Root exudates: a pathway for short-term N transfer from clover and ryegrass

The short-term transfer of nitrogen (N) from legumes to grasses was investigated in two laboratory studies. One study was done in pots where the roots of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were allowed to co-exist, and a second study was performed using a micro-lysimeter system designed to maintain nutrient flow from the clover to the grass, whilst removing direct contact between the root systems. The ¹⁵N-dilution technique was used to quantify the transfer of N between species. Levels of ammonia and amino acids were measured in root exudates. The amounts of N transferred were in the same order of magnitude in both the pot and micro-lysimeter experiments. In the micro-lysimeter experiment, 0.076 mg of N were transferred per plant from clover to ryegrass during the course of the experiment. Ammonium exudation was much higher than amino acid exudation. The most abundant amino acids in both clover and ryegrass root exudates were serine and glycine. However, there was no correlation between the free amino acid profile of root extracts and exudates for both plant species: Asparagine was the major amino acid in clover roots, while glutamine, glutamate and aspartate were the major amino acids in ryegrass roots. Comparison of exudates obtained from plants grown in non-sterile or axenic conditions provides evidence of plant origin of ammonium, serine and glycine.     

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。     

Relationships between cell membrane stability, exudate content and infectivity of Bradyrhizobium japonicum strain 639 to boron starved soybean plants

The influence of boron starvation on the root exudates content in soybean seedlings (Glycine max. L. Merr.) and the effect of exudates pretreatment on the pre-infection processes in symbiotic system Br. japonicum strain 636 and soybean were investigated. Root cell membrane stability of boron starved soybean plants (-B) decreased compared to the control. The concentrations of all analyzed metabolites (reducing sugars, free amino acids, organic acids, soluble phenols and total flavonoids) from root exudates of -B plants were lower than the control concentrations. Analysis of polyphenols after HPLC chromatography of root exudates showed significant difference of peak numbers between chromatograms of exudates obtained from boron starved and from control plants. Bacterial culture treatment with root exudates from -B plants showed decreased growth, chemotaxis and attachment ability toward the host root compared to the control exudate treatments. These changes were accompanied by decreased nodulation and acetylene reduction activity of boron starved soybean plants.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi

Root colonization by arbuscular mycorrhizal (AM) fungi reduces stimulation of seed germination of the plant parasite Striga (Orobanchaceae). This reduction can affect not only host plants for Striga, resulting in a lower parasite incidence, but also false hosts or trap crops, which induce suicidal Striga seed germination, thereby diminishing their effectiveness. In order to better understand these AM-induced effects, we tested the influence of root colonization by different AM fungi on the seed-germination activity of root exudates of the Striga hermonthica nonhost plants cowpea and cotton on S. hermonthica. We also tested the effect of AM fungi on the seed-germination activity of the Striga gesnerioides host plant cowpea on S. gesnerioides. Moreover, we studied whether mycorrhization affects the transport of seed-germination activity to above-ground plant parts. Mycorrhization not only resulted in a lower seed germination of S. gesnerioides in the presence of root exudates of the S. gesnerioides host cowpea but also seed germination of S. hermonthica was also lower in the presence of root exudates of the S. hermonthica nonhosts cowpea and cotton. Downregulation of the Striga seed-germination activity occurs not only in root exudates upon root colonization by different AM fungi but also in the compounds produced by stems. The lowered seed-germination activity does not appear to depend on the presence of seed germination inhibitors in the root exudates of mycorrhizal plants. The implication for Striga control in the field is discussed.     

The effect of root exudates from two transgenic insect-resistant cotton lines on the growth of Fusarium oxysporum

The attenuation of disease resistance in transgenic insect-resistant cotton has become one of the important factors restricting cotton production in China. Two transgenic insect-resistant cotton lines and their parental conventional cotton lines were used as the testing materials. The effects of root exudates of these cotton lines on the spore germination and mycelial growth of Fusarium oxysporum were studied and the components, contents of amino acids and sugars were determined. The results showed that the resistance of the two insect-resistant cotton lines to F. oxysporum was inferior to the parental lines, and that their root exudates promoted fungal spore germination and mycelial growth. Considerable differences in the components and contents of both, amino acids and sugars were found between the root exudates of transgenic cotton lines and their parental lines, where the disease indices were highly correlated with the total amount of sugars in the root exudates.     

