Excess cation uptake, and extrusion of protons and organic acid anions by Lupinus albus under phosphorus deficiency

Transcriptomics Provide Insights into Early Responses to Sucrose Signaling in Lupinus albus, a Model Plant for Adaptations to Phosphorus and Iron Deficiency

Phosphorus (P) and iron (Fe) deficiency are major limiting factors for plant productivity worldwide. White lupin (Lupinus albus L.) has become a model plant for understanding plant adaptations to P and Fe deficiency, because of its ability to form cluster roots, bottle-brush-like root structures play an important role in the uptake of P and Fe from soil. However, little is known about the signaling pathways involved in sensing and responding to P and Fe deficiency. Sucrose, sent in increased concentrations from the shoot to the root, has been identified as a long-distance signal of both P and Fe deficiency. To unravel the responses to sucrose as a signal, we performed Oxford Nanopore cDNA sequencing of white lupin roots treated with sucrose for 10, 15, or 20 min compared to untreated controls. We identified a set of 17 genes, including 2 bHLH transcription factors, that were up-regulated at all three time points of sucrose treatment. GO (gene ontology) analysis revealed enrichment of auxin and gibberellin responses as early as 10 min after sucrose addition, as well as the emerging of ethylene responses at 20 min of sucrose treatment, indicating a sequential involvement of these hormones in plant responses to sucrose.

Environmental conditions affect the nutritive value and alkaloid profiles of Lupinus forage: Opportunities and threats for sustainable ruminant systems

The identification of crops that simultaneously contribute to the global protein supply and mitigate the effects of climate change is an urgent matter. Lupins are well adapted to nutrient-poor or contaminated soils, tolerate various abiotic stresses, and present relevant traits for acting as ecosystem engineers. Lupins are best studied for their seeds, but their full foraging potential needs further evaluation. This study evaluated the effects of location and sowing date on forage production, proximate composition, and the detailed mineral and alkaloid profiles of three species of Lupinus (L. albus cv. Estoril, L. angustifolius cv. Tango, and L. luteus cv. Cardiga). Sowing date and location and their interaction with the plant species significantly affected the vast majority of measured parameters, emphasizing the effects of climate and soil conditions on these crops. The relatively high crude protein and in vitro digestibility support the potential of the lupin species studied as sustainable forage protein sources in diets for ruminant animals. The content of individual essential macro and trace elements was below the maximum tolerable levels for cattle and sheep. Lupanine, smipine, and sparteine were the most abundant quinolizidine alkaloids in L. albus cv. Estoril, lupanine, and sparteine in L. angustifolius cv. Tango, and lupinine, gramine, ammodendrine, and sparteine in L. luteus cv. Cardiga. Based on the maximum tolerable levels of total quinolizidine alkaloid intake, the dietary inclusion of forages of L. albus cv. Estoril and L. angustifolius cv. Tango does not pose a risk to the animals, but the high alkaloid content of L. luteus cv. Cardiga may compromise its utilization at high levels in the diet. Overall, the results reveal a high potential for lupins as protein forage sources well adapted to temperate regions and soils with lower fertility, with a relevant impact on livestock sustainability in a climate change era.

Two-dimensional modeling of CO2 mineral trapping through the oxalate‑carbonate pathway: Influence of the root system model

Two-dimensional reactive transport models, one with a simplified root system and the other accounting for dynamically evolving root architecture, were constructed to examine the influence of model complexity on capturing the effect of soil-root dynamics relating to the Oxalate Carbonate Pathway (OCP) of the Iroko tree over 170 years. Oxidation of oxalate from fallen tree tissue by soil bacteria enables local soil pH increase, leading to the sequestration of atmospheric carbon in carbonate minerals (calcite) in the shallow soil surrounding the tree. Simulations of both root models corroborate previous one-dimensional models of the OCP focused on Ca and C mass balance, where high weathering rates of Ca-containing silicate minerals in bedrock, along with contributions from groundwater, provided sufficient Ca for precipitation of observed quantities of calcite. Both simulations demonstrate the development of a distinct high pH zone where oxalate is oxidized, Ca accumulates, and calcite precipitates (OCP zone); and a low pH zone where roots collect Ca, later returned to the top soil as calcium oxalate (Total Root Extent/TRE zone) via litterfall. While the extent of OCP zone development near the ground surface was very similar between simulations, differences in localized root water uptake between the two approaches resulted in variation in water and solute transport and influenced the geometry of the OCP zone at depth, with implications for calcite precipitation in the soil. Trends in CO2 and O2 partial pressures in the OCP zone were mirrored in the TRE zone, suggesting linkage between the two zones with regard to gas transport. Near the end of the tree's lifespan, results indicate that soil permeability decreases due to calcite precipitation may limit O2 ingress and availability in the shallow soil, while trapping CO2 released from the oxidation of organics in the shallow soil, with implications for the long-term sustainability of the OCP itself.

Omics approaches in effective selection and generation of potential plants for phytoremediation of heavy metal from contaminated resources

Soil and water pollution, rapid industrialization, contaminated irrigation-water, increased waste-production and surge in agricultural land leads to the accumulation of Heavy Metals (HM) with time. HM contamination has raised concern over the past years and new remediation strategies are required to deal with it. HM-contaminated soil is often used for the production of food, which makes a gateway for toxic metals into the food-chain, thereby affecting food security and human health. To avoid HM-toxicity, decontamination of important resources is essential. Therefore, exploring phytoremediation for the removal, decomposition and detoxification of hazardous metals from HM-contaminated sites is of great significance. Hyper-accumulator plants can efficiently remove HMs. However, despite many hyper-accumulator plant species, there is a research gap in the studies of phytotechnology. Hence biotechnological efforts advocating omics studies i.e. genomics, transcriptomics, proteomics, metabolomics and phenomics are in order, the purpose being to select and enhance a plant's potential for the process of phytoremediation to be more effective. There is a need to study newly developed high-efficiency hyper-accumulator plants as HM-decontaminator candidates for phytoremediation and phytomining. Therefore, this review focuses on various strategies and bio-technological methods for the removal of HM contaminants from sites, with emphasis on the advancement of phytoremediation, along with applications in cleaning up various toxic pollutants.

Cu Dynamics in the Rhizosphere of Native Tropical Species: Assessing the Potential for Phytostabilization in Mining-Impacted Soils

The use of native plants for reforestation and/or remediation in areas contaminated by mining is a technique with low implantation and maintenance costs. The success of this practice depends on the plant species and geochemical processes at the soil–plant interface (e.g., rhizosphere). This study evaluated the potential of spontaneous species for mobilizing and altering mineral and metal dynamics in the rhizosphere of Cu-rich soils resulting from the abandoned Pedra Verde mine in NE Brazil. Rhizosphere and bulk soil samples were collected from five shrubby/arboreal species. The pH, organic matter content, Cu fractionation, mineralogical characterization, and Cu content in the leaves and roots of all studied species were determined. In addition, the bioaccumulation factor (BCF) and translocation factor (TF) were used to evaluate the potential of these species for Cu hyperaccumulation. The Cu concentration in leaf plant tissues varied from 18 to 34 mg kg−1, and all plants presented TF and BCF < 1, indicating that the species were not Cu hyperaccumulators. However, the root exudates induce mineral dissolution, indicating potential Cu accumulation in the roots (from 36 to 249 mg kg−1). Combretum aff. pisoniodes Taub was the species with the greatest potential for decreasing Cu bioavailability and phytostabilization. Our findings indicate the potential of native Brazilian plants for growth in Cu-contaminated soil. These findings may be used for reforestation programs.

