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There has been an increasing drive toward better valorising raw biological materials in the context of the sustainability of bio‐based industries and the circular economy. As such, microalgae hold the ability to biosynthesise valuable metabolites, which are sought after within the bioenergy, pharmaceuticals, cosmetics or nutrition sectors. Owing to their bioactivities, the xanthophyll pigment fucoxanthin and the omega‐3 polyunsaturated fatty acid eicosapentaenoic acid (EPA) have fostered increasing interests in terms of sustainably refining them from natural sources, such as microalgae. Together with the suitability of individual species to industrial cultivation, a key challenge resides in optimizing the yields of these compounds within the microalgal biomass they are retrieved from. The marine diatom Stauroneis sp. LACW24 was batch cultivated into its stationary phase of growth prior to being subjected at high cell density (1 × 106 cells mL−1) to seven different regimes of light exposure in replenished medium and under nutritional limitation (silica and nitrate) for 12 days. The highest EPA proportions and yields were obtained under blue LED in f/2 medium (16.5% and 4.8 mg g−1, respectively), double the values obtained under red LED illumination. The fucoxanthin yield was the highest when cells were subjected to blue LEDs (5.9 mg g−1), a fourfold increase compared to the nitrogen‐limited treatment under white LEDs. These results indicate that a two‐stage approach to the batch cultivation of this diatom can be used for enhancing the production of the high‐value metabolites fucoxanthin and EPA post‐stationary phase. [ABSTRACT FROM AUTHOR]

Crops grown for bioenergy purposes are a potential alternative to traditional row crops and pasture–hay systems in the North Carolina (NC) Piedmont, but there is limited information available about their biomass yields and water requirements in this region. The goal of this study was to evaluate biomass yield and water‐use efficiency of switchgrass (Panicum virgatum L.), giant miscanthus (Miscanthus × giganteus Greef et Deu.), biomass sorghum (Sorghum bicolor spp.), silage corn (Zea mays L.) and tall fescue (Lolium arundinacea Schreb.). The perennial systems were established in 2012 while annuals were planted each spring. Crop water use was evaluated for the 2016 and 2017 growing seasons using a water balance approach. Giant miscanthus had the highest 2‐yr average biomass yield (29.1 ± 0.8 Mg ha–1) followed by corn (23.6 ± 0.6 Mg ha–1) and biomass sorghum (22.0 ± 1.8 Mg ha–1). Switchgrass and tall fescue had the lowest biomass yields, 14.2 ± 1.9 and 12.5 ± 1.2 Mg ha–1, respectively. Fescue had the highest season‐long water use in both years of the study. Perennial grasses giant miscanthus and switchgrass had similar seasonal water use, but giant miscanthus had higher water‐use efficiency due to greater biomass yields. The annual crops corn and sorghum used less water than the perennial systems because of their shorter growing season, and, consequently, had higher water‐use efficiencies. This information can aid growers when making management decisions about converting land into bioenergy crops. Core Ideas: Miscanthus had the highest biomass yields, followed by corn and sorghum.Annual crops used less water than perennials due to a shorter growing season.High water‐use efficiency (WUE) of annual crops was due to high biomass yields and low cumulative evapotranspiration.Miscanthus WUE was comparable to that of corn and sorghum. [ABSTRACT FROM AUTHOR]

