Basal Root Zones Research Articles

Anatomical and biochemical characterisation of a barrier to radial O2 loss in adventitious roots of two contrasting Hordeum marinum accessions.

A barrier to radial O2 loss (ROL) in roots is an adaptive trait of waterlogging-tolerant plants. Hordeum marinum Huds. is a waterlogging-tolerant species that, in contrast to its waterlogging-sensitive cultivated relatives, forms a tight barrier to ROL in basal root zones. To evaluate the nature of the barrier to ROL in roots, we combined measurements of ROL with histochemical and biochemical studies of two contrasting H. marinum accessions. H21 formed greater aerenchyma (up to 38% of cross-sectional area) and a tight barrier to ROL when grown under deoxygenated stagnant conditions, whereas the barrier was only partially formed in roots of H90 and aerenchyma was up to 26%. A tight barrier to ROL in roots of H21 corresponded with strong suberisation of the exodermis. In agreement with anatomical studies, almost all aliphatic suberin quantities were greater in roots of H21 grown under stagnant conditions compared with roots from aerated controls, and also to those in H90. By contrast to suberin, no differences in root lignification were observed between the two accessions raised in either aerated or stagnant conditions. These findings show that in adventitious roots of H. marinum, suberisation rather than lignification restricts ROL from the basal root zones.

Improvement of salt and waterlogging tolerance in wheat: comparative physiology of Hordeum marinum-Triticum aestivum amphiploids with their H. marinum and wheat parents.

Hordeum marinum Huds. is a waterlogging-tolerant halophyte that has been hybridised with bread wheat (Triticum aestivum L.) to produce an amphiploid containing both genomes. This study tested the hypothesis that traits associated with waterlogging and salinity tolerances would be expressed in H. marinum-wheat amphiploids. Four H. marinum accessions were used as parents to produce amphiploids with Chinese Spring wheat, and their responses to hypoxic and 200mM NaCl were evaluated. Relative growth rate (RGR) in the hypoxic-saline treatment was better maintained in the amphiploids (58-71% of controls) than in wheat (56% of control), but the amphiploids were more affected than H. marinum (68-97% of controls). In hypoxic-saline conditions, leaf Na+ concentrations in the amphiploids were lower than in wheat (30-41% lower) but were 39-47% higher than in the H. marinum parents. A strong barrier to radial oxygen loss formed in basal root zones under hypoxic conditions in two H. marinum accessions; this barrier was moderate in the amphiploids, absent in wheat, and was weaker for the hypoxic-saline treatment. Porosity of adventitious roots increased with the hypoxic treatments; values were 24-38% in H. marinum, 16-27% in the amphiploids and 16% in wheat. Overall, the amphiploids showed greater salt and waterlogging tolerances than wheat, demonstrating the expression of relevant traits from H. marinum in the amphiploids.

Lead, zinc and iron (Fe 2+) tolerances in wetland plants and relation to root anatomy and spatial pattern of ROL

Metal (Pb, Zn and Fe 2+) tolerances, root anatomy and profile of radial oxygen loss (ROL) along the root (i.e., spatial pattern of ROL) were studied in 10 emergent wetland plants. The species studied could be classified into three groups. Group I included Alternanthera philoxeroides, Beckmannia syzigachne, Oenanthe javanica and Polypogon fugax, with high ROL along the whole length of root (‘partial barrier’ to ROL). Group II included Cyperus flabelliformis, Cyperus malaccensis, Juncus effusus, Leersia hexandra and Panicum paludosum, ROL of which was remarkably high just behind the root apex, but decreased significantly at relatively basal regions (‘tight barrier’ to ROL). Group III consisted of only Neyraudia reynaudiana, with extremely low ROL along the length of root. The results indicated that metal tolerance in wetland plants was related to root anatomy and spatial pattern of ROL. Co-evolution of metal (Fe and Zn) tolerance and flood tolerance possibly developed in wetland plants since species showing a ‘tight barrier’ to ROL (a common trait of flood-tolerant species) in basal root zones had higher Fe and Zn tolerances than those showing a ‘partial barrier’. Root anatomy such as lignin and suberin deposition contributed to a ‘tight barrier’ in root and conferred to exclusion ability in tolerant species.

