Structural elements that modulate the substrate specificity of plant purple acid phosphatases: Avenues for improved phosphorus acquisition in crops

Phosphoester bond hydrolysis by a discrete zirconium-oxo cluster: mechanistic insights into the central role of the binuclear ZrIV-ZrIV active site

Effective degradation of non-natural phosphate triesters (PTs) widely used in pesticides and warfare agents is of paramount relevance for human and environmental safety, particularly under acidic conditions where they are highly stable. Here, we present a detailed reactivity and mechanistic study pioneering discrete {Zr6O8} clusters, which are commonly employed as building blocks for Zr-MOFs and as non-classical soluble coordination compounds for the degradation of PTs using the pesticide ethyl paraoxon as a model. Combined computational studies, mechanistic experiments, and EXAFS analysis show that the reactivity of these clusters arises from their ZrIV-ZrIV bimetallic sites, which hydrolyze ethyl paraoxon under acidic conditions through an intramolecular pathway. Remarkably, the energetics of the reaction is dependent on the protonation state of the active sites, and a weakly acidic medium favors the reaction. Moreover, catalyst stability allowed for its recovery and reuse. Such a mechanism is in close analogy to enzymatic reactions and different from that previously reported for Zr-MOFs. These findings outline the potential of MIV-MIV active sites for PT degradation under challenging aqueous acidic conditions and contribute to the development of bioinspired catalysts and materials.

Analysis and profiling of the purple acid phosphatase gene family in wheat (Triticum aestivum L.)

Purple acid phosphatases (PAPs) play a vital role in plant phosphorus nutrition, serving as a crucial family of metallo-phosphoesterase enzymes. This research aimed to identify the PAP genes from the A/B/D genomes of Triticum aestivum to elucidate evolutionary mechanisms of the gene family in plants and provide genomic information for subsequent research on phosphorous-use efficiency in wheat crops. In total, 105 PAP genes (TaPAPs) were identified from the A/B/D genomes by using the Arabidopsis thaliana and Oryza sativa PAP protein sequences as queries for BLASTP against the wheat protein database. The TaPAPs were grouped into six subfamilies, Ia (17), Ib (26), IIa (11), IIb (30), IIIa (12), and IIIb (9), based on their similarities in the structure of genes and the presence of conserved protein motifs. A majority of TaPAPs were derived from tandemly (20) or segmentally (87) duplicated, with the homoeologous chromosomes 5A/B/D harboring the most duplicated PAP genes. Further analysis indicated that TaPAPs were responsible for the modulation of seed, root, and leaf development and hormone synthesis and signaling, as well as plant responses to abiotic stresses, including low temperatures, drought, and anaerobic conditions. Nine TaPAPs (TaPAP9-4A/4B/4D, TaPAP24-6A/6B/6D, and TaPAP28-7A/7B/7D) were constitutively expressed in diverse tissues such as root, shoot, leaf, spike, and seed, while the remaining genes exhibited tissue-specific expression patterns. Concerning the response to phosphate (Pi) deprivation, 57 TaPAPs were highly expressed in roots under Pi stress, including TaPAP31-4A, 4B, and 4D homeologs from the subfamily IIIb. A TaPAP31-4A transgene in A. thaliana promoted plant growth and development while increasing plant resistance to Pi-deficiency stress by enhancing the secretion of phosphatase. These discoveries provide a scientific foundation for comprehending the role of TaPAPs, offering valuable insights for identifying additional candidate genes and fostering the development of new wheat varieties with enhanced tolerance to low phosphorus conditions.

Multi-omics analysis reveals the roles of purple acid phosphatases in organic phosphorus utilization by the tropical legume Stylosanthes guianensis

Stylo (Stylosanthes guianensis) is a tropical legume known for its exceptional tolerance to low phosphate (Pi), a trait believed to be linked to its high acid phosphatase (APase) activity. Previous studies have observed genotypic variations in APase activity in stylo; however, the gene encoding the crucial APase responsible for this variation remains unidentified. In this study, transcriptomic and proteomic analyses were employed to identify eight Pi starvation-inducible (PSI) APases belonging to the purple APase (PAP) family in the roots of stylo and seven in the leaves. Among these PSI-PAPs, SgPAP7 exhibited a significantly positive correlation in its expression levels with the activities of both internal APase and root-associated APase across 20 stylo genotypes under low-Pi conditions. Furthermore, the recombinant SgPAP7 displayed high catalytic activity toward adenosine 5'-diphosphate (ADP) and phosphoenolpyruvate (PEP) in vitro. Overexpression (OE) of SgPAP7 in Arabidopsis facilitated exogenous organic phosphorus utilization. Moreover, SgPAP7 OE lines showed lower shoot ADP and PEP levels than the wild type, implying that SgPAP7 is involved in the catabolism and recycling of endogenous ADP and PEP, which could be beneficial for plant growth in low-Pi soils. In conclusion, SgPAP7 is a key gene with a major role in stylo adaptation to low-Pi conditions by facilitating the utilization of both exogenous and endogenous organic phosphorus sources. It may also function as a PEP phosphatase involved in a glycolytic bypass pathway that minimizes the need for adenylates and Pi. Thus, SgPAP7 could be a promising target for improving tolerance of crops to low-Pi availability.