Pseudomonas aeruginosa uses quorum sensing (QS) to modulate the expression of several virulence factors that enable it to establish severe infections. The QS system in P. aeruginosa is complex, intricate and is dominated by two main N-acyl-homoserine lactone circuits, LasRI and RhlRI. These two QS systems work in a hierarchical fashion with LasRI at the top, directly regulating RhlRI. Together these QS circuits regulate several virulence associated genes, metabolites, and enzymes in P. aeruginosa. Paradoxically, LasR mutants are frequently isolated from chronic P. aeruginosa infections, typically among cystic fibrosis (CF) patients. This suggests P. aeruginosa can undergo significant evolutionary pathoadaptation to persist in long term chronic infections. In contrast, mutations in the RhlRI system are less common. Here, we have isolated a clinical strain of P. aeruginosa from a CF patient that has deleted the transcriptional regulator RhlR entirely. Whole genome sequencing shows the rhlR locus is deleted in PA80 alongside a few non-synonymous mutations in virulence factors including protease lasA and rhamnolipid rhlA, rhlB, rhlC. Importantly we did not observe any mutations in the LasRI QS system. PA80 does not appear to have an accumulation of mutations typically associated with several hallmark pathoadaptive genes (i.e., mexT, mucA, algR, rpoN, exsS, ampR). Whole genome comparisons show that P. aeruginosa strain PA80 is closely related to the hypervirulent Liverpool epidemic strain (LES) LESB58. PA80 also contains several genomic islands (GI's) encoding virulence and/or resistance determinants homologous to LESB58. To further understand the effect of these mutations in PA80 QS regulatory and virulence associated genes, we compared transcriptional expression of genes and phenotypic effects with isogenic mutants in the genetic reference strain PAO1. In PAO1, we show that deletion of rhlR has a much more significant impact on the expression of a wide range of virulence associated factors rather than deletion of lasR. In PA80, no QS regulatory genes were expressed, which we attribute to the inactivation of the RhlRI QS system by deletion of rhlR and mutation of rhlI. This study demonstrates that inactivation of the LasRI system does not impact RhlRI regulated virulence factors. PA80 has bypassed the common pathoadaptive mutations observed in LasR by targeting the RhlRI system. This suggests that RhlRI is a significant target for the long-term persistence of P. aeruginosa in chronic CF patients. This raises important questions in targeting QS systems for therapeutic interventions. [ABSTRACT FROM AUTHOR]

Background: Brown seaweeds are known to be a rich source of fiber with the presence of several non-digestible polysaccharides including laminarin, fucoidan and alginate. These individual polysaccharides have previously been shown to favorably alter the gut microbiota composition and activity albeit the effect of the collective brown seaweed fiber component on the microbiota remains to be determined. Methods: This study investigated the effect of a crude polysaccharide-rich extract obtained from Laminaria digitata (CE) and a depolymerized CE extract (DE) on the gut microbiota composition and metabolism using an in vitro fecal batch culture model though metagenomic compositional analysis using 16S rRNA FLX amplicon pyrosequencing and short-chain fatty acid (SCFA) analysis using GC-FID. Results: Selective culture analysis showed no significant changes in cultured lactobacilli or bifidobacteria between the CE or DE and the cellulose-negative control at any time point measured (0, 5, 10, 24, 36, 48 h). Following metagenomic analysis, the CE and DE significantly altered the relative abundance of several families including Lachnospiraceae and genera including Streptococcus, Ruminococcus and Parabacteroides of human fecal bacterial populations in comparison to cellulose after 24 h. The concentrations of acetic acid, propionic acid, butyric acid and total SCFA were significantly higher for both the CE and DE compared to cellulose after 10, 24, 36 and 48 h fermentation (p < 0.05). Furthermore, the acetate:propionate ratio was significantly reduced (p < 0.05) for both CD and DE following 24, 36 and 48 h fermentation. Conclusion: The microbiota-associated metabolic and compositional changes noted provide initial indication of putative beneficial health benefits of L. digitata in vitro; however, research is needed to clarify if L. digitata-derived fiber can favorably alter the gut microbiota and confer health benefits in vivo. [ABSTRACT FROM AUTHOR]

