Re-sorption of organic components by roots of Zea mays L. and its consequences in the rhizosphere

The re-sorption of carbon compounds from the rhizosphere was investigated using ¹⁴C-labelled glucose, mannose and citric acid. Uptake in roots of 5-day-old, intact Zea mays plants in sterile solution culture was determined over a period of 48 hours. Under optimal growth conditions significant re-absorption of glucose and mannose occurred with the uptake rates being 70.5 and 40.2 g compound g^-1 root DW h^-1, respectively. For glucose and mannose approximately 25% of the ¹⁴C label taken up by the root was recovered inside the plant as low-MW compounds and 33% polymerized into high MW compounds. 42% was respired as ¹⁴C-CO₂. Citric acid by comparison showed little accumulation within plant tissues (11.4%) with most being respired and recovered as ¹⁴C-CO₂ in KOH traps (88%). The uptake rate for citric acid was 34.8 g g^-1 root DW h^-1. Over the 48-hour period a net efflux (i.e. exudation) of labelled plus unlabelled C was observed at a rate of 608 g C g^-1 root DW h^-1 (equivalent to 1520 g glucose/mannose). Of the C released as root exudates, a minimum estimate of the amount of C taken back into the plant was therefore 9.5%. The two main C fluxes within the rhizosphere, namely release of C by the root and uptake by the microorganisms, have been well documented in recent years. It is now apparent however that a third flux term, re-sorption of C by roots, can also be identified. This may play an important but previously overlooked role within the rhizosphere, and further work is needed to determine its significance.A comparison between exudate release in static (permitting accumulation of C) and flowing culture (C removed as it is released) was also made with the respective rates being 15.36 and 45.18 mg C g^-1 root DW in 2 days. The relative important of re-sorption in natural environments and laboratory experiments is discussed.     

Influx and efflux of amino acids from Zea mays L. roots and their implications for N nutrition and the rhizosphere

The aim of the study was to investigate the ability of Zea mays L. roots to regulate the amount of free amino acids present in the rhizosphere. The active uptake of amino acids was shown to conform to Michaelis-Menten kinetics. Comparison of amino acid-N and NO₃-N kinetic parameters and soil solution concentrations showed that root uptake of free amino acids from soil may contribute significantly to a plant's N budget. The influx of amino acids also helps to minimize net C/N losses to the soil, and is therefore important in regulating the size of the rhizosphere microbial population. Experimental data and a computer simulation model of amino acid influx/efflux in a sterile solution culture, showed that roots were capable of re-sorping over 90% of the amino acids previously lost into solution as a result of passive diffusion.     

Bacterial communities associated with the rhizosphere of transgenic Bt 176 maize (Zea mays) and its non transgenic counterpart

The effect of transgenic Bt 176 maize on the rhizosphere bacterial community has been studied with a polyphasic approach by comparing the rhizosphere of Bt maize cultivated in greenhouse with that of its non transgenic counterpart grown in the same conditions. In the two plants the bacterial counts of the copiotrophic, oligotrophic and sporeforming bacteria, and the community level catabolic profiling, showed no significant differences; differences between the rhizosphere and bulk soil bacterial communities were evidenced. Automated ribosomal intergenic spacer analysis (ARISA) showed differences also in the rhizosphere communities at different plant ages, as well as between the two plant types. ARISA fingerprinting patterns of soil bacterial communities exposed to root growth solutions, collected from transgenic and non transgenic plants grown in hydroponic conditions, were grouped separately by principal component analysis suggesting that root exudates could determine the selection of different bacterial communities.     

根分泌物对根际矿物营养及根际微生物的效应

综述了根系分泌物对植物生长的生理生态学效应,并就根系分泌物的定义、产生机制、组成成分和影响因素等方面进行了讨论。指出根系分泌物在缓解低矿物营养胁迫对植株造成的伤害及决定根际微生物的种群密度和数量方面起着重要的作用;根系分泌物的产生机制多样,组成成分复杂,影响因素繁多。对根分泌物的深入研究有助于进一步了解植物体与土壤间进行的生理生化过程及其调控机制。     

Role of root derived organic acids in the mobilization of nutrients from the rhizosphere

The role of organic acids in the mobilization of plant nutrients from the rhizosphere was assessed in seven contrasting soil types. The results indicated that malate was poor at mobilizing micronutrients from all the test soils, whilst citrate was capable of mobilizing significant quantities. Citrate was also capable of mobilizing P from one soil which possessed a large Ca-P fraction. This mobilization of P was due to both the complexing action of the citrate anion and due to the dissolution properties of the protons released from citric acid upon equilibrium with the soil solution. The reaction of citrate with cations was found to be near instantaneous with significant absorption to the solid phase in some soils at low concentrations. Soil decomposition studies indicated that citrate was rapidly broken down in organic soils but was more resistant to degradation in subsoil horizons. It was concluded that organic acids can be expected to be of little consequence in nutrient mobilization from high pH soils, whilst in acid soils they may be involved both in a more general mechanism for micronutrient uptake or as a potential Al detoxification mechanism.Abbreviations C₆H₈O₇ Citric acid and H-citrate indicates - C₆H₅O₇Na₃ whilst Na-citrate indicates     

Organic acids in the rhizosphere of Banksia integrifolia L.f.