AtDUR3 represents the major transporter for high‐affinity urea transport across the plasma membrane of nitrogen‐deficient Arabidopsis roots

Despite the fact that urea is a ubiquitous nitrogen source in soils and the most widespread form of nitrogen fertilizer used in agricultural plant production, membrane transporters that might contribute to the uptake of urea in plant roots have so far been characterized only in heterologous systems. Two T-DNA insertion lines, atdur3-1 and atdur3-3, that showed impaired growth on urea as a sole nitrogen source were used to investigate a role of the H+/urea co-transporter AtDUR3 in nitrogen nutrition in Arabidopsis. In transgenic lines expressing AtDUR3-promoter:GFP constructs, promoter activity was upregulated under nitrogen deficiency and localized to the rhizodermis, including root hairs, as well as to the cortex in more basal root zones. Protein gel blot analysis of two-phase partitioned root membrane fractions and whole-mount immunolocalization in root hairs revealed the plasma membrane to be enriched in AtDUR3 protein. Expression of the AtDUR3 gene in nitrogen-deficient roots was repressed by ammonium and nitrate but induced after supply of urea. Higher accumulation of urea in roots of wild-type plants relative to atdur3-1 and atdur3-3 confirmed that urea was the substrate transported by AtDUR3. Influx of 15N-labeled urea in atdur3-1 and atdur3-3 showed a linear concentration dependency up to 200 microM external urea, whereas influx in wild-type roots followed saturation kinetics with an apparent Km of 4 microM. The results indicate that AtDUR3 is the major transporter for high-affinity urea uptake in Arabidopsis roots and suggest that the high substrate affinity of AtDUR3 reflects an adaptation to the low urea levels usually found in unfertilized soils.

Structural and physiological changes in the roots of tomato plants over‐expressing a basic peroxidase

Previous studies on the tomato (Lycopersicon esculentum Mill.) peroxidase TPX1, including the development of transgenic tomato over‐expressing this gene, supported an involvement of this peroxidase in the synthesis of lignin and suberin. The transgenic plants showed a wilty phenotype at flowering, but the relationship between this role in ligno‐suberization and this phenotype was not clear. In the present study a histological approach and the measurement of water‐related parameters have been performed in order to obtain an insight into the origin of this phenotype. Clear differences between transgenic and non‐transgenic roots were observed in the cross‐sections of the basal root zones where secondary growth was evident. The diameter of the xylem vessel was diminished in the transgenic plants. Total area corresponding to xylem in the basal cross‐sections decreased 3.9 fold in the transgenic roots. In addition, the radial and outer tangential walls of the exodermis cells were more ligno‐suberized in transgenic than in non‐transgenic plants. After fruit set, predawn and midday water potentials were lower in transgenic than in‐non‐transgenic plants. At midday, the stomatal conductance was also lower in the transgenic plants, 494±69 versus 594±60 mmol m−2 s−1. Root hydraulic conductances of the transgenic and non‐transgenic plants were 1.4±0.38 and 3.47±0.19 g water min−1 MPa−1, respectively. The results obtained support that the phenotype is caused by the anatomical differences found in the transgenic roots. These differences would be the cause of a increased resistance to water flow in the roots that would negatively affect the water supply to the shoot and, as a consequence, resulted in a decreased water potential in the leaves.

Microautoradiographic localization of phosphate and carbohydrates in mycorrhizal roots of Populus tremula x Populus alba and the implications for transfer processes in ectomycorrhizal associations.