Plant-microbe Interactions in Rhizospheric soil Treated with Different Quality Organic Residues

The objective of this research was to study plant-microbe interactions in rhizospheric soil treated with different quality organic residues in short-term incubation of the soil and subsequent planting of maize crop. The treatments combinations were, 1) untreated soil (control; CT), 2) groundnut stover (GN) with high nitrogen (N) but low lignin (L) and polyphenol (PP), 3) tamarind leaf and petiole litter (TM) with medium N, L and PP, 4) rice straw (RS) with low N, L and PP but high cellulose, 5) GN+TM, 6) GN+RS and 7) TM+RS. Single and mixed residue additions of GN and TM, both high and intermediate quality, resulted in higher soil microbial properties and nutrients than the application of RS as a low quality. Accordingly, the application of the former group increased microbial abundances (i.e., bacteria, archaea, and fungi), elevated the enzymes related to the decomposition of organic residue (i.e., invertase, protease, phenoloxidase and peroxidase activity), and enhanced soil nutrients and plant growth. The results indicated that the chemical compositions (N, L, and PP) of the organic residues amendment are key factors regulating soil microbial abundance and enzyme activity both in after incorporation and after planting. Moreover, bacterial and archaeal abundance, and microbial activities including soil respiration, invertase, protease, and peroxidase activity in the soil after planting higher than those in the soil before planting.

A transcription factor STOP1-centered pathway coordinates ammonium and phosphate acquisition in Arabidopsis

Phosphorus (P) is an indispensable macronutrient required for plant growth and development. Natural phosphate (Pi) reserves are finite, and a better understanding of Pi utilization by crops is therefore vital for worldwide food security. Ammonium has long been known to enhance Pi acquisition efficiency in agriculture; however, the molecular mechanisms coordinating Pi nutrition and ammonium remains unclear. Here, we reveal that ammonium is a novel initiator that stimulates the accumulation of a key regulatory protein, STOP1, in the nuclei of Arabidopsis root cells under Pi deficiency. We show that Pi deficiency promotes ammonium uptake mediated by AMT1 transporters and causes rapid acidification of the root surface. Rhizosphere acidification-triggered STOP1 accumulation activates the excretion of organic acids, which help to solubilize Pi from insoluble iron or calcium phosphates. Ammonium uptake by AMT1 transporters is downregulated by a CIPK23 protein kinase whose expression is directly modulated by STOP1 when ammonium reaches toxic levels. Taken together, we have identified a STOP1-centered regulatory network that links external ammonium with efficient Pi acquisition from insoluble phosphate sources. These findings provide a framework for developing possible strategies to improve crop production by enhancing the utilization of non-bioavailable nutrients in soil.

Legume-microbiome interactions unlock mineral nutrients in regrowing tropical forests

Legume trees form an abundant and functionally important component of tropical forests worldwide with N2-fixing symbioses linked to enhanced growth and recruitment in early secondary succession. However, it remains unclear how N2-fixers meet the high demands for inorganic nutrients imposed by rapid biomass accumulation on nutrient-poor tropical soils. Here, we show that N2-fixing trees in secondary Neotropical forests triggered twofold higher in situ weathering of fresh primary silicates compared to non-N2-fixing trees and induced locally enhanced nutrient cycling by the soil microbiome community. Shotgun metagenomic data from weathered minerals support the role of enhanced nitrogen and carbon cycling in increasing acidity and weathering. Metagenomic and marker gene analyses further revealed increased microbial potential beneath N2-fixers for anaerobic iron reduction, a process regulating the pool of phosphorus bound to iron-bearing soil minerals. We find that the Fe(III)-reducing gene pool in soil is dominated by acidophilic Acidobacteria, including a highly abundant genus of previously undescribed bacteria, Candidatus Acidoferrum, genus novus. The resulting dependence of the Fe-cycling gene pool to pH determines the high iron-reducing potential encoded in the metagenome of the more acidic soils of N2-fixers and their nonfixing neighbors. We infer that by promoting the activities of a specialized local microbiome through changes in soil pH and C:N ratios, N2-fixing trees can influence the wider biogeochemical functioning of tropical forest ecosystems in a manner that enhances their ability to assimilate and store atmospheric carbon.

Impacts of elevated CO2 on plant resistance to nutrient deficiency and toxic ions via root exudates: A review

Elevated atmospheric CO₂ (eCO₂) concentration can increase root exudation into soils, which improves plant tolerance to abiotic stresses. This review used a meta-analysis to assess effect sizes of eCO₂ on both efflux rates and total amounts of some specific root exudates, and dissected whether eCO₂ enhances plant's resistance to nutrient deficiency and ion toxicity via root exudates. Elevated CO₂ did not affect efflux rates of total dissolved organic carbon, a measure of combined root exudates per unit of root biomass or length, but increased the efflux amount of root systems per plant by 31% which is likely attributed to increased root biomass (29%). Elevated CO₂ increased efflux rates of soluble-sugars, carboxylates, and citrate by 47%, 111%, and 16%, respectively, but did not affect those of amino acids and malate. The increased carbon allocation to roots, increased plant requirements of mineral nutrients, and heightened detoxification responses to toxic ions under eCO₂ collectively contribute to the increased efflux rates despite lacking molecular evidence. The increased efflux rates of root exudates under eCO₂ were closely associated with improved nutrient uptake whilst less studies have validated the associations between root exudates and resistance to toxic ions of plants when grown under eCO₂. Future studies are required to reveal how climate change (eCO₂) affect the efflux of specific root exudates, particularly organic anions, the corresponding nutrient uptake and toxic ion resistance from plant molecular biology and soil microbial ecology perspectives.

Metabolite Profiling and Network Analysis Reveal Coordinated Changes in Low-N Tolerant and Low-N Sensitive Maize Genotypes under Nitrogen Deficiency and Restoration Conditions

Nitrogen (N), applied in the form of a nitrogenous fertilizer, is one of the main inputs for agricultural production. Food production is closely associated with the application of N. However, the application of nitrogenous fertilizers to agricultural fields is associated with heavy production of nitrous oxide because agricultural crops can only utilize 30-40% of applied N, leaving behind unused 60-70% N in the environment. The global warming effect of this greenhouse gas is approximately 300 times more than of carbon dioxide. Under the present scenario of climate change, it is critical to maintain the natural balance between food production and environmental sustainability by targeting traits responsible for improving nitrogen-use-efficiency (NUE). Understanding of the molecular mechanisms behind the metabolic alterations due to nitrogen status needs to be addressed. Additionally, mineral nutrient deficiencies and their associated metabolic networks have not yet been studied well. Given this, the alterations in core metabolic pathways of low-N tolerant (LNT) and low-N sensitive (LNS) genotypes of maize under N-deficiency and their efficiency of recovering the changes upon resupplying N were investigated by us, using the GC-MS and LC-MS based metabolomic approach. Significant genotype-specific changes were noted in response to low-N. The N limitation affected the whole plant metabolism, most significantly the precursors of primary metabolic pathways. These precursors may act as important targets for improving the NUE. Limited availability of N reduced the levels of N-containing metabolites, organic acids and amino acids, but soluble sugars increased. Major variations were encountered in LNS, as compared to LNT. This study has revealed potential metabolic targets in response to the N status, which are indeed the prospective targets for crop improvement.

