Differentiating phosphate-dependent and phosphate-independent systemic phosphate-starvation response networks in Arabidopsis thaliana through the application of phosphite

The functional mechanisms of phosphite and its applications in crop plants

Phosphite (Phi), the reduced form of phosphate (Pi), is characterized by its stability, high solubility, efficient transport, resistance to fixation in soil, and widespread occurrence in natural environments. Although Phi exhibits greater suitability than Pi as a soil fertilizer, it cannot be metabolized by plants. In agricultural applications, Phi serves as a bio-stimulant, fungicide, herbicide, and has other purposes. As a bio-stimulant, Phi has been shown to promote plant growth, enhance stress resistance, and improve fruit quality. Additionally, when used as a fungicide or pesticide, it effectively inhibits the growth of phytopathogens in various crop species. The discovery of the phosphite dehydrogenase (ptxD) gene in microorganisms has significantly expanded the potential applications of Phi, including its use as a herbicide, phosphatic fertilizer, and a selectable chemical for generating marker-free transgenic plants. Therefore, the dual fertilization and weed control system of ptxD/Phi facilitates the utilization of Phi as the sole phosphorus source while concurrently suppressing the evolution of herbicide-resistant weeds in the future. Notably, ptxD also acts as an ideal selectable marker because its resistant is specific to Phi, thereby eliminating the risk of false positive clones. The application of Phi provides a promising strategy for addressing phosphorus resource shortages and improving the efficiency of phosphatic fertilizers in agriculture. Furthermore, Phi is considered an environmentally friendly fertilizer, as it contributes to the mitigation of eutrophication. In prospect, Phi is anticipated to play a significant role as a chemical fertilizer that promotes the sustainable development of agriculture. In this review, we provide a comprehensive analysis of the functional mechanisms of Phi and its current applications in agriculture, with the aim of offering deeper insights into its potential benefits and practical utility.

Phosphate deficiency inducible OsGDPD5 affects root growth by regulating sugar-auxin crosstalk

Glycerophosphodiester phosphodiesterases (GDPDs) enzymes are known to be involved in phospholipids degradation pathways, where glycerophosphodiesters are hydrolyzed to glycerol-3-phosphate (G3P) and corresponding alcohol. In plants, GDPDs are involved in phosphate deficiency adaptive responses and have been shown to impact root length, but the precise mechanism remains unclear. This study focuses on the rice GDPD5 gene and its role in regulating primary root growth. Our research demonstrates that OsGDPD5 encodes a functional GDPD enzyme and could hydrolyze glycerophosphocholine and glycerophosphorylethanolamine. At transcriptional levels, OsGDPD5 is preferentially expressed in the root tip and regulated by transcription factor OsPHR2. We have used CRISPR/Cas9 to generate OsGDPD5 knock-out lines, allowing us to explore its role in root growth. Our findings show that osgdpd5 mutants had a shorter primary root, which could be restored to a normal level by the exogenous application of sugar or G3P. Further, knocking out OsGDPD5 alters endogenous levels of G3P and sugars, affecting auxin biosynthesis in the root and, ultimately, primary root growth. In this manner, OsGDPD5 has a crucial role in regulating physiological processes, specifically sugar and auxin signaling, which are known to be involved in root growth regulation in rice. Our research thus unraveled a link between rice phosphate deficiency-responsive lipid remodeling and root growth via sugar-hormone signaling.

Sink strength, nutrient allocation, cannabinoid yield, and associated transcript profiles vary in two drug-type Cannabis chemovars

Cannabis sativa L. is one of the oldest domesticated crops. Hemp-type cultivars, which predominantly produce non-intoxicating cannabidiol (CBD), have been selected for their fast growth, seed, and fibre production, while drug-type chemovars were bred for high accumulation of tetrahydrocannabinol (THC). We investigated how the generation of CBD-dominant chemovars by introgression of hemp- into drug-type Cannabis impacted plant performance. The THC-dominant chemovar showed superior sink strength, higher flower biomass, and demand-driven control of nutrient uptake. By contrast, the CBD-dominant chemovar hyperaccumulated phosphate in sink organs leading to reduced carbon and nitrogen assimilation in leaves, which limited flower biomass and cannabinoid yield. RNA-seq analyses determined organ- and chemovar-specific differences in expression of genes associated with nitrate and phosphate homeostasis as well as growth-regulating transcription factors that were correlated with measured traits. Among these were genes positively selected for during Cannabis domestication encoding an inhibitor of the phosphate starvation response, SPX DOMAIN GENE3, nitrate reductase, and two nitrate transporters. Altered nutrient sensing, acquisition, or distribution are likely a consequence of adaption to growth on marginal, low-nutrient-input lands in hemp. Our data provide evidence that such ancestral traits may become detrimental for female flower development and consequently overall CBD yield in protected cropping environments.

Using native and synthetic genes to disrupt inositol pyrophosphates and phosphate accumulation in plants

Inositol pyrophosphates are eukaryotic signaling molecules that have been recently identified as key regulators of plant phosphate sensing and homeostasis. Given the importance of phosphate to current and future agronomic practices, we sought to design plants, which could be used to sequester phosphate, as a step in a phytoremediation strategy. To achieve this, we expressed diadenosine and diphosphoinositol polyphosphate phosphohydrolase (DDP1), a yeast (Saccharomyces cerevisiae) enzyme demonstrated to hydrolyze inositol pyrophosphates, in Arabidopsis thaliana and pennycress (Thlaspi arvense), a spring annual cover crop with emerging importance as a biofuel crop. DDP1 expression in Arabidopsis decreased inositol pyrophosphates, activated phosphate starvation response marker genes, and increased phosphate accumulation. These changes corresponded with alterations in plant growth and sensitivity to exogenously applied phosphate. Pennycress plants expressing DDP1 displayed increases in phosphate accumulation, suggesting that these plants could potentially serve to reclaim phosphate from phosphate-polluted soils. We also identified a native Arabidopsis gene, Nucleoside diphosphate-linked moiety X 13 (NUDIX13), which we show encodes an enzyme homologous to DDP1 with similar substrate specificity. Arabidopsis transgenics overexpressing NUDIX13 had lower inositol pyrophosphate levels and displayed phenotypes similar to DDP1-overexpressing transgenics, while nudix13-1 mutants had increased levels of inositol pyrophosphates. Taken together, our data demonstrate that DDP1 and NUDIX13 can be used in strategies to regulate plant inositol pyrophosphates and could serve as potential targets for engineering plants to reclaim phosphate from polluted environments.

