Phosphatase under-producer mutants have altered phosphorus relations

The PAP Gene Family in Tomato: Comprehensive Comparative Analysis, Phylogenetic Relationships and Expression Profiles

Purple acid phosphatase (PAP) plays a vital role in plant phosphate acquisition and utilization, as well as cell wall synthesis and redox reactions. In this study, comprehensive comparative analyses of PAP genes were carried out using the integration of phylogeny, chromosomal localization, intron/exon structural characteristics, and expression profiling. It was shown that the number of introns of the PAP genes, which were distributed unevenly on 12 chromosomes, ranged from 1 to 12. These findings pointed to the existence of complex structures. Phylogenetic analyses revealed that PAPs from tomato, rice, and Arabidopsis could be divided into three groups (Groups I, II, and III). It was assumed that the diversity of these PAP genes occurred before the monocot–dicot split. RNA-seq analysis revealed that most of the genes were expressed in all of the tissues analyzed, with the exception of SlPAP02, SlPAP11, and SlPAP14, which were not detected. It was also found that expression levels of most of the SlPAP gene family of members were changed under phosphorus stress conditions, suggesting potential functional diversification. The findings of this work will help us to achieve a better insight into the function of SlPAP genes in the future, as well as enhance our understanding of their evolutionary relationships in plants.

Gm6PGDH1, a Cytosolic 6-Phosphogluconate Dehydrogenase, Enhanced Tolerance to Phosphate Starvation by Improving Root System Development and Modifying the Antioxidant System in Soybean

Phosphorus plays an important role in plant growth and development, and is an important limiting factor for crop yield. Although previous studies have shown that 6-phosphogluconate dehydrogenase (6PGDH) plays an important role in plant resistance to adversity, its response to low phosphorus (P) stress remains unknown. In this study, we reported the cloning and characterization of a cytosolic 6PGDH gene, Gm6PGDH1, which enhanced the tolerance to phosphate (Pi) starvation by improving root system development and modifying the antioxidant system in transgenic plants. Gm6PGDH1 was highly expressed in the root at full bloom stage, and strongly induced by Pi starvation. The results from intact soybean composite plant and soybean plant, both containing a Gm6PGDH1-overexpressing construct, showed that Gm6PGDH1 was involved in root system development, and subsequently affected P uptake under Pi-deficient conditions. Meanwhile, the accumulation of reactive oxygen species (ROS) in the root tip of transgenic soybean was reduced, and the activity of ROS-scavenging enzymes was enhanced compared with those of the wild type under Pi-deficient conditions. Interestingly, we found that the overexpression of Gm6PGDH1 weakened the response of several other important Pi-answer genes to Pi starvation, such as some purple acid phosphatases (PAPs) and redox-related genes. In addition, the results from a virus-induced gene silencing (VIGS) indicated that Gm6PGDH1 might have functional redundancy in soybean, and the results from a heterogeneous transformation system showed that overexpressing Gm6PGDH1 also enhanced tolerance to Pi starvation in transgenic Arabidopsis. Together, these results suggested the great potential of Gm6PGDH1 in crop breeding for low Pi tolerance.

Spatiotemporal Pattern of Acid Phosphatase Activity in Soils Cultivated With Maize Sensing to Phosphorus-Rich Patches

AIMS

Acid phosphatase (APase) secretion by roots allows plants to mobilize organic phosphorus (P) in low P soils. However, the spatiotemporal dynamics of soil APase activity in response to P-rich patches remain unclear.

METHODS

Here, we grew maize in rhizoboxes with two contrasting soil types and different localized P supplies. In situ soil zymography was applied to examine the spatial-temporal variation of APase activity.

RESULTS

We found P-rich patches can induce the secretion of APase from roots, indicating that even mineral P fertilizers were localized apply, mobilization of soil organic P by roots can also be enhanced; APase hotspot areas and APase activities in the rhizosphere and bulk soil of the same rhizobox showed opposite diurnal rhythms across the whole soil profile. The APase hotspot area was 10-140% larger at noon than at midnight in the rhizosphere, which is consistent with the diurnal rhythm of photosynthesis. In contrast, in bulk soil, the area was 18-200% larger at midnight than at noon, which led to spatiotemporal niche differentiation with regard to the utilization of soil organic P; this alleviated competition between plants and soil microorganisms.

CONCLUSION

Our findings showed that APase secretion of roots was plastic in P-rich patches and showed an opposite diurnal rhythm with soil microorganisms in bulk soil.

Root phenotyping: important and minimum information required for root modeling in crop plants

As plants cannot relocate, they require effective root systems for water and nutrient uptake. Root development plasticity enables plants to adapt to different environmental conditions. Research on improvements in crop root systems is limited in comparison with that in shoots as the former are difficult to image. Breeding more effective root systems is proposed as the "second green revolution". There are several recent publications on root system architecture (RSA), but the methods used to analyze the RSA have not been standardized. Here, we introduce traditional and current root-imaging methods and discuss root structure phenotyping. Some important root structures have not been standardized as roots are easily affected by rhizosphere conditions and exhibit greater plasticity than shoots; moreover, root morphology significantly varies even in the same genotype. For these reasons, it is difficult to define the ideal root systems for breeding. In this review, we introduce several types of software to analyze roots and identify important root parameters by modeling to simplify the root system characterization. These parameters can be extracted from photographs captured in the field. This modeling approach is applicable to various legacy root data stored in old or unpublished formats. Standardization of RSA data could help estimate root ideotypes.

