Functional PTB phosphate transporters are present in streptophyte algae and early diverging land plants

Evolution of phosphate scouting in the terrestrial biosphere

Chemistry assigns phosphorus and its most oxidized form, inorganic phosphate, unique roles for propelling bioenergetics and metabolism in all domains of life, possibly since its very origin on prebiotic Earth. For plants, access to the vital mineral nutrient profoundly affects growth, development and vigour, thus constraining net primary productivity in natural ecosystems and crop production in modern agriculture. Unlike other major biogenic elements, the low abundance and uneven distribution of phosphate in Earth's crust result from the peculiarities of phosphorus cosmochemistry and geochemistry. Here, we trace the chemical evolution of the element, the geochemical phosphorus cycle and its acceleration during Earth's history until the present (Anthropocene) as well as during the evolution and rise of terrestrial plants. We highlight the chemical and biological processes of phosphate mobilization and acquisition, first evolved in bacteria, refined in fungi and algae and expanded into powerful phosphate-prospecting strategies during land plant colonization. Furthermore, we review the evolution of the genetic and molecular networks from bacteria to terrestrial plants, which monitor intracellular and extracellular phosphate availabilities and coordinate the appropriate responses and adjustments to fluctuating phosphate supply. Lastly, we discuss the modern global phosphorus cycle deranged by human activity and the challenges imposed ahead. This article is part of the theme issue 'Evolution and diversity of plant metabolism'.

Arsenic-Hyperaccumulator Pteris vittata Effectively Uses Sparingly-Soluble Phosphate Rock: Rhizosphere Solubilization, Nutrient Improvement, and Arsenic Accumulation

Sparingly-soluble phosphate rock (PR), a raw material for P-fertilizer production, can be effectively utilized by the As-hyperaccumulator Pteris vittata but not most plants. In this study, we investigated the associated mechanisms by measuring dissolved organic carbon (DOC) and acid phosphatase in the rhizosphere, and nutrient uptake and gene expression related to the As metabolism in P. vittata. The plants were grown in a soil containing 200 mg kg-1 As and/or 1.5% PR for 30 days. Compared to the As treatment, the P. vittata biomass was increased by 33% to 4.6 g plant-1 in the As+PR treatment, corresponding to 27% decrease in its frond oxidative stress as measured by malondialdehyde. Due to PR-enhanced DOC production in the rhizosphere, the Ca, P, and As contents in P. vittata fronds were increased by 17% to 9.7 g kg-1, 29% to 5.0 g kg-1, and 57% to 1045 mg kg-1 in the As+PR treatment, thereby supporting its better growth. Besides, PR-induced rhizosphere pH increase from 5.0 to 6.9 promoted greater P uptake by P. vittata probably via upregulating low-affinity P transporters PvPTB1;1/1;2 by 3.7-4.1 folds. Consequently, 29% lower available-P induced the 3.3-fold upregulation of high-affinity P transporter PvPht1;3 in the As+PR treatment, which was probably responsible for the 58% decrease in available-As content in the rhizosphere. Consistent with the enhanced As translocation and sequestration, arsenite antiporters PvACR3/3;3 were upregulated by 1.8-4.4 folds in the As+PR than As treatment. In short, sparingly-soluble PR enhanced the Ca, P, and As availability in P. vittata rhizosphere and improved their uptake via upregulating genes related to As metabolism, suggesting its potential application for improving phytoremediation in As-contaminated soils.