Beneficial Effects of Phosphite in Arabidopsis thaliana Mediated by Activation of ABA, SA, and JA Biosynthesis and Signaling Pathways

Phosphite (Phi) has gained attention in agriculture due to its biostimulant effect on crops. This molecule has been found to benefit plant performance by providing protection against pathogens, improving yield and fruit quality as well as nutrient and water use efficiency. It is still unclear how Phi enhances plant growth and protects against multiple stresses. It has been hypothesized that Phi acts by directly affecting the pathogens and interacting with the plant cellular components and molecular machinery to elicit defense responses. This study elucidates the mechanisms underlying Phi's beneficial effects on plants, revealing their complex interplay with fundamental signaling pathways. An RNA-seq study of Arabidopsis seedlings under optimal and limiting phosphate conditions helped us unveil Phi's role in promoting plant growth by activating the expression of the genes involved in the biosynthesis and signaling pathways associated with abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA). The expression of ABA-related genes, known for their involvement in stress response and development regulation, is triggered by Phi treatment, contributing to enhanced resilience and growth. Simultaneously, the activation of the SA pathway, associated with defense responses, suggests Phi's potential in bolstering plant immunity. Moreover, Phi influences JA biosynthesis and signaling, which are crucial for defense against herbivores and pathogens, thereby strengthening plants' defenses. Our findings reveal a multifaceted mechanism through which Phi benefits Arabidopsis development. Understanding its intricate interplay with key signaling pathways opens avenues for leveraging Phi as a strategic tool to enhance plant resilience, immunity, and growth in agricultural and ecological contexts.

Dynamic interactions between SPX proteins, the ubiquitination machinery, and signalling molecules for stress adaptation at a whole-plant level

The plant macronutrient phosphorus is a scarce resource and plant-available phosphate is limiting in most soil types. Generally, a gene regulatory module called the phosphate starvation response (PSR) enables efficient phosphate acquisition by roots and translocation to other organs. Plants growing on moderate to nutrient-rich soils need to co-ordinate availability of different nutrients and repress the highly efficient PSR to adjust phosphate acquisition to the availability of other macro- and micronutrients, and in particular nitrogen. PSR repression is mediated by a small family of single SYG1/Pho81/XPR1 (SPX) domain proteins. The SPX domain binds higher order inositol pyrophosphates that signal cellular phosphorus status and modulate SPX protein interaction with PHOSPHATE STARVATION RESPONSE1 (PHR1), the central transcriptional regulator of PSR. Sequestration by SPX repressors restricts PHR1 access to PSR gene promoters. Here we focus on SPX4 that primarily acts in shoots and sequesters many transcription factors other than PHR1 in the cytosol to control processes beyond the classical PSR, such as nitrate, auxin, and jasmonic acid signalling. Unlike SPX1 and SPX2, SPX4 is subject to proteasomal degradation not only by singular E3 ligases, but also by SCF-CRL complexes. Emerging models for these different layers of control and their consequences for plant acclimation to the environment will be discussed.

Dissection of the response mechanism of alfalfa under phosphite stress based on metabolomic and transcriptomic data

Phosphite, a reduced form of phosphate, inhibits the growth and even has toxic effect on plants. To learn more about the mechanism of alfalfa responses to phosphite, the morphological and physiological characteristics, and the metabolites and transcript levels were comprehensively analyzed following the exposure of alfalfa seedlings to phosphite and phosphate under greenhouse conditions. The results showed that phosphite inhibited seedling growth and photosynthesis. However, the absorption efficiency of phosphite was higher than that of phosphate in roots, which was supported by increased total phosphorus concentration of 16.29% and 52.30% on days 8 and 12. Moreover, phosphite stress affected the synthesis of lipids and carbohydrates, which were reflected in enhanced glycolipid and sulfolipid in roots and amylose in shoots. Phosphite stress resulted in a decrease in indole acetic acid (IAA) in the whole plant and zeatin in the shoots, which could enable alfalfa to adapt to the phosphite environment. Some genes involved in phosphate starvation response included SPX, phosphate response regulator2, and inorganic phosphate transporter 1-4 (PHT1;4) in roots were affected by phosphite stress. In addition, some genes that are involved in stress responses and DNA repair were induced by phosphite stress. These observations together suggest that alfalfa responds to phosphite stress by inhibiting growth, regulating the genes induced by phosphate starvation, improving oxidative protection, promoting DNA repair, and adjusting the IAA and zeatin signaling transductions. Our findings provide novel insights into the molecular response to phosphite stress in alfalfa.

Monogalactosyl diacylglycerol synthase 3 affects phosphate utilization and acquisition in rice

Galactolipids are essential to compensate for the loss of phospholipids by 'membrane lipid remodelling' in plants under phosphorus (P) deficiency conditions. Monogalactosyl diacylglycerol (MGDG) synthases catalyse the synthesis of MGDG which is further converted into digalactosyl diacylglycerol (DGDG), later replacing phospholipids in the extraplastidial membranes. However, the roles of these enzymes are not well explored in rice. In this study, the rice MGDG synthase 3 gene (OsMGD3) was identified and functionally characterized. We showed that the plant phosphate (Pi) status and the transcription factor PHOSPHATE STARVATION RESPONSE 2 (OsPHR2) are involved in the transcriptional regulation of OsMGD3. CRISPR/Cas9 knockout and overexpression lines of OsMGD3 were generated to explore its potential role in rice adaptation to Pi deficiency. Compared with the wild type, OsMGD3 knockout lines displayed a reduced Pi acquisition and utilization while overexpression lines showed an enhancement of the same. Further, OsMGD3 showed a predominant role in roots, altering lateral root growth. Our comprehensive lipidomic analysis revealed a role of OsMGD3 in membrane lipid remodelling, in addition to a role in regulating diacylglycerol and phosphatidic acid contents that affected the expression of Pi transporters. Our study highlights the role of OsMGD3 in affecting both internal P utilization and P acquisition in rice.

