Scope for genetic manipulation of mineral acquisition in chickpea

Nutrient acquisition in chickpea needs to be efficient, because it is mainly grown as a post-rainy season, rainfed crop, and generally on soils inferior in physical characteristics and poor in fertility. Nutrient deficiencies have been reported to cause yield losses of varying magnitude in chickpea, e.g., 22–50% due to iron (Fe), around 10% due to sub-optimal nodulation and hence nitrogen (N) deficiency, 29–45% due to phosphorus (P), up to 100% due to boron (B), and 16-30% due to sulphur (S). Yield losses due to salinity are equally large but are difficult to estimate because of its heterogeneous occurrence. In chickpea, genotypic differences in morpho-physiological (including root size) and functional (exudates) root traits, and in nodulation capacity for increased nitrogen fixation have been identified. Genotypic differences in response to application of Fe, B and zinc (Zn) have also been found among chickpea genotypes. A drought tolerant chickpea genotype ICC 4958, which has a relatively large root system, acquired more P than other genotypes during the vegetative period in a pot experiment at ICRISAT. The recent thrust on identifying QTLs for root size should facilitate progress in incorporating useful root traits through marker assisted selection in desirable agronomic backgrounds. Selection for nodulation capacity in released cultivars has resulted in high nodulating chickpea genotypes that produced 10% higher yield than the control varieties. Information on targeted crop improvement for higher nutrient-use efficiency for P, S, Zn, B and Fe is not readily available. Methods to screen for tolerance to salinity are available, but sufficiently high levels of tolerance have not yet been found in germplasm or wild relatives of chickpea to warrant breeding for salinity tolerance. Use of alternative approaches, such as mutation to generate genetic diversity or introgression of alien genes from other crops (transgenic) are thus required, and these remain long-term objectives.     

Genotypic variability in phosphorus solubilizing activity of root exudates by pigeonpea grown in low-nutrient environments

A pot experiment confirmed that pigeonpea could efficiently utilize various sources of phosphorus (P) (aluminium phosphate, iron phosphate and apatite), irrespective of genotype. A qualitative assay method for iron (Fe)-P solubilizing activity showed that root exudates collected from P-deficient pigeonpea contained Fe-P solubilizing substances and that they were released mainly from root tips. Citric, malic, malonic, succinic and piscidic acids were identified in root exudates. Citric and piscidic acids release from roots was increased by low-P treatment in all the genotypes tested. The release rates of citric and piscidic acids were affected by the P concentration of shoots rather than that of roots. The pigeonpea roots released approximately 5–100 times more piscidic acid than citric acid depending on P stress status, plant age and genotype. When organic acids were added to Alfisols, citric acid was most capable of mobilizing P from the soil, followed by piscidic acid and malic acid. No correlation was found between genotypic variability in the release rates of citric and piscidic acids from the roots under low-P treatment at hydroponic culture and in the growth and P uptake of plants on Alfisols. Although citric and piscidic acids released from pigeonpea roots may play a partial role in solubilizing unavailable insoluble P in soils, the releases were thought to be an unsatisfactory strategy for explaining genotypic variation in low P availability of pigeonpea.     

Root exudates of wetland plants influenced by nutrient status and types of plant cultivation

The present study investigated the amounts of root exudates and composition of organic acids released from two wetland plants (Typha latifolia and Vetiver zizanioides) under two nutrient treatments: low level (0.786 mM N and 0.032 mM P) and high level (7.86 mM N and 0.32 mM P) and two types of plant cultivation: monoculture and co-culture of the two plants. Low nutrient treatment significantly (p < 0.05) increased the root exudates of T. latifolia during the initial growth period (1-21 d) and those of V. zizanioides and the co-culture during the whole growth period. The concentrations of dissolved organic carbon in the root exudates of the co-culture in the low nutrient treatment were 3.23-7.91 times of those in the high nutrient treatment during the medium growth period (7-28 d). The compositions of organic acids varied between the two plant species and between the two nutrient treatments. The pattern of organic acids was also different between the co-culture and the monoculture. Oxalic acid was by far the major organic acid exuded from the two wetland plants. The present study on root exudates suggests that co-culture of wetland plant species would be more useful in the reclamation of waste water than a monoculture system.     

