Characterization of Boron Deficiency in Soybeans

Boron-deficient soybean roots have a rosette appearance at the terminals caused by the death of the root primordium and the initiation of new root primordia. The root tips show some darkening, swelling, and then a collapse of the tissue. The petiole s of B-deficient plants are very brittle. Physiologically, B-deficiency in HA-soybeans does not appear to involve an interference in the transport of photosynthate (C¹⁴ distribution) or precursors required for citrate synthesis in the roots. More Ca 45 was found in the tops of B-sufficient than B-deficient plants, but the opposite trend existed in the root sap. Boron deficiency symptoms were accentuated by maintaining a relatively low pH of the prenutrient solution by the addition of K salts.     

Organic acids and iron translocation in maize genotypes

Translocation of Fe was studied in WF9 (Fe-efficient) and ys₁/ys₁ (Fe-inefficient) maize (Zea mays L.) genotypes. Iron-deficient WF9 translocated more Fe to the tops than Fe-deficient ys₁/ys₁. Malate and citrate contents of root saps increased nearly 2-fold and aconitate increased over 4-fold in both genotypes as Fe of nutrient solutions increased from 0.1 to 3 milligrams per liter. Relative acid contents in root saps were as follows: malate > aconitate > citrate. Citric acid concentrations in stem exudates were nearly the same as in root sap. Malic acid concentrations were considerably lower in exudates than in root saps, and only a trace of aconitic acid was detected in the exudates. The concentration of Fe was 7-fold higher in exudate of WF9 than in exudate of ys₁/ys₁ and the concentration of exudate P was about the same for both genotypes.     

Iron translocation I. Plant culture, exudate sampling, iron-citrate analysis

Plant culture, exudate sampling, and analytical methods designed to ascertain the form of iron translocated are presented.Restoration of iron to sunflower plants precultured at different Fe levels resulted in exudate iron concentrations ranging from 0.2 to 31 x 10(-5)m. Citrate was from 3 to 89 x 10(-5)m. Iron and citrate were highest in exudates from iron-deficient plants. Citrate/Fe ratios were between 1 and 3 for exudates of deficient plants. Exudate from normal plants gave a citrate/Fe ratio of 15.Malate, iron, and a fraction of the citrate in stem exudates migrated electrophoretically to similar positions in acetate buffer. Extracts of narrow bands from the iron-containing areas gave curves suggesting that citrate bound the iron. Citrate that was not combined with iron migrated in a slower band. The effect of iron on citrate migration was confirmed in several related experiments.The stability of Fe-citrate was demonstrated electrophoretically in malate buffer. Citrate retained iron against malate.Data given in this paper indicate that citrate binds iron in sunflower exudate. The data suggest that citrate carries iron in intact plants.     

Metabolic responses in iron deficient tomato plants

The effects of Fe deficiency on different metabolic processes were characterized in roots, xylem sap and leaves of tomato. The total organic acid pool increased significantly with Fe deficiency in xylem sap and leaves of tomato plants, whereas it did not change in roots. However, the composition of the pool changed with Fe deficiency, with major increases in citrate concentrations in roots (20-fold), leaves (2-fold) and xylem sap (17-fold). The activity of phosphoenolpyruvate carboxylase, an enzyme leading to anaplerotic C fixation, increased 10-fold in root tip extracts with Fe deficiency, whereas no change was observed in leaf extracts. The activities of the organic acid synthesis-related enzymes malate dehydrogenase, citrate synthase, isocitrate dehydrogenase, fumarase and aconitase, as well as those of the enzymes lactate dehydrogenase and pyruvate carboxylase, increased with Fe deficiency in root extracts, whereas only citrate synthase increased significantly with Fe deficiency in leaf extracts. These results suggest that the enhanced C fixation capacity in Fe-deficient tomato roots may result in producing citrate that could be used for Fe xylem transport. Total pyridine nucleotide pools did not change significantly with Fe deficiency in roots or leaves, although NAD(P)H/NAD(P) ratios were lower in Fe-deficient roots than in controls. Rates of O(2) consumption were similar in Fe-deficient and Fe-sufficient roots, but the capacity of the alternative oxidase pathway was decreased by Fe deficiency. Also, increases in Fe reductase activity with Fe deficiency were only 2-fold higher when measured in tomato root tips. These values are significantly lower than those found in other plant species, where Fe deficiency leads to larger increases in organic acid synthesis-related enzyme activities and flavin accumulation. These data support the hypothesis that the extent of activation of different metabolic pathways, including carbon fixation via PEPC, organic acid synthesis-related enzymes and oxygen consumption is different among species, and this could modulate the different levels of efficiency in Strategy I plants.     