Diverse phosphate and auxin transport loci distinguish phosphate tolerant from sensitive Arabidopsis accessions

Phosphorus (P) is an essential element for plant growth often limiting agroecosystems. To identify genetic determinants of performance under variable phosphate (Pi) supply, we conducted genome-wide association studies on five highly predictive Pi starvation response traits in 200 Arabidopsis (Arabidopsis thaliana) accessions. Pi concentration in Pi-limited organs had the strongest, and primary root length had the weakest genetic component. Of 70 trait-associated candidate genes, 17 responded to Pi withdrawal. The PHOSPHATE TRANSPORTER1 gene cluster on chromosome 5 comprises PHT1;1, PHT1;2, and PHT1;3 with known impact on P status. A second locus featured uncharacterized endomembrane-associated auxin efflux carrier encoding PIN-LIKES7 (PILS7) which was more strongly suppressed in Pi-limited roots of Pi-starvation sensitive accessions. In the Col-0 background, Pi uptake and organ growth were impaired in both Pi-limited pht1;1 and two pils7 T-DNA insertion mutants, while Pi -limited pht1;2 had higher biomass and pht1;3 was indistinguishable from wild-type. Copy number variation at the PHT1 locus with loss of the PHT1;3 gene and smaller scale deletions in PHT1;1 and PHT1;2 predicted to alter both protein structure and function suggest diversification of PHT1 is a key driver for adaptation to P limitation. Haplogroup analysis revealed a phosphorylation site in the protein encoded by the PILS7 allele from stress-sensitive accessions as well as additional auxin-responsive elements in the promoter of the "stress tolerant" allele. The former allele's inability to complement the pils7-1 mutant in the Col-0 background implies the presence of a kinase signaling loop controlling PILS7 activity in accessions from P-rich environments, while survival in P-poor environments requires fine-tuning of stress-responsive root auxin signaling.

Improved the Activity of Phosphite Dehydrogenase and its Application in Plant Biotechnology

Phosphorus (P) is a nonrenewable resource, which is one of the major challenges for sustainable agriculture. Although phosphite (Phi) can be absorbed by the plant cells through the Pi transporters, it cannot be metabolized by plant and unable to use as P fertilizers for crops. However, transgenic plants that overexpressed phosphite dehydrogenase (PtxD) from bacteria can utilize phosphite as the sole P source. In this study, we aimed to improve the catalytic efficiency of PtxD from Ralstonia sp.4506 (PtxDR4506), by directed evolution. Five mutations were generated by saturation mutagenesis at the 139th site of PtxD R4506 and showed higher catalytic efficiency than native PtxDR4506. The PtxDQ showed the highest catalytic efficiency (5.83-fold as compared to PtxDR4506) contributed by the 41.1% decrease in the K m and 2.5-fold increase in the k cat values. Overexpression of PtxDQ in Arabidopsis and rice showed increased efficiency of phosphite utilization and excellent development when phosphite was used as the primary source of P. High-efficiency PtxD transgenic plant is an essential prerequisite for future agricultural production using phosphite as P fertilizers.

Review of Phosphite as a Plant Nutrient and Fungicide

Phosphite (Phi)-containing products are marketed for their antifungal and nutritional value. Substantial evidence of the anti-fungal properties of Phi on a wide variety of plants has been documented. Although Phi is readily absorbed by plant leaves and/or roots, the plant response to Phi used as a phosphorus (P) source is variable. Negative effects of Phi on plant growth are commonly observed under P deficiency compared to near adequate plant P levels. Positive responses to Phi may be attributed to some level of fungal disease control. While only a few studies have provided evidence of Phi oxidation through cellular enzymes genetically controlled in plant cells, increasing evidence exists for the potential to manipulate plant genes to enhance oxidation of Phi to phosphate (Pi) in plants. Advances in genetic engineering to sustain growth and yield with Phi + Pi potentially provides a dual fertilization and weed control system. Further advances in genetic manipulation of plants to utilize Phi are warranted. Since Phi oxidation occurs slowly in soils, additional information is needed to characterize Phi oxidation kinetics under variable soil and environmental conditions.

ROS and Oxidative Response Systems in Plants Under Biotic and Abiotic Stresses: Revisiting the Crucial Role of Phosphite Triggered Plants Defense Response

Phosphite (Phi) is a chemical analog of orthophosphate [HPO4 3-]. It is a systemic pesticide generally known to control the prevalence of oomycetes and soil-borne diseases such as Phytophthora, Pythium, and Plasmopora species. Phi can also control disease symptoms and the spread of pathogenic bacteria, fungi, and nematodes. Phi plays critical roles as a fungicide, pesticide, fertilizer, or biostimulator. Overall, Phi can alleviate the severity of the disease caused by oomycete, fungi, pathogenic bacteria, and nematodes (leave, stem, fruit, tuber, and root) in various plants (vegetables, fruits, crops, root/tuber crops, ornamental plants, and forests). Advance research in molecular, physiological, and biochemical approaches has approved the key role of Phi in enhancing crop growth, quantity, and quality of several plant species. Phi is chemically similar to orthophosphate, and inside the cells, it is likely to get involved in different features of phosphate metabolism in both plants and pathogens. In plants, a range of physiobiochemical alterations are induced by plant pathogen stress, which causes lowered photosynthesis activities, enzymatic activities, increased accumulation of reactive oxygen species (ROS), and modification in a large group of genes. To date, several attempts have been made to study plant-pathogen interactions with the intent to minimize the loss of crop productivity. Phi's emerging function as a biostimulant in plants has boost plant yield and tolerance against various stress factors. This review discusses Phi-mediated biostimulant effects against biotic and abiotic stresses.

Pi-starvation induced transcriptional changes in barley revealed by a comprehensive RNA-Seq and degradome analyses

BACKGROUND

Small RNAs (sRNAs) are 20-30 nt regulatory elements which are responsible for plant development regulation and participate in many plant stress responses. Insufficient inorganic phosphate (Pi) concentration triggers plant responses to balance the internal Pi level.

RESULTS

In this study, we describe Pi-starvation-responsive small RNAs and transcriptome changes in barley (Hordeum vulgare L.) using Next-Generation Sequencing (NGS) RNA-Seq data derived from three different types of NGS libraries: (i) small RNAs, (ii) degraded RNAs, and (iii) functional mRNAs. We find that differentially and significantly expressed miRNAs (DEMs, Bonferroni adjusted p-value < 0.05) are represented by 15 molecules in shoot and 13 in root; mainly various miR399 and miR827 isomiRs. The remaining small RNAs (i.e., those without perfect match to reference sequences deposited in miRBase) are considered as differentially expressed other sRNAs (DESs, p-value Bonferroni correction < 0.05). In roots, a more abundant and diverse set of other sRNAs (DESs, 1796 unique sequences, 0.13% from the average of the unique small RNA expressed under low-Pi) contributes more to the compensation of low-Pi stress than that in shoots (DESs, 199 unique sequences, 0.01%). More than 80% of differentially expressed other sRNAs are up-regulated in both organs. Additionally, in barley shoots, up-regulation of small RNAs is accompanied by strong induction of two nucleases (S1/P1 endonuclease and 3'-5' exonuclease). This suggests that most small RNAs may be generated upon nucleolytic cleavage to increase the internal Pi pool. Transcriptomic profiling of Pi-starved barley shoots identifies 98 differentially expressed genes (DEGs). A majority of the DEGs possess characteristic Pi-responsive cis-regulatory elements (P1BS and/or PHO element), located mostly in the proximal promoter regions. GO analysis shows that the discovered DEGs primarily alter plant defense, plant stress response, nutrient mobilization, or pathways involved in the gathering and recycling of phosphorus from organic pools.

