Induction of an extracellular cyclic nucleotide phosphodiesterase as an accessory ribonucleolytic activity during phosphate starvation of cultured tomato cells

PHR1 involved in the regulation of low phosphate-induced leaf senescence by modulating phosphorus homeostasis in Arabidopsis

Phosphorus (P) is a crucial macronutrient for plant growth, development, and reproduction. The effects of low P (LP) stress on leaf senescence and the role of PHR1 in LP-induced leaf senescence are still unknown. Here, we report that PHR1 plays a crucial role in LP-induced leaf senescence, showing delayed leaf senescence in phr1 mutant and accelerated leaf senescence in 35S:PHR1 transgenic Arabidopsis under LP stress. The transcriptional profiles indicate that 763 differentially expressed SAGs (DE-SAGs) were upregulated and 134 DE-SAGs were downregulated by LP stress. Of the 405 DE-SAGs regulated by PHR1, 27 DE-SAGs were involved in P metabolism and transport. PHR1 could bind to the promoters of six DE-SAGs (RNS1, PAP17, SAG113, NPC5, PLDζ2, and Pht1;5), and modulate them in LP-induced senescing leaves. The analysis of RNA content, phospholipase activity, acid phosphatase activity, total P and phosphate content also revealed that PHR1 promotes P liberation from senescing leaves and transport to young tissues under LP stress. Our results indicated that PHR1 is one of the crucial modulators for P recycling and redistribution under LP stress, and the drastic decline of P level is at least one of the causes of early senescence in P-deficient leaves.

Insights into the metabolism, signaling, and physiological effects of 2',3'-cyclic nucleotide monophosphates in bacteria

2',3'-cyclic nucleotide monophosphates (2',3'-cNMPs) have been discovered within both prokaryotes and eukaryotes in the past decade and a half, raising questions about their conserved existence in cells. In plants and mammals, wounding has been found to cause increased levels of 2',3'-cNMPs. Roles for 2',3'-cNMPs in plant immunity suggest that their regulation may be valuable for both plant hosts and microbial pathogens. In support of this hypothesis, a plethora of microbial enzymes have been found with activities related to these molecules. Studies in bacteria suggest that 2',3'-cNMPs are also produced in response to cellular stress and modulate expression of numerous genes. 2',3'-cNMP levels affect bacterial phenotypes, including biofilm formation, motility, and growth. Within E. coli and Salmonella enterica, 2',3'-cNMPs are produced by RNA degradation by RNase I, highlighting potential roles for Type 2 RNases producing 2',3'-cNMPs in a range of organisms. Development of cellular tools to modulate 2',3'-cNMP levels in bacteria has allowed for interrogation of the effects of 2',3'-cNMP concentration on bacterial transcriptomes and physiology. Pull-downs of cellular 2',3'-cNMP binding proteins have identified the ribosome and in vitro studies demonstrated that 2',3'-cNMPs decrease translation, suggesting a direct mechanism for 2',3-cNMP-dependent control of bacterial phenotypes. Future studies dissecting the cellular roles of 2',3'-cNMPs will highlight novel signaling pathways within prokaryotes and which can potentially be engineered to control bacterial physiology.

Extracellular RNA: mechanisms of secretion and potential functions

Extracellular RNA (exRNA) has long been considered as cellular waste that plants can degrade and utilize to recycle nutrients. However, recent findings highlight the need to reconsider the biological significance of RNAs found outside of plant cells. A handful of studies suggest that the exRNA repertoire, which turns out to be an extremely heterogenous group of non-coding RNAs, comprises species as small as a dozen nucleotides to hundreds of nucleotides long. They are found mostly in free form or associated with RNA-binding proteins, while very few are found inside extracellular vesicles (EVs). Despite their low abundance, small RNAs associated with EVs have been a focus of exRNA research due to their putative role in mediating trans-kingdom RNAi. Therefore, non-vesicular exRNAs have remained completely under the radar until very recently. Here we summarize our current knowledge of the RNA species that constitute the extracellular RNAome and discuss mechanisms that could explain the diversity of exRNAs, focusing not only on the potential mechanisms involved in RNA secretion but also on post-release processing of exRNAs. We will also share our thoughts on the putative roles of vesicular and extravesicular exRNAs in plant-pathogen interactions, intercellular communication, and other physiological processes in plants.

Intracellular phosphate recycling systems for survival during phosphate starvation in plants

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is "Pi recycling" in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling.

Transgenic expression of rice OsPHR2 increases phosphorus uptake and yield in wheat

Oryza sativa PHOSPHATE RESPONSE2 (OsPHR2) can promote the uptake and use of phosphorus (P) in rice. We introduced OsPHR2 into the winter wheat (Triticum aestivum L.) variety "Zhengmai0856." OsPHR2 was integrated into the wheat genome with two copy numbers and could be correctly transcribed and expressed. OsPHR2 was mainly expressed in the leaves at the seedling stage. From the jointing to filling stage, OsPHR2 was mainly expressed in the roots, followed by the leaves, with a low expression level in detected the tassels and stems. The transgenic lines exhibited higher P accumulation at each growth stage and increased P uptake intensity in comparison to the wild type under low P and high P conditions. Analysis of the root characteristics showed that the transgenic expression of OsPHR2 increased the maximum root length, total root length, root-to-shoot ratio, and root volume under the conditions of P deficiency or low P. A field experiment showed that the transgenic lines had a higher grain yield than the wild type under low P and high P conditions. The yield of the transgenic lines increased by 6.29% and 3.73% on average compared with the wild type under low P and high P conditions, respectively. Thus, the transgenic expression of OsPHR2 could increase P uptake and yield in wheat, but the effect was more prominent under low P conditions.

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.

Genome-wide identification and characterization of legume T2 Ribonuclease gene family and analysis of GmaRNS9, a soybean T2 Ribonuclease gene, function in nodulation

T2 ribonuclease family (RNaseT2) proteins are secretory and nonspecific endoribonucleases that have a large conserved biological role. Family members of RNaseT2 are found in every organism and carry out important biological functions. However, little is known about the functions of these proteins in legumes, including potential roles in symbiotic nodulation. This study aimed to characterize and perform bioinformatic analysis of RNaseT2 genes in four legume species that their genome was sequenced. In total, 60 RNaseT2 genes were identified and characterized. By analyzing their phylogeny, we divided these RNaseT2 into five clades. Expression analysis of RNaseT2 genes indicated that these genes are expressed in various tissues, and the most expression level was related to the pod, flower, and root. Moreover, GmaRNS9 expression analysis in soybean was consistent with in silico studies and demonstrated that this gene usually has high root tip expression. GmaRNS9 expression was reduced by Bradyrhizobium japonicum inoculation and nodule formation. Reduced expression of this gene was possibly controlled by the GmNARK gene either directly or pleiotropically through increased phosphorus requirements during increased nodulation. However, the nutrient stress (phosphate and nitrate starvation) led to an increase in the expression level of GmRNS9. In silico and quantitative gene expression analyses showed that RNaseT2 genes could play important roles in the growth and development of legumes as well as nodulation.

