Root-soil Interface Research Articles

Utilization of Phytic Acid by Cooperative Interaction in Rhizosphere

Utilization of Phytic Acid by Cooperative Interaction in Rhizosphere

Modelling oxygen transport in soil with plant root and microbial oxygen consumption: depth of oxygen penetration

A set of equations governing oxygen diffusion and consumption in soils has been developed to include microbial and plant-root sinks. The dependent variable is the transformed oxygen concentration, which is the difference between the gaseous concentration and a scaled value of the aqueous oxygen concentration at the root–soil interface. The results show how, as the air-filled porosity decreases, the reduced oxygen flux causes the depth of extinction to decrease. The results also show how the depth of extinction at a particular value of soil water content decreases with increasing temperature, due to increased microbial respiration. The critical value of water content at which the oxygen concentration goes to extinction at a finite depth was compared with alternative calculations with only a microbial sink. By ignoring the feedback of oxygen concentration on root uptake, the alternative calculations yielded substantially higher critical values of water content at all temperatures. Two soil oxygen diffusion coefficient functions from the literature were compared and shown to give significantly different critical values of water content for fine-textured soils, one more realistic than the other. A single relationship between the extinction depth and the ratio of the water content to the critical value was shown to apply for all temperatures and soil textures. The oxygen profiles were used along with a function relating redox potential to oxygen concentration to generate redox potential profiles. This application of the model could be useful in explaining soil biochemical processes in soils. For one such process, denitrification, the depth at which a critical oxygen concentration is reached was calculated as a function of the air-filled porosity and temperature of the soil. The implications of the critical value of soil water content in terms of water-filled pore space and matric potential are discussed in relation to the diffusion coefficient functions and recent literature.

Electrical impedance and capacitance method: A new approach for detection of functional aspects of arbuscular mycorrhizal colonization in maize

Applicability of root electrical impedance (EI) and electrical capacitance (EC) measurements for the detection of arbuscular mycorrhizal fungal (AMF) colonization was studied. AMF-inoculated and non-inoculated maize plants were compared whilst growing them under well-watered and drought-stressed condition with regular measurement of root EI and EC. AMF-colonized maize plants showed lower EI and higher EC than control counterparts independently of moisture condition, indicating an enhanced root–soil interface. Root scanning revealed lower root surface area in mycorrhizal plants than in non-mycorrhizal ones growing under well-watered condition, while in plants exposed to drought no significant difference between root surface areas was detected. Since fungal colonization didn't provoke an increase in root surface area, the higher root–soil interface showed by EI and EC values was apparently due to the increased absorption surface area caused by the growth of AMF hyphae. Consequently, the simple, non-destructive EI and EC method is considered an appropriate technique for in situ investigation of AMF-colonization and function, which may partially substitute the intrusive techniques commonly used in mycorrhizal research.

Do roots mind the gap?

Aims Roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the rootsoil interface. Do gaps actually limit the root water uptake, or do they form after water flow in soil is already limiting? Methods Four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray CT at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential. Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements. Results Transpiration started to decrease at soil matric potential = between �5 kPa and �10 kPa. Air-filled gaps appeared along tap roots between =0�10 kPa and =0�20 kPa. As = decreased below �40 kPa, roots further shrank and gaps expanded to 0.1 to 0.35 mm. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size. Conclusions Gaps formed after the transpiration rate decreased. We conclude that gaps are not the cause but a consequence of reduced water availability for lupins.

Quantification and Modeling of Local Root Water Uptake Using Neutron Radiography and Deuterated Water

