Transcriptional regulation and signaling in phosphorus starvation: what about legumes?

Genetic architecture of phosphorus use efficiency across diverse environmental conditions: insights from maize elite and landrace lines

Phosphorus is an essential nutrient for all crops. Thus, a better understanding of the genetic control of phosphorus use efficiency evident in physiological, developmental, and morphological traits and its environmental plasticity is required to establish the basis for maintaining or enhancing yield while making agriculture more sustainable. In this study, we utilized a diverse panel of maize (Zea mays L.), including 398 elite and landrace lines, phenotyped across three environments and two phosphorus fertilization treatments. We performed genome-wide association mapping for 13 traits, including phosphorus uptake and allocation, that showed a strong environment dependency in their expression. Our results highlight the complex genetic architecture of phosphorus use efficiency as well as the substantial differences between the evaluated genetic backgrounds. Despite harboring more of the identified quantitative trait loci, almost all of the favorable alleles from landraces were found to be present in at least one of the two elite heterotic groups. Notably, we also observed trait-specific genetic control even among biologically related characteristics, as well as a substantial plasticity of the genetic architecture of several traits in response to the environment and phosphorus fertilization. Collectively, our work illustrates the difficulties in improving phosphorus use efficiency, but also presents possible solutions for the future contribution of plant breeding to improve the phosphorus cycle.

Comprehensive evaluation of phosphate deficiency tolerance in common vetch germplasms and the adaption mechanism to phosphate deficiency

Common vetch (Vicia sativa L.) is widely planted as forage, green manure and food. Phosphate (Pi) deficiency is an important constraint for legume crop production. In this study, P-deficiency tolerance in 40 common vetch collections was evaluated under hydroponic condition. The collections were clustered into three groups based on the tolerance level. Physiological responses to P-deficiency in two tolerant collections (418 and 426) in comparison with one sensitive collection (415) were investigated. Greater growth inhibition was observed in sensitive collection compared with two tolerant collections, although the inorganic phosphorus (P) content in sensitive collection was higher than those in tolerant collections. The internal and external purple acid phosphatase activity in plants showed no significant difference between 418 and 415 under low phosphate condition. Transcriptomic analysis in the tolerant collection 426 in response to Pi starvation showed that many common adaptive strategies were applied and PHOSPHATE STARVATION RESPONSE (PHR)-related Pi signaling and transporter genes were altered. VsPHT1.2 had the highest expression level in root among all VsPHT1s, and it was remarkably upregulated after short time of P-deficiency treatment in tolerant collections compared with sensitive collection. In conclusion, common vetch response to P starvation by altering the expressions of core genes involved in Pi transport and signaling, and the elevated expression of VsPHT1.2 gene might contribute to higher Pi acquisition efficiency in P-deficiency tolerant collections.

Chickpea (Cicer arietinum) PHO1 family members function redundantly in Pi transport and root nodulation

Phosphorus (P), a macronutrient, plays key roles in plant growth, development, and yield. Phosphate (Pi) transporters (PHTs) and PHOSPHATE1 (PHO1) are central to Pi acquisition and distribution. Potentially, PHO1 is also involved in signal transduction under low P. The current study was designed to identify and functionally characterize the PHO1 gene family in chickpea (CaPHO1s). Five CaPHO1 genes were identified through a comprehensive genome-wide search. Phylogenetically, CaPHO1s formed two clades, and protein sequence analyses confirmed the presence of conserved domains. CaPHO1s are expressed in different plant organs including root nodules and are induced by Pi-limiting conditions. Functional complementation of atpho1 mutant with three CaPHO1 members, CaPHO1, CaPHO1;like, and CaPHO1;H1, independently demonstrated their role in root to shoot Pi transport, and their redundant functions. To further validate this, we raised independent RNA-interference (RNAi) lines of CaPHO1, CaPHO1;like, and CaPHO1;H1 along with triple mutant line in chickpea. While single gene RNAi lines behaved just like WT, triple knock-down RNAi lines (capho1/like/h1) showed reduced shoot growth and shoot Pi content. Lastly, we showed that CaPHO1s are involved in root nodule development and Pi content. Our findings suggest that CaPHO1 members function redundantly in root to shoot Pi export and root nodule development in chickpea.

