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     

Shoot and root growth of hydroponic maize (Zea mays L.) as influenced by K deficiency

Potassium (K) has major biophysical and biochemical functions in plant physiology. However, plant responses to K deficiency at the whole plant level are not always clearly related to these well-known functions of K at the cellular level. The objective of this study was to investigate the morphological response of maize to increasing K deficiency and test to what extent this morphological response can be interpreted in the light of the simple model proposed by Leigh and Wyn Jones, suggesting that biophysical functions are affected first. Maize was grown in a greenhouse under hydroponic conditions. For half of the plants, K was removed from the nutrient solution from the 4th visible leaf stage. The K content in the starved plants dropped from 100 to 30 mM, and was not fully compensated by an increase in other cations. Leaf elongation rates were reduced on K-deprived plants, whereas axile root elongation rates were slightly increased between 45°C days and 75°C days after starvation, and reduced thereafter. During the first part of the starvation period, i.e. under moderate K deficiency (K concentration above 40 mM), all measured variables suggest that the whole plant response may be interpreted as the consequence of the reduced leaf growth, probably due to insufficient turgor pressure or cell-wall extensibility. This general pattern of response is in agreement with the model of Leigh and Wyn Jones. However, during the second part of the starvation period, i.e. under more severe K deficiency (K concentration below 40 mM), malfunction of additional physiological processes (mostly related to biochemical functions like photosynthetic processes) must be considered to explain the plant morphological response.     

Light and decapitation effects on in vitro rooting in maize root segments

The effects of white light and decapitation on the initiation and subsequent emergence and elongation of lateral roots of apical maize (Zea mays L. cv LG 11) root segments have been examined. The formation of lateral root primordium was inhibited by the white light. This inhibition did not depend upon the presence of the primary root tip. However, root decapitation induced a shift of the site of appearance of the most apical primordium towards the root apex, and a strong disturbance of the distribution pattern of primordium volumes along the root axis. White light had a significant effect neither on the distribution pattern of primordium volumes, nor on the period of primordium development (time interval required for the smallest detectable primordia to grow out as secondary roots). Thus, considering the rooting initiation and emergence, the light effect was restricted to the initiation phase only. Moreover, white light reduced lateral root elongation as well as primary root growth.     

Zeta potential of protoplasts from gravireacting maize roots

Protoplasts were isolated from cortical cells of the elongating zone of maize (Zea mays L. cv. LG 11) roots and submitted to microelectrophoresis. Significant and transient differences in zeta potential between protoplasts from upper and lower root sides were compared with the gravireaction and the differential elongation of these roots. The maximum difference in the zeta potential was obtained between protoplasts from the upper and lower cortical cells after 90 min, exactly the time of gravipresentation for which the maximum rate of gravireaction was observed. In addition, this almost corresponded to the time for which the difference between the elongation rates of upper and lower sides of the extending zone began to increase. Consequently, the changes in the charges of the plasmalemma of the cortical cells from the growing part of roots could be more or less directly related to the root graviresponse.     

Effect of applied and endogenous indol-3-yl-acetic acid on maize root growth

The level of endogenous Indol-3-yl-acetic acid (IAA) measured by gas chromatography-mass spectrometry in the elongating zone of intact primary roots of Zea mays showed a good linear correlation with the growth rate of these roots. When they were treated with IAA, their relative elongation decreased; this indicates a supraoptimal content of endogenous IAA. However, the growth of some of the relatively rapidly extending roots was enhanced by such treatment. Interactions between endogenous and applied IAA in the control of root growth are discussed.Abbreviations GC-MS gas chromatography-mass spectrometry - IAA Indol-3-yl-acetic acid     

Comparison of maize root mucilages isolated from root exudates and root surface extracts by complementary cytological and biochemical investigations