Knowledge of local water fluxes across the soil–root interface is essential to understand and model root water uptake. Despite its importance, there is a lack of direct methods to measure the location of water uptake along roots. The aim of this study was to develop a technique to quantify local flux of water from soil to the roots of living plants. To this end, we used neutron radiography to trace the transport of deuterated water (D2O) into individual roots. We grew lupin (Lupinus albus L.) in 30- by 25- by 1-cm containers filled with a sandy soil, which was partitioned into different compartments using 1-cm-thick layers of coarse sand. We locally injected D2O into a selected soil compartment near the roots of 18-d-old lupin during the day (transpiring) and night (not transpiring). The transport of D2O into roots was then monitored using time-series neutron radiography. The results show that: (i) the transport of D2O into roots was faster during the day than at night; and (ii) during the day, D2O was quickly transported along the roots toward the shoots, while at night this transport was insignificant. The differences between day and night measurements were explained by convective transport of D2O into the roots driven by transpiration. To quantify the local transport of D2O into roots, we developed a simple convection–diffusion model that assumed the endodermis as the main resistance to water transport. The D2O uptake predicted by the model was in agreement with the axial flow within the roots as derived from D2O transport behind the capillary barrier. This new method allows quantification of local water uptake in different parts of the root system.

Selected Metals in Various Fractions of Soil and Fungi in a Swedish Forest

The patterns of uptake and distribution of Co, Ni, Cu, Zn, Cd, and Pb in the soil-mycelium-sporocarps compartments in various transfer steps are presented. I attempted to find out whether there is a difference between the uptake of metals from soil to fungi (mycelium/soil ratio) and transport within fungal thalli (sporocarps/mycelium ratio). The concentration of Cu, Zn, and Cd increased in the order bulk soil < soil-root interface (or rhizosphere) < fungal mycelium < fungal sporocarps. The concentration of Co, Ni, and Pb decreased in the order bulk soil (or rhizosphere) < fungal mycelium < soil-root interface < fungal sporocarps. The uptake of Cu, Zn, and Cd during the entire transfer process in natural conditions between soil and sporocarps occurred against a concentration gradient. Mycorrhizal fungi (mycelium and sporocarps) only absorbed Co, Ni, and Pb but did not accumulate these elements in their thalli. Metal accumulation within fungal mycelium biomass in the top forest soil layer (0–5 cm) may account for about 5% of the total amount of Co, 4% Ni, 7% Cu, 8% Zn, 24% Cd, and 3% Pb.

Ion Transporters and Abiotic Stress Tolerance in Plants.

Adaptation of plants to salt stress requires cellular ion homeostasis involving net intracellular Na+ and Cl− uptake and subsequent vacuolar compartmentalization without toxic ion accumulation in the cytosol. Sodium ions can enter the cell through several low- and high-affinity K+ carriers. Some members of the HKT family function as sodium transporter and contribute to Na+ removal from the ascending xylem sap and recirculation from the leaves to the roots via the phloem vasculature. Na+ sequestration into the vacuole depends on expression and activity of Na+/H+ antiporter that is driven by electrochemical gradient of protons generated by the vacuolar H+-ATPase and the H+-pyrophosphatase. Sodium extrusion at the root-soil interface is presumed to be of critical importance for the salt tolerance. Thus, a very rapid efflux of Na+ from roots must occur to control net rates of influx. The Na+/H+ antiporter SOS1 localized to the plasma membrane is the only Na+ efflux protein from plants characterized so far. In this paper, we analyze available data related to ion transporters and plant abiotic stress responses in order to enhance our understanding about how salinity and other abiotic stresses affect the most fundamental processes of cellular function which have a substantial impact on plant growth development.

Arsenic Localization and Speciation in the Root–Soil Interface of the Desert Plant Prosopis juliflora-velutina

The bioavailability and mobility of arsenic (As) in soils depends on several factors such as pH, organic matter content, speciation, and the concentration of oxides and clay minerals, among others. Plants modify As bioavailability in the rhizosphere; thus, the biogeochemical processes of As in vegetated and non-vegetated soils are different. Changes in As speciation induced by the rhizosphere can be monitored using micro-focused synchrotron-based X-ray fluorescence (μXRF) combined with μX-ray absorption near-edge spectroscopy (μXANES). This research investigated As speciation in the rhizosphere of mesquite (Prosopis juliflora-velutina) plants grown in a sandy clay loam treated with As(III) and As(V) at 40 mg kg(-1). Rhizosphere soil and freeze-dried root tissues of one-month-old plants were analyzed by bulk XAS. Bulk XAS results showed that As(V) was the predominant species in the soil (rhizosphere and non-vegetated), whereas As(III) was dominant in the root tissues from both As(V) and As(III) treated plants. μXAS and μXRF studies of thin sections from resin embedded soil cores revealed the As(III)-S interactions in root tissues and a predominant As-Fe interaction in the soil. This research demonstrated that the combination of bulk XAS and μXAS techniques is a powerful analytical technique for the study of As speciation in soil and plant samples.