Microautoradiographic studies were carried out to examine the distribution and exchange of phosphate and labeled carbohydrates in mycorrhizal roots of Populus tremula x Populus alba L. following application of 33P-orthophosphate (Pi) and 14CO2. Labeled Pi was not homogeneously distributed along the mycorrhizal longitudinal axis. The fungal sheath and the Hartig net contained more 33Pi in the median parts of the root than in the apical or basal root zones, indicating that uptake and transfer of Pi to the host plant was localized mainly in this area. The Pi was translocated by the Hartig net and the interfacial apoplast to the host plant. It was distributed by way of the stele within the plant. Young leaves and meristematic tissue in the shoot tip were the main sinks for Pi. In plants that were left in the dark for 5 days before 33Pi application, the reduced carbohydrate supply caused a decrease in Pi absorption by mycorrhizal roots. Microautoradiography of mycorrhizal roots after assimilation of 14CO2 revealed that: (1) the fungal partner had a high capacity to attract photosynthates; (2) the main transfer of carbohydrates was localized in the median zone of a mycorrhizal root; (3) carbohydrates that were absorbed by the mycorrhizal fungus were translocated to the fungal sheath and were homogeneously distributed; and (4) in the main exchange zone, cortical cell nuclei showed a high sink capacity, indicating increased metabolic activity in these cells. We postulate that (1) the phosphate demand of the host plant regulates absorption of Pi by the fungus, and (2) a bidirectional transfer of carbohydrates and Pi occurs across the same interface structure in ectomycorrhizal roots of Populus.

Changes in growth, porosity, and radial oxygen loss from adventitious roots of selected mono‐ and dicotyledonous wetland species with contrasting types of aerenchyma

ABSTRACTGrowth in stagnant, oxygen‐deficient nutrient solution increased porosity in adventitious roots of two monocotyledonous (Carex acuta and Juncus effusus) and three dicotyledonous species (Caltha palustris, Ranunculus sceleratus and Rumex palustris) wetland species from 10 to 30% under aerated conditions to 20–45%. The spatial patterns of radial oxygen loss (ROL), determined with root‐sleeving oxygen electrodes, indicated a strong constitutive ‘barrier’ to ROL in the basal root zones of the two monocotyledonous species. In contrast, roots of the dicotyledonous species showed no significant ‘barrier’ to ROL when grown in aerated solution, and only a partial ‘barrier’ when grown in stagnant conditions. This partial ‘barrier’ was strongest in C. palustris, so that ROL from basal zones of roots of R. sceleratus and R. palustris was substantial when compared to the monocotyledonous species. ROL from the basal zones would decrease longitudinal diffusion of oxygen to the root apex, and therefore limit the maximum penetration depth of these roots into anaerobic soil. Further studies of a larger number of dicotyledonous wetland species from a range of substrates are required to elucidate the ecophysiological consequences of developing a partial, rather than a strong, ‘barrier’ to ROL.

Chemical composition of apoplastic transport barriers in relation to radial hydraulic conductivity of corn roots (Zea mays L.).

The hydraulic conductivity of roots (Lp(r)) of 6- to 8-d-old maize seedlings has been related to the chemical composition of apoplastic transport barriers in the endodermis and hypodermis (exodermis), and to the hydraulic conductivity of root cortical cells. Roots were cultivated in two different ways. When grown in aeroponic culture, they developed an exodermis (Casparian band in the hypodermal layer), which was missing in roots from hydroponics. The development of Casparian bands and suberin lamellae was observed by staining with berberin-aniline-blue and Sudan-III. The compositions of suberin and lignin were analyzed quantitatively and qualitatively after depolymerization (BF(3)/methanol-transesterification, thioacidolysis) using gas chromatography/mass spectrometry. Root Lp(r) was measured using the root pressure probe, and the hydraulic conductivity of cortical cells (Lp) using the cell pressure probe. Roots from the two cultivation methods differed significantly in (i) the Lp(r) evaluated from hydrostatic relaxations (factor of 1.5), and (ii) the amounts of lignin and aliphatic suberin in the hypodermal layer of the apical root zone. Aliphatic suberin is thought to be the major reason for the hydrophobic properties of apoplastic barriers and for their relatively low permeability to water. No differences were found in the amounts of suberin in the hypodermal layers of basal root zones and in the endodermal layer. In order to verify that changes in root Lp(r) were not caused by changes in hydraulic conductivity at the membrane level, cell Lp was measured as well. No differences were found in the Lp values of cells from roots cultivated by the two different methods. It was concluded that changes in the hydraulic conductivity of the apoplastic rather than of the cell-to-cell path were causing the observed changes in root Lp(r).

Water Transport in Onion (Allium cepa L.) Roots (Changes of Axial and Radial Hydraulic Conductivities during Root Development).