Unregulated GmAGL82 due to Phosphorus Deficiency Positively Regulates Root Nodule Growth in Soybean

Nitrogen fixation, occurring through the symbiotic relationship between legumes and rhizobia in root nodules, is crucial in sustainable agriculture. Nodulation and soybean production are influenced by low levels of phosphorus stress. In this study, we discovered a MADS transcription factor, GmAGL82, which is preferentially expressed in nodules and displays significantly increased expression under conditions of phosphate (Pi) deficiency. The overexpression of GmAGL82 in composite transgenic plants resulted in an increased number of nodules, higher fresh weight, and enhanced soluble Pi concentration, which subsequently increased the nitrogen content, phosphorus content, and overall growth of soybean plants. Additionally, transcriptome analysis revealed that the overexpression of GmAGL82 significantly upregulated the expression of genes associated with nodule growth, such as GmENOD100, GmHSP17.1, GmHSP17.9, GmSPX5, and GmPIN9d. Based on these findings, we concluded that GmAGL82 likely participates in the phosphorus signaling pathway and positively regulates nodulation in soybeans. The findings of this research may lay the theoretical groundwork for further studies and candidate gene resources for the genetic improvement of nutrient-efficient soybean varieties in acidic soils.

Impaired glycosylation of GmPAP15a, a root-associated purple acid phosphatase, inhibits extracellular phytate-P utilization in soybean

Phosphorus (P) is an essential nutrient, but easily fixed in soils. Therefore, most of soil P exists in the form of inaccessible organic phosphorus (Po), particularly phytate-P. Root-associated purple acid phosphatases (PAPs) are considered to play a crucial role in phosphate (Pi) scavenging in soils. However, evidence for regulating root-associated PAPs in utilization of extracellular phytate-P remain largely unknown in plants at both transcriptional and posttranslational levels. In this study, a Pi-starvation responsive GmPAP15a was identified in soybean (Glycine max). Overexpressing GmPAP15a led to significant increases in root-associated phytase activities, as well as total P content when phytate-P was supplied as the sole P resource in soybean hairy roots. Meanwhile, mass spectrometry (MS) analysis showed GmPAP15a was glycosylated at Asn144 and Asn502 , and its glycan structures of N-linked oligosaccharide chains exhibited microheterogeneity. Moreover, two homologues of AtPHR1, GmPHR9 and GmPHR32 were found to activate GmPAP15a transcription through luciferase activity analysis. Taken together, it is strongly suggested that GmPAP15a plays a vital role in phytate-P utilization in soybean, which might be regulated at both transcriptional and glycosylation modification levels. Our results highlight the GmPHR9/GmPHR32-GmPAP15a signalling pathway might present, and control phytate-P utilization in soybean.

The multifaceted nature of plant acid phosphatases: purification, biochemical features, and applications

Acid phosphatases (EC 3.1.3.2) are the enzymes that catalyse transphosphorylation reactions and promotes the hydrolysis of numerous orthophosphate esters in acidic media, as a crucial element for the metabolism of phosphate in tissues. Inorganic phosphate (Pi) utilisation and scavenging, as well as the turnover of Pi-rich sources found in plant vacuoles, are major processes in which intracellular and secretory acid phosphatases function. Therefore, a thorough understanding of these enzymes' structural characteristics, specificity, and physiochemical properties is required to comprehend the function of acid phosphatases in plant energy metabolism. Furthermore, acid phosphatases are gaining increasing importance in industrial biotechnology due to their involvement in transphosphorylation processes and their ability to reduce phosphate levels in food products. Hence, this review aims to provide a comprehensive overview of the purification methods employed for isolating acid phosphatases from diverse plant sources, as well as their structural and functional properties. Additionally, the review explores the potential applications of these enzymes in various fields.

