Organic acids promote the uptake of lanthanum by barley roots

Organic acids play an important role in metal uptake by, and accumulation in, plants. However, the relevant mechanisms remain obscure. Acetic, malic and citric acids increased the uptake of lanthanum (La) by barley (Hordeum vulgare) roots and enhanced La content in shoots under hydroponic conditions. Concentration-dependent net La influx in the absence and presence of organic acids yielded nonsaturating kinetic curves that could be resolved into linear and saturable components. The saturable component followed Michaelis-Menten kinetics. The K(m) values were similar; however, the V(max) values in the presence of acetic, malic and citric acids were 4.3, 2.8, 1.5-times that of the control, respectively. Enhanced uptake of La by organic acids was mediated mainly, but not solely, by Ca(2+) channels. X-ray absorption spectroscopic techniques provided evidence of La-oxygen environment and established that La(III) was coordinated to 11 oxygen atoms that are likely to be involved in the binding of La(III) to barley roots via carboxylate groups and hydration of La(III).     

Organic acids enhance the uptake of lead by wheat roots

The uptake and bioavailability of lead (Pb) in soil–plant systems remain poorly understood. This study indicates that acetic and malic acids enhance the uptake of Pb by wheat (Triticum aestivum L.) roots under hydroponic conditions. The net concentration-dependent uptake influx of Pb in the presence and absence of organic acids was characterized by Michaelis–Menten type nonsaturating kinetic curves that could be resolved into linear and saturable components. Fitted maximum uptake rates (V _max) of the Michaelis–Menton saturable component in the presence of acetic and malic acids were, respectively, 2.45 and 1.63 times those of the control, while the Michaelis–Menten K _m values of 5.5, 3.7 and 2.2 μM, respectively, remained unchanged. Enhanced Pb uptake by organic acids was partially mediated by Ca²⁺ and K⁺ channels, and also depended upon the physiological function of the plasma membrane P-type ATPase. Uptake may have been further enhanced by an effectively thinner unstirred layer of Pb adjacent to the roots, leading to more rapid diffusion towards roots. X-ray absorption spectroscopic studies provided evidence that the coordination environment of Pb in wheat roots was similar to that of Pb(CH₃COO)₂·3H₂O in that one Pb atom was coordinated to four oxygen atoms via the carboxylate group.     

秋茄(Kandelia candel (L))根系分泌低分子量有机酸及其对重金属生物有效性的影响

应用电感耦合等离子体质谱(ICP-Ms)与高效液相色谱(HPLC)分别对福建漳江口红树林湿地不同土壤结构(砂质与泥质)根际与非根际沉积物中重金属(Cu,Pb,Cd,Zn)含量,以及生长于砂质与泥质滩涂上的红树植物秋茄(Kandelia candel(L)Druce))幼苗根系分泌物中的低分子量有机酸进行测定。在室内模拟秋茄根系分泌的低分子量有机酸,作为重金属提取剂提取沉积物中可溶解态与碳酸盐结合态重金属,并与欧盟标准物质局(BCR)连续提取法相比较,探讨红树根系分泌的低分子量有机酸对红树林沉积物重金属生物有效性的影响。研究结果表明:漳江口红树林泥质沉积物中重金属含量大于砂质沉积物,根际沉积物大于非根际沉积物。两样地沉积物中重金属的赋存形态主要以铁锰氧化物结合态为主,根际沉积物中可交换态与碳酸盐结合态重金属含量均大于非根际沉积物。秋茄根系分泌低分子量有机酸为甲酸,丁酸,苹果酸,柠檬酸,乳酸。不同土壤结构对秋茄根系分泌的苹果酸,柠檬酸,乳酸有显著影响(P<0.05)。以低分子量有机酸作为提取剂对沉积物中可溶解态与碳酸盐结合态重金属的提取率表现为:柠檬酸>混合酸>苹果酸>乳酸>乙酸,低分子量有机酸对红树林沉积物重金属的生物有效性有促进作用。     

Enhanced cadmium accumulation in maize roots—the impact of organic acids

Low molecular weight organic acids are important components of root exudates and therefore, knowledge regarding the mechanisms of cadmium (Cd) uptake and distribution within plants under the influence of organic acids, is necessary for a better understanding of Cd behavior in the plant–soil system. In this study, acetic and malic acids increased the uptake of Cd by maize (Zea mays L. cv. TY2) roots and enhanced Cd accumulation in shoots under hydroponic conditions. Concentration-dependent net Cd influx in the presence and absence of organic acids could be resolved into linear and saturable components. The saturable component followed Michaelis–Menten kinetics, which indicated that Cd uptake across the plasma membrane was transporter-mediated. While the K _m values were similar, the V _max values in the presence of acetic and malic acids were respectively 6.0 and 3.0 times that of the control. Zinc transporters were the most probable pathways for Cd accumulation. It was hypothesized that Cd(II)–organic acid complexes associated with the root zone, could decompose and liberate Cd²⁺ for subsequent absorption by maize roots; and that in the layer of the roots or within the root free space, depletion of Cd²⁺ was buffered by the presence of Cd(II)–organic acid complexes. Plant response to elevated Cd levels involved overproduction of organic acids in maize roots as a resistance mechanism to alleviate Cd toxicity.     

Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation

Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.     

Citrate, Malate, and Succinate Concentration in Exudates from P-Sufficient and P-Stressed Medicago sativa L. Seedlings

Under certain stress conditions roots exude organic molecules, which may facilitate the uptake of nutrients. The objective of this research was to identify and measure the effect of low P upon the exudation of organic acids by roots of alfalfa (Medicago sativa L.) seedlings. Surface-sterilized alfalfa seeds were grown aseptically in sterile sand using an apparatus specially designed for the addition of ±P nutrient solutions and for the collection of root exudates. Citric, malic, and succinic acids were detected in the root exudates of 24-day-old alfalfa seedlings. Citrate exudation from the roots of P-stressed alfalfa was 182% that of plants receiving a complete nutrient solution. The increased release of citrate may provide a mechanism by which P-stressed plants enhance the availability of P in the rhizosphere.     

Effects of Low-Molecular-Weight Organic Acids on Gadolinium Accumulation and Transportation in Tomato Plants

Effects of low-molecular-weight organic acids on the accumulation and transportation of gadolinium (Gd) in tomato plants were studied under hydroponic condition. The results indicated that changes of organic acids occurred in the processes of Gd accumulation and transportation in tomato plants which were treated with extraneous Gd solutions. Malic, citric, and succinic acids contributed to both Gd accumulation in roots and transportation in xylem vessels. When Gd was unloaded from the xylem to the leaf cells, formic, lactic, citric, and succinic acids played important roles in Gd accumulation in leaves. When tomato plants were cultured in the uptake solution of Gd-containing malic, citric, or succinic acid for 48 h, the succinic acid in roots and leaves and the malic acid in xylem saps both increased obviously. From the results above, we can conclude that succinic acid had the most important role in Gd accumulation in tomato roots and leaves, while malic acid transported Gd via xylem vessels more effectively.     

DYNAMICS OF THREE ORGANIC ACIDS (MALIC,ACETIC AND SUCCINIC ACID) IN SUNFLOWER EXPOSED TO CADMIUM AND LEAD

Sunflower (Helianthus annuus L.) has been considered as a good candidate for bioaccumulation of heavy metals. In the present study, sunflower was used to enrich the cadmium and lead in sand culture during 90 days. Biomass, Cd and Pb uptake, three organic acids and pH in cultures were investigated. Results showed that the existence of Cd and Pb showed different interactions on the organic acids exudation. In single Cd treatments, malic and acetic acids in Cd10 showed an incremental tendency with time. In the mixed treatments of Cd and Pb, malic acids increased when 10 and 40 mg?L^–1 Cd were added into Pb50, but acetic acids in Pb50 were inhibited by Cd addition. The Cd10 supplied in Pb100 stimulated the secretion of malic and succinic acids. Moreover, the Cd or Pb uptake in sunflower showed various correlations with pH and some organic acids, which might be due to the fact that the Cd and Pb interfere with the organic acids secretion in rhizosphere of sunflower, and the changes of organic acids altered the form and bioavailability of Cd and Pb in cultures conversely.     

Aluminum Tolerance in Wheat (Triticum aestivum L.) (II. Aluminum-Stimulated Excretion of Malic Acid from Root Apices)

We investigated the role of organic acids in conferring Al tolerance in near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at the Al tolerance locus (Alt1). Addition of Al to nutrient solutions stimulated excretion of malic and succinic acids from roots of wheat seedlings, and Al-tolerant genotypes excreted 5- to 10-fold more malic acid than Al-sensitive genotypes. Malic acid excretion was detectable after 15 min of exposure to 200 [mu]M Al, and the amount excreted increased linearly over 24 h. The amount of malic acid excreted was dependent on the external Al concentration, and excretion was stimulated by as little as 10 [mu]M Al. Malic acid added to nutrient solutions was able to protect Al-sensitive seedlings from normally phytotoxic Al concentrations. Root apices (terminal 3-5 mm of root) were the primary source of the malic acid excreted. Root apices of Al-tolerant and Al-sensitive seedlings contained similar amounts of malic acid before and after a 2-h exposure to 200 [mu]M Al. During this treatment, Al-tolerant seedlings excreted about four times the total amount of malic acid initially present within root apices, indicating that continual synthesis of malic acid was occurring. Malic acid excretion was specifically stimulated by Al, and neither La, Fe, nor the absence of Pi was able to elicit this response. There was a consistent correlation of Al tolerance with high rates of malic acid excretion stimulated by Al in a population of seedlings segregating for Al tolerance. These data are consistent with the hypothesis that the Alt1 locus in wheat encodes an Al tolerance mechanism based on Al-stimulated excretion of malic acid.     