Harnessing the Potential of Symbiotic Associations of Plants in Phosphate-Deficient Soil for Sustainable Agriculture

Many plants associate with arbuscular mycorrhizal (AM) fungi for nutrient acquisition, and most legumes also associate with nitrogen-fixing rhizobial bacteria for nitrogen acquisition. The association of plants with AM fungi and rhizobia depends on the perception of lipo-chitooligosaccharides (LCOs) produced by these micro-symbionts. Recent studies reveal that cereals can perceive LCOs better in soil deprived of phosphate (Pi) and nitrogen to activate symbiosis signaling and form efficient AM symbiosis. Nevertheless, the Pi deficiency in the soil hinders the symbiotic association of legumes with rhizobia, ultimately reducing nitrogen fixation. Here, we discuss a mechanistic overview of the factors regulating root nodule symbiosis under Pi-deficient conditions and further emphasize the possible ways to overcome this hurdle. Ignoring the low Pi problem not only can compromise the functionality of the nitrogen cycle by nitrogen fixation through legumes but can also put food security at risk globally. This review aims to bring the scientific community's attention toward the detrimental response of legumes toward Pi-deficient soil for the formation of root nodule symbiosis and hence reduced nitrogen fixation. In this review, we have highlighted the recent studies that have advanced our understanding of these critical areas and discussed some future directions. Furthermore, this review highlights the importance of communicating science with farmers and the agriculture community to fully harness the potential of the symbiotic association of plants in nutrient-deficient soil for sustainable agriculture.

Improving phosphorus acquisition efficiency through modification of root growth responses to phosphate starvation in legumes

Phosphorus (P) is one of the essential macronutrients for plant growth and development, and it is an integral part of the major organic components, including nucleic acids, proteins and phospholipids. Although total P is abundant in most soils, a large amount of P is not easily absorbed by plants. Inorganic phosphate (Pi) is the plant-available P, which is generally immobile and of low availability in soils. Hence, Pi starvation is a major constraint limiting plant growth and productivity. Enhancing plant P efficiency can be achieved by improving P acquisition efficiency (PAE) through modification of morpho-physiological and biochemical alteration in root traits that enable greater acquisition of external Pi from soils. Major advances have been made to dissect the mechanisms underlying plant adaptation to P deficiency, especially for legumes, which are considered important dietary sources for humans and livestock. This review aims to describe how legume root growth responds to Pi starvation, such as changes in the growth of primary root, lateral roots, root hairs and cluster roots. In particular, it summarizes the various strategies of legumes to confront P deficiency by regulating root traits that contribute towards improving PAE. Within these complex responses, a large number of Pi starvation-induced (PSI) genes and regulators involved in the developmental and biochemical alteration of root traits are highlighted. The involvement of key functional genes and regulators in remodeling root traits provides new opportunities for developing legume varieties with maximum PAE needed for regenerative agriculture.

Integrative analysis of the metabolome and transcriptome reveal the phosphate deficiency response pathways of alfalfa

Understanding the mechanisms underlying the responses to inorganic phosphate (Pi) deficiency in alfalfa will help enhance Pi acquisition efficiency and the sustainable use of phosphorous resources. Integrated global metabolomic and transcriptomic analyses of mid-vegetative alfalfa seedlings under 12-day Pi deficiency were conducted. Limited seedling growth were found, including 13.24%, 16.85% and 33.36% decreases in height, root length and photosynthesis, and a 24.10% increase in root-to-shoot ratio on day 12. A total of 322 and 448 differentially abundant metabolites and 1199 and 1061 differentially expressed genes were identified in roots and shoots. Increased (>3.68-fold) inorganic phosphate transporter 1;4 and SPX proteins levels in the roots (>2.15-fold) and shoots (>2.50-fold) were related to Pi absorption and translocation. The levels of phospholipids and Pi-binding carbohydrates and nucleosides were decreased, while those of phosphatases and pyrophosphatases in whole seedlings were induced under reduced Pi. In addition, nitrogen assimilation was affected by inhibiting high-affinity nitrate transporters (NRT2.1 and NRT3.1), and nitrate reductase. Increased delphinidin-3-glucoside might contribute to the gray-green leaves induced by Pi limitation. Stress-induced MYB, WRKY and ERF transcription factors were identified. The responses of alfalfa to Pi deficiency were summarized as local systemic signaling pathways, including root growth, stress-related responses consisting of enzymatic and nonenzymatic systems, and hormone signaling and systemic signaling pathways including Pi recycling and Pi sensing in the whole plant, as well as Pi recovery, and nitrate and metal absorption in the roots. This study provides important information on the molecular mechanism of the response to Pi deficiency in alfalfa.