The hydraulic architecture of developing onion (Allium cepa L. cv Calypso) roots grown hydroponically was determined by measuring axial and radial hydraulic conductivities (equal to inverse of specific hydraulic resistances). In the roots, Casparian bands and suberin lamellae develop in the endodermis and exodermis (equal to hypodermis). Using the root pressure probe, changes of hydraulic conductivities along the developing roots were analyzed with high resolution. Axial hydraulic conductivity (Lx) was also calculated from stained cross-sections according to Poiseuille's law. Near the base and the tip of the roots, measured and calculated Lx values were similar. However, at distances between 200 and 300 mm from the apex, measured values of Lx were smaller by more than 1 order of magnitude than those calculated, probably because of remaining cross walls between xylem vessel members. During development of root xylem, Lx increased by 3 orders of magnitude. In the apical 30 mm (tip region), axial resistance limited water transport, whereas in basal parts radial resistances (low radial hydraulic conductivity, Lpr) controlled the uptake. Because of the high axial hydraulic resistance in the tip region, this zone appeared to be "hydraulically isolated" from the rest of the root. Changes of the Lpr of the roots were determined by measuring the hydraulic conductance of roots of different length and referring these data to unit surface area. At distances between 30 and 150 mm from the root tip, Lpr was fairly constant (1.4 x 10-7 m s-1 MPa-1). In more basal root zones, Lpr was considerably smaller and varied between roots. The low contribution of basal zones to the overall water uptake indicated an influence of the exodermal Casparian bands and/or suberin lamellae in the endodermis or exodermis, which develop at distances larger than 50 to 60 mm from the root tip.

Distribution of Nicotianamine, a Presumed Symplast Iron Transporter, in Different Organs of Sunflower and of a Tomato Wild Type and Its Mutant chloronerva

The nicotianamine (NA) concentration in the shoot apex of Helianthus annuus amounts to the 2- to 16-fold in comparison to older leaves and cotyledons, and that of apical root zones to the 4-fold in relation to the basal root zones of lateral roots of 2nd order.

Estimation of phosphorus uptake capacity by different zones of the primary root of soil‐grown maize (Zea mays L.)

AbstractA method is described which allows determination of the nutrient uptake capacity by different zones of individual roots of soil‐grown plants. Examples are given for phosphorus uptake by different zones of the primary root of maize. Agar strips with labelled phosphorus (32P) are placed on the soil‐root interface of root zones of different age. After 24 h autoradiographs are made of the soil‐root interface and phosphorus uptake rates determined by plant analysis.Along the primary root the relative phosphorus uptake capacity declines per unit root length from 100% in 1 day old root zones to about 25–30% in 26 day old root zones. The change in the extension of the phosphorus (32P) depletion zone along the root axis indicates that the uptake capacity for phosphorus of root hairs was maximal upto 4 days and declined thereafter. The decline in capacity for phosphorus uptake from apical to basal root zones probably reflects the decline in root hair viability. The relative high uptake capacity in basal root zones is therefore at least partially due to the uptake capacity of the cortical cells.

Ion and Water Uptake in Relation to Root Development in Norway Spruce ( Picea abies (L.) Karst.)

Summary Uptake rates of mineral nutrients (K, Mg, Ca) and water along individual non-mycorrhizal roots of three-year-old Norway spruce ( Picea abies (L.) Karst.) were related to the development of the endodermis and of lateral roots. The uptake rate of K was low at the root tip and high in the root elongation zone and in basal zones where laterals were fully developed. Magnesium uptake rates declined from apical to basal root zones. In contrast, Ca and water uptake rates had two maxima, the most distinct at the root tip in the zone where no Casparian band had yet developed, and another, less distinct in the zone where the lateral primordia had broken through the endodermis. The apoplasmic fluorescent dye Tinopal CBS entered the stele only in the latter root zone. The results stress the role of root growth for water and mineral nutrient uptake and the importance of the apoplasmic pathway for the radial transport of calcium and water in basal root zones of Norway spruce. Whether the high uptake rates of Ca and water in apical root zones also reflect an apoplasmic pathway of radial transport is not yet clear.