At present, anti-virulence drugs are being considered as potential therapeutic alternatives and/or adjuvants to currently failing antibiotics. These drugs do not kill bacteria but inhibit virulence factors essential for establishing infection and pathogenesis through targeting non-essential metabolic pathways reducing the selective pressure to develop resistance. We investigated the effect of naturally isolated plant compounds on the repression of the quorum sensing (QS) system which is linked to virulence/pathogenicity in Pseudomonas aeruginosa. Our results show that trans-cinnamaldehyde (CA) and salicylic acid (SA) significantly inhibit expression of QS regulatory and virulence genes in P. aeruginosa PAO1 at sub-inhibitory levels without any bactericidal effect. CA effectively downregulated both the las and rhl QS systems with lasI and lasR levels inhibited by 13- and 7-fold respectively compared to 3- and 2-fold reductions with SA treatment, during the stationary growth phase. The QS inhibitors (QSI) also reduced the production of extracellular virulence factors with CA reducing protease, elastase and pyocyanin by 65%, 22% and 32%, respectively. The QSIs significantly reduced biofilm formation and concomitantly with repressed rhamnolipid gene expression, only trace amount of extracellular rhamnolipids were detected. The QSIs did not completely inhibit virulence factor expression and production but their administration significantly lowered the virulence phenotypes at both the transcriptional and extracellular levels. This study shows the significant inhibitory effect of natural plant-derived compounds on the repression of QS systems in P. aeruginosa. [ABSTRACT FROM AUTHOR]

Rhamnolipid production was monitored for a period of 216 h using different substrates in Pseudomonas aeruginosa PAO1 and Burkholderia thailandensis E264 which showed comparable crude yields attained by both after 216 h. The crude yield for P. aeruginosa, however, was significantly higher at the early stages of fermentation (72 or 144 h). Additionally, P. aeruginosa produced rhamnolipid with odd and even carbon chain lipid moieties using odd carbon chain fatty acid substrates (up to 45.97 and 67.57%, respectively). In contrast, B. thailandensis produced rhamnolipid with predominantly even carbon chain lipid moieties (up to 99.26). These results indicate the use of the fatty acid synthesis (FAS II) pathway as the main source of lipid precursors in rhamnolipid biosynthesis by B. thailandensis. Isotope tracing using 0.25% stearic acid - d35 + 1% glycerol as carbon substrate showed a single pattern of deuterium incorporation: with predominantly less than 15 deuterium atoms incorporated into a single Di-C14-C14 rhamnolipid molecule. This further indicates that the FAS II pathway is the main source of the lipid precursor in rhamnolipid biosynthesis by B. thailandensis. The pathogenicity of these strains was also assessed, and results showed that B. thailandensis is significantly less pathogenic than P. aeruginosa with an LC50 at 24 h > 2500, approximately three logs higher than P. aeruginosa using the Galleria mellonella larva model. [ABSTRACT FROM AUTHOR]

Several approaches can be used to terminate legume cover crops in the spring prior to planting summer crops, but the effect that these methods have on decomposition and nitrogen (N) release dynamics of legume cover-crop roots is poorly understood. The main objectives of this study were to: (i) quantify decomposition andNrelease of roots from pea (Pisum sativum), clover (Trifolium incarnatum) and vetch (Vicia villosa Roth); (ii) determine if roots decompose and release N faster when cover crops are terminated by disking compared with roller-crimping; and (iii) determine if roots decompose and release N faster under higher soil inorganic N levels. Two field experiments were conducted in Goldsboro and Kinston, North Carolina in the summer of 2012. Cover crops at these sites were terminated in spring by disking or roller-crimping and planted to unirrigated corn. Air-dried roots placed in litterbags were buried in their corresponding cover-crop plots and in plots where cover crops had not been grown that had either synthetic N fertilizer added at burial or had no fertilizer addition. Root litterbags were collected over 16 weeks at both sites. Cover-crop plots terminated by disking had up to 117 and 49% higher soil inorganic N than roller-crimped plots in Goldsboro and Kinston, respectively. However, roots did not appear to contribute significantly to these increases, as measured root decomposition and N release was not affected by termination approach at either site. Roots decomposed rapidly at both sites, losing up to 65% of their original biomass within 4 weeks after burial. Root Nrelease was also rapid at both sites, with vetch generally releasing N fastest and clover slowest. It was estimated that cover-crop roots supplied 47-62 and 19-33 kg N ha-1 during the corn cycle in Goldsboro and Kinston, respectively. Our results indicate that under the warm, humid summer conditions of the Southeastern USA, legume cover-crop roots decompose and release N rapidly. [ABSTRACT FROM AUTHOR]