Production of fungal phytases in solid state fermentation and potential biotechnological applications

Phytases are important enzymes used for eliminating the anti-nutritional properties of phytic acid in food and feed ingredients. Phytic acid is major form of organic phosphorus stored during seed setting. Monogastric animals cannot utilize this phytate-phosphorus due to lack of necessary enzymes. Therefore, phytic acid excretion is responsible for mineral deficiency and phosphorus pollution. Phytases have been reported from diverse microorganisms, however, fungal phytases are preferred due to their unique properties. Aspergillus species are the predominant producers of phytases and have been explored widely as compared to other fungi. Solid-state fermentation has been studied as an economical process for the production of phytases to utilize various agro-industrial residues. Mixed substrate fermentation has also been reported for the production of phytases. Physical and chemical parameters including pH, temperature, and concentrations of media components have significantly affected the production of phytases in solid state fermentation. Fungi produced high levels of phytases in solid state fermentation utilizing economical substrates. Optimization of culture conditions using different approaches has significantly improved the production of phytases. Fungal phytases are histidine acid phosphatases exhibiting broad substrate specificity, are relatively thermostable and protease-resistant. These phytases have been found effective in dephytinization of food and feed samples with concomitant liberation of minerals, sugars and soluble proteins. Additionally, they have improved the growth of plants by increasing the availability of phosphorus and other minerals. Furthermore, phytases from fungi have played an important roles in bread making, semi-synthesis of peroxidase, biofuel production, production of myo-inositol phosphates and management of environmental pollution. This review article describes the production of fungal phytases in solid state fermentation and their biotechnological applications.

Interaction of Vanadium Complexes with Proteins: Revisiting the Reported Structures in the Protein Data Bank (PDB) since 2015

The structural determination and characterization of molecules, namely proteins and enzymes, is crucial to gaining a better understanding of their role in different chemical and biological processes. The continuous technical developments in the experimental and computational resources of X-ray diffraction (XRD) and, more recently, cryogenic Electron Microscopy (cryo-EM) led to an enormous growth in the number of structures deposited in the Protein Data Bank (PDB). Bioinorganic chemistry arose as a relevant discipline in biology and therapeutics, with a massive number of studies reporting the effects of metal complexes on biological systems, with vanadium complexes being one of the relevant systems addressed. In this review, we focus on the interactions of vanadium compounds (VCs) with proteins. Several types of binding are established between VCs and proteins/enzymes. Considering that the V-species that bind may differ from those initially added, the mentioned structural techniques are pivotal to clarifying the nature and variety of interactions of VCs with proteins and to proposing the mechanisms involved either in enzymatic inhibition or catalysis. As such, we provide an account of the available structural information of VCs bound to proteins obtained by both XRD and/or cryo-EM, mainly exploring the more recent structures, particularly those containing organic-based vanadium complexes.

Metabolic footprints in phosphate-starved plants

Plants' requirement of Phosphorus (P) as an essential macronutrient is obligatory for their normal growth and metabolism. Besides restricting plants' primary growth, P depletion affects both primary and secondary metabolism and leads to altered levels of sugars, metabolites, amino acids, and other secondary compounds. Such metabolic shifts help plants optimize their metabolism and growth under P limited conditions. Under P deprivation, both sugar levels and their mobilization change that influences the expression of Pi starvation-inducible genes. Increased sugar repartitioning from shoot to root help root growth and organic acids secretion that in turn promotes phosphate (Pi) uptake from the soil. Other metabolic changes such as lipid remodeling or P reallocation from older to younger leaves release the P from its bound forms in the cell. In this review, we summarize the metabolic footprinting of Pi-starved plants with respect to the benefits offered by such metabolic changes to intracellular Pi homeostasis.

Functional assessment of AtPAP17; encoding a purple acid phosphatase involved in phosphate metabolism in Arabidopsis thaliana

PURPOSE

Purple acid phosphatases (PAPs) includ the largest classes of non-specific plant acid phosphatases. Most characterized PAPs were found to play physiological functions in phosphorus metabolism. In this study, we investigated the function of AtPAP17 gene encoding an important purple acid phosphatase in Arabidopsis thaliana.

METHODS

The full-length cDNA sequence of AtPAP17 gene under the control of CaMV-35S promoter was transferred to the A. thaliana WT plant. The generated homozygote AtPAP17-overexpressed plants were compared by the types of analyses with corresponding homozygote atpap17-mutant plant and WT in both + P (1.2 mM) and - P (0 mM) conditions.

RESULTS

In the + P condition, the highest and the lowest amount of Pi was observed in AtPAP17-overexpressed plants and atpap17-mutant plants by 111% increase and 38% decrease compared with the WT plants, respectively. Furthermore, under the same condition, APase activity of AtPAP17-overexpressed plants increased by 24% compared to the WT. Inversely, atpap17-mutant plant represented a 71% fall compared to WT plants. The comparison of fresh weight and dry weight in the studied plants showed that the highest and the lowest amount of absorbed water belonged to OE plants (with 38 and 12 mg plant^-1) and Mu plants (with 22 and 7 mg plant^-1) in + P and - P conditions, respectively.