Kinetic and thermodynamic studies of novel acid phosphates extracted from Cichorium intybus seedlings

Herein for the first time a novel acid phosphatase from the seedlings of Cichorium intybus was purified to homogeneity by using various chromatographic techniques (salt precipitation, ion exchange, size exclusion and affinity chromatography) and thermodynamically characterized. The molecular mass of purified enzyme (66 kDa) was determined by SDS-PAGE under denaturing and non-denaturing conditions and by gel-filtration confirmed as dimer of molecular mass 130 kDa. The Michaelis-Menten (Km) constant for -p-NPP (0.3 mM) and (7.6 μmol/min/mg) Vmax. The enzyme was competitively inhibited by phosphate, molybdate and vanadate. Phenyl phosphate, ɑ and β-glycero-phosphate and-p-NPP were found to be good substrate. When temperature increased from (55 °C to 75 °C), the deactivation rate constant (kd) was increased (0.1 to 4.6 min-1) and half- life was decreased from 630 min to 15 min. Various thermal denaturation parameters; change in enthalpy (ΔH°), change in entropy (ΔS°) and change in free energy (ΔG°) were found 121.93 KJ·mol-1, 72.45 KJ·mol-1 and 98.08 KJ·mol-1 respectively, confirming that acid phosphatase undergoes a significant process of unfolding during deactivation. The biochemical properties of acid phosphatase from C. intybus on the behalf of biological activity and its relationship to pH variations, thermal deactivation and kinetics parameters provide an insight into its novel features.

A novel role of the calcium sensor CBL1 in response to phosphate deficiency in Arabidopsis thaliana

Phosphorus acts as an essential macroelement in plant growth and development. A lack of phosphate (Pi) in arable soil and phosphate fertilizer resources is a vital limiting factor in crop yields. Calcineurin B-like proteins (CBLs) act as one of the most important calcium sensors in plants; however, whether CBLs are involved in Pi deficiency signaling pathway remains largely elusive. In this study, we utilized a reverse genetic strategy to screen Arabidopsis thaliana T-DNA insertion mutants belonging to the CBL family under Pi deficiency conditions. The cbl1 mutant exhibited a relatively tolerant phenotype, with longer roots, lower anthocyanin content, and elevated Pi content under Pi deficiency, and a more sensitive phenotype to arsenate treatment compared with wild-type plants. Moreover, CBL1 was upregulated, and the mutation of CBL1 caused phosphate starvation-induced (PSIs) genes to be significantly induced under Pi deficiency. Histochemical staining demonstrated that the cbl1 mutant has decreased acid phosphatase activity and hydrogen peroxide concentrations under Pi deficiency. Collectively, our results have revealed a novel role of CBL1 in maintaining Pi homeostasis.

An Integrative Systems Perspective on Plant Phosphate Research

The case for improving crop phosphorus-use-efficiency is widely recognized. Although much is known about the molecular and regulatory mechanisms, improvements have been hampered by the extreme complexity of phosphorus (P) dynamics, which involves soil chemistry; plant-soil interactions; uptake, transport, utilization and remobilization within plants; and agricultural practices. The urgency and direction of phosphate research is also dependent upon the finite sources of P, availability of stocks to farmers and reducing environmental hazards. This work introduces integrative systems approaches as a way to represent and understand this complexity, so that meaningful links can be established between genotype, environment, crop traits and yield. It aims to provide a large set of pointers to potential genes and research practice, with a view to encouraging members of the plant-phosphate research community to adopt such approaches so that, together, we can aid efforts in global food security.

Functions and regulation of phosphate starvation-induced secreted acid phosphatases in higher plants

Phosphorus is essential for plant growth and development, but levels of inorganic phosphate (Pi), the major form of phosphorus that plants assimilate, are quite limiting in most soils. To cope with Pi deficiency, plants trigger a suite of adaptive responses, including the induction and secretion of acid phosphatases (APases). In this article, we describe how Pi starvation-induced (PSI) APases are analyzed, and we provide a brief historical review of their identification. We then discuss the current understanding of the functions of PSI-secreted APases and how these APases are regulated at the molecular level. Finally, we provide a perspective on the future direction of research in this field.