CONCLUSIONS

Our results provide comprehensive data to demonstrate complex responses at the RNA level in barley to maintain Pi homeostasis and indicate that barley adapts to Pi-starvation through elicitation of RNA degradation. Novel P-responsive genes were selected as putative candidates to overcome low-Pi stress in barley plants.

Effects of phosphite as a plant biostimulant on metabolism and stress response for better plant performance in Solanum tuberosum

Food availability represents a major worldwide concern due to population growth, increased demand, and climate change. Therefore, it is imperative to identify compounds that can improve crop performance. Plant biostimulants have gained prominence because of their potentials to increase germination, productivity and quality of a wide range of horticultural and agronomic crops. Phosphite (Phi), an analog of orthophosphate, is an emerging biostimulant used in horticulture and agronomy. The aim of this study was to uncover the molecular mechanisms through which Phi acts as a biostimulant with potential effects of overall plant growth. Field and greenhouse experiments, using 4 potato cultivars, showed that following Phi applications, plant performance, including several physio-biochemical traits, crop productivity, and quality traits, were significantly improved. RNA sequencing of control and Phi-treated plants of cultivar Xingjia No. 2, at 0 h, 6 h, 24 h, 48 h, 72 h and 96 h after the Phi application for 24 h revealed extensive changes in the gene expression profiles. A total of 2856 differentially expressed genes were identified, suggesting that multiple pathways of primary and secondary metabolism, such as flavonoids biosynthesis, starch and sucrose metabolism, and phenylpropanoid biosynthesis, were strongly influenced by foliar applications of Phi. GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analyses associated with defense responses revealed significant effects of Phi on a plethora of defense mechanisms. These results suggest that Phi acted as a biostimulant by priming the plants, that was, by triggering dynamic changes in gene expression and modulating metabolic fluxes in a way that allowed plants to perform better. Therefore, Phi usage has the potential to improve crop yield and health, alleviating the challenges posed by the need of feeding a growing world population, while minimizing the agricultural impact on human health and environment.

Phosphate and phosphite have a differential impact on the proteome and phosphoproteome of Arabidopsis suspension cell cultures

Phosphorus absorbed in the form of phosphate (H₂ PO₄^- ) is an essential but limiting macronutrient for plant growth and agricultural productivity. A comprehensive understanding of how plants respond to phosphate starvation is essential for the development of more phosphate-efficient crops. Here we employed label-free proteomics and phosphoproteomics to quantify protein-level responses to 48 h of phosphate versus phosphite (H₂ PO₃^- ) resupply to phosphate-deprived Arabidopsis thaliana suspension cells. Phosphite is similarly sensed, taken up and transported by plant cells as phosphate, but cannot be metabolized or used as a nutrient. Phosphite is thus a useful tool for differentiating between non-specific processes related to phosphate sensing and transport and specific responses to phosphorus nutrition. We found that responses to phosphate versus phosphite resupply occurred mainly at the level of protein phosphorylation, complemented by limited changes in protein abundance, primarily in protein translation, phosphate transport and scavenging, and central metabolism proteins. Altered phosphorylation of proteins involved in core processes such as translation, RNA splicing and kinase signaling was especially important. We also found differential phosphorylation in response to phosphate and phosphite in 69 proteins, including splicing factors, translation factors, the PHT1;4 phosphate transporter and the HAT1 histone acetyltransferase - potential phospho-switches signaling changes in phosphorus nutrition. Our study illuminates several new aspects of the phosphate starvation response and identifies important targets for further investigation and potential crop improvement.

Potassium and phosphorus transport and signaling in plants

Nitrogen (N), potassium (K), and phosphorus (P) are essential macronutrients for plant growth and development, and their availability affects crop yield. Compared with N, the relatively low availability of K and P in soils limits crop production and thus threatens food security and agricultural sustainability. Improvement of plant nutrient utilization efficiency provides a potential route to overcome the effects of K and P deficiencies. Investigation of the molecular mechanisms underlying how plants sense, absorb, transport, and use K and P is an important prerequisite to improve crop nutrient utilization efficiency. In this review, we summarize current understanding of K and P transport and signaling in plants, mainly taking Arabidopsis thaliana and rice (Oryza sativa) as examples. We also discuss the mechanisms coordinating transport of N and K, as well as P and N.

Spatial Profiles of Phosphate in Roots Indicate Developmental Control of Uptake, Recycling, and Sequestration

The availability of inorganic phosphate (Pi) limits plant growth and crop productivity on much of the world's arable land. To better understand how plants cope with deficient and variable supplies of this essential nutrient, we used Pi imaging to spatially resolve and quantify cytosolic Pi concentrations and the respective contributions of Pi uptake, metabolic recycling, and vacuolar sequestration to cytosolic Pi homeostasis in Arabidopsis (Arabidopsis thaliana) roots. Microinjection coupled with confocal microscopy was used to calibrate a FRET-based Pi sensor to determine absolute, rather than relative, Pi concentrations in live plants. High-resolution mapping of cytosolic Pi concentrations in different cells, tissues, and developmental zones of the root revealed that cytosolic concentrations varied between developmental zones, with highest levels in the transition zone, whereas concentrations were equivalent in epidermis, cortex, and endodermis within each zone. Pi concentrations in all zones were reduced, at different rates, by Pi starvation, but the developmental pattern of Pi concentration persisted. Pi uptake, metabolic recycling, and vacuolar sequestration were distinguished in each zone by using cyanide to block Pi assimilation in wild-type plants and a vacuolar Pi transport mutant, and then measuring the subsequent change in cytosolic Pi concentration over time. Each of these processes exhibited distinct spatial profiles in the root, but only vacuolar Pi sequestration corresponded with steady-state cytosolic Pi concentrations. These results highlight the complexity of Pi dynamics in live plants and revealed developmental control of root Pi homeostasis, which has potential implications for plant sensing and signaling of Pi.

Characterization of STP4 promoter in Indian mustard Brassica juncea for use as an aphid responsive promoter

OBJECTIVE

Brassica juncea, a major oilseed crop, suffers substantial yield losses due to infestation by mustard aphids (Lipaphis erysimi). Unavailability of resistance genes within the accessible gene pool underpins significance of the transgenic strategy in developing aphid resistance. In this study, we aimed for the identification of an aphid-responsive promoter from B. juncea, based on the available genomic resources.

RESULTS

A monosaccharide transporter gene, STP4 in B. juncea was activated by aphids and sustained increased expression as the aphids colonized the plants. We cloned the upstream intergenic region of STP4 and validated its stand-alone aphid-responsive promoter activity. Further, deletion analysis identified the putative cis-elements important for the aphid responsive promoter activity.