Summary A strategy to obtain fractions enriched in mucilages secreted by root caps or produced by the rhizodermis of axenicallygrown maize seedlings is proposed. It involves a two-step procedure allowing the successive collection of root exudates and surface extracts from the same set of intact, sterile maize plants. Cytological controls were performed at each phase of collection. Whereas root cap mucilage is easily collected in water after one day's extraction, under conditions favouring secretory activity, rhizodermal mucilage remains tightly adherent to the root surface. It can be better extracted using neutral saline buffer assisted by gentle shaking at low temperature. Acidic saline buffer is unsuitable as it induces cell lysis and release of cell wall components.Biochemical analyses confirm that fractions enriched in root cap mucilage contain very high levels of fucose and galactose, high levels of arabinose, xylose and glucose and trace amounts of mannose. Fractions enriched in rhizodermal mucilage contain large amounts of glucose, moderate amounts of arabinose, xylose, mannose and galactose and trace levels of fucose. Isoelectric focusing and SDS-PAGE indicate that there are numerous similarities in the protein composition of materials enriched in root cap mucilages from root exudates or aqueous root surface extracts. However, specific protein bands that could be characteristic of rhizodermal mucilage are obtained using neutral saline buffer extracts. According to these biochemical data, the two-step procedure used in the present study appears to be useful for further biochemical characterization of both types of mucilages.Abbreviations BSA bovine serum albumin - BSTFA N,O-bis (trimethylsilyl)-trifluoroacetamide - DTT dithiothreitol - i. d. internal diameter - MW molecular weight - PATAg periodic acid-thiosemicarbazide-silver proteinate - PVPP polyvinylpolypyrrolidone - RE root exudates - RSE root surface extracts - TMCS trimethylchlorosilane - TMS trimethylsilyl     

Water relations of growing pea epicotyl segments

The water relations of growing epicotyl segments of pea (Pisum sativum L.) were studied using the miniaturized pressure probe. For epidermal cells stationary turgor pressures of P=5 to 9 bar and half-times of water exchange of individual cells T _1/2=1 to 27 s were found. The volumetric clastic modulus () of epidermal cells varied from 12 to 200 bar and the hydraulic conductivity, Lp=0.2 to 2·10^-6 cm s^-1 bar^-1. For cortical cells P=5 to 11 bar, T _1/2=0.3 to 1 s, Lp=0.4 to 9·10^-5 cm s^-1 bar^-1 and =6 to 215 bar. The T _1/2 of cortical cells was extremely low and the Lp rather high as compared to other higher plant cells. The T _1/2-values of cortical cells were sometimes observed to change from short to substantially longer values (T _1/2=3 to 20 s). Both short and long pressure relaxations showed all the characteristics of non-artifactual curves. The change is apparently due to an increase in Lp and not , but the reason for the change in cell permeability to water is not known.In osmotic exchange experiments on peeled segments using solutions of different solutes, the half-time of osmotic water exchange for the whole segment was approximately 60 s. Water exchange occurred too quickly to be rate controlled by solute diffusion in the wall space. The data suggest that the short T _1/2-values in the cortical cells are the physiologically relevant ones for the intact tissue and that a considerable component of water transport occurs in the cell-to-cell pathway, although unstirred layer effects at the boundary between the segment and solution may influence the measured half-time. Using the theory of Molz and Boyer (1978, Plant Physiol. 62, 423–429), the gradient in water potential necessary to maintain the uptake of water for cell enlargement can be calculated from the measured diffusivities to be approximately 0.2 and 1 bar for growth rates of 1% h^-1 and 5% h^-1, respectively. Thus, although the T _1/2-values are short and Lp rather high, there may be a significant osmotic disequilibrium in the most rapidly growing tissue and as a consequence the influence of water transport on the growth rate cannot be excluded.Abbreviations P turgor pressure - T _1/2 half-time of water exchange of individual cell - Lp hydraulic conductivity - volumetric elastic modulus - t _1/2 average half-time of water exchange of tissue     

Rapid,localized light effect on root growth in maize

A method using optical microfibers permitted localized exposure of the cap or the elongating part of growing maize (Zea mays L.) roots to white light. When the cap was illuminated, a strong and very rapid inhibition of the elongation rate of the roots was found. When the light microbeam was directed at the elongating region, the roots continued to grow at the same rate as before the illumination.     

