Light-dependent activation of phosphoenolpyruvate carboxylase by reversible phosphorylation in cluster roots of white lupin plants: diurnal control in response to photosynthate supply

Metabolic footprints in phosphate-starved plants

Plants' requirement of Phosphorus (P) as an essential macronutrient is obligatory for their normal growth and metabolism. Besides restricting plants' primary growth, P depletion affects both primary and secondary metabolism and leads to altered levels of sugars, metabolites, amino acids, and other secondary compounds. Such metabolic shifts help plants optimize their metabolism and growth under P limited conditions. Under P deprivation, both sugar levels and their mobilization change that influences the expression of Pi starvation-inducible genes. Increased sugar repartitioning from shoot to root help root growth and organic acids secretion that in turn promotes phosphate (Pi) uptake from the soil. Other metabolic changes such as lipid remodeling or P reallocation from older to younger leaves release the P from its bound forms in the cell. In this review, we summarize the metabolic footprinting of Pi-starved plants with respect to the benefits offered by such metabolic changes to intracellular Pi homeostasis.

Potential metabolic mechanisms for inhibited chloroplast nitrogen assimilation under high CO2

Improving photosynthesis is considered a major and feasible option to dramatically increase crop yield potential. Increased atmospheric CO2 concentration often stimulates both photosynthesis and crop yield, but decreases protein content in the main C3 cereal crops. This decreased protein content in crops constrains the benefits of elevated CO2 on crop yield and affects their nutritional value for humans. To support studies of photosynthetic nitrogen assimilation and its complex interaction with photosynthetic carbon metabolism for crop improvement, we developed a dynamic systems model of plant primary metabolism, which includes the Calvin-Benson cycle, the photorespiration pathway, starch synthesis, glycolysis-gluconeogenesis, the tricarboxylic acid cycle, and chloroplastic nitrogen assimilation. This model successfully captures responses of net photosynthetic CO2 uptake rate (A), respiration rate, and nitrogen assimilation rate to different irradiance and CO2 levels. We then used this model to predict inhibition of nitrogen assimilation under elevated CO2. The potential mechanisms underlying inhibited nitrogen assimilation under elevated CO2 were further explored with this model. Simulations suggest that enhancing the supply of α-ketoglutarate is a potential strategy to maintain high rates of nitrogen assimilation under elevated CO2. This model can be used as a heuristic tool to support research on interactions between photosynthesis, respiration, and nitrogen assimilation. It also provides a basic framework to support the design and engineering of C3 plant primary metabolism for enhanced photosynthetic efficiency and nitrogen assimilation in the coming high-CO2 world.

Soybean responds to phosphate starvation through reversible protein phosphorylation

Phosphorus (P) deficiency is considered as a major constraint on crop production. Although a set of adaptative strategies are extensively suggested in soybean (Glycine max) to phosphate (Pi) deprivation, molecular mechanisms underlying reversible protein phosphorylation in soybean responses to P deficiency remains largely unclear. In this study, isobaric tags for relative and absolute quantitation, combined with liquid chromatography and tandem mass spectrometry analysis was performed to identify differential phosphoproteins in soybean roots under Pi sufficient and deficient conditions. A total of 427 phosphoproteins were found to exhibit differential accumulations, with 213 up-regulated and 214 down-regulated. Among them, a nitrate reductase, GmNR4 exhibiting increased phosphorylation levels under low Pi conditions, was further selected to evaluate the effects of phosphorylation on its nitrate reductase activity and subcellular localization. Mutations of GmNR4 phosphorylation levels significantly influenced its activity in vitro, but not for its subcellular localization. Taken together, identification of differential phosphoproteins reveled the complex regulatory pathways for soybean adaptation to Pi starvation through reversible protein phosphorylation.