CONCLUSION

The identified STP4 promoter can potentially be used for driving high level aphid-inducible expression of transgenes in plants. Use of aphid-responsive promoter instead of constitutive promoters can potentially reduce the metabolic burden of transgene-expression on the host plant.

SPX4 Acts on PHR1-Dependent and -Independent Regulation of Shoot Phosphorus Status in Arabidopsis

Phosphorus (P) is an essential macronutrient for all living organisms and limits plant growth. Four proteins comprising a single SYG1/Pho81/XPR1 (SPX) domain, SPX1 to SPX4, are putative phosphate-dependent inhibitors of Arabidopsis (Arabidopsis thaliana) PHOSPHATE STARVATION RESPONSE1 (PHR1), the master transcriptional activator of phosphate starvation responses. This work demonstrated that SPX4 functions as a negative regulator not only of PHR1-dependent but also of PHR1-independent responses in P-replete plants. Transcriptomes of P-limited spx4 revealed that, unlike SPX1 and SPX2, SPX4 modulates the shoot phosphate starvation response but not short-term recovery after phosphate resupply. In roots, transcriptional regulation of P status is SPX4 independent. Genes misregulated in spx4 shoots intersect with both PHR1-dependent and PHOSPHATE2-dependent signaling networks associated with plant development, senescence, and ion/metabolite transport. Gene regulatory network analyses suggested that SPX4 interacts with transcription factors other than PHR1, such as SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 and ARABIDOPSIS NAC DOMAIN CONTAINING PROTEIN55, known regulators of shoot development. Transient expression studies in protoplasts indicated that PHR1 retention in the cytosol by SPX4 occurs in a dose- and P-status-dependent manner. Using a luciferase reporter in vivo, SPX4 expression kinetics and stability revealed that SPX4 is a short-lived protein with P-status-dependent turnover. SPX4 protein levels were quickly restored by phosphate resupply to P-limited plants. Unlike its monocot ortholog, AtSPX4 was not stabilized by the phosphate analog phosphite, implying that intracellular P status is sensed by its SPX domain via phosphate-rich metabolite signals.

Two bifunctional inositol pyrophosphate kinases/phosphatases control plant phosphate homeostasis

Many eukaryotic proteins regulating phosphate (Pi) homeostasis contain SPX domains that are receptors for inositol pyrophosphates (PP-InsP), suggesting that PP-InsPs may regulate Pi homeostasis. Here we report that deletion of two diphosphoinositol pentakisphosphate kinases VIH1/2 impairs plant growth and leads to constitutive Pi starvation responses. Deletion of phosphate starvation response transcription factors partially rescues vih1 vih2 mutant phenotypes, placing diphosphoinositol pentakisphosphate kinases in plant Pi signal transduction cascades. VIH1/2 are bifunctional enzymes able to generate and break-down PP-InsPs. Mutations in the kinase active site lead to increased Pi levels and constitutive Pi starvation responses. ATP levels change significantly in different Pi growth conditions. ATP-Mg2+ concentrations shift the relative kinase and phosphatase activities of diphosphoinositol pentakisphosphate kinases in vitro. Pi inhibits the phosphatase activity of the enzyme. Thus, VIH1 and VIH2 relay changes in cellular ATP and Pi concentrations to changes in PP-InsP levels, allowing plants to maintain sufficient Pi levels.

The PHOSPHATE1 genes participate in salt and Pi signaling pathways and play adaptive roles during soybean evolution

BACKGROUND

The PHOSPHATE1 (PHO1) gene family plays diverse roles in inorganic phosphate (Pi) transfer and signal transduction, and plant development. However, the functions and diversification of soybean PHO1 family are poorly understood.

RESULTS

Cultivated soybean (Glycine max) was domesticated from wild soybean (Glycine soja). To illuminate their roles in this evolutionary process, we comparatively investigated the G. max PHO1 genes (GmPHO1) in Suinong 14 (SN14) and G. soja PHO1 genes (GsPHO1) in ZYD00006 (ZYD6). The sequences of the orthologous Gm-GsPHO1 pairs were grouped into two Classes. The expression of Class I in both SN14 and ZYD6 was widely but relatively high in developing fruits, whereas Class II was predominantly expressed in the roots. The whole family displayed diverse response patterns to salt stresses and Pi-starvation in roots. Between SN14 and ZYD6, most PHO1 genes responded similarly to salinity stresses, and half had sharp contrasts in response to Pi-starvation, which corroborated the differential response capacities to salinity and low-Pi stress between SN14 and ZYD6. Furthermore, in transgenic Arabidopsis plants, most Class II members and GmPHO1;H9 from Class I could enhance salt tolerance, while only two Class II genes (GmPHO1;H4 and GmPHO1;H8) differently altered sensitivity to Pi-starvation. The expression of critical genes was accordingly altered in either salt or Pi signaling pathways in transgenic Arabidopsis plants.

CONCLUSIONS

Our work identifies some PHO1 genes as promising genetic materials for soybean improvement, and suggests that expression variation is decisive to functional divergence of the orthologous Gm-GsPHO1 pairs, which plays an adaptive role during soybean evolution.

An Integrative Systems Perspective on Plant Phosphate Research

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

The Local Phosphate Deficiency Response Activates Endoplasmic Reticulum Stress-Dependent Autophagy

Inorganic phosphate (Pi) is often a limiting plant nutrient. In members of the Brassicaceae family, such as Arabidopsis (Arabidopsis thaliana), Pi deprivation reshapes root system architecture to favor topsoil foraging. It does so by inhibiting primary root extension and stimulating lateral root formation. Root growth inhibition from phosphate (Pi) deficiency is triggered by iron-stimulated, apoplastic reactive oxygen species generation and cell wall modifications, which impair cell-to-cell communication and meristem maintenance. These processes require LOW PHOSPHATE RESPONSE1 (LPR1), a cell wall-targeted ferroxidase, and PHOSPHATE DEFICIENCY RESPONSE2 (PDR2), the single endoplasmic reticulum (ER)-resident P5-type ATPase (AtP5A), which is thought to control LPR1 secretion or activity. Autophagy is a conserved process involving the vacuolar degradation of cellular components. While the function of autophagy is well established under nutrient starvation (C, N, or S), it remains to be explored under Pi deprivation. Because AtP5A/PDR2 likely functions in the ER stress response, we analyzed the effect of Pi limitation on autophagy. Our comparative study of mutants defective in the local Pi deficiency response, ER stress response, and autophagy demonstrated that ER stress-dependent autophagy is rapidly activated as part of the developmental root response to Pi limitation and requires the genetic PDR2-LPR1 module. We conclude that Pi-dependent activation of autophagy in the root apex is a consequence of local Pi sensing and the associated ER stress response, rather than a means for systemic recycling of the macronutrient.