Variations of dissolved organic matter and Cu fractions in rhizosphere soil induced by the root activities of castor bean

The root soil interaction affects metal bioavailability in the rhizosphere, thus impacting the uptake and accumulation of metals by plants. In this study, a greenhouse experiment using a root-bag technique for castor bean plants was conducted to determine the i) rhizosphere effect on the fractions of Cu, and ii) the characteristics of dissolved organic matter (DOM) in the rhizosphere soil. Results showed that the Cu concentration in the leaves, stems, and roots was 15.41, 6.71, and 47.85 mg kg-1, respectively, in the control and reached up to 96.5, 254.9, and 3204 mg kg-1 in Cu400 treatment, respectively. After cultivating castor bean plants, the concentration of acid exchangeable Cu in rhizosphere soil was higher than that in the bulk soil for the same Cu addition, whereas the concentrations of reducible Cu, oxidizable Cu, and residual Cu in the rhizosphere soil were all lower than those in the bulk soil, respectively. In comparison to the bulk soil, the pH decreased while the total nitrogen and total carbon concentrations both increased in the rhizosphere soil. Moreover, the concentrations of total low molecular weight organic acids (LMWOAs) and total amino acids in the rhizosphere soil of the Cu treatments increased by between 15.18% to 47.17% and 36.35%-200%, respectively with respect to the control. The less complex DOM with a high LMWOAs concentration in the rhizosphere soil shifted the soil Cu from a relative stable fraction to available fractions.

Potentially Toxic Element Availability and Risk Assessment of Cadmium Dietary Exposure after Repeated Croppings of Brassica juncea in a Contaminated Agricultural Soil

Phytoextraction of potentially toxic elements (PTEs) is eco-friendly and cost-effective for remediating agricultural contaminated soils, but plants can only take up bioavailable forms of PTEs, thus meaning that bioavailability is the key for the feasibility of this technique. With the aims to assess the phytoextraction efficiency on an agricultural soil contaminated by Cr, Zn, Cd, and Pb and the changes induced by plants in PTE bioavailability and in human health risk due to dietary exposure, in this work we carried out a mesocosm experiment with three successive croppings of Brassica juncea, each followed by Rocket salad as bioindicator. Brassica juncea extracted more Zn and Cd than Cr and Pb, significantly reducing, after three repeated croppings, the bioavailable element concentrations in soil as a result of plant uptake and soil pH changes. For Cd, this reduction did not bring the bioavailable amounts obtained by soil extraction with NH4NO3 below the trigger value of 0.1 mg kg−1 set by some European countries. Nevertheless, the Hazard Quotient for Cd in Rocket salad decreased across three repeated croppings of Brassica juncea. This indicated the beginning of a re-equilibration process between soil PTE forms of different bioavailability, that are in a dynamic equilibrium, thus stressing the need to monitor the possible regeneration of the most readily bioavailable pool.

Impact of phenanthrene on primary metabolite profiling in root exudates and maize mucilage

This study was conducted to assess the impact of polycyclic aromatic hydrocarbon on the composition of rhizodeposits. Maize was submitted to increasing phenanthrene (PHE) concentrations in the substrate (0, 25, 50, and 100 mg PHE.kg^-1 of dry sand). After 6 weeks of cultivation, two types of rhizodeposit solution were collected. The first one, called rhizospheric sand extract, resulted from the extraction of root adhering sand in order to collect mucilage and associated compounds. The second one, the diffusate solution, was collected by the diffusion of exudates from roots soaked in water. The impact of phenanthrene on maize morphology and functioning was measured prior to the analysis of the main components of the rhizodeposit solutions, by measuring total carbon, protein, amino acid, and sugars as well as by determining about 40 compounds using GC-MS and LC-MS. As maize exposure to PHE increased, different trends were observed in the two rhizodeposit solutions. In the diffusate solution, we measured a global increase of metabolites exudation like carbohydrates, amino acids, and proteins except for some monoglycerides and organic acids which exudation decreased in the presence of PHE. In the rhizospheric sand extract, we witnessed a decrease in carbohydrates and amino acids secretion as well as in fatty and organic acids when plants were exposed to PHE. Many of the compounds measured, like organic acids, carbohydrates, amino acids, or fatty acids, could directly or indirectly drive PAHs availability in soils with particular consequences for their degradation.

An investigation of the effects of capping on internal phosphorus release from sediments under rooted macrophytes (Phragmites australis) revegetation

In eutrophic lake restorations, in situ capping is an often considered method to control sediment internal phosphorus (P) pollution for mitigating eutrophication status. Subsequent aquatic macrophyte revegetation can directly derive P from the sediment for growth. However, the effects of capping with clean soils on internal P release from sediments under rooted aquatic macrophyte revegetation are still unclear. In the present study, the influences of sediment P remobilization by P. australis revegetation on P inactivation by capping were investigated based on an entire growth simulation study. Our findings showed during the growth of P. australis, tests conducted on total phosphorous (TP), calcium-bound P (Ca-P), loosely bound P (loose-P), organic P (Org-P), and iron-adsorbed P (Fe-P) found significant changes (p < 0.001). Specifically, the mean contents of TP and Ca-P decreased by 291.1 and 224.2 mg kg^-1, respectively, while those of Fe-P increased from 26.4 to 124.8 mg kg^-1. In addition, sediment mobile-P contents increased coincidentally with the growth of P. australis during the whole course of experiment. Further analysis indicated calculated diffusion fluxes of soluble reactive phosphorus (SRP) generally increased with incubation time, although capping effectively induced the reduction of SRP concentration in pore water and its release to waters. Therefore, sediment P remobilization by P. australis revegetation was able to enhance P lability in lake sediments, with intermediate activation ability compared to other correlated water bodies. This phenomenon was most likely attributed to solubilization of sediment P by organic acids secreted from P. australis rhizosphere. Overall, sediment P remobilization by rooted macrophytes is unfavorable for capping to control internal P release to water column during eutrophic lake restorations.

Soil geochemical factors regulate Cd accumulation by metal hyperaccumulating Noccaea caerulescens (J. Presl & C. Presl) F.K. Mey in field-contaminated soils

Cadmium contamination in soil is a substantial global problem, and of significant concern due to high food-chain transfer. Cadmium hyperaccumulators are of particular interest because of their ability to tolerate and take up significant amounts of heavy metal pollution from soils. One particular plant, Noccaea caerulescens (formerly, Thlaspi caerulescens), has been extensively studied in terms of its capacity to accumulate heavy metals (specifically Zn and Cd), though these studies have primarily utilized hydroponic and metal-spiked model soil systems. We studied Cd and nutrient uptake by two N. caerulescens ecotypes, Prayon (Zn-only hyperaccumulator) and Ganges (Zn- and Cd-hyperaccumulator) in four long-term field-contaminated soils. Our data suggest that individual soil properties such as total soil Cd, Zn:Cd molar ratio, or soil pH do not accurately predict Cd uptake by hyperaccumulating plants. Additionally, total Cd uptake by the hyperaccumulating Ganges ecotype was substantially less than its physiological capacity, which is likely due to Cd-containing solid phases (primarily iron oxides) and pH that play an important role in regulating and limiting Cd solubility. Increased P accumulation in the Ganges leaves, and greater plant Fe accumulation from Cd-containing soils suggests that rhizosphere alterations via proton, and potentially organic acid, secretion may also play a role in nutrient and Cd acquisition by the plant roots. The current study highlights the role that soil geochemical factors play in influencing Cd uptake by hyperaccumulating plants. While these plants may have high physiological potential to accumulate metals from contaminated soils, individual soil geochemical factors and the plant-soil interactions in that soil will dictate the actual amount of phytoextractable metal. This underlines the need for site-specific understanding of metal-containing solid phases and geochemical properties of soils before undertaking phytoextraction efforts.