A metabolomics insight into the Cyclic Nucleotide Monophosphate signaling cascade in tomato under non-stress and salinity conditions

Cyclic Nucleotides Monophosphate (cNMP) are key signalling compounds whose role in plant cell signal transduction is still poorly understood. In this work we used sildenafil, a phosphodiesterase (PDE) inhibitor used in human, to amplify the signal cascade triggered by cNMP using tomato as model plant. Metabolomics was then used, together with plant growth and root architecture parameters, to unravel the changes elicited by PDE inhibition either under non-stress and 100 mM NaCl salinity conditions. The PDE inhibitor elicited a significant increase in biomass (+62 %) and root length (+56 %) under no stress conditions, and affected root architecture in terms of distribution over diameter classes. Together with cGMP, others cNMP were modulated by the treatment. Moreover, PDE inhibition triggered a broad metabolic reprogramming involving photosynthesis and secondary metabolism. A complex crosstalk network of phytohormones and other signalling compounds could be observed in treated plants. Nonetheless, metabolites related to redox imbalance processes and NO signalling could be highlighted in tomato following PDE application. Despite salinity damped down the growth-promoting effects of sildenafil, interesting implications in plant mitigation to stress-related detrimental effects could be observed.

Ustilago maydis secreted T2 ribonucleases, Nuc1 and Nuc2 scavenge extracellular RNA

Ustilago maydis genome codes for many secreted ribonucleases. The contribution of two among these belonging to the T2 family (Nuc1 and Nuc2) in the pathogen virulence, has been assessed in this study. The nuc1 and nuc2 deletion mutants showed not only reduced pathogenicity compared to the SG200 WT strain but also exhibited significant delay in the completion of the pathogenic lifecycle. Both the proteins were also tested for their nucleolytic activities towards RNA substrates from maize and yeast. This also yielded valuable insights into the ability of the ribonucleases to utilise extracellular RNA as a nutrient source. Our study therefore established a role of two T2 type secreted ribonucleases of a phytopathogen in the acquisition of nutrient for the first time. This study also provides evidence that maize apoplast contains RNA, which can be utilised as a substrate by both Nuc1 and Nuc2.

Downstream Targets of Cyclic Nucleotides in Plants

Efficient integration of various external and internal signals is required to maintain adaptive cellular function. Numerous distinct signal transduction systems have evolved to allow cells to receive these inputs, to translate their codes and, subsequently, to expand and integrate their meanings. Two of these, cyclic AMP and cyclic GMP, together referred to as the cyclic nucleotide signaling system, are between them. The cyclic nucleotides regulate a vast number of processes in almost all living organisms. Once synthesized by adenylyl or guanylyl cyclases, cyclic nucleotides transduce signals by acting through a number of cellular effectors. Because the activities of several of these effectors are altered simultaneously in response to temporal changes in cyclic nucleotide levels, agents that increase cAMP/cGMP levels can trigger multiple signaling events that markedly affect numerous cellular functions. In this mini review, we summarize recent evidence supporting the existence of cNMP effectors in plant cells. Specifically, we highlight cAMP-dependent protein kinase A (PKA), cGMP-dependent kinase G (PKG), and cyclic nucleotide phosphodiesterases (PDEs). Essentially this manuscript documents the progress that has been achieved in recent decades in improving our understanding of the regulation and function of cNMPs in plants and emphasizes the current gaps and unanswered questions in this field of plant signaling research.

Phosphate scouting by root tips

Chemistry assigns phosphate (Pi) dominant roles in metabolism; however, it also renders the macronutrient a genuinely limiting factor of plant productivity. Pi bioavailability is restricted by low Pi mobility in soil and antagonized by metallic toxicities, which force roots to actively seek and selectively acquire the vital element. During the past few years, a first conceptual outline has emerged of the sensory mechanisms at root tips, which monitor external Pi and transmit the edaphic cue to inform root development. This review highlights new aspects of the Pi acquisition strategy of Arabidopsis roots, as well as a framework of local Pi sensing in the context of antagonistic interactions between Pi and its major associated metallic cations, Fe³⁺ and Al³⁺.

In Search of Enzymes with a Role in 3', 5'-Cyclic Guanosine Monophosphate Metabolism in Plants

In plants, nitric oxide (NO)-mediated 3', 5'-cyclic guanosine monophosphate (cGMP) synthesis plays an important role during pathogenic stress response, stomata closure upon osmotic stress, the development of adventitious roots and transcript regulation. The NO-cGMP dependent pathway is well characterized in mammals. The binding of NO to soluble guanylate cyclase enzymes (GCs) initiates the synthesis of cGMP from guanosine triphosphate. The produced cGMP alters various cellular responses, such as the function of protein kinase activity, cyclic nucleotide gated ion channels and cGMP-regulated phosphodiesterases. The signal generated by the second messenger is terminated by 3', 5'-cyclic nucleotide phosphodiesterase (PDEs) enzymes that hydrolyze cGMP to a non-cyclic 5'-guanosine monophosphate. To date, no homologues of mammalian cGMP-synthesizing and degrading enzymes have been found in higher plants. In the last decade, six receptor proteins from Arabidopsis thaliana have been reported to have guanylate cyclase activity in vitro. Of the six receptors, one was shown to be a NO dependent guanylate cyclase enzyme (NOGC1). However, the role of these proteins in planta remains to be elucidated. Enzymes involved in the degradation of cGMP remain elusive, albeit, PDE activity has been detected in crude protein extracts from various plants. Additionally, several research groups have partially purified and characterized PDE enzymatic activity from crude protein extracts. In this review, we focus on presenting advances toward the identification of enzymes involved in the cGMP metabolism pathway in higher plants.

