Roots ofZea Research Articles

Impact of agricultural practices on the Zea mays L. endophytic community.

Agricultural practices are known to alter bulk soil microbial communities, but little is known about the effect of such practices on the plant endophytic community. We assessed the influence of long-term applications (20 years) of herbicides and different fertilizer types on the endophytic community of maize plants grown in different field experiments. Nested PCR-denaturing gradient gel electrophoresis (DGGE) analyses targeting general bacteria, type I or II methanotrophs, actinomycetes, and general fungi were used to fingerprint the endophytic community in the roots of Zea mays L. Low intraplant variability (reproducible DGGE patterns) was observed for the bacterial, type I methanotroph, and fungal communities, whereas the patterns for endophytic actinomycetes exhibited high intraplant variability. No endophytic amplification product was obtained for type II methanotrophs. Cluster and stability analysis of the endophytic type I methanotroph patterns differentiated maize plants cultivated by using mineral fertilizer from plants cultivated by using organic fertilizer with a 100% success rate. In addition, lower methanotroph richness was observed for mineral-fertilized plants than for organically fertilized plants. The use of herbicides could not be traced by fingerprinting the endophytic type I methanotrophs or by evaluating any other endophytic microbial group. Our results indicate that the effect of agrochemicals is not limited to the bulk microbial community but also includes the root endophytic community. It is not clear if this effect is due to a direct effect on the root endophytic community or is due to changes in the bulk community, which are then reflected in the root endophytic community.

Expansion dynamics, metabolite composition and substance transfer of the primary root growth zone ofZea maysL. grown in different external nutrient availabilities

ABSTRACTA combined analysis of growth and metabolite composition was performed in primary roots ofZea maysL. (Var. Alexander). The seedlings were hydroponically cultivated either in pure water or in complete nutrient solution. The overall root growth performance was similar in both treatments. Yet, digital image sequence processing methods resolved, that growth distribution and oscillatory movements within the growth zone depended strongly on external nutrient availability. Metabolite concentration profiles were similar in both treatments for most investigated metabolites, indicating a thorough mobilization of nutrient resources from the seed, but concentrations of glutamine, glutamic acid, NO3–, NH4+, malate and citrate showed pronounced differences between treatments. No diurnal variations in metabolite concentrations were found. Deposition rate profiles were in general more similar to relative elemental growth rate profiles than concentration profiles and were not affected by the treatment. Major ions were deposited maximally in front of the centre of growth activity, while greatest hexose deposition was found behind that. Relative to their abundance in the root growth zone, net rates of transfer from mature tissue were highest for sucrose, glutamic acid and aspartic acid, whereas glucose, fructose and most amino acids inversely showed high net rates of transfer out of the root growth zone, indicating a high catabolic rate for those substances there. NO3–, but not other nutrients, was transferred to a great extent from the root growth zone to the mature tissue in nutrient solution. Overall, the results show, that a careful analysis of growth dynamics allows quantifying and interpreting a number of important flux parameters in the growing organ and that the performance of the primary root does not depend strongly on external nutrient availability.

An Automata-theoretical Model of Meristem Development as Applied to the Primary Root ofZea maysL.

Observations were made of the sequence of division within the cellular packets (groups of cells of common descent) which comprise the cell files that run the length of the central cortex of the primary root meristem ofZea mays.These sequences, and also the relative lengths of the cells within the packets recorded at various times during root growth, indicate that cell-file development can be expressed using one, or a limited number, of deterministic ‘bootstrap’ L-systems which assign different lifespans to sister cells of successive cell generations. The outcome is a regular pattern of divisions from which daughter cells emerge usually with unequal, but definite, lengths. In the immediately post-germination stage of root growth, one division pathway is especially common in the cortex and generates sequences of unequal daughters having a particular basi-apical orientation. Later in root growth, the cellular pattern in the cortex indicates that this pathway is replaced by another where unequal divisions are not so marked, but which nevertheless continues to maintain a regular arrangement of differently sized cells. This latter pathway is characteristic of a zone close to the initial cells of the cortex. It is present at all stages of root growth and spreads along the length of the cortex as the descendants of these initials proliferate. The development of the whole cortical cell file can be simulated from knowledge of the growth functions of the bootstrap systems. The files so generated contain all the observed cell patterns. The growth functions also predict the sequence in which cells cease dividing near the proximal margin of the meristem, but for this it is necessary to incorporate a counter for the number of divisions that will be accomplished in the cell file. Cytological requirements for the propagation of unequal divisions, together with a consideration of the nature of the division counter, as well as the significance of the switch in division pathways encountered during early root growth, are discussed in the context of this deterministic model of cell division.