Role of proteinaceous amino acids released in root exudates in nutrient acquisition from the rhizosphere

The role of proteinaceous amino acids in rhizosphere nutrient mobilization was assessed both experimentally and theoretically. The degree of adsorption onto the soil's solid phase was dependent on both the amino acid species and on soil properties. On addition of amino acids to both soil and freshly precipitated Fe(OH)₃, no detectable mobilization of nutrients (K, Na, Ca, Mg, Cu, Mn, Zn, Fe, S, P, Si and Al) was observed, indicating a very low complexation ability of the acidic, neutral and basic amino acids. This was supported by results from a solution equilibria computer model which also predicted low levels of amino acid complexation with solutes present in the soil solution. On comparison with the Fe(OH)₃ and equilibria data obtained for the organic acid, citrate, it was concluded that amino acids released into the rhizosphere have a limited role in the direct acquisition of nutrients by plants. The effectiveness of root exudates such as amino acids, phytosiderophores and organic acids in nutrient mobilization from the rhizosphere is discussed with reference to rhizosphere diffusion distances, microbial degradation, rate of complexation and the root's capacity to recapture exudate-metal complexes from the soil.     

Root exudates and arsenic accumulation in arsenic hyperaccumulating Pteris vittata and non-hyperaccumulating Nephrolepis exaltata

This study compared the roles of root exudates collected from two fern species, the As hyperaccumulating Chinese Brake fern (Pteris vittata L.) and the As-sensitive Boston fern (Nephrolepis exaltata L.), on As-mobilization of two As minerals (aluminum arsenate and iron arsenate) and a CCA (chromated copper arsenate)-contaminated soil as well as plant As accumulation. Chinese Brake fern exuded 2 times more dissolved organic carbon (DOC) than Boston fern and the difference was more pronounced under As stress. The composition of organic acids in the root exudates for both ferns consisted mainly of phytic acid and oxalic acid. However, Chinese Brake fern produced 0.46 to 1.06 times more phytic acid than Boston fern under As stress, and exuded 3–5 times more oxalic acid than Boston fern in all treatments. Consequently, root exudates from Chinese Brake fern mobilized more As from aluminum arsenate (3–4 times), iron arsenate (4–6 times) and CCA-contaminated soil (6–18 times) than Boston fern. Chinese Brake fern took up more As and translocated more As to the fronds than Boston fern. The molar ratio of P/As in the roots of Chinese Brake fern was greater than in the fronds whereas the reverse was observed in Boston fern. These results suggested that As-mobilization from the soil by the root exudates (enhancing plant uptake), coupled with efficient As translocation to the fronds (keeping a high molar ratio of P/As in the roots), are both important for As hyperaccumulation by Chinese Brake fern.     

Spatial and temporal variation in citrate and malate exudation and tissue concentration as affected by P stress in roots of white lupin

Exudation of carboxylates represents one the most efficient strategies used by P-starved white lupin (Lupinus albus L.) to acquire phosphorus from sparingly soluble sources. This exudation occurs through proteoid root clusters, with citrate being the predominant organic acid released. The occasional detection of malate in whole root exudates suggests that this acid would also be released, but from tissues other than root clusters. To investigate the spatial and temporal pattern of exudation, citrate and malate exudation and concentration were measured in whole roots and root sections of white lupin, from seedling emergence to plant senescence due to P starvation. Both organic acids were detected in whole root exudates of P-stressed plants, and they were released at similar rates throughout the experiment. Malate was predominantly exuded from apices of both seedling taproots and proteoid roots, whereas citrate exudation was restricted to proteoid root clusters. Studies directed to address the association between carboxylate exudation and concentration in proteoid root clusters showed a non-linear response for citrate, within the range of 7 to 23 mol g^–1 fresh weight. This association was further assessed by altering citrate concentration in the whole root. Adding P to 24-day-old P-starved plants reduced citrate concentration and exudation to the level of the control P-fed plants, demonstrating that citrate exudation and concentration are associated. Malate exudation and concentration did not correlate significantly. Results indicate that citrate release by P-starved white lupin would occur whenever a certain threshold of citrate concentration is attained, and that the sites, the rates and the span of transient exudation depend on the physiological age of the tissue.     