Diversity of low-molecular weight organic acids synthesized by <Emphasis Type="Italic">Salix</Emphasis> growing in soils characterized by different Cu,Pb and Zn concentrations

The aim of the study was to evaluate the biosynthesis and exudation of 10 low-molecular weight organic acids (LMWOAs) into the rhizosphere with a simultaneous analysis of the acid contents in the roots and leaves of 9 Salix taxa growing on two experimental areas, differing in their concentrations of copper (Cu), lead (Pb) and zinc (Zn) in the soil (Area 1—low, Area 2—high concentration). The obtained results reveal a significant difference in the phytoextraction of the tested Salix taxa for the analysed metals in both areas. The highest contents of Cu, Pb and Zn were observed for all Salix collected from Area 2, especially in S. × smithiana roots (116 ± 8.76, 87.84 ± 7.30 and 203.42 ± 14.62 mg kg^?1 DW, respectively). The results obtained in Area 2 also revealed acidification of the rhizosphere and a higher concentration of acids, mainly oxalic, malic, malonic, acetic and citric acids. Contents of oxalic, malic, acetic and citric acids increased in the roots of Salix taxa from Area 2, while in the leaves formic and succinic acids were also present. S. × smithiana was the taxon with the highest concentration of acids in the rhizosphere and roots (73.48 ± 6.77 and 49.79 ± 2.65 μM 100 g^?1 DW, respectively), while in leaves a higher content was observed for S. alba and S. viminalis ‘PR’ taxa (78.12 ± 3.95 and 71.12 ± 3.75 μM 100 g^?1 DW, respectively).     

缺磷白羽扇豆排根与非排根区根尖分泌有机酸的比较

采用根系分泌有机酸原位收集方法及高效液相色谱技术分析了供磷及缺磷后不同时间白羽扇豆(LupinusalbusL .)非排根区根尖和排根分泌有机酸的种类和数量 ,以及相应的根尖、排根组织 ,茎木质部、韧皮部汁液中有机酸含量的变化。结果表明 :(1)缺磷能够诱导白羽扇豆根系产生大量排根 ,根系的有机酸分泌量也明显增加。 (2 )无论在供磷或缺磷条件下 ,排根与非排根区根尖组织中的有机酸种类相同 ,但排根主要分泌柠檬酸和苹果酸 ,而非排根区根尖主要分泌苹果酸和乙酸。 (3)缺磷后非排根区根尖分泌苹果酸的量增加 ,至第 17天达到高峰 ;排根开始分泌柠檬酸的时间相对较晚。缺磷后排根分泌柠檬酸的量随缺磷时间的延长不断增加。 (4 )在缺磷的排根与非排根区根尖组织和茎木质部伤流液中含有大量柠檬酸和苹果酸 ,但在茎韧皮部汁液中则几乎检测不到这两种有机酸。上述结果表明 ,尽管排根和非排根区根尖组织中的有机酸种类相同 ,但它们向外分泌的有机酸种类不同。缺磷后排根及非排根区根尖增加向外分泌的有机酸主要在根中合成     

Root exudation pattern of Typha latifolia L. plants after copper exposure

Aims

Typha latifolia L. is an aquatic plant that has been widely exploited for the aims of phytoremediation. The main reason why we have chosen this plant species for the current study is its capacity to accumulate and detoxify heavy metals. The main topic of the investigation focused on the root uptake of copper (II) nitrate and copper (II) sulfate and the impact of different chemical copper species on the excreted organic acids.

Methods

Oxalic, malic, acetic and lactic acids were determined using capillary electrophoresis; a comparison between the concentration and the time course during 7 days of treatment was performed.

Results

There is a correlation between the total copper (II) sulfate concentration in the roots and the total amount of the excreted organic acids. In addition to that organic acids are involved in the detoxification mechanisms of Typha latifolia for copper (II) nitrate and copper (II) sulfate.

Conclusions

Different from so far investigated plant species the highest amounts of organic acids are excreted from T. latifolia roots not in the first hours after treatment, but up to 7 days later.     

