The influence of mechanical resistance and phosphate supply on morphology and function of corn roots

Conclusions The influence of mechanical soil resistance on shoot growth can be explained qualitatively by its influence on root morphology. When relating ion uptake quantitatively to a certain root parameter (length, surface area, weight), availability of nutrients in the different soil layers must be taken into consideration.     

Auxin-induced changes in cell wall extensibility of maize roots

The rheological properties of corn (Zea mays L. cv. Garant) root elongation zones were investigated by means of a computer-controlled extensiometer. Creep closely followed a logarithmic time function, which was used to quantify creep activity. Pretreatment with auxin, which inhibits extension growth in roots, lowered the creep activity and the apparent plastic extensibility. While the time course of the inhibition of apparent plastic extensibility lagged behind the cessation of elongation growth, the drop in creep activity matched the growth inhibition more closely. Creep activity and apparent plastic extensibility were not significantly affected by pH. These data support the view that the auxin-induced cell wall stiffening (e.g. by cross-linking processes), while causal for the growth inhibition, is not brought about by a cell wall alkalinization. Received: 10 December 1996 / Accepted: 19 August 1997     

Cell wall-associated malate dehydrogenase activity from maize roots

Isolated cell walls from maize (Zea mays L.) roots exhibited ionically and covalently bound NAD-specific malate dehydrogenase activity. The enzyme catalyses a rapid reduction of oxaloacetate and much slower oxidation of malate. The kinetic and regulatory properties of the cell wall enzyme solubilized with 1 M NaCl were different from those published for soluble, mitochondrial or plasma membrane malate dehydrogenase with respect to their ATP, Pi, and pH dependence. Isoelectric focusing of ionically-bound proteins and specific staining for malate dehydrogenase revealed characteristic isoforms present in cell wall isolate, different from those present in plasma membranes and crude homogenate. Much greater activity of cell wall-associated malate dehydrogenase was detected in the intensively growing lateral roots compared to primary root with decreased growth rates. Presence of Zn²⁺ and Cu²⁺ in the assay medium inhibited the activity of the wall-associated malate dehydrogenase. Exposure of maize plants to excess concentrations of Zn²⁺ and Cu²⁺ in the hydroponic solution inhibited lateral root growth, decreased malate dehydrogenase activity and changed isoform profiles. The results presented show that cell wall malate dehydrogenase is truly a wall-bound enzyme, and not an artefact of cytoplasmic contamination, involved in the developmental processes, and detoxification of heavy metals.     

Some observations of the effects of mechanical impedance upon the ultrastructure of the root caps of barley

Summary The root apex of barley,Hordeum vulgare cv. Proctor, is a structure which undergoes a number of gross morphological and ultrastructural changes from the normal patterns of development when grown under a small degree of applied mechanical constraint (2 × 10⁴ Pa.). The root cap is generally smaller and thus does not confer to the root meristem the same degree of protection as caps growing in an uncompacted medium. Associated with this loss of peripheral cells is a reduction in the volume of mucigel in contact with the root apex.In many impeded caps, the planes of division in the calyptrogen are often neither transverse nor longitudinal. There is a reduction in both the number of amyloplasts and starch grains per amyloplast in the columella, but any statolith function of these must not be impaired since the root remains geotropically responsive. The patterns of accumulation of polysaccharide in the walls of peripheral cells as a result of Golgi activity are modified by mechanical impedance.     

Differential growth of georeacting maize roots

The growth rate of the two sides of 10-mm apical segments prepared from primary roots and of intact primary roots of maize has been analyzed in both vertical and horizontal positions, using a filming method allowing continuous growth recording. The data showed that the georeaction began by a decrease in the overall elongation rate of the roots. This inhibition is effective on the lower side of the bending zone, where the growth is practically stopped during the period of maximum rate of geocurvature. In contrast, the growth is slightly enhanced on the upper part of the elongating zone.     

Influence of mechanical impedance on root exudation of maize seedlings at two development stages

Studies were undertaken to evaluate the effects of mechanical impedance on root exudation by maize (Zea mays L., var Dea) and to examine the importance of these effects in relation to the stage of plant development. Plants were grown under sterile and hydroponic conditions. Mechanical impedance was simulated using glass beads of 1 mm diameter. This treatment was compared with a control without beads. Results demonstrated that plant growth was influenced by mechanical impedance. Mechanical impedance markedly affected the growth of the shoot, whether this was measured as leaf area or total dry matter. Besides increasing root/shoot biomass ratios, mechanical impedances also stimulated root exudation of organic and inorganic compounds. Stressed plants lost more nitrogenous compounds than control plants. Otherwise, the percentage of released carbon decreased. Depending on the developmental stage of the plant, there was a large variation in the magnitude and time course on mechanical impedance effects. The effects of mechanical impedance persist and accentuate with time.     