Characterization of phosphate absorption in maize root cortex segments

Uptake of phosphate ions by 1 mm segments of isolated maize root cortex layers was studied. Cortex segments (from roots of 8 days old maize plants) absorb phosphate ions from 1 mM KH₂PO₄ in 0.2 mM CaSCO₄ at the average rate of 34.3 ±3.2 μg P_i g^?1 (fr. m.) h^?1,i.e. 0.35± 0.02 μmol P_i g^?1 (fr. m.) h^?1. Phosphate uptake considerably increases after a certain period of “augmentation”,i.e. washing in aerated 0.2 mM CaSO₄. This increase is completely blocked by the presence of 10 μg ml^?1 cycloheximide. The relation of uptake rate to phosphate concentration in the medium was shown to have 3 phases in the concentration range of 0.02 - 40 mM. Transition points were found between 0.8–1 mM and 10–20 mM. Following K_m and V_max values were found: K_m[mM] : 0.37 - 3.82 - 27.67 V_max[μg P_i g^?1 (fr. m.) h^?1] : 3.33 - 39.40 - 66.67 We have found no sharp pH optimum for phosphate uptake. It proceeds at almost constant rate till pH 6.0 and then the uptake rate drops with increasing pH. At low phosphate concentrations (1 mM) the lowest uptake rate was found at 5 and 13 °C, while the uptake is higher at 5 °C than at 13 °C at phosphate concentrations higher than 1 mM. At these concentrations uptake rate at 35 °C is lower than at 25 °C. Phosphate uptake considerably decreased in anaerobic conditions. DNP and iodoacetate (0.1 mM) completely blocked phosphate uptake from 1 mM KH₂PO₄, while uptake from 5 and 10 mM KH₂PO₄ was left unaffected by these substances. The inhibitors of active - SH groups NEM and PCMB inhibited phosphate uptake: 10^?3 M NEM by 81.6%, 10⁴ M NEM by 42% and 10^?4 M PCMB by 42%.     

Differential growth and hormone redistribution in gravireacting maize roots

When growing roots are placed in a horizontal position gravity induces a positive curvature. It is classically considered to be the consequence of a faster elongation rate by the upper side compared to the lower side. A critical examination indicates that the gravireaction is caused by differential cell extension depending on several processes. Some of the endogenous regulators which may control the growth and gravitropism of elongating roots are briefly presented. The growth inhibitors produced or released from the root cap move preferentially in a basipetal direction and accumulate in the lower side of the elongation zone of horizontally maintained roots. The identity of these compounds is far from clear, but one of these inhibitors could be abscisic acid (ABA). However, indol-3y1 acetic acid (IAA) is also important for root growth and gravitropism. ABA may interact with IAA. Two other aspects of root cell extension have also to be carefully considered. An elongation gradient measured from the tip to the base of the root was found to be important for the growth of both vertical and horizontal gravireactive roots. It was changed significantly during the gravipresentation and can be considered as the origin of the differential elongation. Sephadex beads have been used as both growth markers and as monitors of surface pH changes when they contain some pH indicator. This technique has shown that the distribution of cell extension along the main root axis is related to a pH gradient, the proton efflux being larger for faster growing parts of roots. A lateral movement of calcium is obtained when Ca2+ is applied across the tips of horizontally placed roots with a preferential transport towards the lower side. Endogenous calcium, which may accumulate inside the endoplasmic reticulum of some cap cells, may also act in the gravireception. These observations and several others strongly suggest that calcium may play an essential role in controlling root growth and several steps of the root gravireaction.     

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.     

Effect of cadmium on growth, proton extrusion and membrane potential in maize coleoptile segments

Cd accumulation, its effects on elongation growth of maize coleoptile segments, pH changes of their incubation medium and the membrane potential of parenchymal cells were studied. The Cd content increased significantly with exposure to increasing cadmium concentrations. Coleoptile segments accumulated the metal more efficiently in the range 10–100 μM Cd, than in the range 100–1000 μM Cd. Cd at concentrations higher than 1.0 μM produced a significant inhibition of both growth and proton extrusion. 100 μM Cd caused depolarization of the plasma membrane (PM) potential in parenchymal cells. The simultaneous treatment of maize coleoptile segments by indole-3-acetic acid (IAA) and Cd, counteracted the toxic effect of Cd on growth. Moreover, our data also showed that 100 μM Cd suppressed the characteristic IAA-induced hyperpolarization of the membrane potential, causing membrane depolarization. These results indicate that the toxic effect of Cd on growth of maize coleoptile segments might be, at least in part, caused via reduced PM H⁺-ATPase activity.     