Analyses of Root-secreted Acid Phosphatase Activity in Arabidopsis

Induction and secretion of acid phosphatase (APase) is a universal adaptive response of higher plants to low-phosphate stress ( Tran et al., 2010 ). The intracellular APases are likely involved in the remobilization and recycling of phosphate (Pi) from intracellular Pi reserves, whereas the extracellular or secreted APases are believed to release Pi from organophosphate compounds in the rhizosphere. The phosphate starvation-induced secreted APases can be released into the rhizosphere or retained on root surfaces (root-associated APases). In this article, we describe the protocols for analyzing root-secreted APase activity in the model plant Arabidopsis thaliana (Arabidopsis). In Arabidopsis, the activity of both root-associated APases and APases that are released into the rhizosphere can be quantified based on their ability to cleave a synthesized substrate, para-nitrophenyl-phosphate (pNPP), which releases a yellow product, para-nitrophenol (pNP) ( Wang et al., 2011 and 2104). The root-associated APase activity can also be directly visualized by applying a chromogenic substrate, 5-bromo-4-chloro-3-indolyl-phosphate (BCIP), to the root surface ( Lloyd et al., 2001 ; Tomscha et al., 2004 ; Wang et al., 2011 and 2014) whereas the isozymes of APases that are released into rhizosphere can be profiled using an in-gel assay (Trull and Deikman, 1998; Tomscha et al., 2004 ; Wang et al., 2011 and 2014). The protocol for analysis of intracellular APase activity in Arabidopsis has been previously described (Vicki and William, 2013).

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.

OsWRKY74, a WRKY transcription factor, modulates tolerance to phosphate starvation in rice

The WRKY transcription factor family has 109 members in the rice genome, and has been reported to be involved in the regulation of biotic and abiotic stress in plants. Here, we demonstrated that a rice OsWRKY74 belonging to group III of the WRKY transcription factor family was involved in tolerance to phosphate (Pi) starvation. OsWRKY74 was localized in the nucleus and mainly expressed in roots and leaves. Overexpression of OsWRKY74 significantly enhanced tolerance to Pi starvation, whereas transgenic lines with down-regulation of OsWRKY74 were sensitive to Pi starvation. Root and shoot biomass, and phosphorus (P) concentration in rice OsWRKY74-overexpressing plants were ~16% higher than those of wild-type (WT) plants in Pi-deficient hydroponic solution. In soil pot experiments, >24% increases in tiller number, grain weight and P concentration were observed in rice OsWRKY74-overexpressing plants compared to WT plants when grown in P-deficient medium. Furthermore, Pi starvation-induced changes in root system architecture were more profound in OsWRKY74-overexpressing plants than in WT plants. Expression patterns of a number of Pi-responsive genes were altered in the OsWRKY74-overexpressing and RNA interference lines. In addition, OsWRKY74 may also be involved in the response to deficiencies in iron (Fe) and nitrogen (N) as well as cold stress in rice. In Pi-deficient conditions, OsWRKY74-overexpressing plants exhibited greater accumulation of Fe and up-regulation of the cold-responsive genes than WT plants. These findings highlight the role of OsWRKY74 in modulation of Pi homeostasis and potential crosstalk between P starvation and Fe starvation, and cold stress in rice.

Arabidopsis phosphatase under-producer mutants pup1 and pup3 contain mutations in the AtPAP10 and AtPAP26 genes

Production and secretion of acid phosphatases (APases) is a hallmark adaptive response of plants to phosphate (Pi) deprivation. Researchers have long hypothesized that Pi starvation-induced APases are involved in internal Pi recycling and remobilization as well as in external Pi utilization. Two phosphatase under-producer (pup) mutants, pup1 and pup3, were previously isolated in Arabidopsis. Characterization of these 2 pup mutants provided the first genetic evidence for the above hypothesis. To date, however, the molecular lesions in these 2 pup mutants remain unknown. In this work, we demonstrate that pup1 and pup3 contain point mutations in the Arabidopsis purple acid phosphatase gene AtPAP10 and AtPAP26, respectively. Our results answer a long-standing question about the molecular identity of the PUP1 and PUP3 genes and corroborate the conclusions from previous studies regarding the function of AtPAP10 and AtPAP26 in plant acclimation to Pi deprivation.

Activity of the brassinosteroid transcription factors BRASSINAZOLE RESISTANT1 and BRASSINOSTEROID INSENSITIVE1-ETHYL METHANESULFONATE-SUPPRESSOR1/BRASSINAZOLE RESISTANT2 blocks developmental reprogramming in response to low phosphate availability

Plants feature remarkable developmental plasticity, enabling them to respond to and cope with environmental cues, such as limited availability of phosphate, an essential macronutrient for all organisms. Under this condition, Arabidopsis (Arabidopsis thaliana) roots undergo striking morphological changes, including exhaustion of the primary meristem, impaired unidirectional cell expansion, and elevated density of lateral roots, resulting in shallow root architecture. Here, we show that the activity of two homologous brassinosteroid (BR) transcriptional effectors, BRASSINAZOLE RESISTANT1 (BZR1) and BRASSINOSTEROID INSENSITIVE1-ETHYL METHANESULFONATE-SUPPRESSOR1 (BES1)/BZR2, blocks these responses, consequently maintaining normal root development under low phosphate conditions without impacting phosphate homeostasis. We show that phosphate deprivation shifts the intracellular localization of BES1/BZR2 to yield a lower nucleus-to-cytoplasm ratio, whereas replenishing the phosphate supply reverses this ratio within hours. Phosphate deprivation reduces the expression levels of BR biosynthesis genes and the accumulation of the bioactive BR 28-norcastasterone. In agreement, low and high BR levels sensitize and desensitize root response to this adverse condition, respectively. Hence, we propose that the environmentally controlled developmental switch from deep to shallow root architecture involves reductions in BZR1 and BES1/BZR2 levels in the nucleus, which likely play key roles in plant adaptation to phosphate-deficient environments.