Complete mechanical impedance increases the turgor of cells in the apex of pea roots

In this paper we describe an experimental approach which allows turgor (p) in an impeded root to be measured without the need to remove the root from the impeding environment. The maximum axial growth pressure (σ_max) generated by completely impeded pea (Pisum sativum L.) roots was measured using a novel apparatus incorporating a force transducer. The apparatus was designed so that it was possible to gain access to the impeded root with the microcapillary of a pressure probe and so obtain in situ measurements of P. Turgor in cells in the apical region of impeded roots was 0.78 MPa, compared with 0.55 MPa in unimpeded roots. In impeded roots, σ_max was 0.52 MPa, showing that the pressure component resulting from cell wall tension (W, where W=P–σ) decreased from 0.55 to 0.26 MPa as the roots became impeded. When impeded roots were removed from the apparatus, there was no decrease in P over the following 90 min. Impedance did not cause P to change in the non-elongating part of the roots further from the apex.     

Auxin action on proton influx in corn roots and its correlation with growth

At concentrations inhibitory to the elongation of corn (Zea mays L.) roots, the auxins, indole-3-acetic acid (IAA) and α-naphthaleneacetic acid (α-NAA), cause an increase in the pH of the bathing medium; this increase occurs with an average latent period shorter than the latent period for the inhibitory effect of these auxins on elongation. Indole-2-carboxylic acid, an inactive structural analogue of IAA, and β-naphthaleneacetic acid, an inactive analogue of α-NAA, affect neither growth nor the pH of the medium. Since acid pH is known to promote and basic pH to inhibit root elongation, the data are consistent with the hypothesis that hormone-induced modification of cell-wall pH plays a role in the control of elongation of roots, as has been proposed for elongation of stems and coleoptiles.     

Root growth regulation and gravitropism in maize roots does not require the epidermis

We have earlier published observations showing that endogenous alterations in growth rate during gravitropism in maize roots (Zea mays L.) are unaffected by the orientation of cuts which remove epidermal and cortical tissue in the growing zone (Björkman and Cleland, 1988, Planta 176, 513–518). We concluded that the epidermis and cortex are not essential for transporting a growth-regulating signal in gravitropism or straight growth, nor for regulating the rate of tissue expansion. This conclusion has been challenged by Yang et al. (1990, Planta 180, 530–536), who contend that a shallow girdle around the entire perimeter of the root blocks gravitropic curvature and that this inhibition is the result of a requirement for epidermal cells to transport the growth-regulating signal. In this paper we demonstrate that the entire epidermis can be removed without blocking gravitropic curvature and show that the position of narrow girdles does not affect the location of curvature. We therefore conclude that the epidermis is not required for transport of a growth-regulating substance from the root cap to the growing zone, nor does it regulate the growth rate of the elongating zone of roots.     

A model for water transport in the stele of plant roots

Summary The mechanism of water movement across roots is, as yet, not well understood. Some workable black box theories have already been proposed. They, however, assumed unrealistic cell membranes with low values of , or were based on a poor anatomical knowledge of roots. The role of root stele in solute and water transport seems to be especially uncertain. An attempted explanation of the nature of root exudation and root pressure by applying the apoplast canal theory (Katou andFurumoto 1986 a, b) to transport in the root stele is given. The canal equations are solved for boundary conditions based on anatomical and physiological knowledge of the root stele. It is found that the symplast cell membrane, cell wall and net solute transport into the wall apoplast are the essential constituents of the canal system. Numerical analysis shows that the canal system enables the coupled transport of solutes and water into a xylem vessel, and the development of root pressure beyond the level predicted by the osmotic potential difference between the ambient medium and the exudate. Observations on root exudation and root pressure previously reported seem to be explained quite well. It is concluded that the movement of water in the root stele although apparently active is essentially osmotic.Abbreviations J _v ^ex volume exudation per root surface - J₀ non-osmotic exudation - L^r overall radial hydraulic conductivity of an excised root - reflection coefficient - C_s difference in the osmotic concentration between the bathing medium and the exudate - R gas constant - T absolute temperature - C_K molar concentration of K⁺ - C_Cl molar concentration of Cl^– - C_j molar concentration of ion species j - P_j membrane permeability of ion j - z_j valence of ion j - F Faraday constant - V_ix intracellular electric potential with reference to the canal     

Cell-specific expression of two arabinogalactan protein epitopes recognized by monoclonal antobodies JIM8 and JIM13 in maize roots