This mini-review is concerned with the characterization of synthetic and natural product pharmaceuticals by functional group analysis using electrospray ionization-ion trap mass spectrometry. Studies of the electrospray ionization–ion trap mass spectrometry behavior of selected synthetic and natural product pharmaceuticals of low molecular mass have shown certain characteristic fragmentations in that functional groups are generally cleaved from ring systems either as even electron inorganic or organic molecules such as H2O, CO, CO2, alcohols, carboxylic acids, or as odd electron entities such as methoxy and methyl. This is particularly energetically favorable in the former case with the standard heat of formation at 298.15 K of the ejected neutral molecule having a relatively high negative value. Fragmentation of pharmaceuticals involving the generation of odd electron entities such as the methyl radical is structure dependent and is to a large extent correlated with the stability of the newly formed free radical ions. The fragmentation data therefore provides useful information on the structure of these pharmaceuticals, their degradation products and metabolites. In some cases, the cleaving of functional groups may parallel the production of certain metabolites. In addition, electrospray ionization-ion trap mass spectrometry data on even and odd electron mass losses may be cross-referenced with data obtained from unknown analytes such as bioactive molecules isolated from natural sources which can then be of value in their structural characterization, assisting in the dereplication process. This mini-review on functional group analysis of selected pharmaceuticals by electrospray ionization-ion trap mass spectrometry provides examples of drug characterization following cleavage from ring systems of even electron molecules (e.g., hypnotic drugs, coumarins, phloroglucinols, and polyphenols) and odd electron entities (e.g., the alkaloid tetrandrine). Parallels between the cleaving of functional groups as even electron mass losses and the metabolic reactions of the macrocyclic antibiotic rifapentine are also discussed. [ABSTRACT FROM AUTHOR]

While qualitative studies have identified chlorogenic acids in antioxidant extracts, particularly ethyl acetate- derived extracts, of Taraxacum officinale, quantitative analysis of these phenolic compounds remains largely unreported for this species. In this study, bioactivity-guided fractionation of an antioxidant crude ethyl acetate extract (DPPH = 295.481 ± 0.955 mg TE g-1 extract) from T. officinale root resulted in a number of reverse-phase fractions that demonstrated high antioxidant activity (DPPH = 1058.733- 1312.136 mg TE g-1 extract), stronger than that of the synthetic antioxidant Trolox®. UPLC-MS/MS screening of these fractions for the presence of selected mono- and di-caffeoylquinic acids revealed large quantities of 1,5-dicaffeoylquinic acid present in several fractions (853.052-907.324 μg mg-1), respectively. Due to the antioxidant potency and high levels of 1,5-dicaffeoylquinic acid observed in these fractions, it was concluded that specifically this chlorogenic acid derivative is a major contributor to the antioxidant efficacy of dandelion root. [ABSTRACT FROM AUTHOR]