盐碱胁迫下湖南稷子苗期根系分泌物代谢组学

盐碱胁迫下植物根系分泌物包含丰富生化信息并具有重要生态作用。为了探讨耐盐碱牧草湖南稷子(Echinochloa frumentacea) 在盐碱胁迫下根系分泌物组成,揭示其在盐碱胁迫下的生理及生态作用,以湖南稷子为试验对象,在人工气候室开展水培试验,并在苗期分别进行中性盐(NaCl+Na₂SO₄ 100 mmol/L)、碱性盐(NaCl+NaHCO₃ 100 mmol/L)和碱(Na₂CO₃+NaHCO₃ 50 mmol/L)处理。在处理3 d后,利用液质联用仪(LC-MS/MS)检测对照组和处理组根系分泌物的化合物成分。结果表明,盐碱胁迫下湖南稷子根系分泌物共有334种化合物。依据正交偏最小二乘法判别分析(OPLS、|DA),重要值(VIP)得分及t检验的P值, 发现对照比SaSo100(碱性盐处理 100 mmol/L),对照比Soda50(碱处理50 mmol/L)和对照比Salt100(中性盐处理100 mmol/L)分别有22、15和21个差异根系分泌物。其中碱性盐和碱处理下根系分泌物组成相近,包括脂质、酚酸,生物碱,苯酞类,氨基糖,萜类,醌类,氨基酸及其衍生物;中性盐处理下有脂质、酚酸,生物碱,苯酞类,萜类。京都基因与基因组百科全书注释及富集发现,盐碱胁迫下根系分泌物不仅含有三羧酸循环代谢产生的碳水化合物、核苷酸,氨基酸,脂肪酸,类脂和维生素等物质,而且与瓦博格效应、膜运输,信号传导以及遗传信息处理等途径有关。研究表明,湖南稷子通过根系分泌物渗出,调节自身代谢物浓度,加强或改变碳同化、呼吸作用、信号传导等提高对盐碱胁迫的适应性。     

4种植物幼苗根茎叶及根系分泌物中低分子量有机酸的特征

该研究建立了植物根茎叶及根系分泌物中有机酸的离子色谱分析测定方法,并测定了4种不同植物幼苗根茎叶及根系分泌物中低分子量有机酸的组成,为揭示逆境胁迫下植物体内有机酸的作用提供依据。结果表明:离子色谱分析法对植物有机酸的加标回收率为91.10%~105.42%,检测限为0.12~0.36mg/L,方法线性关系良好(R2=0.965 3~0.998 8);4种植物根茎叶及根系分泌物中都可以检测出草酸、柠檬酸、苹果酸、丁二酸和酒石酸,其中草酸、柠檬酸和苹果酸为优势酸;有机酸的组成和含量具有物种以及器官的差异性;根系分泌物中的有机酸与根茎叶中有机酸的相关性也因种属差异而不同。这为研究逆境胁迫下植物器官及根系分泌物提供了可靠方法。     

Root exudates of plants

Summary The release of substances from wheat roots was found to be directly related to the growth of the root system. Plants whose root system did not grow released almost no exudates.When exudate concentration in the vicinity of the roots was lowered by frequent replacements of the nutrient solution or by a simultaneous cultivation of exudate-utilizing bacteria, the release of exudates was enhanced. In axenic wheat cultures, the amount of exudates during a 12-day cultivation with 2- or 4-day intervals between medium replacements represented 50% of root dry weight and 12% of whole plant dry weight.Wheat plants cultivated in the presence of the bacteriumPseudomonas putida released up to double the amount of exudates compared with axenic variants.     

Seasonal acquisition of mineral nutrients by a chalcid gall on lowbush blueberry

Seasonal accumulation of various mineral nutrients by galls of the chalcidHemadas nubilipennis on lowbush blueberry were examined in various phases of gall development. Levels of copper, nickel iron and zinc were higher in tissues of galls in initiation phase compared to tips of ungalled shoots and in general were higher than galls in growth and maturation phases. Levels of manganese and magnesium in gall tissues decrease throughout the season compared to ungalled shoots. Levels of calcium are less in gall tissue in initiation phase compared to ungalled shoots, and elevated in maturation phase. Patterns of mineral concentration within galls at various phases of development are related to known physiological roles of minerals in plant nutrition; however we conclude that patterns are variable and their significance in gall physiology requires further study.     