Summary The cell-specific expression of two arabinogalactan protein (AGP) epitopes recognized by monoclonal antibodies JIM8 and JIM13 is reported in maize roots. Employing immunofluorescence and immunogold electron microscopy, the JIM8 antibody was shown to label exclusively protophloem sieve elements, while the JIM13 antibody labelled sieve elements very strongly and adjacent pericycle and companion cells, as well as sloughing root cap cells less strongly. Since the labelling of sieve elements with JIM8 antibody was specific and did not spread to other cell types during root development, it is concluded that this AGP epitope can serve as a specific marker of these specialized cells within the maize root. In the case of the AGP epitope recognized by JIM13 antibody, part of the immunofluorescence label was also found to be associated with cytoplasmic strands in the pericycle and sloughing root cap cells. Immunogold-labelling of sieve elements revealed the association of both AGP epitopes (JIM8 and JIM13) with cortical sieve element reticulum and plasma membranes. Labelling of sieve element reticulum was prominent at its domains of adhesion to the plasma membrane, P-type plastids, and mitochondria. Based on our subcellular studies, we propose a new function of AGP epitopes in endomembrane recognition and adhesion within the sieve elements of maize roots.Abbreviations AGP arabinogalactan protein - SER sieve element reticulum     

Inhibition of maize root growth by high nitrate supply is correlated with reduced IAA levels in roots

The plant root system is highly sensitive to nutrient availability and distribution in the soil. For instance, root elongation is inhibited when grown in high nitrate concentrations. To decipher the mechanism underlying the nitrate-induced inhibition of root elongation, the involvement of the plant hormone auxin in nitrate-dependent root elongation of maize was investigated. Root growth, nitrogen and nitrate concentrations, and indole-3-acetic acid (IAA) concentrations in roots and in phloem exudates of maize grown under varying nitrate concentrations were analyzed. Total N and nitrate concentrations in shoots and roots increased and elongation of primary, seminal and crown roots were inhibited with increasing external nitrate from 0.05 to 5 mM. High nitrate-inhibited root growth resulted primarily from the reduced cell elongation and not from changes in meristem length. IAA concentrations in phloem exudates reduced with higher nitrate supply. Inhibition of root growth by high nitrate was closely related to the reduction of IAA levels in roots, especially in the sections close to root tips. Exogenous NAA and IAA restored primary root growth in high nitrate concentrations. It is concluded that the inhibitory effect of high nitrate concentrations on root growth may be partly attributed to the decrease in auxin concentrations of roots.     

Cell wall-bound trans- and cis-ferulic acids in growing maize roots

The levels of cell wall-bound trans - and cis -ferulic acids in roots of dark grown Zea mays cv. LG11 plants were measured. They were quantified after alkaline hydrolysis of purified cell walls by reversed phase HPLC using trans -cinnamic acid as internal standard. The total amount of ferulic acid ( trans - and cis -ferulic acid) in the root base was 3–4 times higher than in the root tip. Cis -ferulic acid represented between 2% (tip) and 18% (base) of the total ferulic acid content. The total content of trans - and cis -ferulic acids was approximately the same in the stele and the cortex, but the level of cis -ferulic acid in the stele was 5–6 times higher than in the cortex. Trans - and cis -ferulic acid levels as well as the percentage of cis -ferulic acid in the elongation zone were steady between 48 and 96 h after the beginning of germination. Slowly growing roots contained more wall-bound ferulic acids, particularly cis -ferulic acid, than fast growing roots. This relationship was found in the differentiation zone but not in the elongation zone. The importance of cell wall-bound trans - and cis -ferulic acids is discussed in the context of root growth and differentiation.     

Importance of the cap in maize root growth

A large population of primary roots of Zea mays (cv. LG 11) was selected for uniform length at zero time. Their individual growth rates were measured over an 8-h period in the vertical position (in humid air, darkness). Three groups of these roots with significantly different growth rates were then chosen and their cap length was measured. It was found that slowly growing roots had long caps whereas rapidly growing roots had short caps. The production by the cap cells of basipetally transported growth inhibitors was tested (biologically by the curvature of half-decapped roots) and found to be significantly higher for longer root caps than that for shorter ones.     