Phosphate nutrition: improving low-phosphate tolerance in crops

Phosphorus is an essential nutrient that is required for all major developmental processes and reproduction in plants. It is also a major constituent of the fertilizers required to sustain high-yield agriculture. Levels of phosphate--the only form of phosphorus that can be assimilated by plants--are suboptimal in most natural and agricultural ecosystems, and when phosphate is applied as fertilizer in soils, it is rapidly immobilized owing to fixation and microbial activity. Thus, cultivated plants use only approximately 20-30% of the applied phosphate, and the rest is lost, eventually causing water eutrophication. Recent advances in the understanding of mechanisms by which wild and cultivated species adapt to low-phosphate stress and the implementation of alternative bacterial pathways for phosphorus metabolism have started to allow the design of more effective breeding and genetic engineering strategies to produce highly phosphate-efficient crops, optimize fertilizer use, and reach agricultural sustainability with a lower environmental cost. In this review, we outline the current advances in research on the complex network of plant responses to low-phosphorus stress and discuss some strategies used to manipulate genes involved in phosphate uptake, remobilization, and metabolism to develop low-phosphate-tolerant crops, which could help in designing more efficient crops.

Auxin response factor (OsARF12), a novel regulator for phosphate homeostasis in rice (Oryza sativa)

Phosphorus (P) is crucial nutrient element for crop growth and development. However, the network pathway regulating homeostasis of phosphate (Pi) in crops has many molecular breeding unknowns. Here, we report that an auxin response factor, OsARF12, functions in Pi homeostasis. Measurement of element content, quantitative reverse transcription polymerase chain reaction analysis and acid phosphatases (APases) activity assay showed that the osarf12 mutant and osarf12/25 double mutant with P-intoxicated phenotypes had higher P concentrations, up-regulation of the Pi transporter encoding genes and increased APase activity under Pi-sufficient/-deficient (+Pi/-Pi, 0.32/0 mM NaH2 PO4) conditions. Transcript analysis revealed that Pi-responsive genes--Phosphate starvation (OsIPS)1 and OsIPS2, SYG1/Pho81/XPR1(OsSPX1), Sulfoquinovosyldiacylglycerol 2 (OsSQD2), R2R3 MYB transcription factor (OsMYB2P-1) and Transport Inhibitor Response1 (OsTIR1)--were more abundant in the osarf12 and osarf12/25 mutants under +Pi/-Pi conditions. Knockout of OsARF12 also influenced the transcript abundances of the OsPHR2 gene and its downstream components, such as OsMiR399j, OsPHO2, OsMiR827, OsSPX-MFS1 and OsSPX-MFS2. Results from -Pi/1-naphthylphthalamic acid (NPA) treatments, and auxin reporter DR5::GUS staining suggest that root system alteration and Pi-induced auxin response were at least partially controlled by OsARF12. These findings enrich our understanding of the biological functions of OsARF12, which also acts in regulating Pi homeostasis.

Understanding plant responses to phosphorus starvation for improvement of plant tolerance to phosphorus deficiency by biotechnological approaches

In both prokaryotes and eukaryotes, including plants, phosphorus (P) is an essential nutrient that is involved in various biochemical processes, such as lipid metabolism and the biosynthesis of nucleic acids and cell membranes. P also contributes to cellular signaling cascades by function as mediators of signal transduction and it also serves as a vital energy source for a wide range of biological functions. Due to its intensive use in agriculture, P resources have become limited. Therefore, it is critically important in the future to develop scientific strategies that aim to increase P use efficiency and P recycling. In addition, the biologically available soluble form of P for uptake (phosphate; Pi) is readily washed out of topsoil layers, resulting in serious environmental pollution. In addition to this environmental concern, the wash out of Pi from topsoil necessitates a continuous Pi supply to maintain adequate levels of fertilization, making the situation worse. As a coping mechanism to P stress, plants are known to undergo drastic cellular changes in metabolism, physiology, hormonal balance and gene expression. Understanding these molecular, physiological and biochemical responses developed by plants will play a vital role in improving agronomic practices, resource conservation and environmental protection as well as serving as a foundation for the development of biotechnological strategies, which aim to improve P use efficiency in crops. In this review, we will discuss a variety of plant responses to low P conditions and various molecular mechanisms that regulate these responses. In addition, we also discuss the implication of this knowledge for the development of plant biotechnological applications.