CONCLUSION

The lack of AtPAP17 gene in the A. thaliana genome led to a remarkable reduction in the development of root biomass. Thus, AtPAP17 could have an important role in the root but not shoot developmental and structural programming. Consequently, this function enables them to absorb more water and eventually associated with more phosphate absorption.

Optimization of an α-aminonaphthylmethylphosphonic acid inhibitor of purple acid phosphatase using rational structure-based design approaches

Purple acid phosphatases (PAPs) are ubiquitous binuclear metallohydrolases that have been isolated from various animals, plants and some types of fungi. In humans and mice, elevated PAP activity in osteoclasts is associated with osteoporosis, making human PAP an attractive target for the development of anti-osteoporotic drugs. Based on previous studies focusing on phosphonate scaffolds, as well as a new crystal structure of a PAP in complex with a derivative of a previously synthesized α-aminonaphthylmethylphosphonic acid, phosphonates 24-40 were designed as new PAP inhibitor candidates. Subsequent docking studies predicted that all of these compounds are likely to interact strongly with the active site of human PAP and most are likely to interact strongly with the active site of pig PAP. The seventeen candidates were synthesized with good yields and nine of them (26-28, 30, 33-36 and 38) inhibit in the sub-micromolar to nanomolar range against pig PAP, with 28 and 35 being the most potent mammalian PAP inhibitors reported with K_i values of 168 nM and 186 nM, respectively. This study thus paves the way for the next stage of drug development for phosphonate inhibitors of PAPs as anti-osteoporotic agents.

Genome-Wide Identification of SPX Family Genes and Functional Characterization of PeSPX6 and PeSPX-MFS2 in Response to Low Phosphorus in Phyllostachys edulis

Moso bamboo (Phyllostachys edulis) is the most widely distributed bamboo species in the subtropical regions of China. Due to the fast-growing characteristics of P. edulis, its growth requires high nutrients, including phosphorus. Previous studies have shown that SPX proteins play key roles in phosphorus signaling and homeostasis. However, the systematic identification, molecular characterization, and functional characterization of the SPX gene family have rarely been reported in P. edulis. In this study, 23 SPXs were identified and phylogenetic analysis showed that they were classified into three groups and distributed on 13 chromosomes. The analysis of conserved domains indicated that there was a high similarity between PeSPXs among SPX proteins in other species. RNA sequencing and qRT-PCR analysis indicated that PeSPX6 and PeSPX-MFS2, which were highly expressed in roots, were clearly upregulated under low phosphorus. Co-expression network analysis and a dual luciferase experiment in tobacco showed that PeWRKY6 positively regulated the PeSPX6 expression, while PeCIGR1-2, PeMYB20, PeWRKY6, and PeWRKY53 positively regulated the PeSPX-MFS2 expression. Overall, these results provide a basis for the identification of SPX genes in P. edulis and further exploration of their functions in mediating low phosphorus responses.

Structural and Functional Insight into the Mechanism of the Fe-S Cluster-Dependent Dehydratase from Paralcaligenes ureilyticus

Enzyme-catalyzed reaction cascades play an increasingly important role for the sustainable manufacture of diverse chemicals from renewable feedstocks. For instance, dehydratases from the ilvD/EDD superfamily have been embedded into a cascade to convert glucose via pyruvate to isobutanol, a platform chemical for the production of aviation fuels and other valuable materials. These dehydratases depend on the presence of both a Fe-S cluster and a divalent metal ion for their function. However, they also represent the rate-limiting step in the cascade. Here, catalytic parameters and the crystal structure of the dehydratase from Paralcaligenes ureilyticus (PuDHT, both in presence of Mg²⁺ and Mn²⁺ ) were investigated. Rate measurements demonstrate that the presence of stoichiometric concentrations Mn²⁺ promotes higher activity than Mg²⁺ , but at high concentrations the former inhibits the activity of PuDHT. Molecular dynamics simulations identify the position of a second binding site for the divalent metal ion. Only binding of Mn²⁺ (not Mg²⁺ ) to this site affects the ligand environment of the catalytically essential divalent metal binding site, thus providing insight into an inhibitory mechanism of Mn²⁺ at higher concentrations. Furthermore, in silico docking identified residues that play a role in determining substrate binding and selectivity. The combined data inform engineering approaches to design an optimal dehydratase for the cascade.