Comparative transcriptomic of Stevia rebaudiana provides insight into rebaudioside D and rebaudioside M biosynthesis

Rebaudioside D (Reb D) and rebaudioside M (Reb M) are commercially important low/no-calorie natural sweeteners. However, they are present in a minor proportion of all steviol glycosides (SGs) in Stevia rebaudiana Bertoni (S. rebaudiana). Strain-dependent deviation in Reb D and Reb M biosynthesis is one key breach for breeding of S. rebaudiana, which has not been studied at the transcriptional level. Herein, five different S. rebaudiana varieties with distinct SGs contents, one cultivar having high stevioside content (HST), one cultivar having high Reb A content (HRA) and three cultivars having high Reb D and Reb M content (HDM₁, HDM₂, HDM₃), were selected for RNA-seq analysis. In total, 131,655 de novo assembled unigenes were found in the RNA-seq data. According to Reb D and Reb M content divergence of S. rebaudiana accessions, 2186 differentially expressed genes (DEGs) were selected as potential genes related to Reb D and Reb M biosynthesis. Weighted Gene Co-expression Network Analysis (WGCNA) was used to explore the genes associated with the Reb D and Reb M biosynthesis. The unigenes from the positively associated turquoise module formed a layered co-expression network. There are 7 UDP-dependent glycosyltransferases (UGT) and 76 transcription factors (TFs) distributing at different regions which represented varying coherence of Reb D and Reb M biosynthesis. Particularly, two TFs having a strong correlation with two UGTs in the network were also discovered. The present study provided a comprehensive insight into networks for regulation of Reb D and Reb M contents in S. rebaudiana.

Ca14-3-3 Interacts With CaWRKY58 to Positively Modulate Pepper Response to Low-Phosphorus Starvation

Low-phosphorus stress (LPS) and pathogen attack are two important stresses frequently experienced by plants in their natural habitats, but how plant respond to them coordinately remains under-investigated. Here, we demonstrate that CaWRKY58, a known negative regulator of the pepper (Capsicum annuum) response to attack by Ralstonia solanacearum, is upregulated by LPS. Virus-induced gene silencing (VIGS) and overexpression of CaWRKY58 in Nicotiana benthamiana plants in combination with chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays (EMSA) demonstrated that CaWRKY58 positively regulates the response of pepper to LPS by directly targeting and regulating genes related to phosphorus-deficiency tolerance, including PHOSPHATE STARVATION RESPONSE1 (PHR1). Yeast two-hybrid assays revealed that CaWRKY58 interacts with a 14-3-3 protein (Ca14-3-3); this interaction was confirmed by pull-down, bimolecular fluorescence complementation (BiFC), and microscale thermophoresis (MST) assays. The interaction between Ca14-3-3 and CaWRKY58 enhanced the activation of PHR1 expression by CaWRKY58, but did not affect the expression of the immunity-related genes CaNPR1 and CaDEF1, which are negatively regulated by CaWRKY58 in pepper upon Ralstonia solanacearum inoculation. Collectively, our data indicate that CaWRKY58 negatively regulates immunity against Ralstonia solanacearum, but positively regulates tolerance to LPS and that Ca14-3-3 transcriptionally activates CaWRKY58 in response to LPS.

Cell Wall Proteins Play Critical Roles in Plant Adaptation to Phosphorus Deficiency

Phosphorus is one of the mineral nutrient elements essential for plant growth and development. Low phosphate (Pi) availability in soils adversely affects crop production. To cope with low P stress, remodeling of root morphology and architecture is generally observed in plants, which must be accompanied by root cell wall modifications. It has been documented that cell wall proteins (CWPs) play critical roles in shaping cell walls, transmitting signals, and protecting cells against environmental stresses. However, understanding of the functions of CWPs involved in plant adaptation to P deficiency remains fragmentary. The aim of this review was to summarize advances in identification and functional characterization of CWPs in responses to P deficiency, and to highlight the critical roles of CWPs in mediating root growth, P reutilization, and mobilization in plants.