Effect of root exudates on the spore germiantion of rhizosphere and rhizoplane mycoflora of chilli (Capsicum annuum L.) cultivars

Summary From root exudates of three cultivars of chilli (Capsicum annuum L.) 12 amino acids and 7 sugars were detected. Methionine, d-1- phenylalanine, citrulline and d-xylose were detected only from the root exudates of resdistant cultivars. The root exudates of resistant variety inhibited spore germination of the pathogen (Fusarium oxysporum f. sp.capsici), but that of susceptible variety enhanced spore germiantion of the same. Spore germiantion of antagonistic fungi (Trichderma viride andAspergillus sydowi) was also influenced by the root exudates of resistant and susceptible varieties, but the influence was different.Spore germiantion of a number of rhizosphere fungi was studied and in general root exudate of susceptible cultivar enhanced spore germiantion of majority of fungi, but spore germination of antagonistic fungi against the pathogen was inhibited. However, root exudate of resistant cultivar stimulated spore germination of antagonistic fungi.     

Amino acids in root exudates of cotton

Summary Cotton-field soils from a wilt-free zone (KPT) and from a wilt-sick zone (PLD), and root exudates of diploid and amphidiploid cotton strains grown in KPT and PLD soils were analyzed for 13 free amino acids employing microbiological assay techniques. Test organisms used wereLactobacillus arabinosus 17/5 andLeuconostoc mesenteroides P. 60.Arginine, proline, lysine, serine, glycine, cystine, and tyrosine occurred in both the soils; amounts of these were higher in PLD soil than in KPT soil.Seven to eleven amino acids were found in the root exudates of cotton. Aspartic acid, threonine (in one), phenylalanine and leucine were found in root exudates but not in the control soils. Diploid strains were found to exude greater amounts of amino acids than the amphidiploid strains. Comparing the effect of the two soils tested, it was found that amino acids in exudates of plants grown in PLD soil were higher except in the case of one diploid strain.In the inoculated series, amino acids in the exudates were markedly lower than the respective healthy controls in the diploid strains. In the exudates of amphidiploid strains also, amino acid contents were reduced but to a lesser extent.Significance of occurrence of cystine in the exudates of some strains is indicated.Part of Doctoral thesis, University of Madras.     

Nitrogen deficiency induced changes of free amino acids and coumarin contents in the leaves of Matricaria chamomilla

Through o-hydroxycinnamic acids, the biosynthesis of coumarins is connected with aromatic amino acid metabolism and nitrogen uptake. Therefore the quantitative changes in levels of some free amino acids and coumarins (herniarin and its glucosidic precursors (Z) - and (E)-2-β-D-glucopyranosyloxy-4-methoxycinnamic acids; umbelliferone) in the leaf rosettes of chamomile (Matricaria chamomilla L.) subjected to nitrogen deficiency were studied. Nitrogen content decreased in the leaf rosettes and in the roots of N-deficient plants during the course of the experiment, but these plants produced significantly higher root biomass. Among secondary metabolites, the sum of 2-β-D-glucopyranosyloxy-4-methoxycinnamic acids increased sharply, herniarin increased slowly and the content of umbelliferone was low in N-deficient plants. We have concluded that nitrogen deficiency is not an inducing factor for stress accumulation of herniarin and umbelliferone. A decrease in levels of all detected amino acids, besides histidine, was found. Within aromatic amino acids, tyrosine was the most abundant. The content of free phenylalanine was significantly lower in both, control and N-deficient plants when compared to the content of tyrosine. In this view, the increase of herniarin glucosidic precursors is apparently due to enhancing phenylalanine ammonia-lyase activity under nitrogen deficiency and nitrogen-free carbon skeletons are shunted in to the phenylpropanoid metabolism, including biosynthesis of (Z)-and (E)-2-β-D-glucopyranosyloxy-4-methoxycinnamic acids.     