Overexpression of OsPAP10a, a root-associated acid phosphatase, increased extracellular organic phosphorus utilization in rice

Phosphorus (P) deficiency is a major limitation for plant growth and development. Among the wide set of responses to cope with low soil P, plants increase their level of intracellular and secreted acid phosphatases (APases), which helps to catalyze inorganic phosphate (Pi) hydrolysis from organo-phosphates. In this study we characterized the rice (Oryza sativa) purple acid phosphatase 10a (OsPAP10a). OsPAP10a belongs to group Ia of purple acid phosphatases (PAPs), and clusters with the principal secreted PAPs in a variety of plant species including Arabidopsis. The transcript abundance of OsPAP10a is specifically induced by Pi deficiency and is controlled by OsPHR2, the central transcription factor controlling Pi homeostasis. In gel activity assays of root and shoot protein extracts, it was revealed that OsPAP10a is a major acid phosphatase isoform induced by Pi starvation. Constitutive overexpression of OsPAP10a results in a significant increase of phosphatase activity in both shoot and root protein extracts. In vivo root 5-bromo-4-chloro-3-indolyl-phosphate (BCIP) assays and activity measurements on external media showed that OsPAP10a is a root-associated APase. Furthermore, overexpression of OsPAP10a significantly improved ATP hydrolysis and utilization compared with wild type plants. These results indicate that OsPAP10a can potentially be used for crop breeding to improve the efficiency of P use.

Identification of soybean purple acid phosphatase genes and their expression responses to phosphorus availability and symbiosis

BACKGROUND AND AIMS

Purple acid phosphatases (PAPs) are members of the metallo-phosphoesterase family and have been known to play important roles in phosphorus (P) acquisition and recycling in plants. Low P availability is a major constraint to growth and production of soybean, Glycine max. Comparative studies on structure, transcription regulation and responses to phosphate (Pi) deprivation of the soybean PAP gene family should facilitate further insights into the potential physiological roles of GmPAPs.

METHODS

BLAST searches were performed to identify soybean PAP genes at the phytozome website. Bioinformatic analyses were carried out to investigate their gene structure, conserve motifs and phylogenetic relationships. Hydroponics and sand-culture experiments were carried out to obtain the plant materials. Quantitative real-time PCR was employed to analyse the expression patterns of PAP genes in response to P deficiency and symbiosis.

KEY RESULTS

In total, 35 PAP genes were identified from soybean genomes, which can be classified into three distinct groups including six subgroups in the phylogenetic tree. The expression pattern analysis showed flowers possessed the largest number of tissue-specific GmPAP genes under normal P conditions. The expression of 23 GmPAPs was induced or enhanced by Pi starvation in different tissues. Among them, nine GmPAP genes were highly expressed in the Pi-deprived nodules, whereas only two GmPAP genes showed significantly increased expression in the arbuscular mycorrhizal roots under low-P conditions.

CONCLUSIONS

Most GmPAP genes are probably involved in P acquisition and recycling in plants. Also we provide the first evidence that some members of the GmPAP gene family are possibly involved in the response of plants to symbiosis with rhizobia or arbuscular mycorrhizal fungi under P-limited conditions.

The Arabidopsis purple acid phosphatase AtPAP10 is predominantly associated with the root surface and plays an important role in plant tolerance to phosphate limitation

Induction of secreted acid phosphatase (APase) is a universal response of higher plants to phosphate (Pi) limitation. These enzymes are thought to scavenge Pi from organophosphate compounds in the rhizosphere and thus to increase Pi availability to plants when Pi is deficient. The tight association of secreted APase with the root surface may make plants more efficient in the utilization of soil Pi around root tissues, which is present in organophosphate forms. To date, however, no systematic molecular, biochemical, and functional studies have been reported for any of the Pi starvation-induced APases that are associated with the root surface after secretion. In this work, using genetic and molecular approaches, we identified Arabidopsis (Arabidopsis thaliana) Purple Acid Phosphatase10 (AtPAP10) as a Pi starvation-induced APase that is predominantly associated with the root surface. The AtPAP10 protein has phosphatase activity against a variety of substrates. Expression of AtPAP10 is specifically induced by Pi limitation at both transcriptional and posttranscriptional levels. Functional analyses of multiple atpap10 mutant alleles and overexpressing lines indicated that AtPAP10 plays an important role in plant tolerance to Pi limitation. Genetic manipulation of AtPAP10 expression may provide an effective means for engineering new crops with increased tolerance to Pi deprivation.

Inorganic phosphate as an important regulator of phosphatases

Cellular metabolism depends on the appropriate concentration of intracellular inorganic phosphate (Pi). Pi starvation-responsive genes appear to be involved in multiple metabolic pathways, implying a complex Pi regulation system in microorganisms and plants. A group of enzymes is required for absorption and maintenance of adequate phosphate levels, which is released from phosphate esters and anhydrides. The phosphatase system is particularly suited for the study of regulatory mechanisms because phosphatase activity is easily measured using specific methods and the difference between the repressed and derepressed levels of phosphatase activity is easily detected. This paper analyzes the protein phosphatase system induced during phosphate starvation in different organisms.