Water leachates of the proteoid root layer of a mature stand of Banksia integrifolia were analysed for low molecular weight (LMW) organic acids by GC, HPLC and colorimetric techniques. Large amounts of organic acids (2500 g in 100 mL of leachate) were found in the proteoid root layer compared to the surrounding soil and leaf litter (230 g in 100 mL of leachate). Citric acid represented approximately 50% of the total organic acids leached, malic acid approximately 18%, and aconitic acid constituted approximately 17%. Concentration of citric acid in the proteoid root layer may enhance the availability of phosphorus for plant uptake.     

Potential errors caused by roots in analyses of rhizosphere soil

Roots contain high concentrations of many elements, and have the potential to interfere with measurements of chemical change in rhizosphere soil. To assess potential interferences, maize (Zea mays L.) roots (free of soil) and soil (free of roots) were extracted separately with several extractants commonly used to assess the status of soil nutrients. The maize roots were grown within filter envelopes which prevented direct contact with soil, but permitted passage of mineral nutrients and water from the adjacent soil. Water, ammonium acetate (pH 7), DTPA (pH 7.3), Morgan's solution (pH 4.8), and dilute HCl were used as extractants. Most elements were released readily into soluble forms from roots killed by freezing to lyse the cells. Significantly lower amounts were extracted from fresh roots, with the greatest differences between fresh and killed roots for the extractants H₂O and DTPA, which were the mildest in terms of acidity and salt concentration. Extraction of P from the fresh roots by H₂O and HCL was particularly low. Contamination of rhizosphere samples with root materials would almost certainly prevent the accurate measurement of water-soluble P, K, Mn, Zn, Cu, and Na in the slightly alkaline soil used in this experiment. Large errors would be likely also for P, Mn, and Cu extracted by ammonium acetate. The DTPA extractant is normally used only for micronutrient metals or heavy metals, and the small amounts of these elements released by roots should not contribute to significant error. With Morgan's solution, errors would likely be large only for P. Dilute HCl is a reasonably strong extractant for many elements in soil, and major errors from roots contained in rhizosphere samples are unlikely. The relatively high probability of errors in extractions of soluble elements from rhizosphere soil is unfortunate, because these elements are among the most readily available to plants and the most likely to be altered by the normal activities of roots.     

Wetting and drying cycles in the maize rhizosphere under controlled conditions. Mechanics of the root-adhering soil

Mechanical properties of the topsoil (sandy Podsol and silty Luvisol, FAO) adhering to maize (Zea mays L.) roots and its bulk soil counterpart were studied as a function of soil texture and final soil water suction at harvest, with three soil water suction values of approximately 30, 50 and 60 kPa. Two scales of observation were also selected: the whole soil:root system and the root-adhering soil aggregates. Three methods were used to characterize the stability of the soil:root system: mechanical shaking in air, and dispersion by low-power ultrasonication, with or without preliminary immersion of the soil:root system in water. Soil disruption kinetics, which were fitted with first-order kinetics equations, were analyzed and discussed. For example, silty soil ultrasonication kinetics, without preliminary water-immersion, could be divided into two parts: the first faster part, which was characterized by a mean rate K value of 6.8–7.2 mJ^-1, is attributed to soil slaking, whereas the second slower part, which was characterized by a mean rate K value of 1.5–1.6 mJ^-1, was attributed to the rupture of the `firmly root-adhering soil' from the roots. A clear plant effect was observed for both aggregate tensile strength and friability, with higher aggregate strength for the root-adhering silty soil (450–500 kPa) than for its bulk silty soil counterpart (410–420 kPa), and lower friability (coefficient of variation of the aggregate strength) for the root-adhering silty soil (e.g. 67% at a soil water suction value of 30 kPa) than for its bulk silty soil counterpart (e.g. 49% at asoil water suction value of 30 kPa). These effects were attributed to root exudation, which was significantly higher for the driest silty topsoil than for the wetter ones. In conclusion, the mechanical properties of the silty topsoil adhering to the maize roots are attributed to both physical and biological interactions occurring in the maize rhizosphere. This revised version was published online in June 2006 with corrections to the Cover Date.     