铝胁迫下小麦根部苹果酸和柠檬酸的直接测定

铝的有害性严重制约了约占世界可耕地面积40％的酸性土壤中的作物生产。从植物的培养液巾发现从根部分泌出的有机酸与铝结合从而实现无毒化是其抗铝逆性机理的重要依据。而本文直接测定了铝胁迫下培育的小麦根中的铝和有机酸含量,确认了主要积累的是苹果酸和柠檬酸。发现随着提高培养液中的铝浓度,根部的铝含量也相应增加。同时,虽然根中的柠檬酸含量无明显变化,但苹果酸被诱导增加。通过对有机酸与铝的络合能力的调查,探讨了对植物抗铝逆性强弱的影响。     

Deciphering the Mechanisms of Endophyte-Mediated Biofortification of Fe and Zn in Wheat

An investigation was carried out to understand the mechanism(s) underlying enhanced Fe or Zn uptake in low Fe–Zn accumulator wheat genotype 4HPYT-414, due to inoculation of siderophore-producing and zinc-solubilizing endophytes—Arthrobacter sulfonivorans DS-68 and Arthrobacter sp. DS-179. Root anatomical features, using transmission electron microscopy (TEM), qualitative and quantitative aspects of production of organic acids and sugars in root exudates, and expression of TaZIP genes were analysed to relate to endophyte-mediated higher concentrations of Fe and Zn in the roots and shoots of wheat plants. TEM studies revealed that the endodermis, cortical region, root hair extension, xylem and xylem vessels, pericycle and vascular bundles were more pronounced and thicker in inoculated treatments, as compared to control. The organic acid profile of root exudates revealed five types of organic acids, with citric acid being predominant. Inoculation of A. sulfonivorans and Arthrobacter sp. brought about 5- and eightfold increases in the amounts of acids, respectively, as compared to control, particularly citric acid, succinic acid and acetic acid. Among the four TaZIP genes targeted, expression was achieved only for TaZIP3 and TaZIP7 genes, which showed 1–2 fold increases in the inoculated treatments. The results clearly indicated that the endophyte-mediated overexpression of TaZIP3 and TaZIP7 genes in roots and shoots, and the observed anatomical and exudate changes were acting synergistically in facilitating higher Fe and Zn translocation in roots and shoots.     

Symplastic and apoplastic uptake and root to shoot translocation of nickel in wheat as affected by exogenous amino acids

This study investigated the effect of exogenous amino acids on apoplastic and symplastic uptake and root to shoot translocation of nickel (Ni) in two wheat cultivars. Seedlings of a bread (Triticum aestivum cv. Back Cross) and a durum wheat cultivar (T. durum cv. Durum) were grown in a modified Johnson nutrient solution and exposed to two levels (50 and 100 μM) of histidine, glycine, and glutamine. Application of amino acids resulted in increasing symplastic to apoplastic Ni ratio in roots of both wheat cultivars, although glutamine and glycine were more effective than histidine under our experimental conditions. The amino acid used in the present study generally increased the relative transport of Ni from the roots to shoots in both wheat cultivars. Higher amounts of Ni were translocated to wheat shoots in the presence of histidine than the other amino acids studied, which indicated that histidine was more effective in translocation of Ni from roots to shoots. Amino acids used in the present study largely increased root symplastic Ni, but shoot Ni accumulation was much lower than the total Ni accumulation in roots, indicating a large proportion of Ni was retained or immobilized in wheat roots (either in the apoplastic or symplastic space), with only a very small fraction of Ni being translocated from the root to the shoot. According to the results, glutamine and glycine were more effective than histidine in enhancing the symplastic to apoplastic Ni ratio in the roots, while more Ni was translocated from the roots to the shoots in the presence of histidine.     

有机物质对土壤镉有效性的影响研究综述

土壤中的低分子有机酸和腐殖酸对镉的有效性有重要作用。根系分泌的低分子量有机酸能提高土壤镉的可提取率、移动性和生物有效性,但是更大分子量的有机酸EDTA却抑制植物对镉的吸收,腐殖酸促进土壤镉的溶解性;因腐殖酸组分和环境条件,腐殖酸能够促进也能抑制土壤镉活性,因此有必要深入研究影响腐殖酸固定镉的因素,以达到利用有机质抑制土壤镉活性的目的。     

Accumulation and transport of nickel in relation to organic acids in ryegrass and maize grown with different nickel levels