Characterization of Sym plasmids of Rhizobium leguminosarum strains able to nodulate Pisum sativum cv Afghanistan

Rhizobium leguminosarum strains that can form nodules on Pisum sativum cv. Afghanistan have been reported as uncommon in Europe, North America and Africa [11, 12]. The organization of the nodulation regions of the symbiotic plasmids of five strains of R. leguminosarum originating from Denmark [9], which can nodulate P. sativum cv. Afghanistan, was compared with that of a Turkish strain (TOM [18]) by DNA hybridizations. Four of the five Danish strains were found to be very similar to the Turkish strain with respect to the overall organizations of their respective nodulation regions.     

Determination of low molecular weight dicarboxylic acids and organic functional groups in rhizosphere and bulk soils of Tsuga and Yushania in a temperate rain forest

Low molecular weight organic acids (LMWOAs) derived from root exudates, decomposing organic matter, and other sources are important ligands. The species of these LMWOAs in the Tsuga rhizosphere soil (TRS), and Yushania rhizosphere soil (YRS), and bulk soil (BS) from an alpine forest region were identified. LMWOA and organic functional groups were used to those fresh twigs and leaves, litters, and roots as comparison. The objectives of this study were to (i) develop a method that could be used to determine LMWOAs in soil solution by gas chromatography (GC), (ii) assess methods for processing LMWOAs in soil samples, and (iii) determine the relative proportions of organic carbon functional groups in the TRS, YRS and BS, and fresh plant materials with¹³C nuclear magnetic resonance (¹³C NMR) analysis. The proportion of organic acid contents followed the order of YRS > TRS > BS, and also showed significant differences (P < 0.05) from GC analysis. The amounts of malonic, fumaric and succinic acids in the YRS samples were greater than in the TRS and BS. Samples analyzed after 1 month of deep freeze storage (–24°C) showed no signs of decomposition. The proportion of organic functional groups in the rhizosphere and bulk soils quantified by ¹³C NMR analyses followed the general order: alkyl-C > O-alkyl-C > N-alkyl-C > acetal-C > aromatic-C > carboxylic-C > phenolic-C.     

Root excretion of carboxylic acids and protons in phosphorus-deficient plants

Phosphorus deficiency-induced metabolic changes related to exudation of carboxylic acids and protons were compared in roots of wheat (Triticum aestivum L. cv Haro), tomato (Lycopersicon esculentum L., cv. Moneymaker), chickpea (Cicer arietinum) and white lupin (Lupinus albus L. cv. Amiga), grown in a hydroponic culture system. P deficiency strongly increased the net release of protons from roots of tomato, chickpea and white lupin, but only small effects were observed in wheat. Release of protons coincided with increased exudation of carboxylic acids in roots of chickpea and white lupin, but not in those of tomato and wheat. P deficiency-induced exudation of carboxylic acids in chickpea and white lupin was associated with a larger increase of carboxylic acid concentrations in the roots and lower accumulation of carboxylates in the shoot tissue compared to that in wheat and tomato. - Citric acid was one of the major organic acids accumulated in the roots of all investigated species in response to P deficiency, and this was associated with increased activity and enzyme protein levels of PEP carboxylase, which is required for biosynthesis of citrate. Accumulation of citric acid was most pronounced in the roots of P-deficient white lupin, chickpea and tomato. Increased PEP carboxylase activity in the roots of these plants coincided with decreased activity of aconitase, which is involved in the breakdown of citric acid in the TCA cycle. In the roots of P-deficient wheat plants, however, the activities of both PEP carboxylase and aconitase were enhanced, which was associated with little accumulation of citric acid. The results suggest that P deficiency-induced exudation of carboxylic acids depends on the ability to accumulate carboxylic acids in the root tissue, which in turn is determined by biosynthesis, degradation and partitioning of carboxylic acids or related precursors between roots and shoot. In some plant species such as white lupin, there are indications for a specific transport mechanism (anion channel), involved in root exudation of extraordinary high amounts of citric acid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of rhizobial lipo-oligosacharides in root nodule formation on leguminous plants