Molecular Mechanisms of Phosphorus Metabolism and Transport during Leaf Senescence

Leaf senescence, being the final developmental stage of the leaf, signifies the transition from a mature, photosynthetically active organ to the attenuation of said function and eventual death of the leaf. During senescence, essential nutrients sequestered in the leaf, such as phosphorus (P), are mobilized and transported to sink tissues, particularly expanding leaves and developing seeds. Phosphorus recycling is crucial, as it helps to ensure that previously acquired P is not lost to the environment, particularly under the naturally occurring condition where most unfertilized soils contain low levels of soluble orthophosphate (Pi), the only form of P that roots can directly assimilate from the soil. Piecing together the molecular mechanisms that underpin the highly variable efficiencies of P remobilization from senescing leaves by different plant species may be critical for devising effective strategies for improving overall crop P-use efficiency. Maximizing Pi remobilization from senescing leaves using selective breeding and/or biotechnological strategies will help to generate P-efficient crops that would minimize the use of unsustainable and polluting Pi-containing fertilizers in agriculture. This review focuses on the molecular mechanisms whereby P is remobilized from senescing leaves and transported to sink tissues, which encompasses the action of hormones, transcription factors, Pi-scavenging enzymes, and Pi transporters.

NnSR1, a class III non-S-RNase specifically induced in Nicotiana alata under phosphate deficiency, is localized in endoplasmic reticulum compartments

A combined strategy of phosphate (Pi) remobilization from internal and external RNA sources seems to be conserved in plants exposed to Pi starvation. Thus far, the only ribonucleases (RNases) reported to be induced in Nicotiana alata undergoing Pi deprivation are extracellular S-like RNase NE and NnSR1. NnSR1 is a class III non S-RNase of unknown subcellular location. Here, we examine the hypothesis that NnSR1 is an intracellular RNase derived from the self-incompatibility system with specific expression in self-incompatible Nicotiana alata. NnSR1 was not induced in self-compatible Nicotiana species exposed to Pi deprivation. NnSR1 conjugated with a fluorescent protein and transiently expressed in Arabidopsis protoplasts and Nicotiana leaves showed that the fusion protein co-localized with an endoplasmic reticulum (ER) marker. Subcellular fractionation by ultracentrifugation of roots exposed to Pi deprivation revealed that the native NnSR1 migrated in parallel with the BiP protein, a typical ER marker. To our knowledge, NnSR1 is the first class III RNase reported to be localized in ER compartments. The induction of NnSR1 was detected earlier than the extracellular RNase NE, suggesting that intracellular RNA may be the first source of Pi used by the cell under Pi stress.

NnSR1, a class III non-S-RNase constitutively expressed in styles, is induced in roots and stems under phosphate deficiency in Nicotiana alata

BACKGROUND AND AIMS

Non-S-ribonucleases (non-S-RNases) are class III T2 RNases constitutively expressed in styles of species with S-RNase-based self-incompatibility. So far, no function has been attributed to these RNases. The aim of this work is to examine if NnSR1, a non-S-RNase from Nicotiana alata, is induced under conditions of phosphate (Pi) deprivation. The hypothesis is that under Pi-limited conditions, non-S-RNase functions may resemble the role of S-like RNases. To date, the only RNases reported to be induced by Pi deficiency are class I and class II S-like RNases, which are phylogenetically different from the class III clade of RNases.

METHODS

Gene and protein expression of NnSR1 were assayed in plants grown hydroponically with and without Pi, by combining RT-PCR, immunoblot and enzymatic activity approaches.

KEY RESULTS

NnSR1 transcripts were detected in roots 7 d after Pi deprivation and remained stable for several days. Transcript expression was correlated based on Pi availability in the culture medium. Antiserum against a peptide based on a hypervariable domain of NnSR1 recognized NnSR1 in roots and stems but not leaves exposed to Pi shortage. NnSR1 was not detected in culture medium and was pelleted with the microsomal fraction, suggesting that it was membrane-associated or included in large compartments. The anti-NnSR1 inhibited selectively the enzymatic activity of a 31-kDa RNase indicating that NnSR1 was induced in an enzymatically active form.

CONCLUSIONS

The induction of NnSR1 indicates that there is a general recruitment of all classes of T2 RNases in response to Pi shortage. NnSR1 appears to have regained ancestral functions of class III RNases related to strategies to cope with Pi limitation and also possibly with other environmental challenges. This constitutes the first report for a specific function of class III RNases other than S-RNases.

The secreted purple acid phosphatase isozymes AtPAP12 and AtPAP26 play a pivotal role in extracellular phosphate-scavenging by Arabidopsis thaliana

Orthophosphate (P(i)) is an essential but limiting macronutrient for plant growth. Extensive soil P reserves exist in the form of organic P (P(o)), which is unavailable for root uptake until hydrolysed by secretory acid phosphatases (APases). The predominant purple APase (PAP) isozymes secreted by roots of P(i)-deficient (-P(i)) Arabidopsis thaliana were recently identified as AtPAP12 (At2g27190) and AtPAP26 (At5g34850). The present study demonstrated that exogenous P(o) compounds such as glycerol-3-phosphate or herring sperm DNA: (i) effectively substituted for P(i) in supporting the P nutrition of Arabidopsis seedlings, and (ii) caused upregulation and secretion of AtPAP12 and AtPAP26 into the growth medium. When cultivated under -P(i) conditions or supplied with P(o) as its sole source of P nutrition, an atpap26/atpap12 T-DNA double insertion mutant exhibited impaired growth coupled with >60 and >30% decreases in root secretory APase activity and rosette total P(i) concentration, respectively. Development of the atpap12/atpap26 mutant was unaffected during growth on P(i)-replete medium but was completely arrested when 7-day-old P(i)-sufficient seedlings were transplanted into a -P(i), P(o)-containing soil mix. Both PAPs were also strongly upregulated on root surfaces and in shoot cell-wall extracts of -P(i) seedlings. It is hypothesized that secreted AtPAP12 and AtPAP26 facilitate the acclimation of Arabidopsis to nutritional Pi deficiency by: (i) functioning in the rhizosphere to scavenge P(i) from the soil's accessible P(o) pool, while (ii) recycling P(i) from endogenous phosphomonoesters that have been leaked into cell walls from the cytoplasm. Thus, AtPAP12 and AtPAP26 are promising targets for improving crop P-use efficiency.