Characterization of the soybean GmALMT family genes and the function of GmALMT5 in response to phosphate starvation

A potential mechanism to enhance utilization of sparingly soluble forms of phosphorus (P) is the root secretion of malate, which is mainly mediated by the ALMT gene family in plants. In this study, a total of 34 GmALMT genes were identified in the soybean genome. Expression patterns diverged considerably among GmALMTs in response to phosphate (Pi) starvation in leaves, roots and flowers, with expression altered by P availability in 26 of the 34 GmALMTs. One root-specific GmALMT whose expression was significantly enhanced by Pi-starvation, GmALMT5, was studied in more detail to determine its possible role in soybean P nutrition. Analysis of GmALMT5 tissue expression patterns, subcellular localization, and malate exudation from transgenic soybean hairy roots overexpressing GmALMT5, demonstrated that GmALMT5 is a plasma membrane protein that mediates malate efflux from roots. Furthermore, both growth and P content of transgenic Arabidopsis overexpressing GmALMT5 were significantly increased when sparingly soluble Ca-P was used as the external P source. Taken together, these results indicate that members of the soybean GmALMT gene family exhibit diverse responses to Pi starvation. One member of this family, GmALMT5, might contribute to soybean P efficiency by enhancing utilization of sparingly soluble P sources under P limited conditions.

Model-Based Analysis of the Long-Term Effects of Fertilization Management on Cropland Soil Acidification

Agricultural soil acidification in China is known to be caused by the over-application of nitrogen (N) fertilizers, but the long-term impacts of different fertilization practices on intensive cropland soil acidification are largely unknown. Here, we further developed the soil acidification model VSD+ for intensive agricultural systems and validated it against observed data from three long-term fertilization experiments in China. The model simulated well the changes in soil pH and base saturation over the last 20 years. The validated model was adopted to quantify the contribution of N and base cation (BC) fluxes to soil acidification. The net NO₃^- leaching and NO₄⁺input accounted for 80% of the proton production under N application, whereas one-third of acid was produced by BC uptake when N was not applied. The simulated long-term (1990-2050) effects of different fertilizations on soil acidification showed that balanced N application combined with manure application avoids reduction of both soil pH and base saturation, while application of calcium nitrate and liming increases these two soil properties. Reducing NH₄⁺ input and NO₃^- leaching by optimizing N management and increasing BC inputs by manure application thus already seem to be effective approaches to mitigating soil acidification in intensive cropland systems.

Stability of Chloropyromorphite in Ryegrass Rhizosphere as Affected by Root-Secreted Low Molecular Weight Organic Acids

Understanding the stability of chloropyromorphite (CPY) is of considerable benefit for improving risk assessment and remediation strategies in contaminated water and soil. The stability of CPY in the rhizosphere of phosphorus-deficient ryegrass was evaluated to elucidate the role of root-secreted low molecular weight organic acids (LMWOAs) on the dissolution of CPY. Results showed that CPY treatments significantly reduced the ryegrass biomass and rhizosphere pH. The presence of calcium nitrate extractable lead (Pb) and phosphorus (P) suggested that CPY in the rhizosphere could be bioavailable, because P and Pb uptake by ryegrass potentially provided a significant concentration gradient that would promote CPY dissolution. Pb accumulation and translocation in ryegrass was found to be significantly higher in P-sufficient conditions than in P-deficient conditions. CPY treatments significantly enhanced root exudation of LMWOAs irrigated with P-nutrient solution or P-free nutrient solution. Oxalic acid was the dominant species in root-secreted LMWOAs of ryegrass under P-free nutrient solution treatments, suggesting that root-secreted oxalic acid may be the driving force of root-induced dissolution of CPY. Hence, our work, provides clarifying hints on the role of LMWOAs in controlling the stability of CPY in the rhizosphere.

Rhizosphere priming effect on soil organic carbon decomposition under plant species differing in soil acidification and root exudation

Effects of rhizosphere properties on the rhizosphere priming effect (RPE) are unknown. This study aimed to link species variation in RPE with plant traits and rhizosphere properties. Four C3 species (chickpea, Cicer arietinum; field pea, Pisum sativum; wheat, Triticum aestivum; and white lupin, Lupinus albus) differing in soil acidification and root exudation, were grown in a C4 soil. The CO2 released from soil was trapped using a newly developed NaOH-trapping system. White lupin and wheat showed greater positive RPEs, in contrast to the negative RPE produced by chickpea. The greatest RPE of white lupin was in line with its capacity to release root exudates, whereas the negative RPE of chickpea was attributed to its great ability to acidify rhizosphere soil. The enhanced RPE of field pea at maturity might result from high nitrogen deposition and release of structural root carbon components following root senescence. Root biomass and length played a minor role in the species variation in RPE. Rhizosphere acidification was shown to be an important factor affecting the magnitude and direction of RPE. Future studies on RPE modelling and mechanistic understanding of the processes that regulate RPE should consider the effect of rhizosphere pH.

Impact of biochar and root-induced changes on metal dynamics in the rhizosphere of Agrostis capillaris and Lupinus albus

Rhizosphere interactions are deemed to play a key role in the success of phytoremediation technologies. Here, the effects of biochar and root-induced changes in the rhizosphere of Agrostis capillaris L. and Lupinus albus L. on metal (Cd, Pb and Zn) dynamics were investigated using a biotest on a 2mm soil layer and a sequential extraction procedure (Tessier's scheme). In the bulk soil, the application of 5% biochar significantly reduced the exchangeable pool of metals primarily due to a liming effect which subsequently promoted the metal shift into the carbonate-bound pool. However, metals were re-mobilized in the rhizosphere of both A. capillaris and L. albus due to root-induced acidification which counteracted the liming effect of biochar. As a result, the concentrations of metals in roots and shoots of both plants were not significantly reduced by the application of biochar. Although the study should be considered a worst-case scenario because experimental conditions induced the intensification of rhizosphere processes, the results highlight that changes in rhizosphere pH can impact the effectiveness of biochar to immobilize metals in soil. Biochar has thus a potential as amendment for reducing metal uptake by plants, provided the acidification of the rhizosphere is minimized.