Phosphate Deficiency Negatively Affects Early Steps of the Symbiosis between Common Bean and Rhizobia

Phosphate (Pi) deficiency reduces nodule formation and development in different legume species including common bean. Despite significant progress in the understanding of the genetic responses underlying the adaptation of nodules to Pi deficiency, it is still unclear whether this nutritional deficiency interferes with the molecular dialogue between legumes and rhizobia. If so, what part of the molecular dialogue is impaired? In this study, we provide evidence demonstrating that Pi deficiency negatively affects critical early molecular and physiological responses that are required for a successful symbiosis between common bean and rhizobia. We demonstrated that the infection thread formation and the expression of PvNSP2, PvNIN, and PvFLOT2, which are genes controlling the nodulation process were significantly reduced in Pi-deficient common bean seedlings. In addition, whole-genome transcriptional analysis revealed that the expression of hormones-related genes is compromised in Pi-deficient seedlings inoculated with rhizobia. Moreover, we showed that regardless of the presence or absence of rhizobia, the expression of PvRIC1 and PvRIC2, two genes participating in the autoregulation of nodule numbers, was higher in Pi-deficient seedlings compared to control seedlings. The data presented in this study provides a mechanistic model to better understand how Pi deficiency impacts the early steps of the symbiosis between common bean and rhizobia.

Metabolomics and Transcriptomics in Legumes Under Phosphate Deficiency in Relation to Nitrogen Fixation by Root Nodules

Phosphate (Pi) deficiency is a critical environmental constraint that affects the growth and development of several legume crops that are usually cultivated in semi-arid regions and marginal areas. Pi deficiency is known to be a significant limitation for symbiotic nitrogen (N2) fixation (SNF), and variability in SNF is strongly interlinked with the concentrations of Pi in the nodules. To deal with Pi deficiency, plants trigger various adaptive responses, including the induction and secretion of acid phosphatases, maintenance of Pi homeostasis in nodules and other organs, and improvement of oxygen (O2) consumption per unit of nodule mass. These molecular and physiological responses can be observed in terms of changes in growth, photosynthesis, and respiration. In this mini review, we provide a brief introduction to the problem of Pi deficiency in legume crops. We then summarize the current understanding of how Pi deficiency is regulated in legumes by changes in the transcriptomes and metabolomes found in different plant organs. Finally, we will provide perspectives on future directions for research in this field.

Interaction and Regulation of Carbon, Nitrogen, and Phosphorus Metabolisms in Root Nodules of Legumes

Members of the plant family Leguminosae (Fabaceae) are unique in that they have evolved a symbiotic relationship with rhizobia (a group of soil bacteria that can fix atmospheric nitrogen). Rhizobia infect and form root nodules on their specific host plants before differentiating into bacteroids, the symbiotic form of rhizobia. This complex relationship involves the supply of C₄-dicarboxylate and phosphate by the host plants to the microsymbionts that utilize them in the energy-intensive process of fixing atmospheric nitrogen into ammonium, which is in turn made available to the host plants as a source of nitrogen, a macronutrient for growth. Although nitrogen-fixing bacteroids are no longer growing, they are metabolically active. The symbiotic process is complex and tightly regulated by both the host plants and the bacteroids. The metabolic pathways of carbon, nitrogen, and phosphate are heavily regulated in the host plants, as they need to strike a fine balance between satisfying their own needs as well as those of the microsymbionts. A network of transporters for the various metabolites are responsible for the trafficking of these essential molecules between the two partners through the symbiosome membrane (plant-derived membrane surrounding the bacteroid), and these are in turn regulated by various transcription factors that control their expressions under different environmental conditions. Understanding this complex process of symbiotic nitrogen fixation is vital in promoting sustainable agriculture and enhancing soil fertility.

Identification of phosphorus deficiency responsive proteins in a high phosphorus acquisition soybean (Glycine max) cultivar through proteomic analysis

As one of the major oil crops, soybean might be seriously affected by phosphorus deficiency on both yield and quality. Understanding the molecular basis of phosphorus uptake and utilization in soybean may help to develop phosphorus (P) efficient cultivars. On this purpose, we conducted a comparative proteomic analysis on a high P acquisition soybean cultivar BX10 under low and high P conditions. A total of 61 unique proteins were identified as putative P deficiency responsive proteins. These proteins were involved in carbohydrate metabolism, protein biosynthesis/processing, energy metabolism, cellular processes, environmental defense/interaction, nucleotide metabolism, signal transduction, secondary metabolism and other metabolism related processes. Several proteins involved in energy metabolism, cellular processes, and protein biosynthesis and processing were found to be up-regulated in both shoots and roots, whereas, proteins involved in carbohydrate metabolism appeared to be down-regulated. These proteins are potential candidates for improving P acquisition. These findings provide a useful starting point for further research that will provide a more comprehensive understanding of molecular mechanisms through which soybeans adapt to P deficiency condition.