Accumulation and distribution of iron, cadmium, lead and nickel in cucumber plants grown in hydroponics containing two different chelated iron supplies

Cucumber plants grown in hydroponics containing 10 μM Cd(II), Ni(II) and Pb(II), and iron supplied as Fe(III) EDTA or Fe(III) citrate in identical concentrations, were investigated by total-reflection X-ray fluorescence spectrometry with special emphasis on the determination of iron accumulation and distribution within the different plant compartments (root, stem, cotyledon and leaves). The extent of Cd, Ni and Pb accumulation and distribution were also determined. Generally, iron and heavy-metal contaminant accumulation was higher when Fe(III) citrate was used. The accumulation of nickel and lead was higher by about 20% and 100%, respectively, if the iron supply was Fe(III) citrate. The accumulation of Cd was similar. In the case of Fe(III) citrate, the total amounts of Fe taken up were similar in the control and heavy-metal-treated plants (27-31 μmol/plant). Further, the amounts of iron transported from the root towards the shoot of the control, lead- and nickel-contaminated plants were independent of the iron(III) form. Although Fe mobility could be characterized as being low, its distribution within the shoot was not significantly affected by the heavy metals investigated.     

大豆和水稻对铝胁迫响应的生理机制

通过水培方式,研究了铝处理对双子叶植物大豆和单子叶植物水稻根系生长、养分吸收、根系内含物及根分泌物的影响.结果表明,低铝 (10 μmol·L^-1)浸种刺激大豆种子萌发和根系的生长,对水稻无明显促进作用.铝处理增加了两种作物根系对P的吸收,降低K、Ca、Mg的吸收.水稻比大豆根系积累较少的Al和更多的P.铝胁迫条件下,大豆和水稻根系内源可溶性蛋白含量升高、可溶性酚含量下降、可溶性糖含量先上升后下降,且大豆根系内源柠檬酸含量下降明显.与大豆相比,水稻积累较低的柠檬酸和较高的可溶性蛋白、可溶性酚,但两者可溶性糖没有差异.铝处理增加大豆根系柠檬酸、可溶性蛋白、可溶性酚、可溶性糖的分泌量,且大豆分泌量显著高于水稻.在铝处理条件下,大豆根系具有较高的阳离子交换量(CEC),而水稻的CEC较低.这说明大豆和水稻对铝胁迫具有不同的生理反应,水稻的高耐铝性可能与其较高的磷吸收和较低的CEC有关,而与其根系分泌物的关系不大.     

The Iron-Deficiency Induced Phenolics Accumulation May Involve in Regulation of Fe(III) Chelate Reductase in Red Clover

Although considerable researches have been conducted on the physiological responses to plant iron (Fe) deficiency stress in dicotyledonous plants, much still needs to be learned about the regulation of these processes. In the present research, red clover was used to investigate the role of root phenolics accumulation in regulating Fe-deficiency induced Fe(III) chelate reductase (FCR). The root FCR activity, IAA and phenolics accumulation, and also the phenolics secretion were greatly increased by the Fe deficiency treatment. The application of TIBA (2,3,5-triiodobenoic acid) to the stem, an IAA polar transport inhibitor, which could decrease IAA accumulation in root, significantly inhibited the FCR activity, but did not effect root phenolics accumulation and secretion, suggesting that IAA itself did not involve in root phenolics accumulation and secretion. In contrast, the Fe deficiency treatment significantly decreased the root IAA-oxidase activity. Interestingly the phenolics extracted from roots inhibited IAA-oxidase activity in vitro, and this inhibition was greater with phenolics extracted from roots of Fe deficient plants than that from Fe sufficient plants, indicating that the Fe deficiency-induced IAA-oxidase inhibition probably caused by the phenolics accumulation in Fe deficient roots. Based on these observations, we propose a model where under Fe deficiency stress in dicots, an increase in root phenolics concentrations plays a role in regulating root IAA levels through an inhibition of root IAA oxidase activity. This response, leads to, or at least partially leads to an increase in root IAA levels, which in turn help induce increased root FCR activity.Key Words: Fe deficiency, ferric chelate reductase, phenolics, Trifolium pretense     