Biomimetics for purple acid phosphatases: A historical perspective

Biomimetics hold potential for varied applications in biotechnology and medicine but have also attracted particular interest as benchmarks for the functional study of their more complex biological counterparts, e.g. metalloenzymes. While many of the synthetic systems adequately mimic some structural and functional aspects of their biological counterparts the catalytic efficiencies displayed are mostly far inferior due to the smaller size and the associated lower complexity. Nonetheless they play an important role in bioinorganic chemistry. Numerous examples of biologically inspired and informed artificial catalysts have been reported, designed to mimic a plethora of chemical transformations, and relevant examples are highlighted in reviews and scientific reports. Herein, we discuss biomimetics of the metallohydrolase purple acid phosphatase (PAP), examples of which have been used to showcase synergistic research advances for both the biological and synthetic systems. In particular, we focus on the seminal contribution of our colleague Prof. Ademir Neves, and his group, pioneers in the design and optimization of suitable ligands that mimic the active site of PAP.

Adaptive strategies of plants to conserve internal phosphorus under P deficient condition to improve P utilization efficiency

Phosphorus (P) is one of the limiting factors for plant growth and productivity due to its slow diffusion and immobilization in the soil which necessitates application of phosphatic fertilizers to meet the crop demand and obtain maximum yields. However, plants have evolved mechanisms to adapt to low P stress conditions either by increasing acquisition (alteration of belowground processes) or by internal inorganic P (Pi) utilization (cellular Pi homeostasis) or both. In this review, we have discussed the adaptive strategies that conserve the use of P and maintain cellular Pi homeostasis in the cytoplasm. These strategies involve modification in membrane lipid composition, flavanol/anthocyanin level, scavenging and reutilization of Pi adsorbed in cell wall pectin, remobilization of Pi during senescence by enzymes like RNases and purple acid phosphatases, alternative mitochondrial electron transport, and glycolytic pathways. The remobilization of Pi from senescing tissues and its internal redistribution to various cellular organelles is mediated by various Pi transporters. Although much efforts have been made to enhance P acquisition efficiency, an understanding of the physiological mechanisms conserving internal Pi and their manipulation would be useful for plants that can utilize P more efficiently to produce optimum growth per unit P uptake.

Selectivity in Enzymatic Phosphorus Recycling from Biopolymers: Isotope Effect, Reactivity Kinetics, and Molecular Docking with Fungal and Plant Phosphatases

Among ubiquitous phosphorus (P) reserves in environmental matrices are ribonucleic acid (RNA) and polyphosphate (polyP), which are, respectively, organic and inorganic P-containing biopolymers. Relevant to P recycling from these biopolymers, much remains unknown about the kinetics and mechanisms of different acid phosphatases (APs) secreted by plants and soil microorganisms. Here we investigated RNA and polyP dephosphorylation by two common APs, a plant purple AP (PAP) from sweet potato and a fungal phytase from Aspergillus niger. Trends of δ¹⁸O values in released orthophosphate during each enzyme-catalyzed reaction in ¹⁸O-water implied a different extent of reactivity. Subsequent enzyme kinetics experiments revealed that A. niger phytase had 10-fold higher maximum rate for polyP dephosphorylation than the sweet potato PAP, whereas the sweet potato PAP dephosphorylated RNA at a 6-fold faster rate than A. niger phytase. Both enzymes had up to 3 orders of magnitude lower reactivity for RNA than for polyP. We determined a combined phosphodiesterase-monoesterase mechanism for RNA and terminal phosphatase mechanism for polyP using high-resolution mass spectrometry and ³¹P nuclear magnetic resonance, respectively. Molecular modeling with eight plant and fungal AP structures predicted substrate binding interactions consistent with the relative reactivity kinetics. Our findings implied a hierarchy in enzymatic P recycling from P-polymers by phosphatases from different biological origins, thereby influencing the relatively longer residence time of RNA versus polyP in environmental matrices. This research further sheds light on engineering strategies to enhance enzymatic recycling of biopolymer-derived P, in addition to advancing environmental predictions of this P recycling by plants and microorganisms.

Hydrolytic activity of new bioinspired MnIIIMnII and FeIIIMnII complexes as mimetics of PAPs: Biological and environmental interest