Graviresponsiveness and abscisic-acid content of roots of carotenoid-deficient mutants of Zea mays L.

The abscisic-acid (ABA) content of roots of the carotenoid-deficient w-3, vp-5, and vp-7 mutants of Z. mays was analyzed using gas chromatography-mass spectrometry with an analysis sensitivity of 6 ng ABA g^–1 fresh weight (FW). Roots of normal seedlings of the same lines were characterized by the following amounts of ABA (as ng ABA g^–1 FW,±standard deviation): w-3, 279±43; vp-5, 237±26; vp-7, 338±61. We did not detect any ABA in roots of any of the mutants. Thus, the lack of carotenoids in these mutants correlated positively with the apparent absence of ABA. Primary roots of normal and mutant seedlings were positively gravitropic, with no significant differences in the curvatures of roots of normal as compared with mutant seedlings. These results indicate that ABA 1) is synthesized in maize roots via the carotenoid pathway, and 2) is not necesary for positive gravitropism by primary roots of Z. mays.Abbreviation ABA abscisic acid     

Temperature dependence of trans plasma membrane electron transport in Zea mays L. roots

Intact Zea mays L. cv. Golden Bantam seedlings which were not cold adapted were exposed to various temperatures. Trans plasma membrane potential difference was measured in a temperature range from 0 to 40 °C using intracellular microelectrodes. The depolarization caused by electron transfer across the PM to artificial external electron acceptors was investigated. Active membrane potential increased with temperature in the range from 0 to 15 °C but was independent of temperature above 20 °C. Depolarization caused by the non-membrane-permeating electron acceptors hexacyanoferrate III (HCF III) and hexabromoiridate IV (HBIIV) took place over the whole temperature range investigated. The effect of HBI IV increased up to 10 °C whereas the HCF III effects increased up to 25 °C.     

Action of malformin A1 on gravitropic curvature in primary roots of maize (Zea mays L.)

Malformins, a small family of cyclic pentapeptides, are active plant growth regulators isolated from the fungusAspergillus niger. We purified malformin A₁ from the crude malformin A mixture, and studied its action in the gravitropic response of maize roots. Intact primary roots that had been pretreated vertically with malformin A₁ were placed in a humidified box in the horizontal position. Positive curvature (downward) was inhibited in the pretreated roots compared with the control. In addition, we measured the lateral transport of IAA in primary roots. Roots pretreated with malformin A, did not show asymmetric distribution of IAA between the upper and lower sides of the elongation zone. Malformin A, also stimulated ethylene production in maize root segments. Our results had suggested that malformin A₁ might inhibit the lateral transport of IAA across the roots from the upper to the lower side because of an increased level of ethylene. Therefore, we placed more IAA on the upper side at the initial phase of gravistimulation. These results were consistent with malformin A₁-pretreated roots showing inhibited positive gravitropic curvature.     

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.     

Isolation and characterization of Azotobacter chroococcum from the roots of Zea mays

Abstract Bacteria showing rapid growth on a nitrogenfree medium and acetylene-reducing activity were isolated from maize roots collected from agricultural soils in Spain. The isolates were Gram-negative motile rods and were identified as Azotobacter chroococcum . Acetylene-reducing activity and microbial counts were determined on root segments from 7- and 30-day-old plants. Rates obtained were in the range of 0.0053–0.848 nmol C₂H₂· g⁻¹· h⁻¹. Root populations were 1.4–6.0 × 10⁴ micro-organisms · g⁻¹. These results showed that there was an association between A. chroococcum strains and roots of maize planted in some Spanish soils.     

Brassinosteroids are inherently biosynthesized in the primary roots of maize, Zea mays L

GC-MS analysis revealed that primary roots of maize contain 6-deoxocathasterone, 6-deoxoteasterone and 6-deoxotyphasterol. These brassinosteroids, and the previously identified campesterol, campestanol, 6-deoxocastasterone and castasterone, in the roots are members of a biosynthetic pathway to castasterone, namely the late C-6 oxidation pathway, suggesting that its biosynthetic pathway is operative in the roots. To verify this, a cell-free enzyme extract was prepared from maize roots, and enzymatic conversions from campesterol to castasterone through the aforementioned sterols and brassinosteroids were examined. The presence for the biosynthetic sequences, campesterol-->24-methylcholest-4-en-3beta-ol-->24-methylcholest-4-en-3-one-->24-methylcholest-5 alpha-cholestan-3-one-->campestanol and 6-deoxoteasterone-->6-deoxo-3-dehydroteasterone-->6-deoxotyphasterol-->6-deoxocastasterone-->castasterone were demonstrated. These results indicate that maize roots contain a complete set of enzymes involved in the late C-6 oxidation pathway, thereby demonstrating that endogenous brassinosteroids are biosynthesized in the roots.     

