not only leads to soil infertility but also rapid depletion of non-renewable P reservoirs. Phytase/phosphatase enzyme is essential to liberate P from soils by plants and from seeds by monogastric animals. Phytases are kind of phosphatases which can hydrolyse the indigestible phytate into inorganic Phosphate (Pi) and lower myo-inositol. There are several approaches to mitigate the problems associated with phytate indigestibility. One of the best possible solutions is engineering crops to produce heterologous phytase to improve P utilization by monogastric animals, plant nutrition and sustainable ecological developments. Previously published reviews were focused on either soil phytate or seed-phytate, related issues, but this review will address both the problems as well as phytate related ecological problems. This review summarizes the overall view of engineered phytase crops and their role in sustainable agriculture, animal nutrition and ecological development.

β-Propeller phytases: Diversity, catalytic attributes, current developments and potential biotechnological applications

Phytases are phosphatases which stepwise remove phosphates from phytic acid or its salts. β-Propeller phytase (BPPhy) belongs to a special class of microbial phytases that is regarded as most diverse, isolated and characterized from different microbes, mainly from Bacillus spp. BPPhy class is unique for its Ca²⁺-dependent catalytic activity, strict substrate specificity, active at neutral to alkaline pH and high thermostability. Numerous sequence and structure based studies have revealed unique attributes and catalytic properties of this class, as compared to other classes of phytases. Recent studies including cloning and expression and genetic engineering approaches have led to improvements in BPPhy which provide an opportunity for extended utilization of this class of phytases in improving animal nutrition, human health, plant growth promotion, and environmental protection, etc. This review describes the sources and diversity of BPPhy genes, biochemical properties, Ca²⁺ dependence, current developments in structural elucidation, heterogeneous expression and catalytic improvements, and multifarious applications of BPPhy.

Response-based selection of barley cultivars and legume species for complementarity: Root morphology and exudation in relation to nutrient source

Phosphorus (P) and nitrogen (N) use efficiency may be improved through increased biodiversity in agroecosystems. Phenotypic variation in plants' response to nutrient deficiency may influence positive complementarity in intercropping systems. A multicomponent screening approach was used to assess the influence of P supply and N source on the phenotypic plasticity of nutrient foraging traits in barley (H. vulgare L.) and legume species. Root morphology and exudation were determined in six plant nutrient treatments. A clear divergence in the response of barley and legumes to the nutrient treatments was observed. Root morphology varied most among legumes, whereas exudate citrate and phytase activity were most variable in barley. Changes in root morphology were minimized in plants provided with ammonium in comparison to nitrate but increased under P deficiency. Exudate phytase activity and pH varied with legume species, whereas citrate efflux, specific root length, and root diameter lengths were more variable among barley cultivars. Three legume species and four barley cultivars were identified as the most responsive to P deficiency and the most contrasting of the cultivars and species tested. Phenotypic response to nutrient availability may be a promising approach for the selection of plant combinations for minimal input cropping systems.

Phosphate, phytate and phytases in plants: from fundamental knowledge gained in Arabidopsis to potential biotechnological applications in wheat

Phosphorus (P) is an essential macronutrient for all living organisms. In plants, P is taken up from the rhizosphere by the roots mainly as inorganic phosphate (Pi), which is required in large and sufficient quantities to maximize crop yields. In today's agricultural society, crop yield is mostly ensured by the excessive use of Pi fertilizers, a costly practice neither eco-friendly or sustainable. Therefore, generating plants with improved P use efficiency (PUE) is of major interest. Among the various strategies employed to date, attempts to engineer genetically modified crops with improved capacity to utilize phytate (PA), the largest soil P form and unfortunately not taken up by plants, remains a key challenge. To meet these challenges, we need a better understanding of the mechanisms regulating Pi sensing, signaling, transport and storage in plants. In this review, we summarize the current knowledge on these aspects, which are mainly gained from investigations conducted in Arabidopsis thaliana, and we extended it to those available on an economically important crop, wheat. Strategies to enhance the PA use, through the use of bacterial or fungal phytases and other attempts of reducing seed PA levels, are also discussed. We critically review these data in terms of their potential for use as a technology for genetic manipulation of PUE in wheat, which would be both economically and environmentally beneficial.