Iron-Stress Response in Tomato (Lycopersicon esculentum) 1. Sites of Fe Reduction, Absorption and Transport

T3238fer (Fe-inefficient) and T3238FER (Fe-efficient) tomato plants differ in their ability to utilize Fe and therefore can be used as test genotypes to locate sites of Fe uptake or to characterize changes that occur in roots in response to Fe stress (Fe deficiency). T3238fer does not respond to Fe stress. Release of hydrogen ions and reduction of Fe³⁺ to Fe²⁺ are two primary responses of T3238FER roots to Fe stress. Fe reduction sites were predominately in the young lateral roots, and between the regions of root elongation and maturation of the primary root. The use of BDPS (bathophenanthrolinedisulfonate) to trap Fe²⁺ did not affect the release of H⁺ ions or reduction by T3238FER roots. BPDS did not decrease Fe uptake until it exceeded the Fe concentration in the nutrient solution. A sevenfold increase in BPDS caused a threefold decrease in Fe taken up by the plant. Fe³⁺ is reduced to Fe²⁺ at root sites accessible to BPDS. Adding Zn decreased the response to Fe stress. Iron stress initiates the development of lateral roots, and we propose that most Fe enters the plant through these roots. The iron moves through protoxylem into the metaxylem of the primary root and then to the top of the plant as Fe citrate. Root environmental factors that are competitive or inhibit Fe-stress response, or genotypes that fail to respond to Fe stress, contribute to the development of Fe deficiency in plants.     

Accumulation of apoplastic iron in plant roots : a factor in the resistance of soybeans to iron-deficiency induced chlorosis?

We hypothesized that the resistance of Hawkeye (HA) soybean (Glycine max L.) to iron-deficiency induced chlorosis (IDC) is correlated to an ability to accumulate a large pool of extracellular-root iron which can be mobilized to shoots as the plants become iron deficient. Iron in the root apoplast was assayed after efflux from the roots of intact plants in nutrient solution treated with sodium dithionite added under anaerobic conditions. Young seedlings of HA soybean accumulated a significantly larger amount of extracellular iron in their roots than did either IDC-susceptible PI-54619 (PI) soybean or IDC-resistant IS-8001 (IS) sunflower (Helianthus annus L.). Concurrently, HA soybean had much higher concentrations of iron in their shoots than either PI soybean or IS sunflower. The concentration of iron in the root apoplast and in shoots of HA soybean decreased sharply within days after the first measurements of extracellular root iron were made, in both +Fe and −Fe treatments. The accumulation of short-term iron reserves in the root apoplast and translocation of iron in large quantities to the shoot may be important characteristics of IDC resistance in soybeans.     

Effect of iron on the transport of citrate into the xylem of soybeans and tomatoes

Iron transport in soybeans (Glycine max [L] Merr.) and tomatoes (Lycopersicum esculentum) is controlled by factors that are altered manyfold as the plant experiences an iron stress (deficiency). Depending on their response to an Fe stress, plants in this study are classed (a) Fe-inefficient or (b) Fe-efficient. The Fe-efficient plants transport more Fe and concomitantly more citrate than the Fe-inefficient plants.     

Mechanism of Fe uptake by the leaf symplast: Is Fe inactivation in leaf a cause of Fe deficiency chlorosis?