Bacterial inoculation of maize affects carbon allocation to roots and carbon turnover in the rhizosphere

To assess the influence of bacteria inoculation on carbon flow through maize plant and rhizosphere,¹⁴C allocation after¹⁴CO₂ application to shoots over a 5-day period was determined. Plants were grown on C- and N-free quartz sand in two-compartment pots, separating root and shoot space. While one treatment remained uninoculated, treatments two and three were inoculated withPantoea agglomerans (D5/23) andPseudomonas fluorescens (Ps I A12), respectively, five days after planting. Bacterial inoculation had profound impacts on carbon distribution within the system. Root/rhizosphere respiration was increased and more carbon was allocated to roots of plants being inoculated. After five days of¹⁴CO₂ application, more ethanol-soluble substances were found in roots of inoculated treatments and lower rhizodeposition indicated intensive C turnover in the rhizosphere. In both inoculated treatments the intensity of photosynthesis measured as net-CO₂-assimilation rates were increased when compared to the uninoculated plants. However, high C turnover in the rhizosphere reduced shoot growth of D5/23 inoculated plants, with no effect on shoot growth of Ps I A12 inoculated plants. A separation of labeled compounds in roots and rhizodeposition revealed that neutral substances (sugars) constituted the largest fraction. The relative fractions of sugars, amino acids and organic acids in roots and rhizodeposition suggest that amino acid exudation was particularly stimulated by bacterial inoculation and that turnover of this substance group is high in the rhizosphere.     

Distribution of growth and proton efflux in gravireactive roots of maize (Zea mays L.)

Roots of Zea mays were maintained in a vertical orhorizontal position and the local elongation rate and H⁺ fluxes were measured using Sephadex beads containing a pH indicator. When the roots were kept horizontally, the growth of the lower side was strongly inhibited and that of the upper side slightly stimulated as compared with vertical roots. The H⁺ extrusion, which was greatest in the elongation zone, was strongly inhibited on the lower side and slightly stimulated on the upper side as compared with vertical roots.     

Cytochemical and X-ray microanalytical studies of calcium in the extending zone, particularly in the rhizodermis of Zea mays roots

Calcium was detected in CaCl (10 m M )-pretreated roots of Zea mays L. (cv. LG 11) by means of electron probe X-ray microanalysis with energy-dispersive spectrometry (EDS) using embedded samples, fixed by the antimonate staining procedure. A high level of calcium was found where large amounts of antimonate precipitates were observed by light or transmission electron microscopy. In the elongation zone, after 20 h in humid air following a 2 h CaCl₂ pretreatment, the level of calcium was higher in trichoblasts than in atrichoblasts. In these cells it was detected mainly in the walls and nucleus, and antimonate staining was observed in the walls. Abundant precipitates containing calcium were associated with the nucleus, vacuoles, mitochondria and endoplasmic reticulum of non-differentiated cells, whereas they were confined to the walls of these cells just after the CaCl₂ pretreatment. The involvement of calcium in the formation of root hairs is discussed.     

Mobilization of cadmium and other metals from two soils by root exudates of Zea mays L., Nicotiana tabacum L. and Nicotiana rustica L.

Soluble root exudates were collected from three plants (Nicotiana tabacum L., Nicotiana rustica L. and Zea mays L.), grown under axenic and hydroponic conditions, in order to study their metal-solubilizing ability for Cd and other cations (Cu, Fe, Mn, Ni, Zn). Nicotiana spp. and Zea mays L. root exudates differed markedly in C/N ratio, sugars vs. amino acids ratio and organic acids content. Metals from two soils were extracted with either root exudate solutions, containing equal amounts of organic carbon, or distilled water as control. In the presence or absence of root exudates, the solubility of Fe and Mn was much higher than of the four other metals tested. Root exudates increased the solubilities of Mn and Cu, whereas those of Ni and Zn were not affected. Root exudates of Nicotiana spp. enhanced the solubility of Cd. The extent of Cd extraction by root exudates (N. tabacum L. N. rustica L. Zea mays L.) was similar to the order of Cd bioavailability to these three plants when grown on soil. An increase in Cd solubility in the rhizosphere of apical root zones due to root exudates is likely to be an important cause of the relatively high Cd accumulation in Nicotiana spp.