Coordination compounds that mimic Purple Acid Phosphatases (PAPs) have drawn attention in the bioinorganic field due to their capacity to cleave phosphodiester bonds. However, their catalytic activity upon phosphate triesters is still unexplored. Thus, we report the synthesis and characterization of two binuclear complexes, [Mn^IIMn^III(L₁)(OAc)₂]BF₄ (1) and [Mn^IIFe^III(L₁)(OAc)₂]BF₄ (2) (H₂L₁ = 2-[N,N-bis-(2- pyridilmethyl)aminomethyl]-4-methyl-6-[N-(2-hydroxy-3-formyl-5-methylbenzyl)-N-(2-pyridylmethyl)aminomethyl]phenol), their hydrolytic activity and antioxidant potential. The complexes were fully characterized, including the X-Ray diffraction (XRD) of 1. Density functional theory (DFT) calculations were performed to better understand their electronic and structural properties and phosphate conjugates. The catalytic activity was analyzed for two model substrates, a diester (BDNPP) and a triester phosphate (DEDNPP). The results suggest enhancement of the hydrolysis reaction by 170 to 1500 times, depending on the substrate and complex. It was possible to accompany the catalytic reaction of DEDNPP hydrolysis by phosphorus nuclear magnetic resonance (³¹P NMR), showing that both 1 and 2 are efficient catalysts. Moreover, we also addressed that 1 and 2 present a relevant antioxidant potential, protecting the yeast Saccharomyces cerevisiae, used as eukaryotic model of study, against the exposure of cells to acute oxidative stress.

Advances in chilling injury of postharvest fruit and vegetable: Extracellular ATP aspects

Due to the global use of cold chain, the development of postharvest technology to reduce chilling injury (CI) in postharvest fruits and vegetables during storage and transport is needed urgently. Considerable evidence shows that maintaining intracellular adenosine triphosphate (iATP) in harvested fruits and vegetables is beneficial to inhibiting CI occurrence. Extracellular ATP (eATP) is a damage-associated signal molecule and plays an important role in CI of postharvest fruits and vegetables through its receptor and subsequent signal transduction under low-temperature stress. The development of new aptasensors for the simultaneous determination of eATP level allows for better understanding of the roles of eATP in a myriad of responses mediated by low-temperature stress in relation to the chilling tolerance of postharvest fruits and vegetables. The multiple biological functions of eATP and its receptors in postharvest fruits and vegetables were attributed to interactions with reactive oxygen species (ROS) and nitric oxide (NO) in coordination with phytohormones and other signaling molecules via downstream physiological activities. The complicated interconnection among eATP in relation to its receptors, eATP/iATP homeostasis, ROS, NO, and heat shock proteins triggered by eATP recognition has been emphasized. This paper reviews recent advances in the beneficial effects of energy handling, outlines the production and homeostasis of eATP, discusses the possible mechanism of eATP and its receptors in chilling tolerance, and provides future research directions for CI in postharvest fruits and vegetables during low-temperature storage.

Structure of a cereal purple acid phytase provides new insights to phytate degradation in plants

Grain phytate, a mixed metal ion salt of inositol hexakisphosphate, accounts for 60%-80% of stored phosphorus in plants and is a potent antinutrient of non-ruminant animals including humans. Through neofunctionalization of purple acid phytases (PAPhy), some cereals such as wheat and rye have acquired particularly high mature grain phytase activity. As PAPhy activity supplies phosphate, liberates metal ions necessary for seedling emergence, and obviates antinutrient effects of phytate, its manipulation and control are targeted crop traits. Here we show the X-ray crystal structure of the b2 isoform of wheat PAPhy induced during germination. This high-resolution crystal structure suggests a model for phytate recognition that, validated by molecular dynamics simulations, implicates elements of two sequence inserts (termed PAPhy motifs) relative to a canonical metallophosphoesterase (MPE) domain in forming phytate-specific substrate specificity pockets. These motifs are well conserved in PAPhys from monocot cereals, enzymes which are characterized by high specificity for phytate. Tested by mutagenesis, residues His229 in PAPhy motif 4 and Lys410 in the MPE domain, both conserved in PAPhys, are found to strongly influence phytase activity. These results explain the observed phytase activity of cereal PAPhys and open the way to the rational engineering of phytase activity in planta.

Editing of the starch branching enzyme gene SBE2 generates high-amylose storage roots in cassava

The production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme gene SBE2 was firstly achieved. High-amylose cassava (Manihot esculenta Crantz) is desirable for starch industrial applications and production of healthier processed food for human consumption. In this study, we report the production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme 2 (SBE2). Mutations in two targeted exons of SBE2 were identified in all regenerated plants; these mutations, which included nucleotide insertions, and short or long deletions in the SBE2 gene, were classified into eight mutant lines. Three mutants, M6, M7 and M8, with long fragment deletions in the second exon of SBE2 showed no accumulation of SBE2 protein. After harvest from the field, significantly higher amylose (up to 56% in apparent amylose content) and resistant starch (up to 35%) was observed in these mutants compared with the wild type, leading to darker blue coloration of starch granules after quick iodine staining and altered starch viscosity with a higher pasting temperature and peak time. Further ¹H-NMR analysis revealed a significant reduction in the degree of starch branching, together with fewer short chains (degree of polymerization [DP] 15-25) and more long chains (DP>25 and especially DP>40) of amylopectin, which indicates that cassava SBE2 catalyzes short chain formation during amylopectin biosynthesis. Transition from A- to B-type crystallinity was also detected in the starches. Our study showed that CRISPR/Cas9-mediated mutagenesis of starch biosynthetic genes in cassava is an effective approach for generating novel varieties with valuable starch properties for food and industrial applications.