The mechanism of iron (Fe) uptake from the leaf apoplast into leaf mesophyll cells was studied to evaluate the putative Fe inactivation as a possible cause of Fe deficiency chlorosis. For this purpose, sunflower (Helianthus annuus L.) and faba bean plants (Vicia faba L.) were precultured with varied Fe and bicarbonate (HCO ₃ ^- ) supply in nutrient solution. After 2–3 weeks preculture, Fe^III reduction and ⁵⁹Fe uptake by leaf discs were measured in solutions with Fe supplied as citrate or synthetic chelates in darkness. The data clearly indicate that Fe^III reduction is a prerequisite for Fe uptake into leaf cells and that the Fe nutritional status of plants does not affect either process. In addition, varied supply of Fe and HCO ₃ ^- to the root medium during preculture had no effect on pH of the xylem sap and leaf apoplastic fluid. A varied pH of the incubation solution had no significant effect on Fe^III reduction and Fe uptake by leaf discs in the physiologically relevant pH range of 5.0–6.0 as measured in the apoplastic leaf fluid. It is concluded that Fe inactivation in the leaf apoplast is not a primary cause of Fe deficiency chlorosis induced by bicarbonate. This revised version was published online in June 2006 with corrections to the Cover Date.     

OsNRAMP5, a major player for constitutive iron and manganese uptake in rice

Manganese (Mn) and iron (Fe) are essential mineral micronutrients for plants and their deficiency and or toxicity represents a serious agricultural problem. In rice the information about genes involved in Mn uptake from soil is scarce. Recently, we showed that OsNRAMP5 is a plasma membrane protein involved in Mn and Fe transport. The concentration of Mn in roots, shoots and xylem sap of OsNRAMP5 RNAi (OsNRAMP5i) plants was significantly reduced compared with WT plants. The expression of OsNRAMP5 is not controlled by Fe deficiency in root and was also observed in pistil, ovary, lemma and palea. These data show that rice would utilize OsNRAMP5 for constitutive Fe and Mn uptake, while OsNRAMP5 would also play a role in Fe and Mn transport during flowering and seed development.     

Zusammenhänge zwischen der Eisenaufnahme und dem Eisentransport in der Pflanze

Zusammenfassung Junge Sonnenblumenpflanzen nahmen 8 Std. lang nicht markiertes und danach 15 Std. lang mit Fe⁵⁹ markiertes Eisen in Form von Fe-Chlorid, Fe-EDTA oder Fe-Citrat auf. 1. Bei allen drei Eisenformen absorbierten die Pflanzenwurzeln mehr Eisen aus der N?hrl?sung mit 0,1 ppm Fe als aus der mit 1,0 ppm Fe. 2. Die Wurzeln der mit Fe-Chlorid ern?hrten Pflanzen enthalten den h?chsten Gehalt an Fe⁵⁹. 3. Das Stengelexsudat der 0,1 ppm Fe-Reihe enthielt weniger Fe⁵⁹ als das Exsudat der 1,0 ppm Fe-Reihe. 4. Eine sichere Beziehung zwischen der Ern?hrung mit den verschiedenen drei Eisenformen und der ausgeschiedenen Exsudatmenge konnte nicht festgestellt werden. Ebenfalls bestand kein Zusammenhang zwischen der ausgeschiedenen Exsudatmenge und ihrem Fe⁵⁹-Gehalt. 5. Der Gehalt des Stengelexsudats an Fe⁵⁹ war bei der Ern?hrung mit Fe-EDTA und Fe-Citrat h?her als bei Fe-Chlorid.

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Relationships between iron uptake and iron transport in plants

Summary The uptake of unlabelled iron for 8 h and of Fe⁵⁹, given as Fe chloride, Fe EDTA and Fe citrate for 15 h, was investigated in young sunflower plants. 1. In all three iron forms the roots absorbed more iron from the nutrient solution with 0.1 ppm Fe than with 1.0 ppm Fe. 2. The roots of those plants, supplied with Fe-chloride contained the highest amount of Fe⁵⁹. 3. The stem exudate of plants, given a nutrient solution with 0.1 ppm Fe contained less Fe⁵⁹ than the exudate of plants, grown in a nutrient solution containing 1.0 ppm Fe. 4. No close relationship between the nutrition with the three iron forms and the produced amount of exudate could be established. Furthermore no correlation between the produced amount of exudate and its Fe⁵⁹ content could be found. 5. The content of Fe⁵⁹ in the stem exudate was higher when Fe EDTA and Fe citrate were given as compared with Fe chloride.