The effect of aluminium on phosphate uptake by three isolated ectomycorrhizal fungi

Three wheat cultivars with different tolerances against free aluminium were grown monoxenically in association with Azospirillum brasilense. In situ nitrogen fixation, measured with the acetylene reduction assay, was higher by the aluminium-tolerant cultivars than by the sensitive cultivar. The transfer of fixed nitrogen to the host plant, determined by the ¹⁵N dilution technique, was also significantly higher in the aluminium-resistant wheat plants. The total accumulation of fixed nitrogen in the host plants due to an A. brasilense inoculation varied from approximately 13% to 17% of the total nitrogen in the root and 2.9% to 3.9% of the nitrogen in the shoot.The quantity and quality of exudates released in liquid nutrient solution were analysed separately for two of the wheat cultivars, one aluminium-tolerant and one aluminium-sensitive. After 29 days of growth the aluminium-tolerant plants exudated significantly higher total amounts of carbon than aluminium-sensitive plants. No differences between the two cultivars existed in the carbon exudation rate per gram dry root.Much higher concentrations of low molecular dicarboxylic acids i.e. succinic, malic and oxalic acid, were found in the exudates of aluminium-tolerant plants. Dicarboxylic acids are potential chelating compounds for positively charged metals such as aluminium and they may play an important role in protecting the plant against aluminium incorporation. They are also very suitable substrates for Azospirillum spp. It is therefore suggested that these factors may be causing the higher associative nitrogen fixation rates which was found in the aluminium-tolerant wheat cultivars.     

Manganese reduction in the rhizosphere of mycorrhizal and nonmycorrhizal maize

The influence of rhizosphere microorganisms and vesicular-arbuscular (VA) mycorrhiza on manganese (Mn) uptake in maize (Zea mays L. cv. Tau) plants was studied in pot experiments under controlled environmental conditions. The plants were grown for 7 weeks in sterilized calcareous soil in pots having separate compartments for growth of roots and of VA mycorrhizal fungal hyphae. The soil was left either uninoculated (control) or prior to planting was inoculated with rhizosphere microorganisms only (MO-VA) or with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe] (MO+VA). Mycorrhiza treatment did not affect shoot dry weight, but root dry weight was slightly inhibited in the MO+VA and MO-VA treatments compared with the uninoculated control. Concentrations of Mn in shoots decreased in the order MO-VA > MO+VA > control. In the rhizosphere soil, the total microbial population was higher in mycorrhizal (MO+VA) than nonmycorrhizal (MO-VA) treatments, but the proportion of Mn-reducing microbial populations was fivefold higher in the nonmycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The most important microbial group taking part in the reduction of Mn was fluorescent Pseudomonas. Mycorrhizal treatment decreased not only the number of Mn reducers but also the release of Mn-solubilizing root exudates, which were collected by percolation from maize plants cultivated in plastic tubes filled with gravel quartz sand. Compared with mycorrhizal plants, the root exudates of nonmycorrhizal plants had two fold higher capacity for reduction of Mn. Therefore, changes in both rhizosphere microbial population and root exudation are probably responsible for the lower acquisition of Mn in mycorrhizal plants.     

Amino acids as determinants of host preference for the xylem feeding leafhopper,Homalodisca coagulata (Homoptera: Cicadellidae)

Summary Homalodisca coagulata is a highly polyphagous xylem feeder with distinct seasonal patterns in it's selection of host plants. These patterns were examined in relation to the amino acid content of the xylem for four common host species; Lagerstroemia indica, Baccharis halimifolia, Prunus persica, and Prunus salicina. Xylem fluid was collected from each host species at times when numbers of feeding leafhoppers were both low and high. In each case, concentrations of amino acids were greatest when numbers were high. Similarly, comparisons between host species at given times showed that concentrations of amino acids were positively correlated with host selection. In a second study, amino acids of xylem were manipulated by budding scions of a non-preferred host (P. persica) on rootstocks of preferred (P. salinica) and non-preferred (P. persica) hosts. Morphology and phenology of the budded trees were similar to that of the scion species yet the xylem composition of amino acids was primarily dependent on the rootstock. Concentrations of amino acids and the preference of leafhoppers were roughly two-fold greater for scions of the preferred than the non-preferred rootstock. In both studies, amides (glutamine plus asparagine) were the amino acids most highly correlated with host selection. These compounds are the predominant amino acids in xylem fluid, have high nitrogen to carbon ratios, and account for a high percentage of the caloric value in xylem fluid. Many of the less abundant amino acids were positively correlated with host preference, but the correlations were less consistent and correlation coefficients were generally lower.Florida Agricultural Experiment Station Journal Series No. 9672     