Novel Phosphate Transporter-B PvPTB1;1/1;2 Contribute to Efficient Phosphate Uptake and Arsenic Accumulation in As-Hyperaccumulator Pteris vittata

Arsenic (As) contamination in soil poses a potential threat to human health via crop uptake. As-hyperaccumulator Pteris vittata serves as a model plant to study As uptake and associated mechanisms. This study focuses on a novel P/AsV transport system mediated by low-affinity phosphate transporter-B 1 family (PTB1) in P. vittata. Here, we identified two plasma-membrane-localized PTB1 genes, PvPTB1;1/1;2, in vascular plants for the first time, which were 4.4-40-fold greater in expression in P. vittata than in other Pteris ferns. Functional complementation of a yeast P-uptake mutant and enhanced P accumulation in transgenic Arabidopsis thaliana confirmed their role in P uptake. Moreover, the expression of PvPTB1;1/1;2 facilitated the transport and accumulation of As in both yeast and A. thaliana shoots, demonstrating a comparable AsV uptake capacity. Microdissection-qPCR analysis and single-cell transcriptome analysis collectively suggest that PvPTB1;1/1;2 are specifically expressed in the epidermal cells of P. vittata roots. PTB1 may play a pivotal role in efficient P recycling during phytate secretion and hydrolysis in P. vittata roots. In summary, the dual P transport mechanisms consisting of high-affinity Pht1 and low-affinity PTB1 may have contributed to the efficient P/As uptake in P. vittata, thereby contributing to efficient phytoremediation for As-contaminated soils.

Chloroplast phosphate transporter CrPHT4-7 regulates phosphate homeostasis and photosynthesis in Chlamydomonas

In eukaryotic cells, phosphorus is assimilated and utilized primarily as phosphate (Pi). Pi homeostasis is mediated by transporters that have not yet been adequately characterized in green algae. This study reports on PHOSPHATE TRANSPORTER 4-7 (CrPHT4-7) from Chlamydomonas reinhardtii, a member of the PHT4 transporter family, which exhibits remarkable similarity to AtPHT4;4 from Arabidopsis (Arabidopsis thaliana), a chloroplastic ascorbate transporter. Using fluorescent protein tagging, we show that CrPHT4-7 resides in the chloroplast envelope membrane. Crpht4-7 mutants, generated by the CRISPR/Cas12a-mediated single-strand templated repair, show retarded growth, especially in high light, reduced ATP level, strong ascorbate accumulation, and diminished non-photochemical quenching in high light. On the other hand, total cellular phosphorous content was unaffected, and the phenotype of the Crpht4-7 mutants could not be alleviated by ample Pi supply. CrPHT4-7-overexpressing lines exhibit enhanced biomass accumulation under high light conditions in comparison with the wild-type strain. Expressing CrPHT4-7 in a yeast (Saccharomyces cerevisiae) strain lacking Pi transporters substantially recovered its slow growth phenotype, demonstrating that CrPHT4-7 transports Pi. Even though CrPHT4-7 shows a high degree of similarity to AtPHT4;4, it does not display any substantial ascorbate transport activity in yeast or intact algal cells. Thus, the results demonstrate that CrPHT4-7 functions as a chloroplastic Pi transporter essential for maintaining Pi homeostasis and photosynthesis in C. reinhardtii.

Citronellol biosynthesis in pelargonium is a multistep pathway involving progesterone 5β-reductase and/or iridoid synthase-like enzymes

Citronellol is a pleasant-smelling compound produced in rose (Rosa spp.) flowers and in the leaves of many aromatic plants, including pelargoniums (Pelargonium spp.). Although geraniol production has been well studied in several plants, citronellol biosynthesis has been documented only in crab-lipped spider orchid (Caladenia plicata) and its mechanism remains open to question in other species. We therefore profiled 10 pelargonium accessions using RNA sequencing and gas chromatography-MS analysis. Three enzymes from the progesterone 5β-reductase and/or iridoid synthase-like enzymes (PRISE) family were characterized in vitroand subsequently identified as citral reductases (named PhCIRs). Transgenic RNAi lines supported a role for PhCIRs in the biosynthesis of citronellol as well as in the production of mint-scented terpenes. Despite their high amino acid sequence identity, the 3 enzymes showed contrasting stereoselectivity, either producing mainly (S)-citronellal or a racemate of both (R)- and (S)-citronellal. Using site-directed mutagenesis, we identified a single amino acid substitution as being primarily responsible for the enzyme's enantioselectivity. Phylogenetic analysis of pelargonium PRISEs revealed 3 clades and 7 groups of orthologs. PRISEs from different groups exhibited differential affinities toward substrates (citral and progesterone) and cofactors (NADH/NADPH), but most were able to reduce both substrates, prompting hypotheses regarding the evolutionary history of PhCIRs. Our results demonstrate that pelargoniums evolved citronellol biosynthesis independently through a 3-step pathway involving PRISE homologs and both citral and citronellal as intermediates. In addition, these enzymes control the enantiomeric ratio of citronellol thanks to small alterations of the catalytic site.