Water stress and indol-3yl-acetic acid content of maize roots

Water-stress conditions were applied to the apical 12 mm of intact or excised roots ofZea mays L. (cv. LG 11) using mannitol solutions (0 to 0.66 M) and changes in weight, water content, growth and IAA level of these roots were investigated. With increasing stress a decrease in growth, correlated with an increased IAA level, was observed. The largest increase in IAA (about 2.7-fold) was found in the apical 5 mm of the root and was obtained under a stress corresponding to an osmotic potential of −1.39 MPa in the solution. This stress led to an isotonic state in the cells after 1 h. When the duration of water stress (−1.09 MPa) was increased to 2 or 3 h, no further increase in the IAA content was observed in the root segments. This indicated that there was no correlation between a hypothetical passive penetration of mannitol in the cells and IAA content. Indol-3yl-acetic acid rose to the same level in excised as in intact roots. In both cases, IAA accumulation was apparently independent of the hydrolysis of the conjugated form. The caryopsis and shoot seem not to be necessary to induce the increase of the IAA level in the roots during water stress (−1.09 MPa). Therefore, there seems to be a high rate of IAA biosynthesis in excised maize roots under water-stress conditions. Exodiffusion of IAA was observed during an immersion in either buffer or stress (−1.09 MPa) solution. In both cases, this IAA efflux into the medium represented about 50% of the endogenous level. Considering the present results, IAA appears to play an important part in the regulation of maize root metabolism and growth under water deficiency.

Distribution of IAA and ABA in gravistimulated primary roots ofZea mays. L.

The transport of14C-IAA and14C-ABA applied exogenously to root cap toward the elongation zone was investigated in gravi- and light-stimulated primary roots ofZea mays L. cv. Golden Cross Bantam 70. No significant difference of either IAA or ABA in radioactivities was observed between upper and lower halves of elongation zones during the latent period (0–60 min after the stimulation) of gravitropic response. When quantitative analysis of endogenous IAA and ABA by an internal standard method was carried out 60 min after gravi- and/or light-stimulation, no asymmetric redistribution of either IAA or ABA was observed between upper and lower halves of elongation zones. Light irradiation increased by 20% the contents of ABA in elongation zones. These results suggest that although both IAA and ABA are basipetally transportable and can transmit their information to the elongation zone during a latent period we cannot explain the gravitropic curvature by their redistributions between the two (upper and lower) halves of primary roots ofZea. On the basis of results from the present work and previous papers, the distribution of IAA and ABA in gravistimulatedZea roots is discussed.

The action of divalen Zn, Cd, Hg, Cu and Pb ions on the ATPase activity of a plasma membrane fraction isolated from roots ofZea mays

The effects of the divalent metal ions Zn, Cd, Hg, Cu and Pb on the ATPase activity of a plasma membrane fraction isolated from roots ofZea mays have been investigated. When Mg-ions (3 mM), with or without K-ions (50mM) are included in the reaction medium, inhibition of ATPase activity was found in all cases, the relative order of the inhibitors over the concentration range 10 to 100μM, being Hg>>Cu∼Cd>Zn∼Pb. Below 1.0μM only Hg caused substantial inhibition. In the absence of Mg ions, Zn and to a lesser extent Cd, activated the enzyme up to a concentration of 1 mM, activity being further stimulated in the presence of K-ions (50mM). No activation of ATPase activity was found for Hg, Cu or Pb. It was concluded that Zn-ATP and Cd-ATP are both alternative substrates for the enzyme. Further experiments showed that both Km and Vmax for the substrates Zn-ATP and Cd-ATP are very much lower than for the usual substrate Mg-ATP. These present results are discussed in relation to the known actions of these divalent cations on the trans-root potential and H-ion efflux in excised maize roots (Kennedy and Gonsalves, 1987).