Stingless bees accumulate deposits of plant resins that are mixed with beeswax to produce propolis. Previous studies have reported anti-microbial constituents of stingless bee ( Tetragonula carbonaria) propolis from East Australia, but several components remained to be characterized. In the search of natural products yet unreported for Australian propolis, four bee deposit-resins of T. carbonaria bees were analysed by gas and liquid chromatography mass spectrometry with accurate mass measurements. Ethanolic extracts of the deposit-resins were tested in vitro against Staphylococcus aureus ATCC 25983 and Pseudomonas aeruginosa ATCC 27853 by the agar diffusion method. Phloroglucinols, flavonoids and isoprenoids were identified in samples. The crude extracts showed strong anti-staphylococcal effects but were less active against the Gram-negative bacterium. The diagnostic data enabled the identification of markers that can be used for profiling other Australian propolis sources and to target the isolation of bioactive phloroglucinols in future studies against antibiotic resistant S. aureus strains. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Phlorotannins are a group of complex polymers of phloroglucinol (1,3,5-trihydroxybenzene) unique to macroalgae. These phenolic compounds are integral structural components of the cell wall in brown algae, but also play many secondary ecological roles such as protection from UV radiation and defense against grazing. This study employed Ultra Performance Liquid Chromatography (UPLC) with tandem mass spectrometry to investigate isomeric complexity and observed differences in phlorotannins derived from macroalgae harvested off the Irish coast (Fucus serratus, Fucus vesiculosus, Himanthalia elongata and Cystoseira nodicaulis). Antioxidant activity and total phenolic content assays were used as an index for producing phlorotannin fractions, enriched using molecular weight cut-off dialysis with subsequent flash chromatography to profile phlorotannin isomers in these macroalgae. These fractions were profiled using UPLC-MS with multiple reaction monitoring (MRM) and the level of isomerization for specific molecular weight phlorotannins between 3 and 16 monomers were determined. The majority of the low molecular weight (LMW) phlorotannins were found to have a molecular weight range equivalent to 4-12 monomers of phloroglucinol. The level of isomerization within the individual macroalgal species differed, resulting in substantially different numbers of phlorotannin isomers for particular molecular weights. F. vesiculosus had the highest number of isomers of 61 at one specific molecular mass, corresponding to 12 phloroglucinol units (PGUs). These results highlight the complex nature of these extracts and emphasize the challenges involved in structural elucidation of these compounds. [ABSTRACT FROM AUTHOR]

The efficiency of solid-liquid extraction ( SLE) and pressurised liquid extraction ( PLE) for the recovery of antioxidant and polyphenols from the Irish macroalgae, Fucus serratus, Laminaria digitata, Gracilaria gracilis and Codium fragile, was assessed using the 2,2-diphenyl-1-picrylhydrazyl ( DPPH) and ferric reducing antioxidant power ( FRAP) assays and the Folin- Ciocalteu total phenol content ( TPC) assay. Fucus serratus had TPC and antioxidant activities thirty times higher than the other species. Solid-liquid extraction cold water ( CWSLE) had the highest TPC (81.17 μg GAE mg−1 sample) derived from F. serratus, compared with the TPC of 61.12 μg GAE mg−1 sample for the corresponding PLE extract. For both SLE and PLE extracts, low TPC levels were observed in L. digitata, G. gracilis and C. fragile. The majority of SLE extracts possessed higher FRAP and DPPH activities compared with their PLE counterparts. This study indicates that the high temperatures and pressures in PLE did not enhance the antioxidant activities relative to conventional SLE extraction. [ABSTRACT FROM AUTHOR]

Novel dairy products were produced by the addition of seaweed extracts to milk. Ascophyllum nodosum 100% water ( AN100) or 80% ethanol ( AN80e) and Fucus vesiculosus 60% ethanol ( FV60e) extracts were prepared. AN80e and FV60e (0.25 and 0.5%) milk had higher ( P < 0.05) '−a*' and 'b*' values. FV60e (0.25%) and phloroglucinol ( Phl) (0.5%) milk had lower ( P < 0.05) lipid oxidation. Control and AN100 milk were accepted by sensory panellists. Seaweed extracts were stable (2,2-diphenyl-1-picrylhydrazyl, DPPH) in milk. Milk and digestates exhibited DPPH and ferrous-ion-chelating activities and did not affect cellular antioxidant activity or protect against DNA damage. [ABSTRACT FROM AUTHOR]