Proteomic analysis of secreted proteins from aseptically grown rice

Plants are known to secrete a variety of compounds into the rhizosphere. These compounds are thought to play important roles in the regulation of soil chemical properties and soil microorganisms. To determine the composition of proteins secreted from rice roots, aseptic hydro culture was performed, and the collected proteins were analyzed. Over 100 proteins were identified; most were identified using the rice database (RAP-DB), and about 60% of the identified proteins were suspected to have a signal peptide. Functional categorization suggested that most were secondary metabolism- and defense-related proteins. Pathogenesis- and stress-related proteins were the major proteins found in the bathing solution under aseptic conditions. Thus, we propose that rice plants constitutively secrete a large variety of proteins to protect their roots against abiotic and/or biotic stresses in the environment.

Identification of rice purple acid phosphatases related to phosphate starvation signalling

Purple acid phosphatases (PAPs) are a family of metallo-phosphoesterases involved in a variety of physiological functions, especially phosphate deficiency adaptations in plants. We identified 26 putative PAP genes by a genome-wide analysis of rice (Oryza sativa), 24 of which have isolated EST sequences in the dbEST database. Amino acid sequence analysis revealed that 25 of these genes possess sets of metal-ligating residues typical of known PAPs. Phylogenetic analysis classified the 26 rice and 29 Arabidopsis PAPs into three main groups and seven subgroups. We detected transcripts of 21 PAP genes in roots or leaves of rice seedlings. The expression levels of ten PAP genes were up-regulated by both phosphate deprivation and over-expression of the transcription factor OsPHR2. These PAP genes all contained one or two OsPHR2 binding elements in their promoter regions, implying that they are directly regulated by OsPHR2. Both acid phosphatase (AP) and surface secretory acid phosphatase (SAP) activity assays showed that the up-regulation of PAPs by Pi starvation, OsPHR2 over-expression, PHO2 knockout or OsSPX1 RNA interference led to an increase in AP and SAP activity in rice roots. This study reveals the potential for developing technologies for crop improvement in phosphorus use efficiency.

Analysis of the contribution of acid phosphatase to P efficiency in Brassica napus under low phosphorus conditions

To understand whether genotypic variation in acid phosphatase (APase) activity in rapeseed (Brassica napus L.) induced by phosphorus (P) deficiency has impact on P efficiency, soil APase activity in the rhizosphere for rapeseed P-efficient genotype 102 and P-inefficient genotype 105 was measured against organic and inorganic P sources in the pot experiment, and the activities of root-secreted APase and leaf intracellular APase were investigated in different P-starvation periods in the nutrient solution. Higher activity of root-secreted APase in B. napus was induced under low P conditions. However, P nutrition and P uptake efficiency of the plants supplied with organic P were not directly related to the activity of root-secreted APase due to several confounding factors affecting APase availability. The higher activity of leaf APase improved P remobilization in plants and played important roles in enhancing P use efficiency, shown by the significant correlation between leaf APase activity and P use efficiency in a rapeseed recombinant inbred population of 135 lines.

The dual-targeted purple acid phosphatase isozyme AtPAP26 is essential for efficient acclimation of Arabidopsis to nutritional phosphate deprivation

Induction of intracellular and secreted acid phosphatases (APases) is a widespread response of orthophosphate (Pi)-starved (-Pi) plants. APases catalyze Pi hydrolysis from a broad range of phosphomonoesters at an acidic pH. The largest class of nonspecific plant APases is comprised of the purple APases (PAPs). Although the biochemical properties, subcellular location, and expression of several plant PAPs have been described, their physiological functions have not been fully resolved. Recent biochemical studies indicated that AtPAP26, one of 29 PAPs encoded by the Arabidopsis (Arabidopsis thaliana) genome, is the predominant intracellular APase, as well as a major secreted APase isozyme up-regulated by -Pi Arabidopsis. An atpap26 T-DNA insertion mutant lacking AtPAP26 transcripts and 55-kD immunoreactive AtPAP26 polypeptides exhibited: (1) 9- and 5-fold lower shoot and root APase activity, respectively, which did not change in response to Pi starvation, (2) a 40% decrease in secreted APase activity during Pi deprivation, (3) 35% and 50% reductions in free and total Pi concentration, respectively, as well as 5-fold higher anthocyanin levels in shoots of soil-grown -Pi plants, and (4) impaired shoot and root development when subjected to Pi deficiency. By contrast, no deleterious influence of AtPAP26 loss of function occurred under Pi-replete conditions, or during nitrogen or potassium-limited growth, or oxidative stress. Transient expression of AtPAP26-mCherry in Arabidopsis suspension cells verified that AtPAP26 is targeted to the cell vacuole. Our results confirm that AtPAP26 is a principal contributor to Pi stress-inducible APase activity, and that it plays an important role in the Pi metabolism of -Pi Arabidopsis.