Radial transport of water across cortical sleeves of excised roots ofZea mays L.

The stationary radial volume flows across maize (Zea mays L.) root segments without steles ("sleeves") were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences (Δ π) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied (δ π=±0.5 MPa), corresponding to volume-flow densities of jv, s=±7·10(-8) m·s(-1). Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220-1234), values for the sleeve hydraulic conductivity of about 1·10(-7) m·s(-1) MPa(-1) were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.

Rectification of radial water flow in the hypodermis of nodal roots ofZea mays

Rectification of radial water flow in the hypodermis of nodal roots ofZea mays

The development of structural changes in epidermal cells of Maize Roots During Water Stress

Ultrastructural changes following increasing periods of water stress induced by means of polyethylene glycol 4000 (from 5 min to 18 h) were investigated in young epidermal cells of the primary roots ofZea mays. The sensitivity of the individual cell components to water stress was considered according to the time sequence in which their alterations appeared.Endoplasmic reticulum (ER) and mitochondria proved to be the most sensitive cell components, their structure changing after 5 min of water stress. By 8 h of stress, the condensation of nuclear chromatin in some cells was apparent, preceding polyribosome degradation but not all the other changes of the stressed cells. Similar types of structural alterations appeared slightly earlier in the more differentiated epidermal cells. The rapid changes in the structure of ER and mitochondria coincided with the rapid decrease of water potential of the root tips and the immediate cessation of the root growth.

Total and CO-reactive heme content of actinorhizal nodules and the roots of some non-nodulated plants

The concentration of total and CO-reactive heme was measured in actinorhizal nodules from six different genera. This gave the upper limit to hemoglobin concentration in these nodules. Quantitative extraction of CO-reactive heme was achieved under anaerobic conditions in a buffer equilibrated with CO and containing Triton X-100. The concentration of CO-reactive heme in nodules of Casuarina and Myrica was approximately half of that found in legume nodules, whereas in Comptonia, Alnus and Ceanothus the concentrations of heme were about 10 times lower than in legume nodules. There was no detectable CO-reactive heme in Datisca nodules, but low concentrations were detected in roots of all non-nodulating plants examined, includingZea mays. Difference spectra of CO treated minus dithionite-reduced extracts displayed similar wavelengths of maximal and minimal light absorption for all extracts, and were consistent with those of a hemoglobin. The concentration of CO-reactive heme was not correlated to the degree to which CO inhibited nitrogenase activity nor was it affected by reducing the oxygen concentration in the rooting zone. However, there was a positive correlation between heme concentration and suberization or lignification of the walls of infected host cells. These observations demonstrate that, unlike legume nodules, high concentrations of heme or hemoglobin are not needed for active nitrogen fixation in most actinorhizal nodules. Nonetheless, a significant amount of CO-reactive heme is found in the nodules of Alnus, Comptonia, and Ceanothus, and in the roots ofZea mays. The identity and function of this heme is unknown.

Analysis of growth patterns during gravitropic curvature in roots of Zea mays by use of a computer-based video digitizer.

A computer-based video digitizer system is described which allows automated tracking of markers placed on a plant surface. The system uses customized software to calculate relative growth rates at selected positions along the plant surface and to determine rates of gravitropic curvature based on the changing pattern of distribution of the surface markers. The system was used to study the time course of gravitropic curvature and changes in relative growth rate along the upper and lower surface of horizontally-oriented roots of maize (Zea mays L.). The growing region of the root was found to extend from about 1 mm behind the tip to approximately 6 mm behind the tip. In vertically-oriented roots the relative growth rate was maximal at about 2.5 mm behind the tip and declined smoothly on either side of the maximum. Curvature was initiated approximately 30 min after horizontal orientation with maximal (50 degrees) curvature being attained in 3 h. Analysis of surface extension patterns during the response indicated that curvature results from a reduction in growth rate along both the upper and lower surfaces with stronger reduction along the lower surface.