Flavonoid, Nitrate and Glucosinolate Concentrations in Brassica Species Are Differentially Affected by Photosynthetically Active Radiation, Phosphate and Phosphite

We evaluated the effects of phosphate (Pi-deficiency: 0.1 mM; Pi-sufficiency: 0.5 mM), phosphite (low-Phi: 0.1 mM; medium-Phi: 0.5 mM; and high-Phi: 2.5 mM), and two mean daily photosynthetically active radiations (lower PAR: 22.2 mol ⋅ m^-2 ⋅ d^-1; higher PAR: 29.7 mol ⋅ m^-2 ⋅ d^-1), as well as their interactions, on flavonoid, nitrate and glucosinolate (GL) concentrations and growth characteristics in hydroponically grown Brassica campestris cv. Mibuna Early and Brassica juncea cv. Red Giant. As expected, higher PAR increased dry matter and contrariwise decreased number of leaves but only in B. campestris. Total flavonoid and individual flavonoid compounds increased with the higher PAR value in B. campestris. Pi-sufficiency resulted in a lower quercetin concentration in both species, the isorhamnetin and total flavonoid concentrations in B. campestris, and the cyanidin concentration in B. juncea, in comparison to Pi-deficiency. Similarly, Pi-sufficient plants exhibited lower GL concentration, especially alkyl-GLs in B. campestris and alkenyl-GLs and an aryl-GL in B. juncea. Pi did not affect the nitrate concentration in either species, and nor did Phi influence the flavonoid concentrations in either species. In B. campestris, medium Phi (0.5 mM) increased the 1-methoxyindol-3-ylmethyl GL concentration by 28.3%, as compared to that observed at low Phi. In B. juncea, high Phi level increased the but-3-enyl-GL concentration by 18.9%, in comparison to values recorded at medium Phi. B. campestris plants exposed to higher PAR increased total flavonoids concentration. In both Brassica species, higher PAR stimulated the alkyl-, alkenyl-, and indole-GLs. The interaction of lower PAR and increasing Phi significantly decreased flavonoid concentration in B. juncea, whereas increasing Phi at higher PAR increased such concentration in this species. The same combination reduced the concentration of 2-phenylethyl- and indol-3-ylmethyl-GL in B. juncea. The highest indol-3-ylmethyl-GL concentration was observed when Pi was deficient combined with medium Phi in B. juncea. Thus, PAR, Pi and Phi may modulate flavonoid, GL and nitrate concentrations in Brassica species, which may be a useful tool to improve the nutraceutical quality of these leafy vegetables if properly managed.

Cytokinin at the Crossroads of Abiotic Stress Signalling Pathways

Cytokinin is a multifaceted plant hormone that plays major roles not only in diverse plant growth and development processes, but also stress responses. We summarize knowledge of the roles of its metabolism, transport, and signalling in responses to changes in levels of both macronutrients (nitrogen, phosphorus, potassium, sulphur) and micronutrients (boron, iron, silicon, selenium). We comment on cytokinin's effects on plants' xenobiotic resistance, and its interactions with light, temperature, drought, and salinity signals. Further, we have compiled a list of abiotic stress-related genes and demonstrate that their expression patterns overlap with those of cytokinin metabolism and signalling genes.

Survival of European Ash Seedlings Treated with Phosphite after Infection with the Hymenoscyphus fraxineus and Phytophthora Species

The European Fraxinus species are threatened by the alien invasive pathogen Hymenoscyphus fraxineus, which was introduced into Poland in the 1990s and has spread throughout the European continent, causing a large-scale decline of ash. There are no effective treatments to protect ash trees against ash dieback, which is caused by this pathogen, showing high variations in susceptibility at the individual level. Earlier studies have shown that the application of phosphites could improve the health of treated seedlings after artificial inoculation with H. fraxineus. Three-year-old F. excelsior seedlings were inoculated with the following pathogens: a H. fraxineus, Phytophthora species mixture (P. plurivora, P. megasperma, and P. taxon hungarica), in combination with two pathogens and mock-inoculated as the control, and then either watered or treated with ammonium phosphite (Actifos). Results showed significant differences in the survival of seedlings and symptoms of disease development among the treatments. Chlorophyll-a fluorescence parameters indicated a decrease in photosynthetic efficiency in infected plants, suggesting that they were under strong biotic stress, but none of the parameters could be used as a reliable bioindicator for ash decline disease. The application of Actifos enhanced the production of triterpenes (ursolic and oleanolic acid), and decreased the production of phenols (tyrosol) and sterols (β-sitosterol) in seedlings infected with H. fraxineus. Treatment with Actifos caused seedlings to enhance their response to pathogen(s) attack and increase their survival probability.

Design of synthetic bacterial communities for predictable plant phenotypes

Specific members of complex microbiota can influence host phenotypes, depending on both the abiotic environment and the presence of other microorganisms. Therefore, it is challenging to define bacterial combinations that have predictable host phenotypic outputs. We demonstrate that plant-bacterium binary-association assays inform the design of small synthetic communities with predictable phenotypes in the host. Specifically, we constructed synthetic communities that modified phosphate accumulation in the shoot and induced phosphate starvation-responsive genes in a predictable fashion. We found that bacterial colonization of the plant is not a predictor of the plant phenotypes we analyzed. Finally, we demonstrated that characterizing a subset of all possible bacterial synthetic communities is sufficient to predict the outcome of untested bacterial consortia. Our results demonstrate that it is possible to infer causal relationships between microbiota membership and host phenotypes and to use these inferences to rationally design novel communities.

Phosphite: a novel P fertilizer for weed management and pathogen control

The availability of orthophosphate (Pi) is a key determinant of crop productivity because its accessibility to plants is poor due to its conversion to unavailable forms. Weed's competition for this essential macronutrient further reduces its bio-availability. To compensate for the low Pi use efficiency and address the weed hazard, excess Pi fertilizers and herbicides are routinely applied, resulting in increased production costs, soil degradation and eutrophication. These outcomes necessitate the identification of a suitable alternate technology that can address the problems associated with the overuse of Pi-based fertilizers and herbicides in agriculture. The present review focuses on phosphite (Phi) as a novel molecule for its utility as a fertilizer, herbicide, biostimulant and biocide in modern agriculture. The use of Phi-based fertilization will help to reduce the consumption of Pi fertilizers and facilitate weed and pathogen control using the same molecule, thereby providing significant advantages over current orthophosphate-based fertilization.