Overexpressing AtPAP15 enhances phosphorus efficiency in soybean

Low phosphorus (P) availability is a major constraint to crop growth and production, including soybean (Glycine max), on a global scale. However, 50% to 80% of the total P in agricultural soils exists as organic phosphate, which is unavailable to plants unless hydrolyzed to release inorganic phosphate. One strategy for improving crop P nutrition is the enhanced activity of acid phosphatases (APases) to obtain or remobilize inorganic phosphate from organic P sources. In this study, we overexpressed an Arabidopsis (Arabidopsis thaliana) purple APase gene (AtPAP15) containing a carrot (Daucus carota) extracellular targeting peptide in soybean hairy roots and found that the APase activity was increased by 1.5-fold in transgenic hairy roots. We subsequently transformed soybean plants with AtPAP15 and studied three homozygous overexpression lines of AtPAP15. The three transgenic lines exhibited significantly improved P efficiency with 117.8%, 56.5%, and 57.8% increases in plant dry weight, and 90.1%, 18.2%, and 62.6% increases in plant P content, respectively, as compared with wild-type plants grown on sand culture containing phytate as the sole P source. The transgenic soybean lines also exhibited a significant level of APase and phytase activity in leaves and root exudates, respectively. Furthermore, the transgenic lines exhibited improved yields when grown on acid soils, with 35.9%, 41.0%, and 59.0% increases in pod number per plant, and 46.0%, 48.3%, and 66.7% increases in seed number per plant. Taken together, to our knowledge, our study is the first report on the improvement of P efficiency in soybean through constitutive expression of a plant APase gene. These findings could have significant implications for improving crop yield on soils low in available P, which is a serious agricultural limitation worldwide.

Molecular regulators of phosphate homeostasis in plants

An appropriate cellular phosphate (Pi) concentration is indispensable for essential physiological and biochemical processes. To maintain cellular Pi homeostasis, plants have developed a series of adaptive responses to facilitate external Pi acquisition and to limit Pi consumption and to adjust Pi recycling internally when the Pi supply is inadequate. Over the past decade, significant progress has been made toward understanding such regulation at the molecular level. In this review, the focus is on the molecular regulators that mediate cellular Pi concentrations. The regulators are introduced and organized according to their original identification procedures, by the forward genetic approach of mutant screening or by reverse genetic analysis. These genes are involved in Pi uptake, allocation or remobilization or are upstream regulators, such as transcriptional factors or signalling molecules. In the future, integration of current knowledge and exploration of new technology is expected to offer new insights into molecular mechanisms that maintain Pi homeostasis.

Signaling components involved in plant responses to phosphate starvation

Phosphorus is one of the macronutrients essential for plant growth and development. Many soils around the world are deficient in phosphate (Pi) which is the form of phosphorus that plants can absorb and utilize. To cope with the stress of Pi starvation, plants have evolved many elaborate strategies to enhance the acquisition and utilization of Pi from the environment. These strategies include morphological, biochemical and physiological responses which ultimately enable plants to better survive under low Pi conditions. Though these adaptive responses have been well described because of their ecological and agricultural importance, our studies on the molecular mechanisms underlying these responses are still in their infancy. In the last decade, significant progresses have been made towards the identification of the molecular components which are involved in the control of plant responses to Pi starvation. In this article, we first provide an overview of some major responses of plants to Pi starvation, then summarize what we have known so far about the signaling components involved in these responses, as well as the roles of sugar and phytohormones.

Phosphate homeostasis and root development in Arabidopsis are synchronized by the zinc finger transcription factor ZAT6

Phosphorus availability is limited in many natural ecosystems. Plants adapt to phosphate (Pi) deficiency by complex molecular processes. There is growing evidence suggesting that transcription factors are key components in the regulation of these processes. In this study, we characterized the function of ZAT6 (zinc finger of Arabidopsis 6), a cysteine-2/histidine-2 zinc finger transcription factor that is responsive to Pi stress. ZAT6 is induced during Pi starvation and localizes to the nucleus. While the RNAi suppression of ZAT6 appeared to be lethal, its overexpression affects root development and retards seedling growth as a result of decreased Pi acquisition. The ZAT6 overexpression also resulted in altered root architecture of older plants, with consequent changes in Pi acquisition. These results indicate that ZAT6 regulates root development independent of the Pi status of the plant, thereby influencing Pi acquisition and homeostasis. In addition, the expression of several Pi starvation-responsive genes was decreased in ZAT6 overexpressing plants, thereby confirming the role of ZAT6 in regulating Pi homeostasis. This study thus indicates that ZAT6 is a repressor of primary root growth and regulates Pi homeostasis through the control of root architecture. To our knowledge, ZAT6 is the first cysteine-2/histidine-2 zinc finger transcription factor reported to regulate root development and nutrient stress responses.