The secretion of the bacterial phytase PHY-US417 by Arabidopsis roots reveals its potential for increasing phosphate acquisition and biomass production during co-growth

Phytic acid (PA) is a major source of inorganic phosphate (Pi) in the soil; however, the plant lacks the capacity to utilize it for Pi nutrition and growth. Microbial phytases constitute a group of enzymes that are able to remobilize Pi from PA. Thus, the use of these phytases to increase the capacity of higher plants to remobilize Pi from PA is of agronomical interest. In the current study, we generate transgenic Arabidopsis lines (ePHY) overexpressing an extracellular form of the phytase PHY-US417 of Bacillus subtilis, which are characterized by high levels of secreted phytase activity. In the presence of PA as sole source of Pi, while the wild-type plants show hallmark of Pi deficiency phenotypes, including the induction of the expression of Pi starvation-induced genes (PSI, e.g. PHT1;4) and the inhibition of growth capacity, the ePHY overexpressing lines show a higher biomass production and no PSI induction. Interestingly, when co-cultured with ePHY overexpressors, wild-type Arabidopsis plants (or tobacco) show repression of the PSI genes, improvement of Pi content and increases in biomass production. In line with these results, mutants in the high-affinity Pi transporters, namely pht1;1 and pht1;1-1;4, both fail to accumulate Pi and to grow when co-cultured with ePHY overexpressors. Taken together, these data demonstrate the potential of secreted phytases in improving the Pi content and enhancing growth of not only the transgenic lines but also the neighbouring plants.

BOTRYTIS-INDUCED KINASE1, a plasma membrane-localized receptor-like protein kinase, is a negative regulator of phosphate homeostasis in Arabidopsis thaliana

BACKGROUND

Plants have evolved complex coordinated regulatory networks to cope with deficiency of phosphate (Pi) in their growth environment; however, the detailed molecular mechanisms that regulate Pi sensing and signaling pathways are not fully understood yet. We report here that the involvement of Arabidopsis BIK1, a plasma membrane-localized receptor-like protein kinase that plays critical role in immunity, in Pi starvation response.

RESULTS

qRT-PCR analysis revealed that expression of BIK1 was induced by Pi starvation and GUS staining indicated that the BIK1 promoter activity was detected in root, stem and leaf tissues of plants grown in Pi starvation condition, demonstrating that BIK1 is responsive to Pi starvation stress. The bik1 plants accumulated higher Pi content in root and leaf tissues and exhibited altered root architecture such as shorter primary roots, longer and more root hairs and lateral roots, as compared with those in the wild type plants, when grown under Pi sufficient and deficient conditions. Increased anthocyanin content and acid phosphatase activity, reduced accumulation of reactive oxygen species and downregulated expression of Pi starvation-induced genes including PHR1, WRKY75, AT4, PHT1;2 and PHT1;4 were observed in bik1 plants grown under Pi deficient condition. Furthermore, the expression of PHO2 was downregulated while the expression of miRNA399a and miRNA399d, which target to PHO2, was upregulated in bik1 plants, compared to the wild type plants, when grown under Pi deficient condition.

CONCLUSION

Our results demonstrate that BIK1 is a Pi starvation-responsive gene that functions as a negative regulator of Pi homeostasis in Arabidopsis.

P accumulation and physiological responses to different high P regimes in Polygonum hydropiper for understanding a P-phytoremediation strategy

Phosphorus (P) accumulators used for phytoremediation vary in their potential to acquire P from different high P regimes. Growth and P accumulation in Polygonum hydropiper were both dependent on an increasing level of IHP (1–8 mM P) and on a prolonged growth period (3-9 weeks), and those of the mining ecotype (ME) were higher than the non-mining ecotype (NME). Biomass increments in root, stem, and leaf of both ecotypes were significantly greater in IHP relative to other organic P (Po) sources (G1P, AMP, ATP), but lower than those in inorganic P (Pi) treatment (KH₂PO₄). P accumulation in the ME exceeded the NME from different P regimes. The ME demonstrated higher root activity compared to the NME grown in various P sources. Acid phosphatase (Apase) and phytase activities in root extracts of both ecotypes grown in IHP were comparable to that in Pi, or even higher in IHP. Higher secreted Apase and phytase activities were detected in the ME treated with different P sources relative to the NME. Therefore, the ME demonstrates higher P-uptake efficiency and it is a potential material for phytoextraction from P contaminated areas, irrespective of Pi or Po contamination.