The Arabidopsis thaliana phosphate starvation responsive gene AtPPsPase1 encodes a novel type of inorganic pyrophosphatase

BACKGROUND

Low inorganic phosphate (Pi) availability triggers metabolic responses to maintain the intracellular phosphate homeostasis in plants. One crucial adaptive mechanism is the immediate cleavage of Pi from phosphorylated substrates; however, phosphohydrolases that function in the cytosol and putative substrates have not been characterized yet. One candidate gene is Arabidopsis thaliana At1g73010 encoding an uncharacterized enzyme with homology to the haloacid dehalogenase (HAD) superfamily.

METHODS AND RESULTS

This work reports the molecular cloning of At1g73010, its expression in Escherichia coli, and the enzymatic characterisation of the recombinant protein (33.5 kD). The Mg²(+)-dependent enzyme named AtPPsPase1 catalyzes the specific cleavage of pyrophosphate (K(m) 38.8 μM) with an alkaline catalytic pH optimum. Gel filtration revealed a tetrameric structure of the soluble cytoplasmic protein. Modelling of the active site and assay of the recombinant protein variant D19A demonstrated that the enzyme shares the catalytic mechanism of the HAD superfamily including a phosphorylated enzyme intermediate.

CONCLUSIONS

The tight control of AtPPsPase1 gene expression underlines its important role in the Pi starvation response and suggests that cleavage of pyrophosphate is an immediate metabolic adaptation reaction.

GENERAL SIGNIFICANCE

The novel enzyme, the first pyrophosphatase in the HAD superfamily, differs from classical pyrophosphatases with respect to structure and catalytic mechanism. The enzyme function could be used to discover unknown aspects of pyrophosphate metabolism in general.

Biochemical and molecular characterization of AtPAP12 and AtPAP26: the predominant purple acid phosphatase isozymes secreted by phosphate-starved Arabidopsis thaliana

Plant purple acid phosphatases (PAPs) belong to a large multigene family whose specific functions in Pi metabolism are poorly understood. Two PAP isozymes secreted by Pi-deficient (-Pi) Arabidopsis thaliana were purified from culture filtrates of -Pi suspension cells. They correspond to an AtPAP12 (At2g27190) homodimer and AtPAP26 (At5g34850) monomer composed of glycosylated 60 and 55 kDa subunit(s), respectively. Each PAP exhibited broad pH activity profiles centred at pH 5.6, and overlapping substrate specificities. Concanavalin-A chromatography resolved a pair of secreted AtPAP26 glycoforms. AtPAP26 is dual targeted during Pi stress because it is also the principal intracellular (vacuolar) PAP up-regulated by -Pi Arabidopsis. Differential glycosylation appears to influence the subcellular targeting and substrate selectivity of AtPAP26. The significant increase in secreted acid phosphatase activity of -Pi seedlings was correlated with the appearance of immunoreactive AtPAP12 and AtPAP26 polypeptides. Analysis of atpap12 and atpap26 T-DNA mutants verified that AtPAP12 and AtPAP26 account for most of the secreted acid phosphatase activity of -Pi wild-type seedlings. Semi-quantitative RT-PCR confirmed that transcriptional controls exert little influence on the up-regulation of AtPAP26 during Pi stress, whereas AtPAP12 transcripts correlate well with relative levels of secreted AtPAP12 polypeptides. We hypothesize that AtPAP12 and AtPAP26 facilitate Pi scavenging from soil-localized organophosphates during nutritional Pi deprivation.

DNA is taken up by root hairs and pollen, and stimulates root and pollen tube growth

Phosphorus (P) enters roots as inorganic phosphate (P(i)) derived from organic and inorganic P compounds in the soil. Nucleic acids can support plant growth as the sole source of P in axenic culture but are thought to be converted into P(i) by plant-derived nucleases and phosphatases prior to uptake. Here, we show that a nuclease-resistant analog of DNA is taken up by plant cells. Fluorescently labeled S-DNA of 25 bp, which is protected against enzymatic breakdown by its phosphorothioate backbone, was taken up and detected in root cells including root hairs and pollen tubes. These results indicate that current views of plant P acquisition may have to be revised to include uptake of DNA into cells. We further show that addition of DNA to P(i)-containing growth medium enhanced the growth of lateral roots and root hairs even though plants were P replete and had similar biomass as plants supplied with P(i) only. Exogenously supplied DNA increased length growth of pollen tubes, which were studied because they have similar elongated and polarized growth as root hairs. Our results indicate that DNA is not only taken up and used as a P source by plants, but ironically and independent of P(i) supply, DNA also induces morphological changes in roots similar to those observed with P limitation. This study provides, to our knowledge, first evidence that exogenous DNA could act nonspecifically as signaling molecules for root development.

T2 Family ribonucleases: ancient enzymes with diverse roles

Ribonucleases of the T2 family are found in the genomes of protozoans, plants, bacteria, animals and viruses. A broad range of biological roles for these ribonucleases have been suggested, including scavenging of nucleic acids, degradation of self-RNA, serving as extra- or intracellular cytotoxins, and modulating host immune responses. Recently, RNaseT2 family members have been implicated in human pathologies such as cancer and parasitic diseases. Interestingly, certain functions of RNaseT2 family members are independent of their nuclease activity, suggesting that these proteins have additional functions. Moreover, humans lacking RNASET2 manifest a defect in neurological development, perhaps due to aberrant control of the immune system. We review the basic structure and function of RNaseT2 family members and their biological roles.

Kinetic metabolic modelling for the control of plant cells cytoplasmic phosphate

A previously developed kinetic metabolic model for plant metabolism was used in a context of identification and control of intracellular phosphate (Pi) dynamics. Experimental data from batch flask cultures of Eschscholtiza californica cells was used to calibrate the model parameters for the slow dynamics (growth, nutrition, anabolic pathways, etc.). Perturbation experiments were performed using a perfusion small-scale bioreactor monitored by in vivo(31)P NMR. Parameter identification for Pi metabolism was done by measuring the cells dynamic response to different inputs for extracellular Pi (two pulse-response experiments and a step-response experiment). The calibrated model can describe Pi translocation between the cellular pools (vacuole and cytoplasm). The effect of intracellular Pi management on ATP/ADP and phosphomonoesters concentrations is also described by the model. The calibrated model is then used to develop a control strategy on the cytoplasmic Pi pool. From the identification of the systems dynamics, a proportional-integral controller was designed and tuned. The closed-loop control was implemented in the small-scale NMR bioreactor and experimental results were in accordance with model predictions. Thus, the calibrated model is able to predict cellular behaviour for phosphate metabolism and it was demonstrated that it is possible to control the intracellular level of cytoplasmic Pi in plant cells.