Distribution and redistribution of extension growth along vertical and horizontal gravireacting maize roots

Horizontal primary roots of Zea mays L. were photographed during the course of their gravireaction and during a preceding growth period in the vertical orientation. The displacement, by root elongation, of marker particles on the root surface was recorded. The particle-displacement rates were used to estimate the distribution of elemental elongation rates along opposite sides of the growing root apex. In the temperature range 21–25°C there was a stimulation of local elongation rates along the upper side of a gravireacting root and a reduction (and sometimes a cessation) of elongation along the lower side. Elemental elongation rates have been related to the development of root curvature, and the magnitude of the differential growth between upper and lower sides required for a particular rate of bending has also been estimated. The results complement, and are compatible with, findings relating to the distribution of certain endogenous growth regulators believed to participate in the gravireaction.Abbreviation RELEL relative elemental rate of elongation     

Acid growth effects in maize roots: Evidence for a link between auxin-economy and proton extrusion in the control of root growth

The role of proton excretion in the growth of apical segments of maize roots has been examined. Growth is stimulated by acidic buffers and inhibited by neutral buffers. Organic buffers such as 2[N-morpholino] ethane sulphonic acid (MES) — 2-amino-2-(hydroxymethyl)propane-1,3 diol (Tris) are more effective than phosphate buffers in inhibiting growth. Fusicoccin(FC)-induced growth is also inhibited by neutral buffers. The antiauxins 4-chlorophenoxyisobutyric acid (PCIB) and 2-(naphthylmethylthio) propionic acid (NMSP) promote growth and H⁺-excretion over short time periods; this growth is also inhibited by neutral buffers. We conclude that growth of maize roots requires proton extrusion and that regulation of root growth by indol-3yl-acetic acid (IAA) may be mediated by control of this proton extrusion.Abbreviations IAA indol-3yl-acetic acid - ABA abscisic acid - FC fusicoccin - PCIB 4-chlorophenoxy-isobutyric acid - MES 2(N-morpholino)ethane sulphonic acid - Tris 2-amino-2-(hydroxymethyl) propane-1,3-diol - NMSP 2-(naphthylmethylthio)propionic acid     

Evaluation of agar and agarose gels for studying mechanical impedance in rice roots

Agar and agarose gels were evaluated as systems to mechanically impede roots of rice (Oryza sativa L.). Two-layer gels were used so that seedlings established in a layer of weak gel (0.35% weight/volume) and then grew downwards to encounter a treatment gel of up to 5.0% (w/v). Agarose gels were stronger than agar gels of the same concentration, reaching a maximum penetrometer resistance of 1.2 MPa at a concentration of 5.0%, compared to 0.3 MPa with agar. The 5.0% agar gel stimulated elongation of the seminal axis by 40% in seedlings of variety TN1 (compared with elongation in the 0.2% gel), but decreased it by 15% in the variety Lac 23. Although increasing agarose concentration decreased seminal axis elongation in both varieties, the seminal axis did not reach the lower layer of treatment gel when the concentration of the treatment gel was greater than 2.0%. The decreased root elongation was therefore a non-mechanical inhibition. In experiments conducted using a different batch of agarose, these inhibitory effects were not seen and strong agarose gels stimulated seminal axis elongation. It was concluded that the agar and agarose gel systems studied were unsuitable for studying the effect of mechanical impedance on the elongation of rice roots and that great care should be taken in interpreting the results of experiments using gels as a growth medium. This revised version was published online in June 2006 with corrections to the Cover Date.     

The case for multinet growth in growing walls of plant cells

The basis of multinet gwoth, the multinet growth hypothesis, is examined in view of recent criticisms. It is shown that the strain across a growing wall may be calculated by simple means and the expected reorientations are deduced (a) for a wall in which the microfibrils of the innermost wall lamella always lie helically with the same pitch and (b) in which the microfibrils lie at random. Calculations are presented both for cells increasing in length only and for cells also increasing in breadth. Both the strains and the reorientations are smaller than commonly implied and are too small to be reliably detectable in wall sections. Observations on wall sections cannot therefore be accepted as proof that microfibril reorientation has not occured and it is concluded that the multinet growth hypothesis still stands as applying both to parenchyma and to collenchyma cells. In view of the wide dispersity in the structure of the walls of growing cells, it is recommended that the qualifying multinet should be dropped and replaced by passive reorientation.Abbreviation MGH multinet growth hypothesis     

A xyloglucan endotransglucosylase/hydrolase involves in growth of primary root and alters the deposition of cellulose in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Xyloglucan endotransglucosylase/hydrolases (XTHs) are a class of enzymes that mediate the construction and restructure of the cellulose/xyloglucan framework by splitting and reconnecting xyloglucan molecule cross-linking among cellulose microfibrils. Remodification of cellulose microfibrils within cell-wall matrices is realized to be one of the most critical steps in the regulation of cells expansion in plants. Thirty-three XTH genes have been found in Arabidopsis thaliana but their roles remain unclear. AtXTH21 (At2g18800), an Arabidopsis XTH gene that mainly expresses in root and flower, exhibits different expression profiles from other XTH members under hormone treatment. We examined loss-of-function mutants using T-DNA insertion lines and overexpression lines and found that the AtXTH21 gene played a principal role in the growth of the primary roots by altering the deposition of cellulose and the elongation of cell wall.