Insights of intracellular/intercellular phosphate transport and signaling in unicellular green algae and multicellular land plants

Phosphorus (P) is an essential element for plant growth and development. Vacuoles play a fundamental role in the storage and remobilization of P in plants, while our understanding of the evolutionary mechanisms of creating and reusing P stores are limited. Besides, we also know very little about the coordination of intercellular P translocation, neither the inorganic phosphate (Pi) signaling nor the Pi transport patterns. Here we summarize recent advances in understanding the core elements involved in cellular and/or subcellular P homeostasis and signaling in unicellular green algae and multicellular land plants. We also propose further work that might help to uncover the high-resolution intracellular and intercellular landscape of Pi distribution and signaling in plants.

Revisiting carbon, nitrogen, and phosphorus metabolisms in microalgae for wastewater treatment

Threats posed to humans - including environmental pollution, water scarcity, food shortages, and resource crises drive a new concept to think about wastewater and its treatment. Wastewater is not only a waste but also a source of energy, renewable and/or non-renewable resources, including water itself. The nutrient in wastewater should not only be removed but also need to be upcycled. Microalgae based wastewater treatment has attracted considerable interests because algae have the potential to efficiently redirect nutrients from wastewater to the accumulated algal biomass. Additionally, microalgae are commercialized in human consumption and animal feed owing to their high content of essential amino and fatty acids, vitamins, and pigments. The whole process establishes a circular economy, totally relying on the ability of microalgae to uptake and store nutrients in wastewater, such as carbon (C), nitrogen (N), and phosphorus (P). It makes the study of the mechanisms underlying the uptake and storage of nutrients in microalgae of great interest. This review specifically aims to summarize C, N, and P metabolisms in microalgae for a better understanding of the microalgae-based wastewater treatment from the nutrient uptake pathway, and examine the key physiological factors or the operating conditions related to nutrient metabolisms that may affect the treatment efficiency. At last, I discuss the potential approaches to enhance the overall treatment performance by adjusting the critical parameters for C, N, and P metabolisms.

Transcriptional and Morpho-Physiological Responses of Marchantia polymorpha upon Phosphate Starvation

Phosphate (Pi) is a pivotal nutrient that constraints plant development and productivity in natural ecosystems. Land colonization by plants, more than 470 million years ago, evolved adaptive mechanisms to conquer Pi-scarce environments. However, little is known about the molecular basis underlying such adaptations at early branches of plant phylogeny. To shed light on how early divergent plants respond to Pi limitation, we analyzed the morpho-physiological and transcriptional dynamics of Marchantia polymorpha upon Pi starvation. Our phylogenomic analysis highlights some gene networks present since the Chlorophytes and others established in the Streptophytes (e.g., PHR1–SPX1 and STOP1–ALMT1, respectively). At the morpho-physiological level, the response is characterized by the induction of phosphatase activity, media acidification, accumulation of auronidins, reduction of internal Pi concentration, and developmental modifications of rhizoids. The transcriptional response involves the induction of MpPHR1, Pi transporters, lipid turnover enzymes, and MpMYB14, which is an essential transcription factor for auronidins biosynthesis. MpSTOP2 up-regulation correlates with expression changes in genes related to organic acid biosynthesis and transport, suggesting a preference for citrate exudation. An analysis of MpPHR1 binding sequences (P1BS) shows an enrichment of this cis regulatory element in differentially expressed genes. Our study unravels the strategies, at diverse levels of organization, exerted by M. polymorpha to cope with low Pi availability.