Replace, reuse, recycle: improving the sustainable use of phosphorus by plants

The 'phosphorus problem' has recently received strong interest with two distinct strands of importance. The first is that too much phosphorus (P) is entering into waste water, creating a significant economic and ecological problem. Secondly, while agricultural demand for phosphate fertilizer is increasing to maintain crop yields, rock phosphate reserves are rapidly declining. Unravelling the mechanisms by which plants sense, respond to, and acquire phosphate can address both problems, allowing the development of crop plants that are more efficient at acquiring and using limited amounts of phosphate while at the same time improving the potential of plants and other photosynthetic organisms for nutrient recapture and recycling from waste water. In this review, we attempt to synthesize these important but often disparate parts of the debate in a holistic fashion, since solutions to such a complex problem require integrated and multidisciplinary approaches that address both P supply and demand. Rapid progress has been made recently in our understanding of local and systemic signalling mechanisms for phosphate, and of expression and regulation of membrane proteins that take phosphate up from the environment and transport it within the plant. We discuss the current state of understanding of such mechanisms involved in sensing and responding to phosphate stress. We also discuss approaches to improve the P-use efficiency of crop plants and future direction for sustainable use of P, including use of photosynthetic organisms for recapture of P from waste waters.

Interactive effects of phosphorus deficiency and exogenous auxin on root morphological and physiological traits in white lupin (Lupinus albus L.)

White lupin (Lupinus albus) exhibits strong root morphological and physiological responses to phosphorus (P) deficiency and auxin treatments, but the interactive effects of P and auxin in regulating root morphological and physiological traits are not fully understood. This study aimed to assess white lupin root traits as influenced by P (0 or 250 μmol L(-1)) and auxin (10(-8) mol L(-1) NAA) in nutrient solution. Both P deficiency and auxin treatments significantly altered root morphological traits, as evidenced by reduced taproot length, increased number and density of first-order lateral roots, and enhanced cluster-root formation. Changes in root physiological traits were also observed, i.e., increased proton, citrate, and acid phosphatase exudation. Exogenous auxin enhanced root responses and sensitivity to P deficiency. A significant interplay exists between P and auxin in the regulation of root morphological and physiological traits. Principal component analysis showed that P availability explained 64.8% and auxin addition 21.3% of the total variation in root trait parameters, indicating that P availability is much more important than auxin in modifying root responses of white lupin. This suggests that white lupin can coordinate root morphological and physiological responses to enhance acquisition of P resources, with an optimal trade-off between root morphological and physiological traits regulated by external stimuli such as P availability and auxin.

Low pH, aluminum, and phosphorus coordinately regulate malate exudation through GmALMT1 to improve soybean adaptation to acid soils

Low pH, aluminum (Al) toxicity, and low phosphorus (P) often coexist and are heterogeneously distributed in acid soils. To date, the underlying mechanisms of crop adaptation to these multiple factors on acid soils remain poorly understood. In this study, we found that P addition to acid soils could stimulate Al tolerance, especially for the P-efficient genotype HN89. Subsequent hydroponic studies demonstrated that solution pH, Al, and P levels coordinately altered soybean (Glycine max) root growth and malate exudation. Interestingly, HN89 released more malate under conditions mimicking acid soils (low pH, +P, and +Al), suggesting that root malate exudation might be critical for soybean adaptation to both Al toxicity and P deficiency on acid soils. GmALMT1, a soybean malate transporter gene, was cloned from the Al-treated root tips of HN89. Like root malate exudation, GmALMT1 expression was also pH dependent, being suppressed by low pH but enhanced by Al plus P addition in roots of HN89. Quantitative real-time PCR, transient expression of a GmALMT1-yellow fluorescent protein chimera in Arabidopsis protoplasts, and electrophysiological analysis of Xenopus laevis oocytes expressing GmALMT1 demonstrated that GmALMT1 encodes a root cell plasma membrane transporter that mediates malate efflux in an extracellular pH-dependent and Al-independent manner. Overexpression of GmALMT1 in transgenic Arabidopsis, as well as overexpression and knockdown of GmALMT1 in transgenic soybean hairy roots, indicated that GmALMT1-mediated root malate efflux does underlie soybean Al tolerance. Taken together, our results suggest that malate exudation is an important component of soybean adaptation to acid soils and is coordinately regulated by three factors, pH, Al, and P, through the regulation of GmALMT1 expression and GmALMT1 function.

How a phosphorus-acquisition strategy based on carboxylate exudation powers the success and agronomic potential of lupines (Lupinus, Fabaceae)

Lupines (Lupinus species; Fabaceae) are an ancient crop with great potential to be developed further for high-protein feed and food, cover crops, and phytoremediation. Being legumes, they are capable of symbiotically fixing atmospheric nitrogen. However, Lupinus species appear to be nonmycorrhizal or weakly mycorrhizal at most; instead some produce cluster roots, which release vast amounts of phosphate-mobilizing carboxylates (inorganic anions). Other lupines produce cluster-like roots, which function in a similar manner, and some release large amounts of carboxylates without specialized roots. These traits associated with nutrient acquisition make lupines ideally suited for either impoverished soils or soils with large amounts of phosphorus that is poorly available for most plants, e.g., acidic or alkaline soils. Here we explore how common the nonmycorrhizal phosphorus-acquisition strategy based on exudation of carboxylates is in the genus Lupinus, concluding it is very likely more widespread than generally acknowledged. This trait may partly account for the role of lupines as pioneers or invasive species, but also makes them suitable crop plants while we reach "peak phosphorus".

An RNA-Seq transcriptome analysis of orthophosphate-deficient white lupin reveals novel insights into phosphorus acclimation in plants

Phosphorus, in its orthophosphate form (P(i)), is one of the most limiting macronutrients in soils for plant growth and development. However, the whole-genome molecular mechanisms contributing to plant acclimation to P(i) deficiency remain largely unknown. White lupin (Lupinus albus) has evolved unique adaptations for growth in P(i)-deficient soils, including the development of cluster roots to increase root surface area. In this study, we utilized RNA-Seq technology to assess global gene expression in white lupin cluster roots, normal roots, and leaves in response to P(i) supply. We de novo assembled 277,224,180 Illumina reads from 12 complementary DNA libraries to build what is to our knowledge the first white lupin gene index (LAGI 1.0). This index contains 125,821 unique sequences with an average length of 1,155 bp. Of these sequences, 50,734 were transcriptionally active (reads per kilobase per million reads ≥ 3), representing approximately 7.8% of the white lupin genome, using the predicted genome size of Lupinus angustifolius as a reference. We identified a total of 2,128 sequences differentially expressed in response to P(i) deficiency with a 2-fold or greater change and P ≤ 0.05. Twelve sequences were consistently differentially expressed due to P(i) deficiency stress in three species, Arabidopsis (Arabidopsis thaliana), potato (Solanum tuberosum), and white lupin, making them ideal candidates to monitor the P(i) status of plants. Additionally, classic physiological experiments were coupled with RNA-Seq data to examine the role of cytokinin and gibberellic acid in P(i) deficiency-induced cluster root development. This global gene expression analysis provides new insights into the biochemical and molecular mechanisms involved in the acclimation to P(i) deficiency.