Difference in Ni tolerance/accumulation in plant genotypes might be used to identify or develop plants for remediation of high Ni soils. Ryegrass was shown to be more sensitive to Ni toxicity and accumulated much more Ni in shoots than maize. The objectives of this study were to examine the relationship of organic acids to Ni accumulation and xylem transport of Ni in ryegrass (Lolium perenne L.) and maize (Zea mays L.). The results showed that accumulation of Ni in shoots was 5 to 7 fold higher in ryegrass than in maize grown at 20 to 80 µM Ni, whereas Ni concentration in ryegrass roots was only 1 to 2 fold higher at 0.1 to 40 µM Ni and 1.5-fold lower at 80 µM Ni than that of maize roots. Xylem transport rates of Ni increased with increasing Ni supply for both species, and were about 2 to 7 times higher in ryegrass than in maize. Shoot concentrations of citric, malic, oxalic and cis-aconitic acids increased at Ni levels above 20 µM, and were about 2 to 6 times higher in ryegrass than in maize. Whereas, maize roots accumulated greater amount of malic, oxalic, and cis-aconitic acids than ryegrass roots, especially at Ni levels of 40-80 µM. The rate of Ni exudation by roots in the two species was significantly correlated with root Ni concentrations. It could be concluded that high Ni accumulation in shoots was closely related to high xylem transport rates of Ni and that the accumulation of organic acids, citric and malic acid in particular. A high root exudate rate of Ni and the enhanced accumulation of organic acids, malic acid in particular, in roots might be among the important factors which are associated with the tolerance of crops to toxic Ni levels.     

The role of organic acids in assisted phytoremediation processes of salt marsh sediments

Some plants have high ability to absorb heavy metals in high concentrations. In this study, Spartina maritima was tested in conjunction with low molecular weight organic acids (LMWOA), in order to evaluate the possible use of this plant in phytoremediation processes in salt marshes. Three different LMWOA (citric acid, malic acid and acetic acid) were applied to contaminated intact cores of S. maritima colonized sediment and several heavy metals (Cd, Zn, Pb, Cu, Cr and Ni) were analyzed in sediment and plant parts. Acetic acid application proved to be the most efficient, enhancing greatly the uptake of all metals analyzed. Citric acid also showed good results, while malic acid proved to be very inefficient in most of the cases. The highest enhancement was observed for Cr with a 10-fold increase of the uptake upon application of acetic acid, while improving the Pb uptake proved to be the most difficult, probably due to its low solubility.     

Pattern of aluminum-induced secretion of organic acids differs between rye and wheat

Al-Induced secretion of organic acids from the roots has been considered as a mechanism of Al tolerance, but the processes leading to the secretion of organic acids are still unknown. In this study, the secretion pattern and alteration in the metabolism of organic acids under Al stress were examined in rye (Secale cereale L. cv King) and wheat (Triticum aestivum L. cv Atlas 66). Al induced rapid secretion of malate in the wheat, but a lag (6 and 10 h for malic and citric acids, respectively) between the exposure to Al and the secretion of organic acids was observed in the rye. The activities of isocitrate dehydrogenase, phosphoenolpyruvate carboxylase, and malate dehydrogenase were not affected by Al in either plant. The activity of citrate synthase was increased by the exposure to Al in the rye, but not in the wheat. The secretion of malate was not suppressed at low temperature in the wheat, but that of citrate was stopped in the rye. The Al-induced secretion of citrate from roots of the rye was inhibited by the inhibitors of a citrate carrier, which transports citrate from the mitochondria to the cytoplasm. All of these results suggest that alteration in the metabolism of organic acids is involved in the Al-induced secretion of organic acids in rye, but only activation of an anion channel seems to be responsible for the rapid secretion of malate in the wheat.     

Evaluation of single chemical extractants for the prediction of heavy metal uptake by barley in soils amended with polluted sewage sludge

The main aim of this study was to compare the suitability of three single chemical extractants [EDTA, CaCl₂ and the low-molecular-weight organic acids solution (LMWOAs)] to estimate Cu, Zn and Ni uptake by barley (Hordeum vulgare) from rhizosphere soils, following a single application of a metal salts-spiked sewage sludge. Thirty-six contrasting soils from different parts of Spain were amended with the same dose (15.71 g dry weight kg^-1) of polluted sewage sludge and sown with barley seeds under greenhouse conditions. Eight weeks after sowing, the plants were harvested and Cu, Zn and Ni were analysed in the roots. Heavy metal uptake was then compared with the theoretically available heavy metals in the rhizosphere soils, assessed by the three single chemical extractants. These three extractants alone failed to predict heavy metal uptake, and soil properties were needed to obtain accurate predictions. Thus, none of the methods tested in this study can be used as a universal soil extraction for estimating Cu, Zn and Ni uptake by barley.