ABSTRACT Introduction Several analytical methods exist for the determination of sulphoraphane or sulphoraphane nitrile from biological matrices and plant extracts. However, no UPLC-MS/MS method exists for the simultaneous detection of both. Objective To develop and validate an UPLC-MS/MS method for the simultaneous analysis of sulphoraphane and sulphoraphane nitrile from Brassica oleracea L. ssp. italica Methods This method was developed utilising an Acquity BEH C8 column with gradient elution combined with tandem mass spectrometry, using positive electrospray ionisation in multiple reaction monitoring mode. Results The retention times for sulphoraphane and sulphoraphane nitrile were 0.4 and 0.6 min respectively, and total run time was 3 min. The method was validated for linearity, sensitivity, precision, accuracy, matrix effects and recovery. The method was employed to determine glucoraphanin hydrolysis products in broccoli and the predominant product was found to vary depending on the variety tested. It was also applied to the accurate determination of sulphoraphane and sulphoraphane nitrile in broccoli samples hydrolysed under different conditions. It was observed that the formation of sulphoraphane and sulphoraphane nitrile was influenced by the temperature of the reaction. Conclusion The validated UPLC-MS/MS method for simultaneous detection of sulphoraphane and sulphoraphane nitrile was shown to be applicable to broccoli plants and is expected to be applicable to other cruciferous sources. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Introduction: There is an increasing effort worldwide to investigate the antioxidant efficacy of traditional medicinal plants, which may be used to alleviate oxidative stress synonymous with the pathophysiology of many metabolic disorders as well as prevent oxidative deterioration of food products. In this study the antioxidant potential of dandelion (Taraxacum officinale) root was evaluated. Methods: The radical scavenging and antioxidant activities of solid-liquid extracts from freeze dried dandelion root was measuring in vitro using the modified DPPH and FRAP colorimetric assay. The phenolic content was also evaluated in vitro using the Folin-Ciocaltteu colorimetric assay. Subsequent identification and quantification of phenolic in active extracts was carried out using UPLC-MS/MS. Results: The ethyl acetate crude extract (E3) demonstrated the highest antioxidant activity for both the DPPH (227.728 ± 11.849 mg TE/g) and FRAP (463.066 ± 3.942 mg TE/g) assays. This extract also contained the highest phenolic content (228.723 ± 2.392 mg GAE/g). UPLC-MS/MS was used to identify and quantify phenolic compounds from E3 extract using an internal library of 36 phenolic standards. In total 18 phenolic compounds were quantified with chlorogenic acid, caffeic acid, syringic and vanillic being the most abundant compounds. The flavonoids apigenin-7-O-glycoside, luteolin-7-O-glycoside and naringenin-7-O-glycoside were detected for the first time in dandelion root. Conclusion: The results of this study suggest that dandelion root is a good source of natural antioxidants and could be used in foods with the potential to delay the onset of diseases linked to metabolic derived reactive oxygen species (ROS). [ABSTRACT FROM AUTHOR]

The efficiencies of pressurised liquid extraction ( PLE) and a traditional solid-liquid extraction ( SLE) at extracting antioxidant polyphenols from Irish macroalgae Ascophyllum nodosum, Pelvetia canaliculata, Fucus spiralis and Ulva intestinalis were compared. PLE was more effective for extracting polyphenols with acetone/water (80:20); however, when food-friendly solvents of ethanol/water (80:20) and water were employed, SLE resulted in higher phenolic content in brown macroalgal extracts. For example, the Fucus spiralis SLE water and ethanol/water extracts displayed total phenolic contents ( TPCs) of 130.58 ± 2.78 and 142.81 ± 1.77 μg phloroglucinol equivalents ( PGE) mg−1 sample, respectively, compared with TPCs of 90.79 ± 1.16 and 124 ± 6.54 μg PGE mg−1 sample for the corresponding PLE extracts. All SLE aqueous ethanolic macroalgal extracts possessed higher DPPH radical scavenging abilities ( RSA) and ferric reducing antioxidant power ( FRAP) than their PLE equivalents . This study indicates that the application of high extraction temperatures (50-200 °C) and pressures (500-3000 psi) used in PLE does not enhance the antioxidant activities of macroalgal extracts relative to SLE extraction. The ability to produce antioxidant food-friendly macroalgal extracts using SLE could represent significant cost reductions on an industrial scale further enhancing the potential of macroalgal polyphenols to be used in functional food preparations. [ABSTRACT FROM AUTHOR]