Synergistic interactions between Glomus mosseae and Bradyrhizobium japonicum in enhancing proton release from nodules and hyphae

Soybean (Glycine max L. Merr.) seedlings were inoculated with Glomus mosseae (GM) and Bradyrhizobium japonicum (BJ) together or separately to study the effect of interactions on net H(+) effluxes of nodules or extraradical hyphae by in vivo vibrating electrode techniques. GM promoted three-fold the H(+) effluxes of nodules on mycorrhizal lateral roots and BJ increased eight-fold the net H(+) effluxes of hyphae developing in the vicinity of nodules on lateral roots. Increments in plant P content were positively and linearly correlated with the net H(+) efflux of nodules and hyphae. It is concluded that increased H(+) effluxes of nodules resulted from enhanced nitrogenase activities induced by the presence of the AM fungus in lateral roots. The results point to additive effects of interactions between mycorrhizal fungi and rhizobia in increasing the extent of acidification of the "nodulesphere" and the hyposphere.

Time and substrate dependent exudation of carboxylates by Lupinus albus L. and Brassica napus L

Root exudates influence significantly physical, chemical and biological characteristics of rhizosphere soil. Their qualitative and quantitative composition is affected by environmental factors such as pH, soil type, oxygen status, light intensity, soil temperature, plant growth, nutrient availability and microorganisms. The aim of the present study was to assess the influence of growth substrate and plant age on the release of carboxylates from Lupinus albus L. and Brassica napus L. Both plant species were studied in continuously percolated microcosms filled with either sand, soil or sand + soil (1:1) mixture. Soil solution was collected every week at 7, 14, 21, 28 and 35 days after planting (DAP). Carboxylate concentrations were determined by reversed-phase liquid chromatography - electrospray ionization - time of flight mass spectrometry (LC-ESI-TOFMS). Oxalate, citrate, succinate, malate and maleate were detected in soil solutions of both plant species. Their concentrations were correlated with the physiological status of the plant and the growth substrate. Oxalate was the predominant carboxylate detected within the soil solution of B. napus plants while oxalate and citrate were the predominant ones found in the soil solutions of L. albus plants. The sampling determination of carboxylates released by plant roots with continuous percolation systems seems to be promising as it is a non-destructive method and allows sampling and determination of soluble low molecular weight organic compounds derived from root exudation as well as the concentration of soluble nutrients, which both might reflect the nutritional status of plants.

Interaction of arsenic and phosphate on their uptake and accumulation in Chinese brake fern

Interactive effects of arsenate (As (V)) and phosphate (Pi) were investigated under hydroponic culture. Arsenic concentrations in fronds and roots of Chinese brake fern (Pteris vittata L.) significantly (p < 0.05) increased with increasing As (V), but decreased (p < 0.05) with increasing Pi in nutrient solution. Phosphate uptake was significantly (p < 0.05) inhibited by 1000 micromol L(-1) As (V). Under 100 micromol L(-1) As (V), frond phosphorus (P) increased at 100 and 1000 micromol L(-1) Pi, and root P increased at 250 micromol L(-1) Pi exposures. Arsenic and P concentrations in fronds and roots of Chinese brake fern were negatively correlated (p < 0.05). Arsenate treatments enhanced As and P transport to fronds, while increasing Pi inhibited their transportation, with highest frond P and As (%) obtained under 100 micromol L(-1) treatment. pH values in nutrient solution increased with increasing exposure time, but decreased with increasing Pi levels. Dissolved organic carbon (DOC) contents (dry weight) in nutrient solution decreased with increasing Pi levels, both for treatments with and without As (V). Arsenate at 1000 micromol L(-1) significantly (p < 0.05) increased DOC contents, especially for treatment without Pi. Six organic acids were detected in root exudates of Chinese brake fern, with oxalic and malic acids being most dominant.

Influence of indian mustard (Brassica juncea) on rhizosphere soil solution chemistry in long-term contaminated soils: a rhizobox study

This study investigated the influence of Indian mustard (Brassica juncea) root exudation on soil solution properties (pH, dissolved organic carbon (DOC), metal solubility) in the rhizosphere using a rhizobox. Measurement was conducted following the cultivation of Indian mustard in the rhizobox filled four different types of heavy metal contaminated soils (two alkaline soils and two acidic soils). The growth of Indian mustard resulted in a significant increase (by 0.6 pH units) in rhizosphere soil solution pH of acidic soils and only a slight increase (< 0.1 pH units) in alkaline soils. Furthermore, the DOC concentration increased by 17-156 mg/L in the rhizosphere regardless of soil type and the extent of contamination, demonstrating the exudation of DOC from root. Ion chromatographic determination showed a marked increase in the total dissolved organic acids (OAs) in rhizosphere. While root exudates were observed in all soils, the amount of DOC and OAs in soil solution varied considerably amongst different soils, resulting in significant changes to soil solution metals in the rhizosphere. For example, the soil solution Cd, Cu, Pb, and Zn concentrations increased in the rhizosphere of alkaline soils compared to bulk soil following plant cultivation. In contrast, the soluble concentrations of Cd, Pb, and Zn in acidic soils decreased in rhizosphere soil when compared to bulk soils. Besides the influence of pH and DOC on metal solubility, the increase of heavy metal concentration having high stability constant such as Cu and Pb resulted in a release of Cd and Zn from solid phase to liquid phase.

Mobilization and acquisition of sparingly soluble P-Sources by Brassica cultivars under P-starved environment II. Rhizospheric pH changes, redesigned root architecture and pi-uptake kinetics

Non-mycorrhizal Brassica does not produce specialized root structures such as cluster or dauciform roots but is an effective user of P compared with other crops. In addition to P-uptake, utilization and remobilization activity, acquisition of orthophosphate (Pi) from extracellular sparingly P-sources or unavailable bound P-forms can be enhanced by biochemical rescue mechanisms such copious H(+)-efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H(+) ATPase and anion channels triggered by P-starvation. To visualize the dissolution of sparingly soluble Ca-phosphate (Ca-P), newly formed Ca-P was suspended in agar containing other essential nutrients. With NH(4)(+) applied as the N source, the precipitate dissolved in the root vicinity can be ascribed to rhizosphere acidification, whereas no dissolution occurred with nitrate nutrition. To observe in situ rhizospheric pH changes, images were recorded after embedding the roots in agar containing bromocresol purple as a pH indicator. P-tolerant cultivar showed a greater decrease in pH than the sensitive cultivar in the culture media (the appearance of typical patterns of various colors of pH indicator in the root vicinity), and at stress P-level this acidification was more prominent. In experiment 2, low P-tolerant class-I cultivars (Oscar and Con-II) showed a greater decrease in solution media pH than low P-sensitive class-II (Gold Rush and RL-18) cultivars, and P-contents of the cultivars was inversely related to decrease in culture media pH. To elucidate P-stress-induced remodeling and redesigning in a root architectural system, cultivars were grown in rhizoboxes in experiment 3. The elongation rates of primary roots increased as P-supply increased, but the elongation rates of the branched zones of primary roots decreased. The length of the lateral roots and topological index values increased when cultivars were exposed to a P-stress environment. To elucidate Pi-uptake kinetics, parameters related to P influx: maximal transport rate (V(max)), the Michaelis-Menten constant (K(m)), and the external concentration when net uptake is zero (C(min)) were tested in experiment 4. Lower K(m) and C(min) values were better indicative of the P-uptake ability of the class-I cultivars, evidencing their adaptability to P-starved environmental cues. In experiment 5, class-I cultivars exuded two- to threefold more carboxylates than class-II cultivars under the P-stress environment. The amount and types of carboxylates exuded from the roots of P-starved plants differed from those of plants grown under P-sufficient conditions. Nevertheless, the exudation rate of both class-I and class-II cultivars decreased with time, and the highest exudation rate was found after the first 4 h of carboxylates collection. Higher P uptake by class-I cultivars was significantly related to the drop in root medium pH, which can be ascribed to H(+)-efflux from the roots supplied with sparingly soluble rock-P and Ca(3)(PO(4))(2). These classical rescue strategies provided the basis of P-solubilization and acquisition from sparingly soluble P-sources by Brassica cultivars to thrive in a typically stressful environment.