During recent years signals leading to the early stages of nodulation of legumes by rhizobia have been identified. Plant flavonoids induce rhizobialnod genes that are essential for nodulation. Most of thenod gene products are involved in the biosynthesis of lipo-oligosaccharide molecules. The commonnodABC genes are minimally required for the synthesis of all lipo-oligosaccharides. Host-specificnod gene products in a givenRhizobium species are responsible for synthesis or addition of various moieties to those basic lipo-oligosaccharide molecules. For example, inR. leguminosarum, thenodFEL operon is involved in the production of lipo-oligosaccharide signals that mediate host specificity. AnodFE-determined highly unsaturated fatty acid (trans-2, trans-4, trans-6, cis-11-octadecatetraenoic acid) is essential for inducing nodule meristems and pre-infection thread structures on the host plantVicia sativa. Lipo-oligosaccharides also trigger autoregulation of nodulation in pea and, if applied in excessive amounts to a legume, can prevent nodulation and thereby might play a role in competition. During our studies on the biosynthesis of lipo-oligosaccharides, we discovered that, besides the lipo-oligosaccharides, other metabolites are synthesizedde novo after induction of thenod genes. These novel metabolites appeared to be phospholipids, containing either one of the three fatty acids which are made by the action of NodFE inR. leguminosarum.     

丛枝菌根真菌对三叶草根系分泌的有机酸组分和含量的影响

比较洗根法、层析纸法和琼脂膜法收集土培条件下生长的菌根化和非菌根化三叶草根分泌物的效果。试验采用三室根箱装置 ,将根系与菌丝生长空间分开 ,三叶草生长 5 6d后 ,打开三室根箱装置 ,由于尼龙网的阻挡作用使根系均匀垫积在尼龙网内侧并形成根垫。分别采用洗根法、层析纸法和琼脂膜法 3种方法收集三叶草根系分泌物 ,并通过高效液相色谱方法测定分泌物中草酸、酒石酸、苹果酸、乳酸、乙酸、顺丁烯二酸、反丁烯二酸、柠檬酸、丁二酸等有机酸的含量。结果表明 :3种收集方法收集的三叶草分泌的有机酸无论在种类上还是在数量上都存在相当大的差别。从检测到的有机酸种类来看 ,琼脂膜法收集检测到苹果酸、乙酸、顺丁烯二酸、柠檬酸、丁二酸和乳酸 6种有机酸 ;洗根法收集的分泌物检测到酒石酸、苹果酸、乙酸、顺丁烯二酸、柠檬酸、丁二酸和乳酸 7种有机酸 ;层析纸法收集的分泌物检测到酒石酸、苹果酸、柠檬酸和乳酸 4种有机酸。从收集到的有机酸数量来看 ,洗根法收集到的有机酸总量为 2 9.97～ 2 32 .7μg/( gfw· 2 h) ;琼脂膜法集到的有机酸总量为 1 .5～ 7.3μg/( cm2· 2 h) ;层析纸法收集的有机酸总量为0 .2 3～ 6.5 8μg/( cm2 · 2 h)。丛枝菌根真菌侵染对三叶草根系分泌的有机酸的组分和含量都有一     

不同列当抗性的向日葵品种根系分泌物对向日葵列当种子萌发的影响

以对向日葵列当抗性不同的向日葵幼苗为材料,利用水培法收集其根系分泌物,经二氯甲烷萃取后结合硅胶柱色谱分离法,研究不同列当抗性的向日葵品种根系分泌物对向日葵列当种子萌发的影响。结果表明:(1)在一定浓度范围内,向日葵根系分泌物对向日葵列当种子萌发具有低浓度促进、高浓度抑制的作用;二氯甲烷萃取的向日葵根系分泌物浓度为0.1mg/mL时,其向日葵列当萌发率高于其他浓度的作用。(2)‘星火大白边’和‘白葵杂9号’根系分泌物提取物作用下向日葵列当萌发率最高分别为46.92%和43.88%,显著高于免疫品种‘MGS’根系分泌物提取物作用下列当的萌发率(27.81%)。(3)根系分泌物提取物正己烷-乙酸乙酯1∶1洗脱组分的萌发刺激活性显著高于其他组分,‘星火大白边’、‘白葵杂9号’和‘MGS’的该组分作用下向日葵列当最高萌发率分别为49.90%、45.66%和30.00%。(4)‘MGS’的根系分泌物提取物与GR24或脱氢木香内酯共同作用时种子萌发率显著低于GR24或脱氢木香内酯的单独作用,说明‘MGS’根系分泌物中可能含有抑制GR24或脱氢木香内酯刺激向日葵列当种子萌发的物质,使其对向日葵列当表现出免疫特性。     