The role of gluconate production by Pseudomonas spp. in the mineralization and bioavailability of calcium-phytate to Nicotiana tabacum

Organic phosphorus (P) is abundant in most soils but is largely unavailable to plants. Pseudomonas spp. can improve the availability of P to plants through the production of phytases and organic anions. Gluconate is a major component of Pseudomonas organic anion production and may therefore play an important role in the mineralization of insoluble organic P forms such as calcium-phytate (CaIHP). Organic anion and phytase production was characterized in 2 Pseudomonas spp. soil isolates (CCAR59, Ha200) and an isogenic mutant of strain Ha200, which lacked a functional glucose dehydrogenase (Gcd) gene (strain Ha200 gcd::Tn5B8). Wild-type and mutant strains of Pseudomonas spp. were evaluated for their ability to solubilize and hydrolyze CaIHP and to promote the growth and assimilation of P by tobacco plants. Gluconate, 2-keto-gluconate, pyruvate, ascorbate, acetate, and formate were detected in Pseudomonas spp. supernatants. Wild-type pseudomonads containing a functional gcd could produce gluconate and mineralize CaIHP, whereas the isogenic mutant could not. Inoculation with Pseudomonas improved the bioavailability of CaIHP to tobacco plants, but there was no difference in plant growth response due to Gcd function. Gcd function is required for the mineralization of CaIHP in vitro; however, further studies will be needed to quantify the relative contribution of specific organic anions such as gluconate to plant growth promotion by soil pseudomonads.

Physiological and Molecular Aspects of Tolerance to Environmental Constraints in Grain and Forage Legumes

Despite the agronomical and environmental advantages of the cultivation of legumes, their production is limited by various environmental constraints such as water or nutrient limitation, frost or heat stress and soil salinity, which may be the result of pedoclimatic conditions, intensive use of agricultural lands, decline in soil fertility and environmental degradation. The development of more sustainable agroecosystems that are resilient to environmental constraints will therefore require better understanding of the key mechanisms underlying plant tolerance to abiotic constraints. This review provides highlights of legume tolerance to abiotic constraints with a focus on soil nutrient deficiencies, drought, and salinity. More specifically, recent advances in the physiological and molecular levels of the adaptation of grain and forage legumes to abiotic constraints are discussed. Such adaptation involves complex multigene controlled-traits which also involve multiple sub-traits that are likely regulated under the control of a number of candidate genes. This multi-genetic control of tolerance traits might also be multifunctional, with extended action in response to a number of abiotic constraints. Thus, concrete efforts are required to breed for multifunctional candidate genes in order to boost plant stability under various abiotic constraints.

The mitochondrial malate dehydrogenase 1 gene GhmMDH1 is involved in plant and root growth under phosphorus deficiency conditions in cotton

Cotton, an important commercial crop, is cultivated for its natural fibers, and requires an adequate supply of soil nutrients, including phosphorus, for its growth. Soil phosporus exists primarily in insoluble forms. We isolated a mitochondrial malate dehydrogenase (MDH) gene, designated as GhmMDH1, from Gossypium hirsutum L. to assess its effect in enhancing P availability and absorption. An enzyme kinetic assay showed that the recombinant GhmMDH1 possesses the capacity to catalyze the interconversion of oxaloacetate and malate. The malate contents in the roots, leaves and root exudates was significantly higher in GhmMDH1-overexpressing plants and lower in knockdown plants compared with the wild-type control. Knockdown of GhmMDH1 gene resulted in increased respiration rate and reduced biomass whilst overexpression of GhmMDH1 gave rise to decreased respiration rate and higher biomass in the transgenic plants. When cultured in medium containing only insoluble phosphorus, Al-phosphorus, Fe-phosphorus, or Ca-phosphorus, GhmMDH1-overexpressing plants produced significantly longer roots and had a higher biomass and P content than WT plants, however, knockdown plants showed the opposite results for these traits. Collectively, our results show that GhmMDH1 is involved in plant and root growth under phosphorus deficiency conditions in cotton, owing to its functions in leaf respiration and P acquisition.