Arabidopsis PAP17 is a dual-localized purple acid phosphatase up-regulated during phosphate deprivation, senescence, and oxidative stress

A 35 kDa monomeric purple acid phosphatase (APase) was purified from cell wall extracts of Pi starved (-Pi) Arabidopsis thaliana suspension cells and identified as AtPAP17 (At3g17790) by mass spectrometry and N-terminal microsequencing. AtPAP17 was de novo synthesized and dual-localized to the secretome and/or intracellular fraction of -Pi or salt-stressed plants, or senescing leaves. Transiently expressed AtPAP17-green fluorescent protein localized to lytic vacuoles of the Arabidopsis suspension cells. No significant biochemical or phenotypical changes associated with AtPAP17 loss of function were observed in an atpap17 mutant during Pi deprivation, leaf senescence, or salinity stress. Nevertheless, AtPAP17 is hypothesized to contribute to Pi metabolism owing to its marked up-regulation during Pi starvation and leaf senescence, broad APase substrate selectivity and pH activity profile, and rapid repression and turnover following Pi resupply to -Pi plants. While AtPAP17 also catalyzed the peroxidation of luminol, which was optimal at pH 9.2, it exhibited a low Vmax and affinity for hydrogen peroxide relative to horseradish peroxidase. These results, coupled with absence of a phenotype in the salt-stressed or -Pi atpap17 mutant, do not support proposals that the peroxidase activity of AtPAP17 contributes to the detoxification of reactive oxygen species during stresses that trigger AtPAP17 up-regulation.

Purple acid phosphatases: roles in phosphate utilization and new emerging functions

Plants strive for phosphorus (P), which is an essential mineral for their life. Since P availability is limiting in most of the world's soils, plants have evolved with a complex network of genes and their regulatory mechanisms to cope with soil P deficiency. Among them, purple acid phosphatases (PAPs) are predominantly associated with P remobilization within the plant and acquisition from the soil by hydrolyzing organic P compounds. P in such compounds remains otherwise unavailable to plants for assimilation. PAPs are ubiquitous in plants, and similar enzymes exist in bacteria, fungi, mammals, and unicellular eukaryotes, but having some differences in their catalytic center. In the recent past, PAPs' roles have been extended to multiple plant processes like flowering, seed development, senescence, carbon metabolism, response to biotic and abiotic stresses, signaling, and root development. While new functions have been assigned to PAPs, the underlying mechanisms remained understood poorly. Here, we review the known functions of PAPs, the regulatory mechanisms, and their relevance in crop improvement for P-use-efficiency. We then discuss the mechanisms behind their functions and propose areas worthy of future research. Finally, we argue that PAPs could be a potential target for improving P utilization in crops. In turn, this is essential for sustainable agriculture.

Rational Design of Potent Inhibitors of a Metallohydrolase Using a Fragment-Based Approach

Metallohydrolases form a large group of enzymes that have fundamental importance in a broad range of biological functions. Among them, the purple acid phosphatases (PAPs) have gained attention due to their crucial role in the acquisition and use of phosphate by plants and also as a promising target for novel treatments of bone-related disorders and cancer. To date, no crystal structure of a mammalian PAP with drug-like molecules bound near the active site is available. Herein, we used a fragment-based design approach using structures of a mammalian PAP in complex with the Maybridge^TM fragment CC063346, the amino acid L-glutamine and the buffer molecule HEPES, as well as various solvent molecules to guide the design of highly potent and efficient mammalian PAP inhibitors. These inhibitors have improved aqueous solubility when compared to the clinically most promising PAP inhibitors available to date. Furthermore, drug-like fragments bound in newly discovered binding sites mapped out additional scaffolds for further inhibitor discovery, as well as scaffolds for the design of inhibitors with novel modes of action.