Distribution and Function of Proteoid Roots and other Root Clusters

Proteoid roots are bottlebrush-like clusters of rootlets which form along lateral roots. They are characteristic of most species of the Proteaceae, which are mainly distributed in Australia and South Africa. Homologous root clusters are present in species of the Casuarinaceae, Mimosaceae, Fabaceae, Myricaceae and Moraceae. Many similarities exist between these species in relation to morphology and function of root clusters. Many are non-mycorrhizal and are highly efficient in phosphorus (P) acquisition. In these species, proteoid roots and proteoid-like root clusters are abundant when grown on infertile soils. Their formation is predominantly affected by the P status of the plants, being induced at low P levels and repressed at high P levels. Proteoid roots and proteoid-like root clusters play an important role in acquisition of P and other mineral nutrients. Although increase in root surface area may be a contributing factor, in many species these roots excrete large amounts of organic acids and phenolics. The excretion of these compounds in a small soil volume gives rise to extensive nutrient mobilization by acidification, reduction and chelation of sparingly soluble forms of P and micronutrients such as Fe and Mn.     

Changes in Azospirillum brasilense motility and the effect of wheat seedling exudates

The rhizobacterium Azospirillum brasilense Sp245 swims, swarms (Swa⁺ phenotype) or, very rarely, migrates with the formation of granular macrocolonies (Gri⁺ phenotype). Our aims were (i) to identify Sp245 mutants that swarm faster than the parent strain or differ from it in the mode of spreading and (ii) to compare the mutants’ responses to wheat seedling exudates. In isotropic liquid media, the swimming speeds of all motile A. brasilense strains were not influenced by the exudates. However, the exudates significantly stimulated the swarming of Sp245. In several Sp245 mutants, the superswarming phenotype was insensitive to local colonial density and to the presence of wheat seedling exudates. An A. brasilense polar-flagellum-defective Gri⁺ mutant BK759.G gave rise to stable Swa⁺⁺ derivatives with restored flagellum production. This transition was concurrent with plasmid rearrangements and was stimulated in the presence of wheat seedling exudates. The swarming rate of the Swa⁺⁺ derivatives of BK759.G was affected by the local density of their colonies but not by the presence of the exudates.     

Aluminum resistance in two cultivars of Zea mays L.: Root exudation of organic acids and influence of phosphorus nutrition

Root exudation of organic acids as Al-chelating compounds and P nutrition have been suggested to play a major role in Al-resistance in higher plants. Effects of Al exposure on maize plant growth, and organic acid root content and root exudation under various levels of P nutrition were examined. Sikuani, a Colombian maize cultivar tolerant to acid soils with high Al saturation, and Corso, a Swiss cultivar, were grown in sterile hydroponic conditions for 21 days. Al-caused inhibition of root growth was lower in Sikuani than in Corso. Al effect on plant growth was decreased with increasing P content in roots. Al content in roots increased with increasing P content and was higher in Sikuani than in Corso. When exposed to Al, the contents in root apices as well as the root exudation of citric and malic acids in Corso and citric, malic and succinic acids in Sikuani increased, and were higher in Sikuani than in Corso. Increased PEP carboxylase (PEPC) activity in root apices after Al exposure partially explained the variations of organic acid content in the roots. These Al-induced changes in PEPC activity, organic acid content and exudation were reduced in plants supplied with higher P concentrations during the 21 days prior to treatment. Increased secretion of organic acids after exposure to Al appeared to be specific to Al and was not totally explained by increased root content in organic acids.