Increased expression of the MYB-related transcription factor, PHR1, leads to enhanced phosphate uptake in Arabidopsis thaliana

Plants have evolved a number of adaptive strategies to cope with fluctuations in phosphorus (P) supply. The current knowledge of the transcriptional regulation of the P-starvation response in plants is limited. However, one MYB-related transcription factor, PHR1, is known to be involved in the P-starvation response. In this paper, we characterize a T-tagged phr1 knockout mutant and a series of transgenic plant lines which over-express PHR1 in wild type (WT) and phr1 mutant background. The knockout mutant has an altered phosphate (Pi) allocation between root and shoot; accumulates less anthocyanins, sugars and starch than P-starved WT; has a lower AGPase activity; and is impaired in induction of a subset of Pi starvation-induced genes. Expression of PHR1 in the phr1 mutant rescues the responsiveness to P-starvation and leads to WT levels of sugars and starch during Pi starvation conditions, confirming the involvement of PHR1 in adjusting carbon metabolism. Over-expression of PHR1 further resulted in a dramatic increase in the microRNA miR399d, and this resulted in changes in the transcript level for the target gene PHO2. Furthermore, over-expression of PHR1 in both WT and phr1 mutant results in strongly increased content of Pi irrespective of P regime. This shows that targeting a key regulatory element in the Pi starvation regulatory network represents a useful approach for molecular breeding of plants towards more efficient Pi uptake and assimilation.

Extracellular Matrix Proteome of Chickpea (Cicer arietinum L.) Illustrates Pathway Abundance, Novel Protein Functions and Evolutionary Perspect

The extracellular matrix (ECM) or cell wall is a dynamic system and serves as the first line mediator in cell signaling to perceive and transmit extra- and intercellular signals in many pathways. Although ECM is a conserved compartment ubiquitously present throughout evolution, a compositional variation does exist among different organisms. ECM proteins account for 10% of the ECM mass, however, comprise several hundreds of different molecules with diverse functions. To understand the function of ECM proteins, we have developed the cell wall proteome of a crop legume, chickpea (Cicer arietinum). This comprehensive overview of the proteome would provide a basis for future comparative proteomic efforts for this important crop. Proteomic analyses revealed new ECM proteins of unknown functions vis-à-vis the presence of many known cell wall proteins. In addition, we report here evidence for the presence of unexpected proteins with known biochemical activities, which have never been associated with ECM.

Loss of At4 function impacts phosphate distribution between the roots and the shoots during phosphate starvation

Plants display a range of adaptive responses to phosphate (Pi) starvation including an increase in the proportion of Pi allocated to the roots, which enhances lateral root development and consequently Pi acquisition. The mechanisms by which plants sense Pi and signal Pi reallocation are largely unknown. Previously, we cloned At4, a gene predicted to contain multiple short open-reading frames (ORFs), whose expression is strongly induced by Pi starvation. At4 is a member of a small gene family whose members, AtIPS1 and two additional genes reported here, At4.1 and At4.2, share little conservation among the predicted ORFs but high conservation of a 22-nt sequence located in the 3' half of the transcript. Here, we show that under Pi-starvation conditions, At4 is expressed in the vascular tissue and transcript levels are regulated by both cytokinin and ABA. at4, an At4 loss-of-function mutant fails to redistribute Pi to the roots correctly in response to Pi deprivation and At4 shoots continue to accumulate a greater proportion of Pi relative to wild type. Consistent with this, the primary root growth rate in at4 is faster than wild type in low-Pi conditions. The conserved sequence found in all members of the At4 gene family hybridizes to a small RNA present in Pi-starved roots. These data support a role for At4 in the internal allocation of Pi and suggest that the At4 gene is not only subject to Pi-starvation-inducible expression, but that transcript levels may be adjusted at a post-transcriptional level by the activity of an miRNA.

Effect of long-term phosphate starvation on the levels and metabolism of purine nucleotides in suspension-cultured Catharanthus roseus cells

The effect of long-term phosphate (Pi) starvation of up to 3 weeks on the levels of purine nucleotides and related compounds was examined using suspension-cultured Catharanthus roseus cells. Levels of adenine and guanine nucleotides, especially ATP and GTP, were markedly reduced during Pi-starvation. There was an increase in the activity of RNase, DNase, 5'- and 3'-nucleotidases and acid phosphatase, which may participate in the hydrolysis of nucleic acids and nucleotides. Accumulation of adenosine, adenine, guanosine and guanine was observed during the long-term Pi starvation. Long-term Pi starvation markedly depressed the flux of transport of exogenously supplied [8-(14)C]adenosine and [8-(14)C]adenine, but these labelled compounds which were taken up by the cells were readily converted to adenine nucleotides even in Pi-starved cells, in which RNA synthesis from these precursors was significantly reduced. The activities of adenosine kinase, adenine phosphoribosyltransferase and adenosine nucleosidase were maintained at a high level in long-term Pi starved cells.

Phosphatase activity in temperate pasture soils: Potential regulation of labile organic phosphorus turnover by phosphodiesterase activity

Phosphatase enzymes regulate organic phosphorus (P) turnover in soil, but a clear understanding remains elusive. To investigate this, phosphomonoesterase and phosphodiesterase activities were determined by using para-nitrophenol (pNP) analogue substrates in a range of temperate pasture soils from England and Wales. Substrate-induced phosphatase activity ranged between 2.62 and 12.19 micromol pNP g-1 soil h-1 for phosphomonoesterase and between 0.25 and 2.24 micromol pNP g-1 soil h-1 for phosphodiesterase. Activities were correlated strongly with soil pH and labile organic P extracted in sodium bicarbonate, although the relationships differed markedly for the two enzymes. Acidic soils contained high phosphomonoesterase activity, low phosphodiesterase activity, and high concentrations of labile organic P, whereas the reverse was true in more neutral soils. As most of the organic P inputs to soil are phosphate diesters, it therefore seems likely that phosphodiesterase activity regulates labile organic P turnover in pasture soils. The low phosphodiesterase activity in acidic soils may be linked to the dominance of fungi or an effect of sorption on the enzyme. These results suggest that greater emphasis should be placed on understanding the role of phosphodiesterase activity in the cycling of soil organic P.