The Pht1;9 and Pht1;8 transporters mediate inorganic phosphate acquisition by the Arabidopsis thaliana root during phosphorus starvation

• The activation of high-affinity root transport systems is the best-conserved strategy employed by plants to cope with low inorganic phosphate (Pi) availability, a role traditionally assigned to Pi transporters of the Pht1 family, whose respective contributions to Pi acquisition remain unclear. • To characterize the Arabidopsis thaliana Pht1;9 transporter, we combined heterologous functional expression in yeast with expression/subcellular localization studies and reverse genetics approaches in planta. Double Pht1;9/Pht1;8 silencing lines were also generated to gain insight into the role of the closest Pht1;9 homolog. • Pht1;9 encodes a functional plasma membrane-localized transporter that mediates high-affinity Pi/H⁺ symport activity in yeast and is highly induced in Pi-starved Arabidopsis roots. Null pht1;9 alleles exhibit exacerbated responses to prolonged Pi limitation and enhanced tolerance to arsenate exposure, whereas Pht1;9 overexpression induces the opposite phenotypes. Strikingly, Pht1;9/Pht1;8 silencing lines display more pronounced defects than the pht1;9 mutants. • Pi and arsenic plant content analyses confirmed a role of Pht1;9 in Pi acquisition during Pi starvation and arsenate uptake at the root-soil interface. Although not affecting plant internal Pi repartition, Pht1;9 activity influences the overall Arabidopsis Pi status. Finally, our results indicate that both the Pht1;9 and Pht1;8 transporters function in sustaining plant Pi supply on environmental Pi depletion.

Water Dynamics of the Root Zone: Rhizosphere Biophysics and Its Control on Soil Hydrology

Approximately 40% of total terrestrial precipitation transits the tiny volume of rhizosphere soil around plant roots before being transpired, making it one of the most hydrologically active regions of the biosphere. This study considers several findings at the root–soil interface that affect our understanding of water retention and flow in the root zone and hence the water relations of all vegetated soil profiles. Imaging methods, including neutron radiography and light transmission, are illuminating the dynamics of water content around plant root systems. These methods, together with studies on samples of mucilage and mucilage‐compound‐amended soils, have provided increasing evidence that rhizosphere hydraulic properties differ from those of bulk soil. Changes in soil structure due to root growth, rhizodeposition, and repeated drying cycles change the pore size distribution and coat soil particles with organic compounds. Some of these compounds exhibit hydrophobic or hydrophilic behavior, depending on the soil water content, giving rise to the hysteretic‐like behavior in the rhizosphere that has been observed in dynamic image sequences. Data from studies that consider the water retention properties of maize (Zea mays L.) mucilage from primary and nodal roots together with polymer gels are compared to consider the likely impact of mucilage on soil water release. Roots often generate and are intimately associated with flow paths for water and solutes in the soil, and vegetation is well known to exert a major influence on catchment hydrology. The potential use of vegetation to manage hydrologic processes at field scale is considered briefly as a way of influencing water outflow rates and engineering soils for particular purposes.

Interactions between exotic invasive plants and soil microbes in the rhizosphere suggest that 'everything is not everywhere'.