Plasma membrane H-ATPase-dependent citrate exudation from cluster roots of phosphate-deficient white lupin

White lupin (Lupinus albus L.) is able to grow on soils with sparingly available phosphate (P) by producing specialized structures called cluster roots. To mobilize sparingly soluble P forms in soils, cluster roots release substantial amounts of carboxylates and concomitantly acidify the rhizosphere. The relationship between acidification and carboxylate exudation is still largely unknown. In the present work, we studied the linkage between organic acids (malate and citrate) and proton exudations in cluster roots of P-deficient white lupin. After the illumination started, citrate exudation increased transiently and reached a maximum after 5 h. This effect was accompanied by a strong acidification of the external medium and alkalinization of the cytosol, as evidenced by in vivo nuclear magnetic resonance (NMR) analysis. Fusicoccin, an activator of the plasma membrane (PM) H+-ATPase, stimulated citrate exudation, whereas vanadate, an inhibitor of the H+-ATPase, reduced citrate exudation. The burst of citrate exudation was associated with an increase in expression of the LHA1 PM H+-ATPase gene, an increased amount of H+-ATPase protein, a shift in pH optimum of the enzyme and post-translational modification of an H+-ATPase protein involving binding of activating 14-3-3 protein. Taken together, our results indicate a close link in cluster roots of P-deficient white lupin between the burst of citrate exudation and PM H+-ATPase-catalysed proton efflux.

Is there a critical level of shoot phosphorus concentration for cluster-root formation in Lupinus albus?

This study examined the effects of localised phosphorus (P) supply on cluster-root formation and citrate exudation in white lupin (Lupinus albus L. cv. Kiev Mutant). White lupin plants were grown in nutrient solutions with a range of P supplies in a split-root system with one root half deprived of P and the other root supplied with 0, 2, 5, 8, 10 or 75 μm P. Plants were also grown in soil with or without organic matter added to the top layer. The proportion of cluster roots as a percentage of the total root biomass decreased similarly on both root halves with increasing P supply in the hydroponic experiments. More than 18% of the P taken up by the P-supplied root halves was incorporated into the P-deprived halves. Irrespective of the P supply or organic matter addition in the experiments, the proportion of cluster roots and the rate of citrate exudation decreased sharply with increasing P concentration in the shoots up to a critical level of 2-3 mg P g^-1 dry weight. In contrast, the rate of proton release was higher in P-deprived root halves than in P-supplied ones. The formation of cluster roots is regulated by shoot P concentration with a critical level of 2-3 mg g^-1. Citrate exudation is predominantly governed by shoot P status, whereas proton release strongly responds to local P supply.

Confirmation and determination of carboxylic acids in root exudates using LC-ESI-MS

Reversed-phase liquid chromatography with UV detection is of limited applicability in the separation and identification of carboxylic acids because of the column's poor separation efficiency and the non-selective nature of the UV detector. To address this issue, RP-LC with electrospray ionization mass spectrometry has been explored for the confirmation and determination of carboxylic acids in plant root exudates, with ESI-MS providing structural information, high selectivity, and high sensitivity. The separation of 10 carboxylic acids (pyruvic, lactic, malonic, maleic, fumaric, succinic, malic, tartaric, trans-aconitic, and citric acid) was performed on a C(18) column using an eluent containing 0.1% (v/v) acetic acid within 10 min, where the acidic eluent not only suppressed the ionization of the carboxylic acids to be retained on the column, but was also compatible with ESI-MS detection. In addition, an additional standard was used to overcome the matrix effect. The results showed that peak areas correlated linearly with the concentration of carboxylic acids over the range 0.05-10 mg/L. The detection limits of target acids (signal-to-noise S/N ratio of 3) ranged from 20 to 30 microg/L. Finally, the proposed method was used for the confirmation and determination of low-molecular-weight carboxylic acids in plant root exudates, and provided a simple analytical procedure, including sample processing, fast separation, and high specificity and sensitivity.

Relationship between lead uptake by lettuce and water-soluble low-molecular-weight organic acids in rhizosphere as influenced by transpiration

The relationship between Pb uptake by leaf lettuce ( Lactuca sativa L.) and water-soluble low-molecular-weight organic acids (LMWOAs) in rhizosphere, as influenced by transpiration (high and low), has been studied. Studies were carried out by culturing lettuce plants grown for 2 weeks in pots filled with quartz sand mixed with anion-exchange resin and then for 30 days in a greenhouse. The potted lettuce plants were subjected to stress by the addition of Pb(NO 3) 2 solutions (100, 200, and 300 mg of Pb L (-1)) and by high and low transpiration treatments for another 10-day period. Blank experiments (without addition of Pb(NO 3) 2 solutions to the pots) were also run. There were no significant differences in the growth of the plants with the addition of Pb(NO 3) 2 solutions in either of the transpirations studies. Uptake of Pb by the shoots and roots of the plants was found to be proportional to the concentration of Pb solutions added, and more accumulation was observed in the roots than in the shoots at the end of days 3 and 10. High transpiration caused more Pb uptake than did low transpiration. One volatile acid (propionic acid) and nine nonvolatile acids (lactic, glycolic, oxalic, succinic, fumaric, oxalacetic, d-tartaric, trans-aconitic, and citric acids) in rhizosphere quartz sand or anion-exchange resin were identified and quantified by gas chromatography analysis with a flame ionization detector. The amount of LMWOAs in rhizosphere quartz sand or anion-exchange resin increased with higher amounts of Pb in quartz sand solution and also with longer duration of the study. The total quantities of the LMWOAs in the rhizosphere quartz sand or anion-exchange resin were significantly higher under high and low transpiration with a 300 mg of Pb L (-1) solution addition at the end of day 10. Compared with our previous related studies (published work), the present study shows that the presence of LMWOAs in rhizosphere does not significantly affect Pb uptake by lettuce plants under high and low transpiration. A physiological mechanism of the roots of lettuce plants governing the relationship between Pb contamination level and quantity of water-soluble LMWOAs in rhizosphere quartz sand and resin, as influenced by transpiration, was proposed.