Genome-Wide Identification, Expression Profiling, and Evolution of Phosphate Transporter Gene Family in Green Algae

Phosphorus (P) is an essential nutrient for plant growth and development. Phosphate transporters (PHTs) are trans-membrane proteins that mediate the uptake and translocation of phosphate (Pi) in green plants. The PHT family including PHT1, PHT2, PHT3 and PHT4 subfamilies are well-studied in land plants; however, PHT genes in green algae are poorly documented and not comprehensively identified. Here, we analyzed the PHTs in a model green alga Chlamydomonas reinhardtii and found 25 putative PHT genes, which can be divided into four subfamilies. The subfamilies of CrPTA, CrPTB, CrPHT3, and CrPHT4 contain four, eleven, one, and nine genes, respectively. The structure, chromosomal distribution, subcellular localization, duplication, phylogenies, and motifs of these genes were systematically analyzed in silico. Expression profile analysis showed that CrPHT genes displayed differential expression patterns under P starvation condition. The expression levels of CrPTA1 and CrPTA3 were down-regulated, while the expression of most CrPTB genes was up-regulated under P starvation, which may be controlled by CrPSR1. The transcript abundance of most CrPHT3 and CrPHT4 genes was not significantly affected by P starvation except CrPHT4-3, CrPHT4-4, and CrPHT4-6. Our results provided basic information for understanding the evolution and features of the PHT family in green algae.

Structure of the sodium-dependent phosphate transporter reveals insights into human solute carrier SLC20

Inorganic phosphate (P_i) is a fundamental and essential element for nucleotide biosynthesis, energy supply, and cellular signaling in living organisms. Human phosphate transporter (hPiT) dysfunction causes numerous diseases, but the molecular mechanism underlying transporters remains elusive. We report the structure of the sodium-dependent phosphate transporter from Thermotoga maritima (TmPiT) in complex with sodium and phosphate (TmPiT-Na/Pi) at 2.3-angstrom resolution. We reveal that one phosphate and two sodium ions (Pi-2Na) are located at the core of TmPiT and that the third sodium ion (Na_fore) is located near the inner membrane boundary. We propose an elevator-like mechanism for sodium and phosphate transport by TmPiT, with the TmPiT-Na/Pi complex adopting an inward occluded conformation. We found that disease-related hPiT variants carry mutations in the corresponding sodium- and phosphate-binding residues identified in TmPiT. Our three-dimensional structure of TmPiT provides a framework for understanding PiT dysfunction and for future structure-based drug design.

Neofunctionalisation of basic helix-loop-helix proteins occurred when embryophytes colonised the land

ROOT HAIR DEFECTIVE SIX-LIKE (RSL) genes control the development of structures from single cells at the surface of embryophytes (land plants) such as rhizoids and root hairs. RSL proteins constitute a subclass (VIIIc) of the basic helix-loop-helix (bHLH) class VIII transcription factor family. The Charophyceae form the only class of streptophyte algae with tissue-like structures and rhizoids. To determine if the function of RSL genes in the control of cell differentiation in embryophytes was inherited from a streptophyte algal ancestor, we identified the single class VIII bHLH gene from the charophyceaen alga Chara braunii (CbbHLHVIII). CbbHLHVIII is sister to the RSL proteins; they constitute a monophyletic group. Expression of CbbHLHVIII does not compensate for loss of RSL functions in Marchantia polymorpha or Arabidopsis thaliana. In C. braunii CbbHLHVIII is expressed at sites of morphogenesis but not in rhizoids. This finding indicates that C. braunii class VIII protein is functionally different from land plant RSL proteins. This result suggests that the function of RSL proteins in cell differentiation at the plant surface evolved by neofunctionalisation in the land plants lineage after its divergence from its last common ancestor with C. braunii, at or before the colonisation of the land by embryophytes.