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Auszug aus der Dissertation des Verfassers.     

Metabolic reprogramming in nodules,roots, and leaves of symbiotic soybean in response to iron deficiency

To elucidate the mechanism of adaptation of leguminous plants to iron (Fe)‐deficient environment, comprehensive analyses of soybean (Glycine max) plants (sampled at anthesis) were conducted under Fe‐sufficient control and Fe‐deficient treatment using metabolomic and physiological approach. Our results show that soybeans grown under Fe‐deficient conditions showed lower nitrogen (N) fixation efficiency; however, ureides increased in different tissues, indicating potential N‐feedback inhibition. N assimilation was inhibited as observed in the repressed amino acids biosynthesis and reduced proteins in roots and nodules. In Fe‐deficient leaves, many amino acids increased, accompanied by the reduction of malate, fumarate, succinate, and α‐ketoglutarate, which implies the N reprogramming was stimulated by the anaplerotic pathway. Accordingly, many organic acids increased in roots and nodules; however, enzymes involved in the related metabolic pathway (e.g., Krebs cycle) showed opposite activity between roots and nodules, indicative of different mechanisms. Sugars increased or maintained at constant level in different tissues under Fe deficiency, which probably relates to oxidative stress, cell wall damage, and feedback regulation. Increased ascorbate, nicotinate, raffinose, galactinol, and proline in different tissues possibly helped resist the oxidative stress induced by Fe deficiency. Overall, Fe deficiency induced the coordinated metabolic reprogramming in different tissues of symbiotic soybean plants.     

Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots

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 microM, 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 microM. 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 microM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 microM 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 microM 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.     

Abscisic acid alleviates iron deficiency by promoting root iron reutilization and transport from root to shoot in Arabidopsis

Abscisic acid (ABA) has been demonstrated to be involved in iron (Fe) homeostasis, but the underlying mechanism is largely unknown. Here, we found that Fe deficiency induced ABA accumulation rapidly (within 6 h) in the roots of Arabidopsis. Exogenous ABA at 0.5 μM decreased the amount of root apoplastic Fe bound to pectin and hemicellulose, and increased the shoot Fe content significantly, thus alleviating Fe deficiency‐induced chlorosis. Exogenous ABA promoted the secretion of phenolics to release apoplastic Fe and up‐regulated the expression of AtNRAMP3 to enhance reutilization of Fe stored in the vacuoles, leading to a higher level of soluble Fe and lower ferric–chelate reductase (FCR) activity in roots. Treatment with ABA also led to increased Fe concentrations in the xylem sap, partially because of the up‐regulation of AtFRD3, AtYSL2 and AtNAS1, genes related to long‐distance transport of Fe. Exogenous ABA could not alleviate the chlorosis of abi5 mutant resulting from the significantly low expression of AtYSL2 and low transport of Fe from root to shoot. Taken together, our data support the conclusion that ABA is involved in the reutilization and transport of Fe from root to shoot under Fe deficiency conditions in Arabidopsis.     

Carboxylate metabolism changes induced by Fe deficiency in barley, a Strategy II plant species