Cluster roots of Embothrium coccineum modify their metabolism and show differential gene expression in response to phosphorus supply

Embothrium coccineum produces cluster roots (CR) to acquire sparingly soluble phosphorus (P) from the soil through the exudation of organic compounds. However, the physiological mechanisms involved in carbon drainage through its roots, as well as the gene expression involved in the biosynthesis of carboxylates and P uptake, have not been explored. In this work, we evaluated the relationship between carboxylate exudation rate and phosphoenolpyruvate carboxylase (PEPC) activity in roots of E. coccineum seedlings grown in a nutrient-poor volcanic substrate. Second, we evaluated CR formation and the expression of genes involved in the production of carboxylates (PEPC) and P uptake (PHT1) in E. coccineum seedlings grown under three different P supplies in hydroponic conditions. Our results showed that the carboxylate exudation rate was higher in CR than in non-CR, which was consistent with the higher PEPC activity in CR. We found higher CR formation in seedlings grown at 5 μM of P supply, concomitant with a higher expression of EcPEPC and EcPHT1 in CR than in non-CR. Overall, mature CR of E. coccineum seedlings growing on volcanic substrates poor in nutrients modify their metabolism compared to non-CR, enhancing carboxylate biosynthesis and subsequent carboxylate exudation. Additionally, transcriptional responses of EcPEPC and EcPHT1 were induced simultaneously when E. coccineum seedlings were grown in P-limited conditions that favored CR formation. Our results showed, for the first time, changes at the molecular level in CR of a species of the Proteaceae family, demonstrating that these root structures are highly specialized in P mobilization and uptake.

Recent insights into the metabolic adaptations of phosphorus-deprived plants

Inorganic phosphate (Pi) is an essential macronutrient required for many fundamental processes in plants, including photosynthesis and respiration, as well as nucleic acid, protein, and membrane phospholipid synthesis. The huge use of Pi-containing fertilizers in agriculture demonstrates that the soluble Pi levels of most soils are suboptimal for crop growth. This review explores recent advances concerning the understanding of adaptive metabolic processes that plants have evolved to alleviate the negative impact of nutritional Pi deficiency. Plant Pi starvation responses arise from complex signaling pathways that integrate altered gene expression with post-transcriptional and post-translational mechanisms. The resultant remodeling of the transcriptome, proteome, and metabolome enhances the efficiency of root Pi acquisition from the soil, as well as the use of assimilated Pi throughout the plant. We emphasize how the up-regulation of high-affinity Pi transporters and intra- and extracellular Pi scavenging and recycling enzymes, organic acid anion efflux, membrane remodeling, and the remarkable flexibility of plant metabolism and bioenergetics contribute to the survival of Pi-deficient plants. This research field is enabling the development of a broad range of innovative and promising strategies for engineering phosphorus-efficient crops. Such cultivars are urgently needed to reduce inputs of unsustainable and non-renewable Pi fertilizers for maximum agronomic benefit and long-term global food security and ecosystem preservation.

Effects of root morphology, respiration and carboxylate exudation on carbon economy in two non-mycorrhizal lupines under phosphorus deficiency

Under phosphorus (P) deficiency, Lupinus albus develops cluster roots that allow efficient P acquisition, while L. angustifolius without cluster roots also grows well. Both species are non-mycorrhizal. We quantitatively examined the carbon budgets to investigate the different strategies of these species. Biomass allocation, respiratory rates, protein amounts and carboxylate exudation rates were examined in hydroponically-grown plants treated with low (1 μM; P1) or high (100 μM; P100) P. At P1, L. albus formed cluster roots, and L. angustifolius increased biomass allocation to the roots. The respiratory rates of the roots were faster in L. albus than in L. angustifolius. The protein amounts of the non-phosphorylating alternative oxidase and uncoupling protein were greater in the cluster roots of L. albus at P1 than in the roots at P100, but similar between the P treatments in L. angustifolius roots. At P1, L. albus exuded carboxylates at a faster rate than L. angustifolius. The carbon budgets at P1 were surprisingly similar between the two species, which is attributed to the contrasting root growth and development strategies. L. albus developed cluster roots with rapid respiratory and carboxylate exudation rates, while L. angustifolius developed a larger root system with slow respiratory and exudation rates.

Leaf Phosphorus Concentration Regulates the Development of Cluster Roots and Exudation of Carboxylates in Macadamia integrifolia

Phosphorus (P) deficiency induces cluster-root formation and carboxylate exudation in most Proteaceae. However, how external P supply regulates these root traits in Macadamia integrifolia remains unclear. Macadamia plants were grown hydroponically with seven P levels to characterize biomass allocation, cluster-root development, and exudation of carboxylates and acid phosphatases. Plant biomass increased with increasing P supply, peaking at 5 μM P, was the same at 5-25 μM P, and declined at 50-100 μM P. Leaf P concentration increased with increasing P supply, but shoot biomass was positively correlated with leaf P concentration up to 0.7-0.8 mg P g^-1 dry weight (DW), and declined with further increasing leaf P concentration. The number of cluster roots declined with increasing P supply, with a critical value of leaf P concentration at 0.7-0.8 mg P g^-1 DW. We found a similar trend for carboxylate release, with a critical value of leaf P concentration at 0.5 mg g^-1 DW, but the activity of acid phosphatases showed a gradually-decreasing trend with increasing P supply. Our results suggest that leaf P concentration regulates the development and functioning of cluster roots, with a critical P concentration of 0.5-0.8 mg g^-1, above which macadamia growth is inhibited.