Genotypic Differences in Aluminium Tolerance of Soybean ( Glycine max L.) as Affected by Ammonium and Nitratenitrogen Nutrition

Soybean ( Glycine max L.) genotypes differed considerably in response of root elongation to aluminium supply in solution culture. Root elongation was more sensitive to Al than nitrate and ammonium net uptake rates, which were higher in Al treated plants. Nitrate reductase activity, which was also higher in Al treated plants in more basal root zones, was unaffected on a much higher activity level in the apical 2 mm root zone. Genotypic differences in Al tolerance were not related to differences in NH 4 + versus NO 3 - uptake, nitrate reductase activity and related changes of pH in the rhizosphere. This was confirmed by a sand culture experiment comparing NH 4 + and NO 3 - nutrition and direct measurement of pH at the root surface along the roots.

Localization of phytosiderophore release and of iron uptake along intact barley roots

Under iron deficiency the release of so‐called phytosiderophores by roots of barley plants (Hordeum vulgare L. cv. Europa) was greater by a factor of 10 to 50 compared to iron‐sufficient plants. This enhanced release occurred particularly in apical zones of the seminal roots and in the lateral root zones. Under iron deficiency, uptake rates for iron, supplied as FeIII phytosiderophore, increased by a factor of ca 5 as compared to iron‐sufficient plants. This enhanced uptake rate for iron was also much more pronounced in apical than in basal root zones. In contrast, with supply of the synthetic iron chelate, FelII EDDHA (ferric diaminoethane‐N, N‐di‐o‐hydroxyphenyl acetic acid), the Fe deficiency‐enhanced uptake rates for iron were only small and similar along the roots, except for the lateral root zones. The high selectivity of barley roots for uptake and translocation of FeIII phytosiderophores compared with FeIII EDDHA is reflected by the fact that, at the same external concentration (2 μM), rates of uptake and translocation of iron from FeIII phytosiderophores were between 100 (Fe‐sufficient) and 1 000 times higher (Fe‐deficient plants) than from FeIII EDDHA. The relatively high rates of uptake and particularly of translocation of iron supplied as FeIII EDDHA in the zone of lateral root formation strongly suggest an apoplastic pathway of radial transport of the synthetic iron chelate into the stele in this root zone.The results demonstrate that apical root zones are the main sites both for Fe deficiency‐enhanced release of phytosiderophores and for uptake and translocation of iron supplied as FeIII phytosiderophores.

Localization and Capacity of Proton Pumps in Roots of Intact Sunflower Plants

Proton extrusion by roots of intact sunflower plants (Helianthus annuus L.) was studied in nutrient solutions or in agar media with a pH indicator. Proton extrusion was enhanced by either iron deficiency, addition of fusicoccin, or single salt solutions of ammonium or potassium salts. The three types of proton extrusion differ in both localization along the roots and capacity. From their sensitivity to ATPase inhibitors it seems justified to characterize them as proton pumps driven by plasma membrane APTases.Enhanced proton extrusion induced by preferential cation uptake from (NH(4))(2)SO(4) or K(2)SO(4) was uniformly distributed over the whole root system. In contrast, the enhancement effect of fusicoccin was confined to the basal root zones and that of iron deficiency to the apical root zones. Also the rates of proton extrusion per unit of root fresh weight differed remarkably and increased in the order: Fusicoccin << K(2)SO(4) < (NH(4))(2)SO(4) < iron deficiency.Under iron deficiency the average values of proton extrusion for the whole root system are 5.6 micromoles H(+) per gram fresh weight per hour; however, for the apical root zones values of about 28 micromoles H(+) can be calculated. This high capacity is most probably related to the iron deficiency-induced formation of rhizodermal transfer cells in the apical root zones. It can be assumed that the various types of root-induced acidification of the rhizosphere are of considerable ecological importance for the plant-soil relationships in general and for mobilization of mineral nutrients from sparingly soluble sources in particular.