Deuterated rhamnolipids were produced using strain AD7 of Pseudomonas aeruginosa, which was progressively adapted to increasing levels of deuterium in D2O and carbon substrates. Fourteen different deuterated rhamnolipid structures, including structural isomers, were produced which is similar to normal protonated structures. There were two main products monorhamnolipid Rha-C10-C10 and dirhamnolipid Rha2-C10-C10. The levels of deuteration varied from 16% with 25% D2O + h-glycerol to 90% with 100% D2O + d-glycerol. When d-tetradecane was used with H2O, virtually all the deuterium appeared in the lipid chains while using h-tetradecane + D2O led to the majority of deuterium in the sugars. The adaptation to growth in deuterium appeared to be metabolic since no genetic changes could be found in the key rhamnolipid biosynthetic genes, the rhamnosyl transferases RhlB and RhlC. Deuterated sophorolipids were similarly produced using Candida bombicola and Candida apicola although in this case, no adaptation process was necessary. Up to 40 different sophorolipids were produced by these yeasts. However, unlike the rhamnolipids, use of D2O did not lead to any deuteration of the lipid chains, but direct incorporation into the lipid was achieved using d-isostearic acid. The results from these experiments show the feasibility of producing deuterated bioactive compounds from microorganisms coupled with the possibility of manipulating the pattern of labelling through judicious use of different deuterated substrates. [ABSTRACT FROM AUTHOR]

The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al3+ activities up to 4.7 μM in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r2=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate. [ABSTRACT FROM AUTHOR]

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity. The influence of Ca and Mg on the production and secretion of citrate and malate, and on Al accumulation by roots was investigated with soybean genotypes Young and PI 416937 which differ in Al tolerance. In the presence of a solution Al3+ activity of 4.6 µM, citrate and malate concentrations of tap root tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 µM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 50-fold. Maximum citrate release occurred within 12 h after adding Mg to solution treatments. Adding 50 µM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 µM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants treated with either 50 µM Mg or 3 mM Ca had similar reductions in Al accumulation at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity. Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimulated production and efflux of citrate by roots. [ABSTRACT FROM PUBLISHER]

The mechanistic basis for cation amelioration of Al rhizotoxicity in soybean was investigated through a series of studies comparing protective effects of Ca and Mg against Al inhibition of root elongation in a background 0.8 mM CaSO4 solution (pH 4.3). A modified Gouy-Chapman-Stern model was used to evaluate the effect of cations on electrical potential and Al3+ activity at root plasma membrane surfaces. Activities of Al3+ up to 4.6 µM in the background solution inhibited soybean tap root elongation by more than 80%. There was little or no response in root elongation when Ca and Mg were added to background solutions in the absence of Al. When added to Al-toxic solutions in the micromolar concentration range, Mg was 100-fold more effective than Ca in alleviating Al toxicity, whereas both cations were equally effective when added in the millimolar concentration range. The protective effect of micromolar additions of Mg on root elongation was specific for Al and it failed to alleviate La rhizotoxicity. In contrast to wheat, Mg amelioration of Al toxicity to soybean root elongation at low Mg concentration could not be explained by changes in potential and Al3+ activity at the root plasma membrane surfaces as predicted by a Gouy-Chapman-Stern model. These results suggest that Mg is not acting as an indifferent cation when present at low concentration and implies the involvement of a mechanism other than pure electrostatic effects at the root surface. [ABSTRACT FROM PUBLISHER]