Effects of phosphate limitation on expression of genes involved in pyrimidine synthesis and salvaging in Arabidopsis

Arabidopsis seedlings grown for 14 d without phosphate (P) exhibited stunted growth and other visible symptoms associated with P deficiency. RNA contents in shoots decreased nearly 90%, relative to controls. In shoots, expression of Pht1;2, encoding an inducible high-affinity phosphate transporter, increased threefold, compared with controls, and served as a molecular marker for P limitation. Transcript levels for five enzymes (aspartate transcarbamoylase, ATCase, EC 2.1.3.2; carbamoyl phosphate synthetase, CPSase, EC 6.3.5.5); UMP synthase, EC 2.4.1.10, EC 4.1.1.23; uracil phosphoribosyltransferase, UPRTase, EC 2.4.2.9; UMP kinase, EC 2.7.1.14) increased 2-10-fold in response to P starvation in shoots. These enzymes, which utilize phosphorylated intermediates at putative regulated steps in de novo synthesis and salvaging pathways leading to UMP and pyrimidine nucleotide formation, appear to be coordinately regulated, at the level of gene expression. This response may facilitate pyrimidine nucleotide synthesis under P limitation in this plant. Expression of P-dependent and P-independent phosphoribosyl pyrophosphate (PRPP) synthases (PRS2 and PRS3, respectively) which provide PRPP, the phosphoribosyl donor in UMP synthesis via both de novo and salvaging pathways, was differentially regulated in response to P limitation. PRS2 mRNA levels increased twofold in roots and shoots of P-starved plants, while PRS3 was constitutively-expressed. PRS3 may play a novel role in providing PRPP to cellular metabolism under low P availability.

Identification and characterization of plant glycerophosphodiester phosphodiesterase

GPX-PDE (glycerophosphodiester phosphodiesterase; EC 3.1.4.46) is a relatively poorly characterized enzyme that catalyses the hydrolysis of various glycerophosphodiesters (glycerophosphocholine, glycerophosphoethanolamine, glycerophosphoglycerol, glycerophosphoserine and bis-glycerophosphoglycerol), releasing sn-glycerol 3-phosphate and the corresponding alcohol. In a previous study, we demonstrated the existence of a novel GPX-PDE in the cell walls and vacuoles of plant cells. Since no GPX-PDE had been identified in any plant organism, the purification of GPX-PDE from carrot cell walls was attempted. After extraction of cell wall proteins from carrot cell suspension cultures with CaCl2, GPX-PDE was purified up to 2700-fold using, successively, ammonium sulphate precipitation, gel filtration and concanavalin A-Sepharose. Internal sequence analysis of a 55 kDa protein identified in the extract following 2700-fold purification revealed strong similarity to the primary sequence of GLPQ, a bacterial GPX-PDE. To confirm the identity of plant GPX-PDE, an Arabidopsis thaliana cDNA similar to that encoding the bacterial GPX-PDE was cloned and overexpressed in a bacterial expression system, and was used to raise antibodies against the putative Arabidopsis thaliana GPX-PDE. Immunochemical assays performed on carrot cell wall proteins extracted by CaCl2 treatment showed a strong correlation between GPX-PDE activity and detection of the 55 kDa protein, validating the identity of the plant GPX-PDE. Finally, various properties of the purified enzyme were investigated. GPX-PDE is a multimeric enzyme, specific for glycerophosphodiesters, exhibiting a K(m) of 36 microM for glycerophosphocholine and active within a wide pH range (from 4 to 10). Since these properties are similar to those of GLPQ, the bacterial GPX-PDE, the similarities between plant and bacterial enzymes are also discussed.

Phosphate transport in Arabidopsis: Pht1;1 and Pht1;4 play a major role in phosphate acquisition from both low- and high-phosphate environments

Of the mineral nutrients essential for plant growth, phosphorus plays the widest diversity of roles and a lack of phosphorus has profound effects on cellular metabolism. At least eight members of the Arabidopsis Pht1 phosphate (Pi) transporter family are expressed in roots and Pht1;1 and Pht1;4 show the highest transcript levels. The spatial and temporal expression patterns of these two genes show extensive overlap. To elucidate the in planta roles of Pht1;1 and Pht1;4, we identified loss-of-function mutants and also created a double mutant, lacking both Pht1;1 and Pht1;4. Consistent with their spatial expression patterns, membrane location and designation as high-affinity Pi transporters, Pht1;1 and Pht1;4 contribute to Pi transport in roots during growth under low-Pi conditions. In addition, during growth under high-Pi conditions, the double mutant shows a 75% reduction in Pi uptake capacity relative to wildtype. Thus, Pht1;1 and Pht1;4 play significant roles in Pi acquisition from both low- and high-Pi environments.

Phosphate or phosphite addition promotes the proteolytic turnover of phosphate-starvation inducible tomato purple acid phosphatase isozymes

Within 48 h of the addition of 2.5 mM phosphate (HPO42-, Pi) or phosphite (H2PO3-, Phi) to 8-day-old Pi-starved (-Pi) tomato suspension cells: (i) secreted and intracellular purple acid phosphatase (PAP) activities decreased by about 12- and 6-fold, respectively and (ii) immunoreactive PAP polypeptides either disappeared (secreted PAPs) or were substantially reduced (intracellular PAP). The degradation of both secreted PAP isozymes was correlated with the de novo synthesis of two extracellular serine proteases having M(r)s of 137 and 121 kDa. In vitro proteolysis of purified secreted tomato PAP isozymes occurred following their 24 h incubation with culture filtrate from Pi-resupplied cells. The results indicate that Pi or Phi addition to -Pi tomato cells induces serine proteases that degrade Pi-starvation inducible extracellular proteins.

Influence of ozone on ribonuclease activity in wheat (Triticum aestivum) leaves

Ribonucleases (RNases) degrade RNA and exert a major influence on gene expression during development and in response to biotic and abiotic stresses. RNase activity typically increases in response to pathogen attack, wounding and phosphate (P(i)) deficiency. Activity also increases during senescence and other programmed cell death processes. The air pollutant ozone (O(3)) often induces injury and accelerated senescence in many plants, but the biochemical mechanisms involved in these responses remain unclear. The objective of this study was to determine whether RNase activity and isozyme expression was stimulated in wheat (Triticum aestivum L.) flag leaves following treatment with O(3). Plants were treated in open-top chambers with charcoal-filtered air (27 nmol O(3) mol(-1)) (control) or non-filtered air plus O(3) (90 nmol O(3) mol(-1)) (O(3)) from seedling to reproductive stage. After exposure for 56 days, RNase activity was 2.1 times higher in flag leaf tissues from an O(3)-sensitive cultivar in the O(3) treatment compared with the control, which generally coincided with foliar injury and lower soluble protein concentration, but not soluble leaf [P(i)]. Soluble [P(i)] in leaf tissue extracts from the O(3) and control treatments was not significantly different. RNase activity gels indicated the presence of three major RNases and two nucleases, and their expression was enhanced by the O(3) treatment. Isozymes stimulated in the O(3) treatment were also stimulated in naturally senescent flag leaf tissues from plants in the control. However, soluble [P(i)] in extracts from naturally senescent flag leaves was 50% lower than that found in green flag leaves in the control treatment. Thus, senescence-like pathological responses induced by O(3) were accompanied by increased RNase and nuclease activities that also were observed in naturally senescent leaves. However, [P(i)] in the leaf tissue samples suggested that O(3)-induced injury and accelerated senescence was atypical of normal senescence processes in that P(i) export was not observed in O(3)-treated plants.