Dissolved Phosphorus Retention in Buffer Strips: Influence of Slope and Soil Type

Phosphorus (P) contributes to eutrophication of surface waters and buffer strips may be implemented to reduce its transfer from agricultural sources to watercourses. This study was conducted to test the hypothesis that soil type and slope influence the retention of dissolved organic P and inorganic orthophosphate in agricultural runoff in 2-m-wide buffer strip soils. A solution, comprised of dissolved orthophosphate and the organic P compounds glucose-1-phosphate, RNA, and inositol hexakisphosphate (1.8 mg L total P) and a chloride tracer, was applied as simulated overland flow to grassland soil blocks (2 m long × 0.5 m wide × 0.35 m deep), containing intact clay or loam soils, at slope angles of 2, 5, and 10°. Phosphorus forms were determined in the surface and subsurface flow from the soil blocks. Slope had no significant effect on the hydrological behavior of the soil blocks or on the retention of any form of P at the water application rate tested. The clay soil retained 60% of the unreactive P and 21% of the reactive P applied. The loam soil retained 74% of the unreactive P applied but was a net source of reactive P (the load increased by 61%). This indicates leaching of native soil P or hydrolysis of organic compounds and complicates our understanding of P retention in buffer strip soils. Our results suggest that a 2-m buffer strip may be more effective for reducing dissolved unreactive P transfers to surface waters than for reducing the eutrophication risk posed by dissolved reactive P.

Strigolactone regulates anthocyanin accumulation, acid phosphatases production and plant growth under low phosphate condition in Arabidopsis

Phosphate is an essential macronutrient in plant growth and development; however, the concentration of inorganic phosphate (Pi) in soil is often suboptimal for crop performance. Accordingly, plants have developed physiological strategies to adapt to low Pi availability. Here, we report that typical Pi starvation responses in Arabidopsis are partially dependent on the strigolactone (SL) signaling pathway. SL treatment induced root hair elongation, anthocyanin accumulation, activation of acid phosphatase, and reduced plant weight, which are characteristic responses to phosphate starvation. Furthermore, the expression profile of SL-response genes correlated with the expression of genes induced by Pi starvation. These results suggest a potential overlap between SL signaling and Pi starvation signaling pathways in plants.

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.

From soil to plant, the journey of P through trophic relationships and ectomycorrhizal association

Phosphorus (P) is essential for plant growth and productivity. It is one of the most limiting macronutrients in soil because it is mainly present as unavailable, bound P whereas plants can only use unbound, inorganic phosphate (Pi), which is found in very low concentrations in soil solution. Some ectomycorrhizal fungi are able to release organic compounds (organic anions or phosphatases) to mobilize unavailable P. Recent studies suggest that bacteria play a major role in the mineralization of nutrients such as P through trophic relationships as they can produce specific phosphatases such as phytases to degrade phytate, the main form of soil organic P. Bacteria are also more effective than other microorganisms or plants at immobilizing free Pi. Therefore, bacterial grazing by grazers, such as nematodes, could release Pi locked in bacterial biomass. Free Pi may be taken up by ectomycorrhizal fungus by specific phosphate transporters and transferred to the plant by mechanisms that have not yet been identified. This mini-review aims to follow the phosphate pathway to understand the ecological and molecular mechanisms responsible for transfer of phosphate from the soil to the plant, to improve plant P nutrition.