The effects of iron (Fe) deficiency on carboxylate metabolism were investigated in barley (Hordeum vulgare L.) using two cultivars, Steptoe and Morex, which differ in their Fe efficiency response. In both cultivars, root extracts of plants grown in Fe-deficient conditions showed higher activities of enzymes related to organic acid metabolism, including citrate synthase, malate dehydrogenase and phosphoenolpyruvate carboxylase, compared to activities measured in root extracts of Fe-sufficient plants. Accordingly, the concentration of total carboxylates was higher in Fe-deficient roots of both cultivars, with citrate concentration showing the greatest increase. In xylem sap, the concentration of total carboxylates was also higher with Fe deficiency in both cultivars, with citrate and malate being the major organic acids. Leaf extracts of Fe-deficient plants also showed increases in citric acid concentration and in the activities of glucose-6-phosphate dehydrogenase and fumarase activities, and decreases in aconitase activity. Our results indicate that changes in root carboxylate metabolism previously reported in Strategy I species also occur in barley, a Strategy II plant species, supporting the existence of anaplerotic carbon fixation via increases in the root activities of these enzymes, with citrate playing a major role. However, these changes occur less intensively than in Strategy I plants. Activities of the anaerobic metabolism enzymes pyruvate decarboxylase and lactate dehydrogenase did not change in barley roots with Fe deficiency, in contrast to what occurs in Strategy I plants, suggesting that these changes may be Strategy I-specific. No significant differences were observed in overall carboxylate metabolism between cultivars, for plants challenged with high or low Fe treatments, suggesting that carboxylate metabolism changes are not behind the Fe-efficiency differences between these cultivars. Citrate synthase was the only measured enzyme with constitutively higher activity in Steptoe relative to Morex leaf extracts.     

硝普钠对缺铁和硝酸盐胁迫下番茄幼苗生长及生理特性的影响

采用营养液培养方法,研究外源NO供体(硝普钠,SNP)对缺铁和硝酸盐胁迫番茄幼苗生长、养分吸收及抗氧化酶活性的影响.结果表明: 处理7 d后,缺铁使番茄幼苗生长受到抑制,叶绿素a、b、类萝卜素含量显著降低,出现明显失绿症状;降低叶片中超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT)的活性,电解质渗漏率、丙二醛含量明显增加,脯氨酸和可溶性糖含量变化不显著,幼苗叶片和根中N、P、K、Ca、Mg、Fe含量比对照处理有不同程度的减少.硝酸盐和缺铁双重胁迫对番茄幼苗生长抑制加剧,叶绿素a、b、类萝卜素含量、SOD、POD和CAT活性显著降低,电解质渗漏率、脯氨酸、可溶性糖和丙二醛含量明显增加;番茄幼苗叶片和根中N、P、Mg、Fe含量显著减少,而K、Ca含量显著增加. 与不添加处理相比,添加0.1 mmol·L^-1 SNP处理使胁迫番茄幼苗的生长抑制明显缓解.添加0.1 mmol·L^-1 SF(亚铁氰化钠)的处理在SOD、POD和CAT等指标上也表现出一定程度的缓解或促进作用,但其他生理指标没有表现出缓解或促进作用,原因是SF中也含有铁离子.     

OsFRDL1 Is a Citrate Transporter Required for Efficient Translocation of Iron in Rice

Multidrug and toxic compound extrusion (MATE) transporters represent a large family in plants, but their functions are poorly understood. Here, we report the function of a rice (Oryza sativa) MATE gene (Os03g0216700, OsFRDL1), the closest homolog of barley (Hordeum vulgare) HvAACT1 (aluminum [Al]-activated citrate transporter 1), in terms of metal stress (iron [Fe] deficiency and Al toxicity). This gene was mainly expressed in the roots and the expression level was not affected by either Fe deficiency or Al toxicity. Knockout of this gene resulted in leaf chlorosis, lower leaf Fe concentration, higher accumulation of zinc and manganese concentration in the leaves, and precipitation of Fe in the root's stele. The concentration of citrate and ferric iron in the xylem sap was lower in the knockout line compared to the wild-type rice. Heterologous expression of OsFRDL1 in Xenopus oocytes showed transport activity for citrate. Immunostaining showed that OsFRDL1 was localized at the pericycle cells of the roots. On the other hand, there was no difference in the Al-induced secretion of citrate from the roots between the knockout line and the wild-type rice. Taken together, our results indicate that OsFRDL1 is a citrate transporter localized at the pericycle cells, which is necessary for efficient translocation of Fe to the shoot as a Fe-citrate complex.