Phosphate starvation triggers distinct alterations of genome expression in Arabidopsis roots and leaves

Arabidopsis genome expression pattern changes in response to phosphate (Pi) starvation were examined during a 3-d period after removal of Pi from the growth medium. Available Pi concentration was decreased after the first 24 h of Pi starvation in roots by about 22%, followed by a slow recovery during the 2nd and 3rd d after Pi starvation, but no significant change was observed in leaves within the 3 d of Pi starvation. Microarray analysis revealed that more than 1,800 of the 6,172 genes present in the array were regulated by 2-fold or more within 72 h from the onset of Pi starvation. Analysis of these Pi starvation-responsive genes shows that they belong to wide range of functional categories. Many genes for photosynthesis and nitrogen assimilation were down-regulated. A complex set of metabolic adaptations appears to occur during Pi starvation. More than 100 genes each for transcription factors and cell-signaling proteins were regulated in response to Pi starvation, implying major regulatory changes in cellular growth and development. A significant fraction of those regulatory genes exhibited distinct or even contrasting expression in leaves and roots in response to Pi starvation, supporting the idea that distinct Pi starvation response strategies are used for different plant organs in response to a shortage of Pi in the growth medium.

Differential expression of the LePS2 phosphatase gene family in response to phosphate availability, pathogen infection and during development

In this study, we report the cloning of the three-member LePS2 gene family of acid phosphatases via subtractive screening of a cDNA library of Pi-starved cultivated tomato cells (Lycopersicon esculentum Mill. cv. Lukullus). As members of the plant Pi-starvation response, LePS2 genes were tightly regulated in cultivated cells and tomato seedlings by Pi availability. The LePS2 enzymes which are most likely expressed in the cytoplasma could be involved in processes that are accompanied by degradation of phosphorylated organic substrates. Independently from exogenous phosphate supply LePS2 expression was detected in tomato endosperm during germination. LePS2 genes were differentially induced after infection with the bacterial pathogen Pseudomonas syringae and in the early stages of flower development. Using RT-PCR it was found that the gene LePS2B was the most abundant transcript in phosphate-depleted cells, but a reduced expression was determined in floral buds and it was not found during pathogen interaction. In this respect, it is interesting that the promoter sequences of the LePS2 genes are also divergent. LePS2 gene products may have functions in developmental processes which are restricted to distinct plant tissues or cell types.

Purification and characterization of two secreted purple acid phosphatase isozymes from phosphate-starved tomato (Lycopersicon esculentum) cell cultures

Two secreted acid phosphatases (SAP1 and SAP2) were markedly up-regulated during Pi-starvation of tomato suspension cells. SAP1 and SAP2 were resolved during cation-exchange FPLC of culture media proteins from 8-day-old Pi-starved cells, and purified to homogeneity and final p-nitrophenylphosphate hydrolyzing specific activities of 246 and 940 micro mol Pi produced.min-1 mg.protein-1, respectively. SDS/PAGE, periodic acid-Schiff staining and analytical gel filtration demonstrated that SAP1 and SAP2, respectively, exist as 84 and 57 kDa glycosylated monomers. SAP1 and SAP2 are purple acid phosphatases (PAPs) as they displayed an absorption maximum at 518 and 538 nm, respectively, and were not inhibited by l-tartrate. The respective sequence of a SAP1 and SAP2 tryptic peptide was very similar to a portion of the deduced sequence of several putative Arabidopsis thaliana PAPs. CNBr peptide mapping indicated that SAP1 and SAP2 are structurally distinct. Both isozymes displayed a pH optimum of approximately pH 5.3 and were heat stable. Although they exhibited wide substrate specificities, the Vmax of SAP2 with various phosphate-esters was significantly greater than that of SAP1. SAP1 and SAP2 were activated by up to 80% by 5 mm Mg2+, and demonstrated potent competitive inhibition by molybdate, but mixed and competitive inhibition by Pi, respectively. Interestingly, both SAPs exhibited significant peroxidase activity, which was optimal at approximately pH 8.4 and insensitive to Mg2+ or molybdate. This suggests that SAP1 and SAP2 may be multifunctional proteins that operate: (a) PAPs that scavenge Pi from extracellular phosphate-esters during Pi deprivation, or (b) alkaline peroxidases that participate in the production of extracellular reactive oxygen species during the oxidative burst associated with the defense response of plants to pathogen infection.

Glycerophosphocholine metabolism in higher plant cells. Evidence of a new glyceryl-phosphodiester phosphodiesterase

Glycerophosphocholine (GroPCho) is a diester that accumulates in different physiological processes leading to phospholipid remodeling. However, very little is known about its metabolism in higher plant cells. (31)P-Nuclear magnetic resonance spectroscopy and biochemical analyses performed on carrot (Daucus carota) cells fed with GroPCho revealed the existence of an extracellular GroPCho phosphodiesterase. This enzymatic activity splits GroPCho into sn-glycerol-3-phosphate and free choline. In vivo, sn-glycerol-3-phosphate is further hydrolyzed into glycerol and inorganic phosphate by acid phosphatase. We visualized the incorporation and the compartmentation of choline and observed that the major choline pool was phosphorylated and accumulated in the cytosol, whereas a minor fraction was incorporated in the vacuole as free choline. Isolation of plasma membranes, culture medium, and cell wall proteins enabled us to localize this phosphodiesterase activity on the cell wall. We also report the existence of an intracellular glycerophosphodiesterase. This second activity is localized in the vacuole and hydrolyzes GroPCho in a similar fashion to the cell wall phosphodiesterase. Both extra- and intracellular phosphodiesterases are widespread among different plant species and are often enhanced during phosphate deprivation. Finally, competition experiments on the extracellular phosphodiesterase suggested a specificity for glycerophosphodiesters (apparent K(m) of 50 microM), which distinguishes it from other phosphodiesterases previously described in the literature.