Novel signals in the regulation of Pi starvation responses in plants: facts and promises

Plants have evolved numerous adaptive developmental and metabolic responses to cope with growth in conditions of limited phosphate (Pi). Regulation of these Pi starvation responses (PSR) at the organism level involves not only cellular Pi perception in different organs, but also inter-organ communication of Pi levels via systemic signaling. Here we summarize recent discoveries on Pi starvation sensing and signaling, with special emphasis on structure-function studies that showed a role for inositol polyphosphates (InsP) as intracellular Pi signals, and on genomic studies that identified a large number of mRNAs with inter-organ mobility, which provide an immense source of potential systemic signals in the control of PSR and other responses.

Improvement in phosphate acquisition and utilization by a secretory purple acid phosphatase (OsPAP21b) in rice

Phosphate (Pi) deficiency in soil system is a limiting factor for rice growth and yield. Majority of the soil phosphorus (P) is organic in nature, not readily available for root uptake. Low Pi-inducible purple acid phosphatases (PAPs) are hypothesized to enhance the availability of Pi in soil and cellular system. However, information on molecular and physiological roles of rice PAPs is very limited. Here, we demonstrate the role of a novel rice PAP, OsPAP21b in improving plant utilization of organic-P. OsPAP21b was found to be under the transcriptional control of OsPHR2 and strictly regulated by plant Pi status at both transcript and protein levels. Biochemically, OsPAP21b showed hydrolysis of several organophosphates at acidic pH and possessed sufficient thermostability befitting for high-temperature rice ecosystems with acidic soils. Interestingly, OsPAP21b was revealed to be a secretory PAP and encodes a distinguishable major APase (acid phosphatase) isoform under low Pi in roots. Further, OsPAP21b-overexpressing transgenics showed increased biomass, APase activity and P content in both hydroponics supplemented with organic-P sources and soil containing organic manure as sole P source. Additionally, overexpression lines depicted increased root length, biomass and lateral roots under low Pi while RNAi lines showed reduced root length and biomass as compared to WT. In the light of these evidences, present study strongly proposes OsPAP21b as a useful candidate for improving Pi acquisition and utilization in rice.

Root Cell-Specific Regulators of Phosphate-Dependent Growth

Cellular specialization in abiotic stress responses is an important regulatory feature driving plant acclimation. Our in silico approach of iterative coexpression, interaction, and enrichment analyses predicted root cell-specific regulators of phosphate starvation response networks in Arabidopsis (Arabidopsis thaliana). This included three uncharacterized genes termed Phosphate starvation-induced gene interacting Root Cell Enriched (PRCE1, PRCE2, and PRCE3). Root cell-specific enrichment of 12 candidates was confirmed in promoter-GFP lines. T-DNA insertion lines of 11 genes showed changes in phosphate status and growth responses to phosphate availability compared with the wild type. Some mutants (cbl1, cipk2, prce3, and wdd1) displayed strong biomass gain irrespective of phosphate supply, while others (cipk14, mfs1, prce1, prce2, and s6k2) were able to sustain growth under low phosphate supply better than the wild type. Notably, root or shoot phosphate accumulation did not strictly correlate with organ growth. Mutant response patterns markedly differed from those of master regulators of phosphate homeostasis, PHOSPHATE STARVATION RESPONSE1 (PHR1) and PHOSPHATE2 (PHO2), demonstrating that negative growth responses in the latter can be overcome when cell-specific regulators are targeted. RNA sequencing analysis highlighted the transcriptomic plasticity in these mutants and revealed PHR1-dependent and -independent regulatory circuits with gene coexpression profiles that were highly correlated to the quantified physiological traits. The results demonstrate how in silico prediction of cell-specific, stress-responsive genes uncovers key regulators and how their manipulation can have positive impacts on plant growth under abiotic stress.

Transcriptome analysis of the Populus trichocarpa-Rhizophagus irregularis Mycorrhizal Symbiosis: Regulation of Plant and Fungal Transportomes under Nitrogen Starvation

Nutrient transfer is a key feature of the arbuscular mycorrhizal (AM) symbiosis. Valuable mineral nutrients are transferred from the AM fungus to the plant, increasing its fitness and productivity, and, in exchange, the AM fungus receives carbohydrates as an energy source from the plant. Here, we analyzed the transcriptome of the Populus trichocarpa-Rhizophagus irregularis symbiosis using RNA-sequencing of non-mycorrhizal or mycorrhizal fine roots, with a focus on the effect of nitrogen (N) starvation. In R. irregularis, we identified 1,015 differentially expressed genes, whereby N starvation led to a general induction of gene expression. Genes of the functional classes of cell growth, membrane biogenesis and cell structural components were highly abundant. Interestingly, N starvation also led to a general induction of fungal transporters, indicating increased nutrient demand upon N starvation. In non-mycorrhizal P. trichocarpa roots, 1,341 genes were differentially expressed under N starvation. Among the 953 down-regulated genes in N starvation, most were involved in metabolic processes including amino acids, carbohydrate and inorganic ion transport, while the 342 up-regulated genes included many defense-related genes. Mycorrhization led to the up-regulation of 549 genes mainly involved in secondary metabolite biosynthesis and transport; only 24 genes were down-regulated. Mycorrhization specifically induced expression of three ammonium transporters and one phosphate transporter, independently of the N conditions, corroborating the hypothesis that these transporters are important for symbiotic nutrient exchange. In conclusion, our data establish a framework of gene expression in the two symbiotic partners under high-N and low-N conditions.

A phosphate starvation-driven bidirectional promoter as a potential tool for crop improvement and in vitro plant biotechnology

Phosphate (Pi)-deficient soils are a major limitant factor for crop production in many regions of the world. Despite that plants have innovated several developmental and biochemical strategies to deal with this stress, there are still massive extensions of land which combine several abiotic stresses, including phosphate starvation, that limit their use for plant growth and food production. In several plant species, a genetic programme underlies the biochemical and developmental responses of the organism to cope with low phosphate (Pi) availability. Both protein- and miRNA-coding genes involved in the adaptative response are transcriptionally activated upon Pi starvation. Several of the responsive genes have been identified as transcriptional targets of PHR1, a transcription factor that binds a conserved cis-element called PHR1-binding site (P1BS). Our group has previously described and characterized a minimal genetic arrangement that includes two P1BS elements, as a phosphate-responsive enhancer (EZ2). Here, we report the engineering and successful use of a phosphate-dependent bidirectional promoter, which has been designed and constructed based on the palindromic sequences of the two P1BS elements present in EZ2. This bidirectional promoter has a potential use in both plant in vitro approaches and in the generation of improved crops adapted to Pi starvation and other abiotic stresses.