Hierarchical Reactivity of Enzyme-Mediated Phosphorus Recycling from Organic Mixtures by Aspergillus niger Phytase

Biological recycling of inorganic phosphorus (P_i) from organic phosphorus (P_o) compounds by phosphatase-type enzymes, including phytases, is an important contributor to the pool of bioavailable P to plants and microorganisms. However, studies of mixed-substrate reactions with these enzymes are lacking. Here, we explore the reactivity of a phytase extract from the fungus Aspergillus niger toward a heterogeneous mixture containing, in addition to phytate, different structures of environmentally relevant P_o compounds such as ribonucleotides and sugar phosphates. Using a high-resolution liquid chromatography-mass spectrometry method to monitor simultaneously the parent P_o compounds and their by-products, we captured sequential substrate-specific evolution of P_i from the mixture, with faster hydrolysis of multiphosphorylated compounds (phytate, diphosphorylated sugars, and di- and tri-phosphorylated ribonucleotides) than hydrolysis of monophosphorylated compounds (monophosphorylated sugars and monophosphorylated ribonucleotides). The interaction mechanisms and energies revealed by molecular docking simulations of each P_o compound within the enzyme's active site explained the substrate hierarchy observed experimentally. Specifically, the favorable orientation for binding of the negatively charged phosphate moieties with respect to the positive potential surface of the active site was important. Collectively, our findings provide mechanistic insights about the broad but hierarchical role of phytase-type enzymes in P_i recycling from the heterogeneous assembly of P_o compounds in agricultural soils or wastes.

Recent insights into the metabolic adaptations of phosphorus-deprived plants

Inorganic phosphate (Pi) is an essential macronutrient required for many fundamental processes in plants, including photosynthesis and respiration, as well as nucleic acid, protein, and membrane phospholipid synthesis. The huge use of Pi-containing fertilizers in agriculture demonstrates that the soluble Pi levels of most soils are suboptimal for crop growth. This review explores recent advances concerning the understanding of adaptive metabolic processes that plants have evolved to alleviate the negative impact of nutritional Pi deficiency. Plant Pi starvation responses arise from complex signaling pathways that integrate altered gene expression with post-transcriptional and post-translational mechanisms. The resultant remodeling of the transcriptome, proteome, and metabolome enhances the efficiency of root Pi acquisition from the soil, as well as the use of assimilated Pi throughout the plant. We emphasize how the up-regulation of high-affinity Pi transporters and intra- and extracellular Pi scavenging and recycling enzymes, organic acid anion efflux, membrane remodeling, and the remarkable flexibility of plant metabolism and bioenergetics contribute to the survival of Pi-deficient plants. This research field is enabling the development of a broad range of innovative and promising strategies for engineering phosphorus-efficient crops. Such cultivars are urgently needed to reduce inputs of unsustainable and non-renewable Pi fertilizers for maximum agronomic benefit and long-term global food security and ecosystem preservation.

Genomic dissection of ROS detoxifying enzyme encoding genes for their role in antioxidative defense mechanism against Tomato leaf curl New Delhi virus infection in tomato

In the present study, genes encoding for six major classes of enzymatic antioxidants, namely superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), Peroxidase (Prx) and glutathione S-transferase (GST) are identified in tomato. Their expression was studied in tomato cultivars contrastingly tolerant to ToLCNDV during virus infection and different hormone treatments. Significant upregulation of SlGR3, SlPrx25, SlPrx75, SlPrx95, SlGST44, and SlGST96 was observed in the tolerant cultivar during disease infection. Virus-induced gene silencing of SlGR3 in the tolerant cultivar conferred disease susceptibility to the knock-down line, and higher accumulation (~80%) of viral DNA was observed in the tolerant cultivar. Further, subcellular localization of SlGR3 showed its presence in cytoplasm, and its enzymatic activity was found to be increased (~65%) during ToLCNDV infection. Knock-down lines showed ~3- and 3.5-fold reduction in GR activity, which altogether underlines that SlGR3 is vital component of the defense mechanism against ToLCNDV infection.

Identification, structure analysis, and transcript profiling of purple acid phosphatases under Pi deficiency in tomato (Solanum lycopersicum L.) and its wild relatives

Purple acid phosphatases (PAPs), a family of metallo-phosphoesterase enzymes, are involved in phosphorus nutrition in plants. In this study, we report that the tomato genome encodes 25 PAP members. Physio-biochemical analyses revealed relatively lower total root-associated acid phosphatase activity in the seedlings of Solanum pimpinellifolium than their cultivated tomato seedlings under Pi deficiency. Scrutiny of their transcript abundance shows that most of PAPs are activated, although to varying levels, under Pi deficiency in tomato. Further investigation demonstrates that the magnitude of induction of phosphate starvation inducible root-associated PAP homologs remains lower in the Pi-starved S. pimpinellifolium seedlings, hence, accounting for the lower acid phosphatase activity in this wild relative. Examination of their amino acid sequences revealed significant variation in their substrate-specificity defining residues. Among all members, only SlPAP15 possesses the critical lysine residue (R337) and atypical REKA motif in its C-terminal region. Homology modeling and docking studies revealed that ADP and ATP are preferred substrates of SlPAP15. We also identified other amino acid residues present in the vicinity of the active site, possibly facilitating such physical interactions. Altogether, the results presented here will help in the functional characterization of these genes in the tomato in the future.