Purple acid phosphatases of Arabidopsis thaliana. Comparative analysis and differential regulation by phosphate deprivation

Purple acid phosphatases (PAPs) are members of the metallo-phosphoesterase family. They are characterized by the presence of seven conserved amino acid residues involved in coordinating the dimetal nuclear center in their reactive site. We compared the 29 PAPs predicted for Arabidopsis thaliana in their varieties of potential metal-ligating residues. Although 24 members possessed sets of metal-ligating residues typical of known PAPs, 1 member lacked four of the seven residues. For the remaining four members, potential metal-ligating residues were generally more similar to those in metal-dependent exonucleases and related proteins. Evidence was obtained for the expression of the majority of the 29 PAPs. To facilitate future investigations, a scheme for naming Arabidopsis PAPs and a system for classifying the 29 PAPs are proposed. The cDNA sequences and the responses to phosphate deprivation of seven Arabidopsis PAPs (AtPAP7-AtPAP13) were characterized. For some AtPAPs analyzed, there were fully processed transcripts as well as splice variants. The splice variants of AtPAP10 were found to associate with polyribosomes and may be translated into a NH(2)-terminal truncated protein. Phylogenetic investigations showed that AtPAPs 7 and 8, together with similar enzymes from other plant species, formed the low molecular weight plant PAP group. Members of this group were more closely related to PAPs from mammalian cells. AtPAPs 9-13, together with kidney bean PAP, formed the high molecular weight PAP group. In phosphate deprivation experiments, gene transcription of AtPAP11 and AtPAP12 was induced and increased, respectively, whereas that of the remaining five AtPAPs was not affected by phosphate deprivation. The present work demonstrates that structure variation and expression regulation of plant PAPs are more complex than previously described and provides a framework for comprehensive molecular genetic and biochemical studies of all Arabidopsis PAPs in the future.

Phosphate sensing in higher plants

Phosphate (Pi) plays a central role as reactant and effector molecule in plant cell metabolism. However, Pi is the least accessible macronutrient in many ecosystems and its low availability often limits plant growth. Plants have evolved an array of molecular and morphological adaptations to cope with Pi limitation, which include dramatic changes in gene expression and root development to facilitate Pi acquisition and recycling. Although physiological responses to Pi starvation have been increasingly studied and understood, the initial molecular events that monitor and transmit information on external and internal Pi status remain to be elucidated in plants. This review summarizes molecular and developmental Pi starvation responses of higher plants and the evidence for coordinated regulation of gene expression, followed by a discussion of the potential involvement of plant hormones in Pi sensing and of molecular genetic approaches to elucidate plant signalling of low Pi availability. Complementary genetic strategies in Arabidopsis thaliana have been developed that are expected to identify components of plant signal transduction pathways involved in Pi sensing. Innovative screening methods utilize reporter gene constructs, conditional growth on organophosphates and the inhibitory properties of the Pi analogue phosphite, which hold the promise for significant advances in our understanding of the complex mechanisms by which plants regulate Pi-starvation responses.

Attenuation of phosphate starvation responses by phosphite in Arabidopsis

When inorganic phosphate is limiting, Arabidopsis has the facultative ability to metabolize exogenous nucleic acid substrates, which we utilized previously to identify insensitive phosphate starvation response mutants in a conditional genetic screen. In this study, we examined the effect of the phosphate analog, phosphite (Phi), on molecular and morphological responses to phosphate starvation. Phi significantly inhibited plant growth on phosphate-sufficient (2 mM) and nucleic acid-containing (2 mM phosphorus) media at concentrations higher than 2.5 mM. However, with respect to suppressing typical responses to phosphate limitation, Phi effects were very similar to those of phosphate. Phosphate starvation responses, which we examined and found to be almost identically affected by both anions, included changes in: (a) the root-to-shoot ratio; (b) root hair formation; (c) anthocyanin accumulation; (d) the activities of phosphate starvation-inducible nucleolytic enzymes, including ribonuclease, phosphodiesterase, and acid phosphatase; and (e) steady-state mRNA levels of phosphate starvation-inducible genes. It is important that induction of primary auxin response genes by indole-3-acetic acid in the presence of growth-inhibitory Phi concentrations suggests that Phi selectively inhibits phosphate starvation responses. Thus, the use of Phi may allow further dissection of phosphate signaling by genetic selection for constitutive phosphate starvation response mutants on media containing organophosphates as the only source of phosphorus.

Two isoforms of a nucleotide-sugar pyrophosphatase/phosphodiesterase from barley leaves (Hordeum vulgare L.) are distinct oligomers of HvGLP1, a germin-like protein

Two isoforms of ADPglucose pyrophosphatase/phosphodiesterase (AGPPase) have been characterized using barley leaves (Hordeum vulgare L.). Whilst one of the isoforms, designated as soluble AGPPase1 (SAGPPase1), is soluble in low ionic strength buffers, the other, SAGPPase2, is extractable using cell wall hydrolytic enzymes or high salt concentration solutions, thus indicating that it is adventitiously bound to the cell wall. Both AGPPase isoforms are highly resistant to SDS, this characteristic being utilized to purify them to homogeneity after zymographic detection of AGPPase activity in SDS-containing gels. N-terminal and internal amino acid sequencing analyses revealed that both SAGPPase1 and SAGPPase2 are distinct oligomers of the previously designated HvGLP1, which is a member of the ubiquitously distributed group of proteins of unknown function designated as germin-like proteins (GLPs).

PHOSPHATE ACQUISITION

Phosphorus is one of the major plant nutrients that is least available in the soil. Consequently, plants have developed numerous morphological, physiological, biochemical, and molecular adaptations to acquire phosphate (Pi). Enhanced ability to acquire Pi and altered gene expression are the hallmarks of plant adaptation to Pi deficiency. The intricate mechanisms involved in maintaining Pi homeostasis reflect the complexity of Pi acquisition and translocation in plants. Recent discoveries of multiple Pi transporters have opened up opportunities to study the molecular basis of Pi acquisition by plants. An increasing number of genes are now known to be activated under Pi starvation. Some of these genes may be involved in Pi acquisition, transfer, and signal transduction during Pi stress. This review provides an overview of plant adaptations leading to enhanced Pi acquisition, with special emphasis on recent developments in the molecular biology of Pi acquisition.