A phosphate starvation-driven bidirectional promoter as a potential tool for crop improvement and in vitro plant biotechnology

Phosphate (Pi)-deficient soils are a major limitant factor for crop production in many regions of the world. Despite that plants have innovated several developmental and biochemical strategies to deal with this stress, there are still massive extensions of land which combine several abiotic stresses, including phosphate starvation, that limit their use for plant growth and food production. In several plant species, a genetic programme underlies the biochemical and developmental responses of the organism to cope with low phosphate (Pi) availability. Both protein- and miRNA-coding genes involved in the adaptative response are transcriptionally activated upon Pi starvation. Several of the responsive genes have been identified as transcriptional targets of PHR1, a transcription factor that binds a conserved cis-element called PHR1-binding site (P1BS). Our group has previously described and characterized a minimal genetic arrangement that includes two P1BS elements, as a phosphate-responsive enhancer (EZ2). Here, we report the engineering and successful use of a phosphate-dependent bidirectional promoter, which has been designed and constructed based on the palindromic sequences of the two P1BS elements present in EZ2. This bidirectional promoter has a potential use in both plant in vitro approaches and in the generation of improved crops adapted to Pi starvation and other abiotic stresses.

Central Metabolic Responses to Ozone and Herbivory Affect Photosynthesis and Stomatal Closure

Plants have evolved adaptive mechanisms that allow them to tolerate a continuous range of abiotic and biotic stressors. Tropospheric ozone (O3), a global anthropogenic pollutant, directly affects living organisms and ecosystems, including plant-herbivore interactions. In this study, we investigate the stress responses of Brassica nigra (wild black mustard) exposed consecutively to O3 and the specialist herbivore Pieris brassicae Transcriptomics and metabolomics data were evaluated using multivariate, correlation, and network analyses for the O3 and herbivory responses. O3 stress symptoms resembled those of senescence and phosphate starvation, while a sequential shift from O3 to herbivory induced characteristic plant defense responses, including a decrease in central metabolism, induction of the jasmonic acid/ethylene pathways, and emission of volatiles. Omics network and pathway analyses predicted a link between glycerol and central energy metabolism that influences the osmotic stress response and stomatal closure. Further physiological measurements confirmed that while O3 stress inhibited photosynthesis and carbon assimilation, sequential herbivory counteracted the initial responses induced by O3, resulting in a phenotype similar to that observed after herbivory alone. This study clarifies the consequences of multiple stress interactions on a plant metabolic system and also illustrates how omics data can be integrated to generate new hypotheses in ecology and plant physiology.

Identification and expression analysis of OsLPR family revealed the potential roles of OsLPR3 and 5 in maintaining phosphate homeostasis in rice

BACKGROUND

Phosphorus (P), an essential macronutrient, is often limiting in soils and affects plant growth and development. In Arabidopsis thaliana, Low Phosphate Root1 (LPR1) and its close paralog LPR2 encode multicopper oxidases (MCOs). They regulate meristem responses of root system to phosphate (Pi) deficiency. However, the roles of LPR gene family in rice (Oryza sativa) in maintaining Pi homeostasis have not been elucidated as yet.

RESULTS

Here, the identification and expression analysis for the homologs of LPR1/2 in rice were carried out. Five homologs, hereafter referred to as OsLPR1-5, were identified in rice, which are distributed on chromosome1 over a range of 65 kb. Phylogenetic analysis grouped OsLPR1/3/4/5 and OsLPR2 into two distinct sub-clades with OsLPR3 and 5 showing close proximity. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed higher expression levels of OsLPR3-5 and OsLPR2 in root and shoot, respectively. Deficiencies of different nutrients ie, P, nitrogen (N), potassium (K), magnesium (Mg) and iron (Fe) exerted differential and partially overlapping effects on the relative expression levels of the members of OsLPR family. Pi deficiency (-P) triggered significant increases in the relative expression levels of OsLPR3 and 5. Strong induction in the relative expression levels of OsLPR3 and 5 in osphr2 suggested their negative transcriptional regulation by OsPHR2. Further, the expression levels of OsLPR3 and 5 were either attenuated in ossiz1 and ospho2 or augmented in rice overexpressing OsSPX1.

CONCLUSIONS

The results from this study provided insights into the evolutionary expansion and a likely functional divergence of OsLPR family with potential roles of OsLPR3 and 5 in the maintenance of Pi homeostasis in rice.

Foliar phosphite application has minor phytotoxic impacts across a diverse range of conifers and woody angiosperms

Phytophthora plant pathogens cause tremendous damage in planted and natural systems worldwide. Phosphite is one of the only effective chemicals to control broad-scale Phytophthora disease. Little work has been done on the phytotoxic effects of phosphite application on plant communities especially in combination with plant physiological impacts. Here, we tested the phytotoxic impact of phosphite applied as foliar spray at 0, 12, 24 and 48 kg a.i. ha(-1) . Eighteen-month-old saplings of 13 conifer and angiosperm species native to New Zealand, and two exotic coniferous species were treated and the development of necrotic tissue and chlorophyll-a-fluorescence parameters (optimal quantum yield, Fv /Fm ; effective quantum yield of photosystem II, ΦPSII ) were assessed. In addition, stomatal conductance (gs ) was measured on a subset of six species. Significant necrosis assessed by digital image analysis occurred in only three species: in the lauraceous canopy tree Beilschmiedia tawa (8-14%) and the understory shrub Dodonaea viscosa (5-7%) across phosphite concentrations and solely at the highest concentration in the myrtaceous pioneer shrub Leptospermum scoparium (66%). In non-necrotic tissue, Fv /Fm , ΦPSII and gs remained unaffected by the phosphite treatment. Overall, our findings suggest minor phytotoxic effects resulting from foliar phosphite application across diverse taxa and regardless of concentration. This study supports the large-scale use of phosphite as a management tool to control plant diseases caused by Phytophthora pathogens in plantations and natural ecosystems. Long-term studies are required to ascertain potential ecological impacts of repeated phosphite applications.

Performance and Limitations of Phosphate Quantification: Guidelines for Plant Biologists

Phosphate (Pi) is a macronutrient that is essential for plant life. Several regulatory components involved in Pi homeostasis have been identified, revealing a very high complexity at the cellular and subcellular levels. Determining the Pi content in plants is crucial to understanding this regulation, and short real-time(33)Pi uptake imaging experiments have shown Pi movement to be highly dynamic. Furthermore, gene modulation by Pi is finely controlled by localization of this ion at the tissue as well as the cellular and subcellular levels. Deciphering these regulations requires access to and quantification of the Pi pool in the various plant compartments. This review presents the different techniques available to measure, visualize and trace Pi in plants, with a discussion of the future prospects.