Expression of Aspergillus nidulans phy gene in Nicotiana benthamiana produces active phytase with broad specificities

A full-length phytase gene (phy) of Aspergillus nidulans was amplified from the cDNA library by polymerase chain reaction (PCR), and it was introduced into a bacterial expression vector, pET-28a. The recombinant protein (rPhy-E, 56 kDa) was overexpressed in the insoluble fraction of Escherichia coli culture, purified by Ni-NTA resin under denaturing conditions and injected into rats as an immunogen. To express A. nidulans phytase in a plant, the full-length of phy was cloned into a plant expression binary vector, pPZP212. The resultant construct was tested for its transient expression by Agrobacterium-infiltration into Nicotiana benthamiana leaves. Compared with a control, the agro-infiltrated leaf tissues showed the presence of phy mRNA and its high expression level in N. benthamiana. The recombinant phytase (rPhy-P, 62 kDa) was strongly reacted with the polyclonal antibody against the nonglycosylated rPhy-E. The rPhy-P showed glycosylation, two pH optima (pH 4.5 and pH 5.5), an optimum temperature at 45~55 °C, thermostability and broad substrate specificities. After deglycosylation by peptide-N-glycosidase F (PNGase-F), the rPhy-P significantly lost the phytase activity and retained 1/9 of the original activity after 10 min of incubation at 45 °C. Therefore, the deglycosylation caused a significant reduction in enzyme thermostability. In animal experiments, oral administration of the rPhy-P at 1500 U/kg body weight/day for seven days caused a significant reduction of phosphorus excretion by 16% in rat feces. Besides, the rPhy-P did not result in any toxicological changes and clinical signs.

Overexpression and functional characterization of an Aspergillus niger phytase in the fat body of transgenic silkworm, Bombyx mori

In a previous study, we isolated 1,119 bp of upstream promoter sequence from Bmlp3, a gene encoding a member of the silkworm 30 K storage protein family, and demonstrated that it was sufficient to direct fat body-specific expression of a reporter gene in a transgenic silkworm, thus highlighting the potential use of this promoter for both functional genomics research and biotechnology applications. To test whether the Bmlp3 promoter can be used to produce recombinant proteins in the fat body of silkworm pupae, we generated a transgenic line of Bombyx mori which harbors a codon-optimized Aspergillus niger phytase gene (phyA) under the control of the Bmlp3 promoter. Here we show that the Bmlp3 promoter drives high levels of phyA expression in the fat body, and that the recombinant phyA protein is highly active (99.05 and 54.80 U/g in fat body extracts and fresh pupa, respectively). We also show that the recombinant phyA has two optimum pH ranges (1.5-2.0 and 5.5-6.0), and two optimum temperatures (55 and 37 °C). The activity of recombinant phyA was lost after high-temperature drying, but treating with boiling water was less harmful, its residual activity was approximately 84% of the level observed in untreated samples. These results offer an opportunity not only for better utilization of large amounts of silkworm pupae generated during silk production, but also provide a novel method for mass production of low-cost recombinant phytase using transgenic silkworms.

Evaluation of phytase producing bacteria for their plant growth promoting activities

Bacterial inoculants are known to possess plant growth promoting abilities and have potential as liquid biofertilizer application. Four phytase producing bacterial isolates (phytase activity in the range of 0.076–0.174 U/mL), identified as Advenella species (PB-05, PB-06, and PB-10) and Cellulosimicrobium sp. PB-09, were analyzed for their plant growth promoting activities like siderophore production, IAA production, HCN production, ammonia production, phosphate solubilization, and antifungal activity. All isolates were positive for the above characteristics except for HCN production. The solubilization index for phosphorus on Pikovskaya agar plates was in the range of 2–4. Significant amount of IAA (7.19 to 35.03 μg/mL) production and solubilized phosphate (189.53 to 746.84 μg/mL) was noticed by these isolates at different time intervals. Besides that, a greenhouse study was also conducted with Indian mustard to evaluate the potential of these isolates to promote plant growth. Effect of seed bacterization on various plant growth parameters and P uptake by plant were used as indicators. The plant growth promoting ability of bacterial isolates in pot experiments was correlated to IAA production, phosphate solubilization, and other in vitro tests. On the basis of present findings, isolate PB-06 was most promising in plant growth promotion with multiple growth promoting characteristics.