Biochemical and molecular characterization of AtPAP26, a vacuolar purple acid phosphatase up-regulated in phosphate-deprived Arabidopsis suspension cells and seedlings

A vacuolar acid phosphatase (APase) that accumulates during phosphate (Pi) starvation of Arabidopsis (Arabidopsis thaliana) suspension cells was purified to homogeneity. The final preparation is a purple APase (PAP), as it exhibited a pink color in solution (A(max) = 520 nm). It exists as a 100-kD homodimer composed of 55-kD glycosylated subunits that cross-reacted with an anti-(tomato intracellular PAP)-IgG. BLAST analysis of its 23-amino acid N-terminal sequence revealed that this PAP is encoded by At5g34850 (AtPAP26; one of 29 PAP genes in Arabidopsis) and that a 30-amino acid signal peptide is cleaved from the AtPAP26 preprotein during its translocation into the vacuole. AtPAP26 displays much stronger sequence similarity to orthologs from other plants than to other Arabidopsis PAPs. AtPAP26 exhibited optimal activity at pH 5.6 and broad substrate selectivity. The 5-fold increase in APase activity that occurred in Pi-deprived cells was paralleled by a similar increase in the amount of a 55-kD anti-(tomato PAP or AtPAP26)-IgG immunoreactive polypeptide and a >30-fold reduction in intracellular free Pi concentration. Semiquantitative reverse transcription-PCR indicated that Pi-sufficient, Pi-starved, and Pi-resupplied cells contain similar amounts of AtPAP26 transcripts. Thus, transcriptional controls appear to exert little influence on AtPAP26 levels, relative to translational and/or proteolytic controls. APase activity and AtPAP26 protein levels were also up-regulated in shoots and roots of Pi-deprived Arabidopsis seedlings. We hypothesize that AtPAP26 recycles Pi from intracellular P metabolites in Pi-starved Arabidopsis. As AtPAP26 also exhibited alkaline peroxidase activity, a potential additional role in the metabolism of reactive oxygen species is discussed.

pho2, a phosphate overaccumulator, is caused by a nonsense mutation in a microRNA399 target gene

We recently demonstrated that microRNA399 (miR399) controls inorganic phosphate (Pi) homeostasis by regulating the expression of UBC24 encoding a ubiquitin-conjugating E2 enzyme in Arabidopsis (Arabidopsis thaliana). Transgenic plants overexpressing miR399 accumulated excessive Pi in the shoots and displayed Pi toxic symptoms. In this study, we revealed that a previously identified Pi overaccumulator, pho2, is caused by a single nucleotide mutation resulting in early termination within the UBC24 gene. The level of full-length UBC24 mRNA was reduced and no UBC24 protein was detected in the pho2 mutant, whereas up-regulation of miR399 by Pi deficiency was not affected. Several characteristics of Pi toxicity in the pho2 mutant were similar to those in the miR399-overexpressing and UBC24 T-DNA knockout plants: both Pi uptake and translocation of Pi from roots to shoots increased and Pi remobilization within leaves was impaired. These phenotypes of the pho2 mutation could be rescued by introduction of a wild-type copy of UBC24. Kinetic analyses revealed that greater Pi uptake in the pho2 and miR399-overexpressing plants is due to increased Vmax. The transcript level of most PHT1 Pi transporter genes was not significantly altered, except PHT1;8 whose expression was enhanced in Pi-sufficient roots of pho2 and miR399-overexpressing compared with wild-type plants. In addition, changes in the expression of several organelle-specific Pi transporters were noticed, which may be associated with the redistribution of intracellular Pi under excess Pi. Furthermore, miR399 and UBC24 were colocalized in the vascular cylinder. This observation not only provides important insight into the interaction between miR399 and UBC24 mRNA, but also supports their systemic function in Pi translocation and remobilization.

pho2, a phosphate overaccumulator, is caused by a nonsense mutation in a microRNA399 target gene

We recently demonstrated that microRNA399 (miR399) controls inorganic phosphate (Pi) homeostasis by regulating the expression of UBC24 encoding a ubiquitin-conjugating E2 enzyme in Arabidopsis (Arabidopsis thaliana). Transgenic plants overexpressing miR399 accumulated excessive Pi in the shoots and displayed Pi toxic symptoms. In this study, we revealed that a previously identified Pi overaccumulator, pho2, is caused by a single nucleotide mutation resulting in early termination within the UBC24 gene. The level of full-length UBC24 mRNA was reduced and no UBC24 protein was detected in the pho2 mutant, whereas up-regulation of miR399 by Pi deficiency was not affected. Several characteristics of Pi toxicity in the pho2 mutant were similar to those in the miR399-overexpressing and UBC24 T-DNA knockout plants: both Pi uptake and translocation of Pi from roots to shoots increased and Pi remobilization within leaves was impaired. These phenotypes of the pho2 mutation could be rescued by introduction of a wild-type copy of UBC24. Kinetic analyses revealed that greater Pi uptake in the pho2 and miR399-overexpressing plants is due to increased Vmax. The transcript level of most PHT1 Pi transporter genes was not significantly altered, except PHT1;8 whose expression was enhanced in Pi-sufficient roots of pho2 and miR399-overexpressing compared with wild-type plants. In addition, changes in the expression of several organelle-specific Pi transporters were noticed, which may be associated with the redistribution of intracellular Pi under excess Pi. Furthermore, miR399 and UBC24 were colocalized in the vascular cylinder. This observation not only provides important insight into the interaction between miR399 and UBC24 mRNA, but also supports their systemic function in Pi translocation and remobilization.