The study of soil biota in the context of exotic plant invasions has led to an explosion in our understanding of the ecological roles of many different groups of microbes that function in roots or at the root-soil interface. Part of this progress has been the emergence of two biogeographic patterns involving invasive plants and soil microbes. First, in their non-native ranges invasive plants commonly interact differently with the same soil microbes than native plants. Second, in their native ranges, plants that are invasive elsewhere commonly interact functionally with soil microbes differently in their home ranges than they do in their non-native ranges. These studies pose a challenge to a long-held paradigm about microbial biogeography - the idea that microbes are not limited by dispersal and are thus free from the basic taxonomic, biogeographical and evolutionary framework that characterizes all other life on Earth. As an analogy, the global distribution of animals that function as carnivores does not negate the fascinating evolutionary biogeographic patterns of carnivores. Other challenges to this notion come from new measurements of genetic differences among microbes across geographic boundaries, which also suggest that meaningful biogeographic patterns exist for microorganisms. We expand this discussion of whether or not 'everything is everywhere' by using the inherently biogeographic context of plant invasions by reviewing the literature on interactions among invasive plants and the microorganisms in the rhizosphere. We find that these interactions can be delineated at multiple scales: from individual plants to continents. Thus the microbes that regulate major aspects of plant biology do not appear to be exempt from the fundamental evolutionary processes of geographical isolation and natural selection. At the important scales of taxonomy, ecotype and ecosystem functions, the fundamental ecology of invaders and soil microbes indicates that everything might not be everywhere.

Effects of arbuscular mycorrhizal fungi on grassland productivity are altered by future climate and below-ground resource availability

Due to their key position at the soil–root interface, arbuscular mycorrhizal fungi (AMF) may play an important role in determining the impact of global change on plant communities. We studied the effects of mycorrhizal colonization and “future climate” on productivity of model grassland ecosystems. In 2007 and 2008, artificially assembled grassland communities, grown on pasteurized soil inoculated with or free from AMF, were exposed to ambient conditions or to combined elevated atmospheric CO2 concentration and temperature. In the full soil accessibility experiment (“full soil”, 2007) plant roots could explore the total soil volume. In the partial root-exclusion experiment (“partial soil”, 2008), a nylon membrane of 25μm mesh size allowed AMF to explore an additional soil volume that was not accessible to the roots. In the latter experiment, planting density was higher which increased the competition for nutrient uptake by roots compared to the “full soil” experiment.The main findings were the following:•“Future climate” had a positive effect on AMF root colonization.•Under “future climate”, the stimulated AMF suppressed root biomass in both experiments.•AMF increased plant nutrient stocks and above-ground biomass, regardless of the climate, but only in the “partial soil” experiment, when plant competition for nutrients likely was higher and AMF could increase the available nutrient pool by exploring an extra root-free soil compartment.

Agroforestry Practices in Arid and Semi - Arid Regions of India: Challenges and Opportunities

The review indicates the potential of Agroforestry Practices to meet out the requirements of food, fodder, fuelwood and other tree products as well as resource conservation in the arid and semi-arid regions of the country. However, it is recommended that there is an urgent need of integration of forestry with agriculture at the same place. Based upon various studies conducted on Agroforestry Practices such as planting methods, fertilization, spacing and pruning schedules for raising some of the promising forest and fruit trees in association with annual and perennial field crops have been developed and standardized for management of land degradation. Successful efforts have been made to stabilize the yields of selected crops under tree canopies through agronomic manipulations and by tree canopy management practices. Supplement income can be obtained without reducing productivity and quality of natural grasslands through the introduction of promising leguminous trees on dry degraded lands. These lands which were producing only 2-3 t/ ha/ year biomass earlier are now producing 8-10 t/ha/year high quality biomass. Soil-root interface studies in terms of sharing of space, water, nutrients and light has been workout. The lands which belong to Classes IV/V-VII can be brought under protective measures through plantation of forest trees are considered very important for Agroforestry System. Fodder trees leaves are valuable feed stuffs to animals during lean period. These can be obtained from the top feed species by lopping management. Besides, these are good source of protein and supply of high energy diet during dry season of the year. Moreover, pods and fruits from planted tree species in Agroforestry System are highly palatable and can substitute in the concentrate feed. Some trees pods can also be used as a human and cattle feed. In general, pods are more nutritious than leaf. Under Agroforestry Systems agricultural crops can be grown successfully when the tree are lopped. The negative effects of Agroforestry trees can be narrowed down by lopping management. The plantation of trees as field boundary serves as shelterbelts and windbreaks to protect the field crops from biotic and abiotic stresses. Short rotation tree species are planted in high density and managed by regular pruning, thinning, felling, copping farming to ensure high productivity and conservation on degraded lands. The degraded lands could be rehabilitated by the use of silvi-pastoral system for continuous supply of fodder, fuelwood and other associated products. The yield of cultivated crops had decreased under trees because of competitions for natural resources like soil, water, light and nutrients. But reduction of fuel, fodder and soil nutrients can compensate this loss. Under horti-pastoral system, fruit trees can be planted very well and seeds of perennial legumes and grasses can be grown in between the rows of fruit trees. Hedgerows intercropping between bushes/ trees and agricultural crops primarily as an alternate land use system for enhancing productivity of whole system. Besides, it also fulfills the demand of fodder, firewood, and control of soil erosion. Alternate strip cropping of grasses and perennial legumes on sloppy degraded lands yielded high dry matter production when vegetative barriers were also used. Thus, in nutshell holistic approaches of Agroforestry Systems in dry fringe in India can be served as a multi purposes. This paper focused especially on the Agroforestry Practices recently conducted in dry parts of the country.