Use of Ethylene-Producing Bacteria for Stimulation of Striga spp. Seed Germination

Striga spp. are obligate root-parasitic flowering plants that threaten cereal and legume production, and consequently human well-being, in Africa. Successful control depends on eliminating the seed reserves of Striga spp. in soil and preventing parasitism. A proven method of eliminating these seed reserves is soil-injection of ethylene gas. This method was used successfully in the United States to control Striga asiatica, but injection of ethylene gas is potentially dangerous, very costly, and generally unsuitable in Africa. The bacterium Pseudomonas syringae pathovar glycinea synthesizes relatively large amounts of ethylene. In this study a laboratory procedure was developed for testing strains of P. syringae pv. glycinea for efficacy in stimulating germination of seeds of Striga spp. The procedure allows comparisons among bacteria, volatile compounds, root exudates, and synthetic stimulants for germination of Striga spp. seeds. Seeds of three Striga spp. were tested over a 10-month period. No seed germination was ever observed with sterile water. When compared across Striga spp. the bacterial strains were consistently better stimulators of germination of seeds of the parasites than ethylene gas or root pieces of a Vigna unguiculata cultivar known to stimulate germination of parasite seeds. The strains were as effective in germinating S. aspera and S. gesnerioides seeds as a synthetic germination stimulant. Our results showing that ethylene-producing bacteria are highly effective in promoting seed germination in Striga spp. suggest that these bacteria may provide a practical means of biological control of Striga spp. in Africa and other locations.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Rhizospheric exudation of Eriophorum vaginatum L. — Potential link to methanogenesis

Root exudates are a direct link between primary production in higher plants and methanogenesis. The relationship has been widely studied on rice paddies, but less is known about its role in wetlands populated by naturally occurring species. This study provides information about the amount and composition of root exudates produced by a widespread mire plant, Eriophorum vaginatum L. For this purpose, E. vaginatum plants were grown in quartz sand in pots from April to October, and root exudates were collected once a month by percolation of the cultivation substrate. In June and October, a set of plants was labelled with ¹⁴CO₂ for two days and subsequently harvested for the determination of dry weight and for root exudates collected by the dipping method. The study supports earlier findings that natural wetland plants can enhance methanogenesis in their rhizosphere via active and seasonally varying exudation, but that the amount of exuded carbon (C) is many times lower than that delivered via litter formation. At both harvests in June and October, the proportion of incorporated radioactivity in shoots, roots and exudates was 92–96%, 4–8%, and 0.2%, respectively. New C was primarily fixed in the metabolically important carbohydrates, as well as acid anions that composed the main compounds of the new exudates. However, microbes seemed to rapidly metabolise the exudates into other substances like acetate. This was the dominant compound in the rhizoplane and rhizosphere, and it was the only detected substance that occurred in higher amounts outside the roots than inside them. Further studies in the field, including the quantification of gaseous end products, are necessary to complete our understanding of the carbon cycling in E. vaginatum-soil-microbe-system.     

Root exudates as mediators of mineral acquisition in low-nutrient environments

Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO₃ ^–, OH^–, H⁺), gaseous molecules (CO₂, H₂), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N₂-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N₂ is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N₂ reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH^– and HCO₃ ^– to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.     

Fractional changes in phenolic acids composition in root nodules of Arachis hypogaea L.

Phenolic acids are active antimicrobial compounds and root signaling molecules that play important roles in plant defense responses. They are generally present in plants as glycosides or esters. A range of soluble and bound phenolic acids were detected in roots and root nodules of Arachis hypogaea L., among which five were identified by high performance liquid chromatography (HPLC) coupled with UV–Vis diode array detector (DAD), viz., p-coumaric acid (p-com), p-hydroxybenzaldehyde (HBAld), p-hydroxybenzoic acid (HBA), caffeic acid (CA) and protocatechuic acid (PA). Para-coumaric acid was constitutively present in all fractions whereas HBA was present in the soluble form only in young nodules. CA and PA were mostly present in the wall bound fraction. The root nodules contain higher concentration of phenolic acids than non-nodulated roots and presence of peroxidase and polyphenol oxidase indicate the metabolism of phenolic acids in roots and root nodules. These results indicate that phenolic acids (p-com and CA) in bound-glycosidic or ester forms were major components in cell wall fortification which provide protection to the root nodule from pathogen attack.     