Ion Transport to the Xylem in Aerenchymatous Roots ofZea maysL.

Ion Transport to the Xylem in Aerenchymatous Roots of<i>Zea mays</i>L.

Enumeration and Localization of N 2 -Fixing Bacteria Associated with Roots of Spartina alterniflora Loisel

Numbers and possible locations of N(2)-fixing bacteria were investigated in roots of Spartina alterniflora Loisel, which support nitrogenase activity in the undisturbed native habitat. N(2)-fixing bacteria were recovered in cultures both from S. alterniflora roots and from the surrounding sediment, and they formed a greater proportion of the bacteria recovered from root homogenates than from salt-marsh sediment. N(2)-fixing bacteria were recovered in high numbers from the rhizoplane of S. alterniflora after roots were treated with 1 or 5% chloramine-T for 1 h or with 1% NaOCl for 1 or 2 h. Immersing S. alterniflora roots in 5% NaOCl for 1 h was more effective in distinguishing bacteria inside the roots since this treatment nearly eliminated N(2)-fixing bacteria recoverable from the rhizoplane, although high numbers of N(2)-fixing bacteria were recovered from homogenates of roots treated with 5% NaOCl for 1 h. However, this treatment was less effective with roots of Zea mays L. (Funks G4646) and Sorghum bicolor (L.) Moench (CK-60 A), indicating that techniques to surface sterilize roots should be evaluated for different plants. Bacteria were observed by light and electron microscopy inter- and intracellularly in the cortex and in the aerenchyma of S. alterniflora roots. This study clearly shows that bacteria, including N(2) fixers, colonize the interior of roots of S. alterniflora growing in a Chesapeake Bay, Maryland, salt marsh.

Tissue specificity of assimilation and transport of nitrogen in the root ofZea mays L. seedlings. III. Interaction of radial and longitudinal transport

Roots obtained from 4-day maize seedlings were allowed to absorb uniformly labelled14C-suorose only with their middle part while the apical and basal parts did not contact the medium. The amino acids and their amides produced in the cortex at the expense of nitrates and the14C-sucrose carbon moved radially to the stele where they were selectively distributed in the polar directions of transport. “Primary” amino acids moved predominantly in the apical direction while amides and basic amino acids were transported mainly in the opposite direction. When the nutrient medium was devoid of nitrogen sources, the amino acids entering the stele from the cortex were transported mostly to the root tip, indicating a significant role of young tissues in consuming organic nitrogen compounds when they are in short supply in the root.

Tissue specificity of assimilation and transport of nitrogen in the root ofZea mays L. seedlings. II. Radial transport of14C-amino acids

The radial transport of organic nitrogen compounds was studied in maize seedling roots in relation to the metabolism of uniformly labelled14C-amino acids (alanine, arginine, dicarboxylic amino acids and their amides) in the cortex zone. Most active metabolism accompanying transport to the stele was observed for14C-glutamic acid of “primary” amino acids and for14C-glutamine of “reserve” nitrogen sources. The transport of14C-asparagine and14C-arginine to the conducting bundles is accompanied by weak metabolism. A distinguishing feature of nitrogen metabolism in the stele is intensive decarboxylation of glutamic acid, formed in the course of radial transport and metabolism, to gamma-amino butyric acid. This process is assisted by a highly active glutamate decarboxylase present in the conducting bundles.