Influence of N deficiency and salinity on metal (Pb, Zn and Cu) accumulation and tolerance by Rhizophora stylosa in relation to root anatomy and permeability

Effects of N deficiency and salinity on root anatomy, permeability and metal (Pb, Zn and Cu) translocation and tolerance were investigated using mangrove seedlings of Rhizophora stylosa. The results showed that salt could directly reduce radial oxygen loss (ROL) by stimulation of lignification within exodermis. N deficiency, oppositely, would reduce lignification. Such an alteration in root permeability may also influence metal tolerance by plants. The data indicated that a moderate salinity could stimulate a lignified exodermis that delayed the entry of metals into the roots and thereby contributed to a higher metal tolerance, while N deficiency would aggravate metal toxicity. The results from sand pot trail further confirmed this issue. This study provides a barrier property of the exodermis in dealing with environments. The plasticity of root anatomy is likely an adaptive strategy to regulate the fluxes of gases, nutrients and toxins at root–soil interface.

How the Rhizosphere May Favor Water Availability to Roots

Recent studies have shown that rhizosphere hydraulic properties may differ from those of the bulk soil. Specifically, mucilage at the root–soil interface may increase the rhizosphere water holding capacity and hydraulic conductivity during drying. The goal of this study was to point out the implications of such altered rhizosphere hydraulic properties for soil–plant water relations. We addressed this problem through modeling based on a steady‐rate approach. We calculated the water flow toward a single root assuming that the rhizosphere and bulk soil were two concentric cylinders having different hydraulic properties. Based on our previous experimental results, we assumed that the rhizosphere had higher water holding capacity and unsaturated conductivity than the bulk soil. The results showed that the water potential gradients in the rhizosphere were much smaller than in the bulk soil. The consequence is that the rhizosphere attenuated and delayed the drop in water potential in the vicinity of the root surface when the soil dried. This led to increased water availability to plants, as well as to higher effective conductivity under unsaturated conditions. The reasons were two: (i) thanks to the high unsaturated conductivity of the rhizosphere, the radius of water uptake was extended from the root to the rhizosphere surface; and (ii) thanks to the high soil water capacity of the rhizosphere, the water depletion in the bulk soil was compensated by water depletion in the rhizosphere. We conclude that under the assumed conditions, the rhizosphere works as an optimal hydraulic conductor and as a reservoir of water that can be taken up when water in the bulk soil becomes limiting.

Acquisition of phosphorus and other poorly mobile nutrients by roots. Where do plant nutrition models fail?

In the context of increasing global food demand, ecological intensification of agroecosystems is required to increase nutrient use efficiency in plants while decreasing fertilizer inputs. Better exploration and exploitation of soil resources is a major issue for phosphorus, given that rock phosphate ores are finite resources, which are going to be exhausted in decades from now on. We review the processes governing the acquisition by plants of poorly mobile nutrients in soils, with a particular focus on processes at the root–soil interface. Rhizosphere processes are poorly accounted for in most plant nutrition models. This lack largely explains why present-day models fail at predicting the actual uptake of poorly mobile nutrients such as phosphorus under low input conditions. A first section is dedicated to biophysical processes and the spatial/temporal development of the rhizosphere. A second section concentrates on biochemical/biogeochemical processes, while a third section addresses biological/ecological processes operating in the rhizosphere. New routes for improving soil nutrient efficiency are addressed, with a particular focus on breeding and ecological engineering options. Better mimicking natural ecosystems and exploiting plant diversity appears as an appealing way forward, on this long and winding road towards ecological intensification of agroecosystems.