Strain-specific salt tolerance and chemotaxis ofAzospirillum brasilense and their associative N-fixation with finger millet in saline calcareous soil

Three salt-tolerantAzospirillum brasilense strains were isolated from the roots of finger millet grown in saline calcareous soil and characterized. The effect of various salts on growth and N₂ase activity of these strains was tested and strain STR1 was found more tolerant at higher concentrations of Cl^-, SO₄ ² and HCO₃ ^-. Bicarbonate was found to be the most toxic. The content and concentrations of root exudates of finger millet genotypes were different and chemotaxis to sugars, amino acids, organic acids and root exudates was strain specific. Under salt stress, significant interactions between strains and genotypes of finger millet resulted in different responses of N₂ase activity, endo- and exorhizospheric population, dry weight of root, shoot and grain yield.     

Effect of nitrate supply on the in-vivo synthesis and distribution of trifollin A,a Rhizobium trifolii-binding lectin,in Trifolium repens seedlings

In-vivo synthesis of the white-clover lectin, trifoliin A, was examined by the incorporation of labeled amino acids into protein during heterotrophic growth of intact Trifolium repens L. seedlings. Lectin synthesis was quantified by measuring the level of labeled protein immunoprecipitated from root exudate, from the hapten (2-deoxyglucose) eluate of the roots, and from root and shoot homogenates. The presence of labeled trifoliin A was confirmed by non-denaturing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by fluorography and comparison with trifoliin A standards. In-vivo-labeled trifoliin A was detected in seedling root homogenate 2 h after the addition of labeled amino acids and on the root surface by 8 h. Incorporation of labeled amino acids into protein and trifoliin A was greatest with 2-d-old seedlings and was greater when the plants were grown continuously in the dark than when they were exposed to 14 h light daily. Significantly more labeled lectin accumulated on the root surface of seedlings grown with 1.5 mM KNO₃ than of seedlings grown either without N or with 15.0 mM KNO₃. The labeled lectin from the root surface in all nitrate treatments and from the rootexudate samples of seedlings grown N-free and with 1.5 mM KNO₃ was fully able to bind to Rhizobium trifolii. In contrast, only 2% of the immunoprecipitable protein found in the root exudate of seedlings grown with 15.0 mM KNO₃ was able to bind to the bacteria. Thus, excess nitrate does not repress the synthesis of trifoliin A in the root, but does affect the distribution and activity of this newly synthesized lectin in a way which reduces its ability to interact with R. trifolii. By using Western blot analysis, much more total trifoliin A is detected in the homogenates of shoots than roots. However, greater than 80% of the total labeled protein and 85–90% of the total labeled lectin were found in the root homogenates of 2-d-old dark-grown seedlings incubated for 5 h with labeled amino acids. In addition, Western blot analysis indicated that the shoot homogenate contained smaller-molecular-weight peptides which reacted with the specific anti-trifoliin A antibody. These studies indicate that stored trifoliin A in the seed is degraded in the shoots during seedling development, while newly synthesized trifoliin A in the roots is excreted to the root surface and external environment.Abbreviations IgG immunoglobulin G - LPS lipopolysaccharide - PBS 10 mM potassium-phosphate buffer, pH 7.0, containing 0.8% NaCl - PBS-T 20 mM phosphate-buffered saline, pH 7.4, containing 0.05% Tween 20 - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis     

Effect of seed and root exudates on the growth ofXanthomonas phaseoli var.fuscans

In a medium containing bean, barley and wheat seed exudates,Xanthomonas phaseoli var.fuscans (Burk.) Starr et Burk. grew substantially better than in that containing root exudates of these plants. When the bacteria were cultivated in a medium containing root exudates of bean plants deprived of cotyledons after eleven days of growth, growth was slower than in the presence of root exudates of control plants. On the other hand, the growth was stimulated in a medium containing root exudates of bean plants deprived of leaves. It was found that seed exudates of these plants contained biologically active peptides stimulating the growth of the microorganism. These peptides were not found in root exudates. These findings suggest a relationship between the survival ofXanthomonas phaseoli var.fuscans in the rhizosphere of bean and the exudation of biologically active peptides originating from the stock substances of seeds and cotyledons.