Tissue specificity of assimilation and transport of nitrogen in the root ofZea mays L. seedlings. I. Transformations of14C- sucrose and the nitrogen metabolism enzymes in cortex and stele

An investigation has been carried out to find out in what compounds carbon is radially transported from stele to the cortex and to study sucrose metabolism in the root. These tissues were fed with14C-sucrose either directly or through the mesocotyle. In the latter case, the bulk of radioactivity, both in the stele and cortex, was concentrated in sucrose rather than in mono saccharides. This corresponds to the radial transport of carbon from stele to cortex predominantly in the form of sucrose. In the case of a direct exposure of stele and cortex,14C-sucrose was utilized for respiration, amino acid biosynthesis, and accumulation of carbon in the form of glucose. The hydrolysis of sucrose to the monosaccharides and subsequent utilization of the latter in respiration was more intensive in the stele. In the cortex, the sucrose carbon was more actively used for amino acid synthesis, owing to a high concentration of nitrate reductase, glutamate dehydrogenase and glutamine synthetase. It was concluded that the cortex is the main issue zone providing nitrogen assimilation in the root.

Fucose in the surface deposits of axenic and field grown roots ofZea mays L.

The location of materials containing terminal fucose residues on the surface of axenic and field grown roots of corn has been determined.

The effect of temperature on the uptake and loss of anions by seedling roots ofZea mays L

Intact maize seedlings were examined for the uptake and leakage of labelled sulphate and phosphate anions affected by temperature. Control plants, grown at 25 °C were compared from the aspects of uptake capacity and leakage with plants incubated in nutrient solutions cooled to 15 °C and 5 °C, respectively. Short time intervals as well as 1–7 d exposure to cooling were used. Already after 1 h exposure at 5 °C and 5 h cooling at 15 °C and at 5 °C, considerable changes were manifested in anion uptake and leakage.The uptake of32P declined more than that of35S. So, after a 30 min uptake interval the uptake of35S decreased at 15 °C to 49.84% and at 5 °C to 6.05% comparing with the uptake at 25 °C, while the uptake of32P declined to 28.64% at 15 °C and to 4.45% at 5 °C. The leakage of both anions was the highest at 25 °C in absolute rates, but relatively most of the uptaken35S and32P was released at 5 °C. Longer exposure to a chilling temperature of 5 °C (1–7 days) resulted in two patterns of sulphate and phosphate uptake.

Pressure-flow characteristics of the roots ofZea mays

Root pressure is generally considered to be osmotic in origin. This explanation is not held by all workers, however. An alternative view contends that while an osmotic component certainly exists, the xylem concentrations are inadequate to account for the total pressure and that therefore the remainder must result from some form of active water pumping (see for example [2,4]). In the ideal method of testing this claim one would allow the root pressure to attain its maximum value, collect a sample of the xylem sap and compare its osmotic pressure to that developed by the root. Unfortunately, this is impossible with present techniques, since to develop maximum pressures requires zero flow, in which case no sap can be collected. One can, however, reduce the flow very nearly to zero and still obtain a small sample for comparison. Such an experiment was performed by collecting 50 µl samples of exudate from each of 22 plants under 2.7 bar pressure and pooling them for analysis. Since the exudation flowrate in this case was reduced to an average of only 7% of the unimpeded rate, the results should represent a close approximation to the ideal case. The osmotic pressure of the exudate collected in this way was found to be 3.2 bar above that of the bathing solution, well in excess of the applied pressure. Thus in the present instance, there is no need to assume that root pressure is generated by any means other than osmotic.

Substrate specificity of peroxidase isoenzymes for hydrogen donors

The investigation of the substrate specificity of the anionic peroxidase isoenzymes, isolated from the zone of differentiation of the primary roots ofZea mays, for some representatives of phenolic compounds and aromatic amines, as hydrogen donors, is reported. The investigation was carried out electrophoretically with peroxidase isoenzymes partially purified by a combination of gel filtration by Sephadex G-25 and Sephadex G-100. A difference in the substrate specificity of the individual isoenzymes is observed. It was established that the anionic peroxidase isoenzymes showed a similarity in total number and relative activity on staining with bivalent phenols and difference on staining with trivalent phenols, as hydrogen donors. A greater number of isoenzymes was stained with benzidine ando-dianisidine and a lesser number witho- andp-phenylendiamine. The substrate specificity of the peroxidase isoenzymes was compared for guaiacol and benzidine. The substrate specificity of peroxidase soenzymes was discussed as regards their diverse role in the plant metabolism.