Actin Capping Protein Is Required for Dendritic Spine Development and Synapse Formation

Dendritic spines serve as the postsynaptic platform for most excitatory synapses in the mammalian brain, and their shape and size are tightly correlated with synaptic strength. The actin cytoskeleton plays a crucial role in the spine structure and its modifications during synapse development and plasticity, but the underlying regulatory mechanisms remain to be elucidated. Here, we report that actin capping protein (CP), a regulator of actin filament growth, plays an essential role for spine development and synapse formation. We found that CP expression in rat hippocampus is elevated at and after the stage of substantial synapse formation. CP knockdown in hippocampal cultures resulted in a marked decline in spine density accompanied by increased filopodia-like protrusions. Moreover, the spines of CP knockdown neurons exhibited an altered morphology, highlighted by multiple thin filopodia-like protrusions emerging from the spine head. Finally, the number of functional synapses was reduced by CP knockdown as evidenced by a reduction in the density of paired presynaptic and postsynaptic markers and in the frequency of miniature EPSCs. These findings indicate that capping of actin filaments by CP represents an essential step for the remodeling of the actin architecture underlying spine morphogenesis and synaptic formation during development.

Assessing the applicability of the earth impedance method for in situ studies of tree root systems.

Several electrical methods have been introduced as non-invasive techniques to overcome the limited accessibility to root systems. Among them, the earth impedance method (EIM) represents the most recent development. Applying an electrical field between a cormus and the rooted soil, the EIM measures the absorptive root surface area (ARSA) from grounding resistance patterns. Allometric relationships suggested that this method was a valuable tool. Crucial assumptions for the applicability of the EIM, however, have not been tested experimentally. Focusing on tree root systems, the present study assesses the applicability of the EIM. Six hypotheses, deduced from the EIM approach, were tested in several experiments and the results were compared with conventional methods. None of the hypotheses could be verified and the results allow two major conclusions. First, in terms of an analogue electrical circuit, a tree-root–soil continuum appears as a serial circuit with xylem and soil resistance being the dominant components. Allometric variation in contact resistance, with the latter being the proxy for root surface area, are thus overruled by the spatial and seasonal variation of soil and xylem resistances. Second, in a tree-root–soil continuum, distal roots conduct only a negligible portion of the electric charge. Most of charge carriers leave the root system in the proximal parts of the root–soil interface.

Cadmium-induced changes in soil biochemical characteristics of oat (Avena sativa L.) rhizosphere during early growth stages

A microcosm was assembled to physically separate soil from roots and was used to study both the impact of living roots on the soil–plant system during early stages of growth and plant responses to abiotic stress. Oat (Avena sativa L.) seedlings were grown in the microcosm unit for 44 days. Twenty-three days after planting, 0.154 mg CdSO4/g dry soil was added. Plants grown in Cd-treated microcosms showed considerable inhibition of shoot growth rates, and leaf chlorophyll content. Soil microbial biomass C and respiration increased with plant age, and most of the measured biochemical indicators decreased with increasing distance from the soil–root interface, thus demonstrating the rhizosphere effect, likely due to the quick assimilation of rhizodeposits by rhizosphere microflora. On the other hand, short-term Cd contamination sometimes had an inhibitory effect on soil respiration, qCO2, ATP content, and phosphatase activity, while stimulating microbial biomass, mainly at the rhizosphere level. The decrease in rhizosphere microbial activity observed after Cd application to soil may be due to a synergic effect of the metal directly on microbial cells and indirectly on plants, which reduced shoot growth rate and chlorophyll content, resulting in decreased availability of root exudates.

Peter Hague Nye. 16 September 1921 — 13 February 2009

Peter Nye was at the forefront of research in plant and soil sciences for over 40 years. He began his career in West Africa in the 1950s, where he did seminal work on the soils under shifting cultivation systems of agriculture. He spent the rest of his career in Oxford, where he pioneered research on the complexities of the plant root–soil interface and mathematical modelling of plant and soil processes.