Genotypic variation in phosphorus utilisation of soybean [Glycine max (L.) Murr.] grown in various sparingly soluble P sources

This study compared the utilisation of sparingly soluble P among soybean genotypes selected from 3 geographical regions with contrasting soil pH. Plants of 5 genotypes from each region were grown for 72 days in soil columns supplied with hydroxyapetite (Ca-P), AlPO4 (Al-P), or FePO4 (Fe-P) at a rate of 25 mg P/kg soil. NaH2PO4 (Na-P) was used as control at the same rate. Shoot weights ranged from 2.4 to 5.9 g/plant. On average, the genotypes selected from calcareous soils of north-west China produced the highest shoot biomass whereas those from neutral soils of the north-east region the least. Root biomass and root surface area followed the same trend. In contrast, the root-to-shoot weight ratio was highest in Fe-P and lowest in Na-P while root length in Na-P and Fe-P was greater than in the other P forms. The genotypes from the north-east region had higher P concentration in the shoot than those from the other 2 regions. Total P uptake ranged from 9.0 to 15.9 mg/plant for Na-P, and from 6.3 to 12.4 for the sparingly soluble P. Average total P uptake was the highest for the genotypes from the north-west region, and was greatest in Na-P and lowest in Fe-P. The genotypes from the north-east region displayed the greatest ability to use sparingly soluble P, and those from the north-west the least relative to Na-P. Total P uptake from the sparingly soluble P correlated highly with plant biomass production, N2 fixation and nodulation, and seed P, while the relative P uptake correlated highly with P concentration in shoots but neither correlated with root carboxylate release nor leachate pH. The results suggest that there is a substantial genotypic variation in utilisation of sparingly soluble P, which is related to early vigour, nodulation, and seed P reserve but not to origin site pH or root exudation.

Triticum aestivum shows a greater biomass response to a supply of aluminium phosphate than Lupinus albus, despite releasing fewer carboxylates into the rhizosphere

The relationship between carboxylate release and the ability of plants to access phosphorus from AlPO4 and to detoxify aluminium was studied by comparing species with a low and high rate of carboxylate release, Triticum aestivum (wheat) and Lupinus albus (white lupin), respectively. Species were supplied with P at 10, 20, 40 or 100 mg P kg-1 sand in the form of sparingly soluble AlPO4 or soluble KH2PO4; control plants did not receive any P. Triticum aestivum was significantly better than L. albus at accessing P from AlPO4, despite accumulating fewer carboxylates in its rhizosphere. Rhizosphere pH of L. albus did not vary with form or level of P supply, while the rhizosphere pH of T. aestivum increased with the level of P supplied. Based on the evidence in the present study, a model is proposed to explain the poor performance of L. albus, whereby the release of carboxylates and associated protons reduces the chelating ability of exuded carboxylates, thus reducing P acquisition and increasing Al toxicity.

A link between citrate and proton release by proteoid roots of white lupin (Lupinus albus L.) grown under phosphorus-deficient conditions?

White lupin (Lupinus albus L.) is able to acclimate to phosphorus deficiency by forming proteoid roots that release a large amount of citric acid, resulting in the mobilization of sparingly soluble soil phosphate in the rhizosphere. The mechanisms responsible for the release of organic acids have not been fully elucidated. In this study, we focused on the link between citrate and malate release and the release of H+ and other inorganic ions by proteoid roots of white lupin. The release of citrate was closely correlated with the release of H+, K+, Na+ and Mg2+, but not with that of Ca2+. The stoichiometric relationships between citrate release and the release of H+, K+, Na+ and Mg2+ were 1 : 1.3, 1 : 2.1, 1 : 1.5 and 1 : 0.47, respectively. Similar correlations were found between exudation of malate and cations. During 30 min incubation, fusicoccin addition stimulated H+ and malate release, but not citrate release. A concomitant stimulation of H+, malate and citrate release was measured after 60 min incubation. Vanadate inhibited the release of H+ and malate, but not that of citrate. Anthracene-9-carboxylic acid, an anion channel blocker, caused a concomitant decrease in release of citrate, malate and H+. We conclude that for export of citrate across the plasma membrane of proteoid root cells, H+ release is not strictly related to citrate release. Other cations such as K+ and Na+ can also serve as counterions for citrate release. In contrast, malate release shows a strong H+ release dependency.

Citrate-permeable channels in the plasma membrane of cluster roots from white lupin

White lupin (Lupinus albus) is well adapted to phosphorus deficiency by developing cluster roots that release large amounts of citrate into the rhizosphere to mobilize the sparingly soluble phosphorus. To determine the mechanism underlying citrate release from cluster roots, we isolated protoplasts from different types of roots of white lupin plants grown in phosphorus-replete (+P) and phosphorus-deficient (-P) conditions and used the patch-clamp technique to measure the whole-cell currents flowing across plasma membrane of these protoplasts. Two main types of anion conductance were observed in protoplasts prepared from cluster root tissue: (1) an inwardly rectifying anion conductance (IRAC) activated by membrane hyperpolarization, and (2) an outwardly rectifying anion conductance (ORAC) that became more activated with membrane depolarization. Although ORAC was an outward rectifier, it did allow substantial inward current (anion efflux) to occur. Both conductances showed citrate permeability, with IRAC being more selective for citrate3- than Cl- (PCit/PCl = 26.3), while ORAC was selective for Cl- over citrate (PCl/PCit = 3.7). Both IRAC and ORAC were sensitive to the anion channel blocker anthracene-9-carboxylic acid. These currents were also detected in protoplasts derived from noncluster roots of -P plants, as well as from normal (noncluster) roots of plants grown with 25 microm phosphorus (+P). No differences were observed in the magnitude or frequency of IRAC and ORAC currents between the cluster roots and noncluster roots of -P plants. However, the IRAC current from +P plants occurred less frequently than in the -P plants. IRAC was unaffected by external phosphate, but ORAC had reduced inward current (anion efflux) when phosphate was present in the external medium. Our data suggest that IRAC is the main pathway for citrate efflux from white lupin roots, but ORAC may also contribute to citrate efflux.

Plant and mycorrhizal regulation of rhizodeposition

The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil-based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474.

Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource

Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants. Contents I. Introduction 424 II. The phosphorus conundrum 424 III. Adaptations to low P 424 IV. Uptake of P 424 V. P deficiency alters root development and function 426 VI. P deficiency modifies carbon metabolism 431 VII. Acid phosphatase 436 VIII. Genetic regulation of P responsive genes 437 IX. Improving P acquisition 439 X. Synopsis 440.

Nylon filter arrays reveal differential gene expression in proteoid roots of white lupin in response to phosphorus deficiency

White lupin (Lupinus albus) adapts to phosphorus deficiency (-P) by the development of short, densely clustered lateral roots called proteoid (or cluster) roots. In an effort to better understand the molecular events mediating these adaptive responses, we have isolated and sequenced 2,102 expressed sequence tags (ESTs) from cDNA libraries prepared with RNA isolated at different stages of proteoid root development. Determination of overlapping regions revealed 322 contigs (redundant copy transcripts) and 1,126 singletons (single-copy transcripts) that compile to a total of 1,448 unique genes (unigenes). Nylon filter arrays with these 2,102 ESTs from proteoid roots were performed to evaluate global aspects of gene expression in response to -P stress. ESTs differentially expressed in P-deficient proteoid roots compared with +P and -P normal roots include genes involved in carbon metabolism, secondary metabolism, P scavenging and remobilization, plant hormone metabolism, and signal transduction.

Cluster roots--an underground adaptation for survival in extreme environments

Cluster roots are a characteristic of members of the Proteaceae and of several other plant species that are adapted to habitats of extremely low soil fertility, usually without formation of mycorrhizal associations. Functionally linked with intense mobilization of nutrients (P, Fe, Zn, Mn) by root-induced chemical changes (pH, root exudates, redox potential) in the rhizosphere, cluster-rooted plant species can serve as model plants to study rhizosphere processes and regulatory aspects of plant adaptations for chemical mobilization of nutrients in the rhizosphere.

PHOSPHATE ACQUISITION

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