Functional Analysis of Four Terpene Synthases in Rose-Scented Pelargonium Cultivars (Pelargonium × hybridum) and Evolution of Scent in the Pelargonium Genus

Pelargonium genus contains about 280 species among which at least 30 species are odorant. Aromas produced by scented species are remarkably diverse such as rose, mint, lemon, nutmeg, ginger and many others scents. Amongst odorant species, rose-scented pelargoniums, also named pelargonium rosat, are the most famous hybrids for their production of essential oil (EO), widely used by perfume and cosmetic industries. Although EO composition has been extensively studied, the underlying biosynthetic pathways and their regulation, most notably of terpenes, are largely unknown. To gain a better understanding of the terpene metabolic pathways in pelargonium rosat, we generated a transcriptome dataset of pelargonium leaf and used a candidate gene approach to functionally characterise four terpene synthases (TPSs), including a geraniol synthase, a key enzyme responsible for the biosynthesis of the main rose-scented terpenes. We also report for the first time the characterisation of a novel sesquiterpene synthase catalysing the biosynthesis of 10-epi-γ-eudesmol. We found a strong correlation between expression of the four genes encoding the respective TPSs and accumulation of the corresponding products in several pelargonium cultivars and species. Finally, using publically available RNA-Seq data and de novo transcriptome assemblies, we inferred a maximum likelihood phylogeny from 270 pelargonium TPSs, including the four newly discovered enzymes, providing clues about TPS evolution in the Pelargonium genus. Notably, we show that, by contrast to other TPSs, geraniol synthases from the TPS-g subfamily conserved their molecular function throughout evolution.

Effect of phosphate nutrition on growth, physiology and phosphate transporter expression of cucumber seedlings

Although abundantly present in soils, inorganic phosphate (Pi) acquisition by plants is highly dependent on the transmembrane phosphate transporter (PT) gene family. Cucumber (Cucumis sativus) requires a large amount of phosphorus (P). The purpose of this study was to isolate the CsPT2-1 from cucumber roots, and to determine the influence of Pi nutrition on cucumber growth, metabolism and transcript levels of CsPT2-1 in tissues. Full length CsPT2-1 was cloned and phylogenetically identified. In two greenhouse experiments, P-deficient seedlings provided with low or high P concentrations were sampled at 10 and 21 days post treatment, respectively. Addition of P dramatically reduced growth of roots but not shoots. Supplying plants with high P resulted in increased total protein in leaves. Acid phosphatase activity increased significantly in leaves at any rate higher than 4 mM P. Increasing P concentration had a notable decrease in glucose concentrations in leaves of plants supplied with >0.5 mM P. In roots, glucose and starch concentrations increased with increasing P supply. Steady-state transcript levels of CsPT2-1 were high in P-deprived roots, but declined when plants were provided >10 mM P. To our knowledge, this is the first report focusing on a PT and its expression levels in cucumber.

Ion and metabolite transport in the chloroplast of algae: lessons from land plants

Chloroplasts are endosymbiotic organelles and play crucial roles in energy supply and metabolism of eukaryotic photosynthetic organisms (algae and land plants). They harbor channels and transporters in the envelope and thylakoid membranes, mediating the exchange of ions and metabolites with the cytosol and the chloroplast stroma and between the different chloroplast subcompartments. In secondarily evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Here, we provide an overview of the current knowledge about ion and metabolite transport in the chloroplast from algae. The main aspects reviewed are localization and activity of the transport proteins from algae and/or of homologues from other organisms including land plants. Most chloroplast transporters were identified in the green alga Chlamydomonas reinhardtii, reside in the envelope and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play role in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and roles in the chloroplast are suggested. A deep knowledge in this field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.