Coimmunoprecipitation of reversibly glycosylated polypeptide with sucrose synthase from developing castor oilseeds

The sucrose synthase (SUS) interactome of developing castor oilseeds (COS; Ricinus communis) was assessed using coimmunoprecipitation (co-IP) with anti-(COS RcSUS1)-IgG followed by proteomic analysis. A 41-kDa polypeptide (p41) that coimmunoprecipitated with RcSUS1 from COS extracts was identified as reversibly glycosylated polypeptide-1 (RcRGP1) by LC-MS/MS and anti-RcRGP1 immunoblotting. Reciprocal Far-western immunodot blotting corroborated the specific interaction between RcSUS1 and RcRGP1. Co-IP using anti-(COS RcSUS1)-IgG and clarified extracts from other developing seeds as well as cluster (proteoid) roots of white lupin and Harsh Hakea consistently recovered 90 kDa SUS polypeptides along with p41/RGP as a SUS interactor. The results suggest that SUS interacts with RGP in diverse sink tissues to channel UDP-glucose derived from imported sucrose into hemicellulose and/or glycoprotein/glycolipid biosynthesis.

Plant roots: understanding structure and function in an ocean of complexity

Background The structure and function of plant roots and their interactions with soil are exciting scientific frontiers that will ultimately reveal much about our natural systems, global water and mineral and carbon cycles, and help secure food supplies into the future. This Special Issue presents a collection of papers that address topics at the forefront of our understanding of root biology.

Scope These papers investigate how roots cope with drought, nutrient deficiencies, toxicities and soil compaction as well as the interactions that roots have with soil microorganisms. Roots of model plant species, annual crops and perennial species are studied in short-term experiments through to multi-year trials. Spatial scales range from the gene up to farming systems and nutrient cycling. The diverse, integrated approaches described by these studies encompass root genetics as applied to soil management, as well as documenting the signalling processes occurring between roots and shoots and between roots and soil.

Conclusions This Special Issue on roots presents invited reviews and research papers covering a span of topics ranging from fundamental aspects of anatomy, growth and water uptake to roots in crop and pasture systems. Understanding root structure and function and adaptation to the abiotic and biotic stresses encountered in field conditions is important for sustainable agricultural production and better management of natural systems.

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

Trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is an essential signal metabolite in plants, linking growth and development to carbon status. The Suc-Tre6P nexus model postulates that Tre6P is both a signal and negative feedback regulator of Suc levels, forming part of a mechanism to maintain Suc levels within an optimal range and functionally comparable to the insulin-glucagon system for regulating blood Glc levels in animals. The target range and sensitivity of the Tre6P-Suc feedback control circuit can be adjusted according to the cell type, developmental stage, and environmental conditions. In source leaves, Tre6P modulates Suc levels by affecting Suc synthesis, whereas in sink organs it regulates Suc consumption. In illuminated leaves, Tre6P influences the partitioning of photoassimilates between Suc, organic acids, and amino acids via posttranslational regulation of phosphoenolpyruvate carboxylase and nitrate reductase. At night, Tre6P regulates the remobilization of leaf starch reserves to Suc, potentially linking starch turnover in source leaves to carbon demand from developing sink organs. Use of Suc for growth in developing tissues is strongly influenced by the antagonistic activities of two protein kinases: SUC-NON-FERMENTING-1-RELATED KINASE1 (SnRK1) and TARGET OF RAPAMYCIN (TOR). The relationship between Tre6P and SnRK1 in developing tissues is complex and not yet fully resolved, involving both direct and indirect mechanisms, and positive and negative effects. No direct connection between Tre6P and TOR has yet been described. The roles of Tre6P in abiotic stress tolerance and stomatal regulation are also discussed.