Physiological and molecular alterations in plants exposed to high [CO2] under phosphorus stress

Atmospheric [CO2] has increased substantially in recent decades and will continue to do so, whereas the availability of phosphorus (P) is limited and unlikely to increase in the future. P is a non-renewable resource, and it is essential to every form of life. P is a key plant nutrient controlling the responsiveness of photosynthesis to [CO2]. Increases in [CO2] typically results in increased biomass through stimulation of net photosynthesis, and hence enhance the demand for P uptake. However, most soils contain low concentrations of available P. Therefore, low P is one of the major growth-limiting factors for plants in many agricultural and natural ecosystems. The adaptive responses of plants to [CO2] and P availability encompass alterations at morphological, physiological, biochemical and molecular levels. In general low P reduces growth, whereas high [CO2] enhances it particularly in C3 plants. Photosynthetic capacity is often enhanced under high [CO2] with sufficient P supply through modulation of enzyme activities involved in carbon fixation such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, high [CO2] with low P availability results in enhanced dry matter partitioning towards roots. Alterations in below-ground processes including root morphology, exudation and mycorrhizal association are influenced by [CO2] and P availability. Under high P availability, elevated [CO2] improves the uptake of P from soil. In contrast, under low P availability, high [CO2] mainly improves the efficiency with which plants produce biomass per unit P. At molecular level, the spatio-temporal regulation of genes involved in plant adaptation to low P and high [CO2] has been studied individually in various plant species. Genome-wide expression profiling of high [CO2] grown plants revealed hormonal regulation of biomass accumulation through complex transcriptional networks. Similarly, differential transcriptional regulatory networks are involved in P-limitation responses in plants. Analysis of expression patterns of some typical P-limitation induced genes under high [CO2] suggests that long-term exposure of plants to high [CO2] would have a tendency to stimulate similar transcriptional responses as observed under P-limitation. However, studies on the combined effect of high [CO2] and low P on gene expression are scarce. Such studies would provide insights into the development of P efficient crops in the context of anticipated increases in atmospheric [CO2].

Two in one sweep: aluminum tolerance and grain yield in P-limited soils are associated to the same genomic region in West African sorghum

BACKGROUND

Sorghum (Sorghum bicolor L. Moench) productivity is severely impeded by low phosphorus (P) and aluminum (Al) toxic soils in sub-Saharan Africa and especially West Africa (WA). Improving productivity of this staple crop under these harsh conditions is crucial to improve food security and farmer's incomes in WA.

RESULTS

This is the first study to examine the genetics underlying sorghum adaptation to phosphorus limitation in a wide range of WA growing conditions. A set of 187 diverse sorghum genotypes were grown in 29 -P and + P field experiments from 2006-2012 in three WA countries. Sorghum grain yield performance under -P and + P conditions was highly correlated (r = 0.85***). Significant genotype-by-phosphorus interaction was detected but with small magnitude compared to the genotype variance component. We observed high genetic diversity within our panel, with rapid linkage disequilibrium decay, confirming recent sequence based studies in sorghum. Using genome wide association mapping based on 220 934 SNPs we identified one genomic region on chromosome 3 that was highly associated to grain yield production. A major Al-tolerance gene in sorghum, SbMATE, was collocated in this region and SbMATE specific SNPs showed very high associations to grain yield production, especially under -P conditions, explaining up to 16% of the genotypic variance.

CONCLUSION

The results suggest that SbMATE has a possible pleiotropic role in providing tolerance to two of the most serious abiotic stresses for sorghum in WA, Al toxicity and P deficiency. The identified SNPs can help accelerate breeding for increased sorghum productivity under unfavorable soil conditions and contribute to assuring food security in WA.