Plant‐induced pH changes in the rhizosphere of “Fe‐efficient” and “Fe‐inefficient” soybean and corn cultivars

Abstract In intact root systems of soybean and corn cultivars differing in Fe efficiency the pH gradients at the soil‐root interface (rhizosphere) have been studied. For this purpose a combination of an agar technique (fluid agar + pH indicator) and a soil culture (sandy soil, pH 6.0) were used. In addition the rhizosphere pH was measured with antimony microelectrodes. With this method, striking differences in rhizosphere pH could be observed both between the cultivars and within a cultivar along the root system. In general, the rhizosphere pH was higher in “Fe‐inefficient”; cultivars compared to “Fe‐efficient”; cultivars. Compared to the bulk soil (pH 6.0) the rhizosphere pH in the basal root zones of Fe‐inefficient cultivars (PI‐54169; ys1/ ys1) increased up to pH 7.5 and pH 6.8, respectively; the corresponding pH values for the Fe‐efficient cultivars (Hawkeye; WF9) were pH 6.8 and pH 5.6, respectively. Within the root system of the same plant the rhizosphere pH of apical zones was always more than one ...

Langstreckentransport von Natrium in Bohnenpflanzen

Long distance transport of sodium in bean plantsIn 8–10 days old bean plants Na+ (22Na) has been applied to either a certain root zone, the stem, or the base and tip respectively of a primary leaf and the long distance transport of Na+ was studied in the following 12–48 h.The long distance transport of Na+ applied to the root zone 9–12 cm behind the tip was strongly restricted towards the shoot and hardly detectable towards the root tip (phloem transport). Presence of K+ in the surrounding solution strongly increased the Na+ efflux from the roots.After leaf application within 48 h 30–40% of the absorbed Na+ had been translocated out of the leaf in direction of the root where, from the basal root zones, intensive Na+ efflux took place. This Na+ efflux was hardly affected by presence of K+ in the external medium and was usually more than 10% of the Na+ taken up by the leaves. From the Na+ taken up by the hypocotyl within 12 h more than 25% had been released from the basal root zones into the nutrient solution. Less than 1% of the Na+ applied either to the leaf or the stem was translocated towards the shoot apex.Separation of the hypocotyl into cortex and stele at the end of the experiment demonstrated the high capacity of the stele for Na+ accumulation. Within the hypocotyl the transfer of Na+ from the stele to the cortex and the phloem seems to be a rapid process whereas the release of Na+ from the phloem into the stele is obviously very restricted. The long distance transport of Na+ within the phloem of the shoot is strictly basipetal and very efficient. Low Na+ contents of bean leaves are therefore the result of several regulating mechanisms: K+ stimulated Na+ efflux in the roots, restricted long distance transport in the xylem due to high Na+ accumulation in the stele, Na+ pumps at the phloem in stem and leaves for phloem loading of Na+, and finally strictly basipetal retranslocation of Na+ in the phloem into the roots and efflux into the solution from basal root zones.

On the Development of Crown Roots in Various Growth Stages of the Rice Plant

It was reported that a shoot of rice plants is composed of shoot units, each of which consists of an apical leaf, a basal bud and an apical and basal root zones, and that the crown root emergence of a shoot unit is usually synchronized with the emergence of the leaf of the third upper shoot unit. The present study was carried out, with early-sown and late-sown rice plants and rice plants with soil kept in constant temperature during the growing period as materials, in order to clarify the pattern of development of crown roots emerged from shoot units of the main stem, with special reference to the rate of elongation and the diameter of crown roots and the acropetal emergence of secondary roots. The rate of elongation of crown roots increased from lower to upper shoot units with the highest rate in the ninth or the tenth unit, upper than which the rate declined upto the eleventh shoot unit which was the highest one with crown root emerged, in both cases of natural and controlled soil temperature conditions. The elongating rate of rapidly growing crown roots of early-sown rice plants at the stage of the thirteenth leaf emerging was found to be bigger than that of crown roots which emerged on the same day of late-sown rice plants at the stage of the tenth leaf emerging. The diameter of crown roots and the rate of acropetal emergence of secondary roots of the former plants were also bigger than those of the latter plants. As to the diameter, the rate of elongation of the primary roots and the rate of acropetal emergence of the secondary roots as associated with the growth of the primary roots, the crown roots from the upper root zone were not superior to those from the lower root zone of the same shoot unit. Furthermore, a high positive correlation was found between the diameter and the rate of elongation in crown roots. From these results, it may be said that the development of crown roots of each shoot units is affected not only by environmental factors but also by the physiological situations of rice plants which depend on the developmental stages.