Two common bean genotypes with contrasting response to phosphorus deficiency show variations in the microRNA 399-mediated PvPHO2 regulation within the PvPHR1 signaling pathway

Crop production of the important legume, the common bean (Phaseolus vulgaris), is often limited by low phosphorus (P) in the soil. The genotypes, BAT477 and DOR364, of the common bean have contrasting responses to P starvation. Plants from the BAT477 P deficiency tolerant genotype showed higher phosphate content and root biomass as compared to the DOR364 plants under P starvation. The PvPHR1 transcription factor-signaling pathway plays an essential role in the response to P starvation. PvPHO2, a negative regulator of this pathway, encodes an ubiquitin E2 conjugase that promotes degradation of P-responsive proteins and is the target gene of PvmiR399. PvPHO2 is downregulated in BAT477 plants under P deficiency, while such a response is not observed in P-starved DOR364 plants. Five putative PvmiR399 binding sites were identified in the 5' UTR region in both genotypes. While four sites showed an identical DNA sequence, the fifth (binding site of PvPHO2 one) showed three base changes and higher complementarity scores in DOR364 as compared to BAT477. Modified 5'RACE experiments indicated that PvmiR399 binding and/or processing was affected in DOR364 P-starved plants. We propose that a less efficient cleavage of the PvPHO2 mRNA directed by PvmiR399 would result in a higher PvPHO2-mediated degradation of P-responsive proteins in the DOR364 genotype with decreased P deficiency tolerance.

Transcriptional regulation of phosphate acquisition by higher plants

Phosphorus (P), an essential macronutrient required for plant growth and development, is often limiting in natural and agro-climatic environments. To cope with heterogeneous or low phosphate (Pi) availability, plants have evolved an array of adaptive responses facilitating optimal acquisition and distribution of Pi. The root system plays a pivotal role in Pi-deficiency-mediated adaptive responses that are regulated by a complex interplay of systemic and local Pi sensing. Cross-talk with sugar, phytohormones, and other nutrient signaling pathways further highlight the intricacies involved in maintaining Pi homeostasis. Transcriptional regulation of Pi-starvation responses is particularly intriguing and involves a host of transcription factors (TFs). Although PHR1 of Arabidopsis is an extensively studied MYB TF regulating subset of Pi-starvation responses, it is not induced during Pi deprivation. Genome-wide analyses of Arabidopsis have shown that low Pi stress triggers spatiotemporal expression of several genes encoding different TFs. Functional characterization of some of these TFs reveals their diverse roles in regulating root system architecture, and acquisition and utilization of Pi. Some of the TFs are also involved in phytohormone-mediated root responses to Pi starvation. The biological roles of these TFs in transcriptional regulation of Pi homeostasis in model plants Arabidopsis thaliana and Oryza sativa are presented in this review.

Characterization of two putative protein phosphatase genes and their involvement in phosphorus efficiency in Phaseolus vulgaris

Protein dephosphorylation mediated by protein phosphatases plays a major role in signal transduction of plant responses to environmental stresses. In this study, two putative protein phosphatases, PvPS2:1 and PvPS2:2 were identified and characterized in bean (Phaseolus vulgaris). The two PvPS2 members were found to be localized to the plasma membrane and the nucleus by transient expression of PvPS2:GFP in onion epidermal cells. Transcripts of the two PvPS2 genes were significantly increased by phosphate (P(i) ) starvation in the two bean genotypes, G19833 (a P-efficient genotype) and DOR364 (a P-inefficient genotype). However, G19833 exhibited higher PvPS2:1 expression levels than DOR364 in both leaves and roots during P(i) starvation. Increased transcription of PvPS2:1 in response to P(i) starvation was further verified through histochemical analysis of PvPS2:1 promoter fusion ß-glucuronidase (GUS) in transgenic Arabidopsis plants. Analysis of PvPS2:1 overexpression lines in bean hairy roots and Arabidopsis showed that PvS2:1 was involved in root growth and P accumulation. Furthermore, expression levels of two P(i) starvation responsive genes were upregulated and the APase activities were enhanced in the overexpressing PvPS2:1 Arabidopsis lines. Taken together, our results strongly suggested that PvPS2:1 positively regulated plant responses to P(i) starvation, and could be further targeted as a candidate gene to improve crop P efficiency.

Changes in expression of soluble inorganic pyrophosphatases of Phaseolus vulgaris under phosphate starvation

Phosphorus is an essential element for all living cells, but its availability is often limiting in the soil. Plants have adapted to such limitation and respond to phosphorus deficiency. The soluble inorganic pyrophosphatases (PPase; EC 3.6.1.1) recycle the pyrophosphate produced by many biosynthetic reactions, and may play a role in the plant adaptation to phosphorus deficiency. In this work, three PPase mRNAs were identified from the Phaseolus vulgaris EST international database and their sequences were corroborated and completed using 3'RACE. After design and validation of the appropriate oligonucleotide primers, the PPase mRNA expression was measured by qRT-PCR in leaves, stems, and roots of bean plants grown with 1mM phosphate or under phosphate starvation. The plant tissues were classified according to their position on the plant, and some physiological signs of stress were recorded. qRT-PCR revealed changes in mRNA expression, but not for all isozymes under analysis, and not for all tissues. In addition, changes in the activity of some PPases were observed in zymograms. Our data are consistent with an important role for pyrophosphate in the adaptation of the plant to phosphate starvation.

Arbuscular mycorrhizal symbiosis elicits proteome responses opposite of P-starvation in SO4 grapevine rootstock upon root colonisation with two Glomus species

Although plant biotisation with arbuscular mycorrhizal fungi (AMF) is a promising strategy for improving plant health, a better knowledge regarding the molecular mechanisms involved is required. In this context, we sought to analyse the root proteome of grapevine rootstock Selection Oppenheim 4 (SO4) upon colonisation with two AMF. As expected, AMF colonisation stimulates plant biomass. At the proteome level, changes in protein amounts due to AMF colonisation resulted in 39 differentially accumulated two-dimensional electrophoresis spots in AMF roots relative to control. Out of them, 25 were co-identified in SO4 roots upon colonisation by Glomus irregulare and Glomus mosseae supporting the existence of conserved plant responses to AM symbiosis in a woody perennial species. Among the 18 proteins whose amount was reduced in AMF-colonised roots were proteins involved in glycolysis, protein synthesis and fate, defence and cell rescue, ethylene biosynthesis and purine and pyrimidine salvage degradation. The six co-identified proteins whose amount was increased had functions in energy production, signalling, protein synthesis and fate including proteases. Altogether these data confirmed that a part of the accommodation program of AMF previously characterized in annual plants is maintained within roots of the SO4 rootstock cuttings. Nonetheless, particular responses also occurred involving proteins of carbon metabolism, development and root architecture, defence and cell rescue, anthocyanin biosynthesis and P remobilization, previously reported as induced upon P-starvation. This suggests the occurrence of P reprioritization upon AMF colonization in a woody perennial plant species with agronomical interest.

Signaling network in sensing phosphate availability in plants

Plants acquire phosphorus in the form of phosphate (Pi), the concentration of which is often limited for plant uptake. Plants have developed diverse responses to conserve and remobilize internal Pi and to enhance Pi acquisition to secure them against Pi deficiency. These responses are achieved by the coordination of an elaborate signaling network comprising local and systemic machineries. Recent advances have revealed several important components involved in this network. Pi functions as a signal to report its own availability. miR399 and sugars act as systemic signals to regulate responses occurring in roots. Hormones also play crucial roles in modulating gene expression and in altering root system architecture. Transcription factors function as a hub to perceive the signals and to elicit steady outputs. In this review, we outline the current knowledge on this subject and present hypotheses pertaining to other potential signals and to the organization and coordination of signaling.

Regulation of OsSPX1 and OsSPX3 on expression of OsSPX domain genes and Pi-starvation signaling in rice

The rice (Oryza sativa L.) genome contains at least six genes exclusively with an SPX (SYG1/PHO81/XPR1) domain at the N-terminal, designated as OsSPX1-6. Here we report the diverse expression patterns of the OsSPX genes in different tissues and their responses to Pi-starvation. Among them, five genes, OsSPX1, 2, 3, 5 and 6 are responsive to Pi-starvation in shoots and/or in roots. The subcellular localization analysis indicates that OsSPX1 and OsSPX2 is exclusively located in nucleus, OsSPX3 in the cytoplasm, and OsSPX4 is a membrane localization protein. OsSPX1 regulates OsSPX2, 3 and 5 at the transcription level and is positively involved in the responses of the genes to Pi-starvation. Overexpression of OsSPX3 downregulates OsSPX5 in shoots under Pi-sufficiency. OsSPX3 negatively regulates the PSI (Pi-starvation induced) gene, OsIPS1 and is involved in the responses of miR399 and OsPHO2 to Pi-starvation. Our results suggest that OsSPX1 may be a regulator involved in the transcriptions of OsSPX2, 3 and 5. OsSPX3 plays a role in OsIPS1/miR399 mediated long distance regulation on OsPHO2. Our results also indicate that OsSPX3 is involved in plant tolerance to Pi-starvation stress.