Mechanical properties and polysaccharide nature of the cell walls of maize root

Changes in mechanical properties and chemical nature of the cell walls of the different zones along elongating maize ( Zea mays L. cv. LG 11) roots were analyzed and the following results were obtained. (1) The apical region 2 to 5 mm from the tip of 15 mm long roots showed rapid elongation whereas the region 8–10 mm from the tip showed very little growth. (2) The minimum stress-relaxation time (To) and the mean stress-relaxation rate (R) of the cell wall were small whereas the maximum stress-relaxation time (Tm) was large in the region where cell elongation was optimum. The To and R increased and the Tm decreased gradually towards the base of the root. (3) The amounts of non-cellulosic polysaccharides of the cell wall were highest in the region 1.5–2.5 mm from the tip, decreasing until 5 mm from the tip, and then increasing towards the base. However, the proportion of this fraction in the total cell wall polysaccharides was highest in the extreme tip (cap and meristem, 0–1 mm) and decreased towards the base. (4) Major neutral sugars constituting the non-cellulosic polysaccharides of the cell wall were xylose, arabinose, galactose and glucose, with minor amounts of rhamnosc. mannose and fucose. The 1–15 mm region was on the whole rich in glucose and xylose and contained arabinose to a lesser extent. However, the chemical nature in the apical region, (0–2 mm, was rather special, being rich in galactose and fucose. (5) The cell wall of maize roots contained, as a whole, only little pectic substances but was high in hemicellulose 1 (rich in xylose, arabinose and glucose) and hemicellulose 2 (rich in glucose and xylose). (6) It appeared that in the elongating region (apical 2 to 5 mm) the cell elongation rate (CET) showed a rather good correlation with the parameters of mechanical properties (To, Tm and R) and with neutral sugar compositions in the non-cellulosic polysaccharides.     

Cryptochrome photoreceptors cry1 and cry2 antagonistically regulate primary root elongation in Arabidopsis thaliana

Cryptochromes are blue-light receptors controlling multiple aspects of plant growth and development. They are flavoproteins with significant homology to photolyases, but instead of repairing DNA they function by transducing blue light energy into a signal that can be recognized by the cellular signaling machinery. Here we report the effect of cry1 and cry2 blue light receptors on primary root growth in Arabidopsis thaliana seedlings, through analysis of both cryptochrome-mutant and cryptochrome-overexpressing lines. Cry1 mutant seedlings show reduced root elongation in blue light while overexpressing seedlings show significantly increased elongation as compared to wild type controls. By contrast, the cry2 mutation has the opposite effect on root elongation growth as does cry1, demonstrating that cry1 and cry2 act antagonistically in this response pathway. The site of cryptochrome signal perception is within the shoot, and the inhibitor of auxin transport, 1-N-naphthylphthalamic acid, abolishes the differential effect of cryptochromes on root growth, suggesting the blue-light signal is transmitted from the shoot to the root by a mechanism that involves auxin. Primary root elongation in blue light may thereby involve interaction between cryptochrome and auxin signaling pathways.     

Effects of a brassinosteroid on growth and electrogenic proton extrusion in maize root segments

In apical or subapical root segments of maize ( Zea mays L. cv. Dekalb XL640), 10⁻⁷–10⁻⁵ M 24-epibrassinolide (BR), a physiologically active synthetic epimer of the pollen hormone brassinolide, induces a significant stimulation of root growth, associated with an increase of acid secretion. The increase in acid secretion is enhanced by the presence of K⁺ in the medium and is accompanied by an early, significant hyperpolarization of the transmembrane electric potential (PD), which is completely suppressed by the addition of the protonophore uncoupler FCCP. Similar effects of BR have earlier been reported for shoots, and also for IAA in shoots. Contrariwise, 10⁻⁸–10⁻⁷ M IAA inhibits acid secretion and depolarizes the PD in the maize root segments. This suggests different pathways for the action of the two different hormones on the proton pump.     

Ethylene inhibitors enhance in vitro root formation from apple shoot cultures

Effects of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and three ethylene inhibitors, AgNO₃, aminoethoxyvinyglycine (AVG) and CoCl₂, on root formation were tested in vitro using shoot cultures of the apple (Malus×domestica Borkh.) cultivar Royal Gala. ACC inhibited root formation by delaying root emergence and increasing callus formation at the bases of shoots. In contrast, ethylene inhibitors promoted root formation. Both AgNO₃ and AVG at the appropriate concentrations increased the percentage of shoots producing roots and reduced callus formation at the base of these shoots. AgNO₃ stimulated root emergence and enhanced root growth, while AVG increased the number of roots per shoot. CoCl₂ slightly increased root number and rooting efficiency. These promotive effects may result from a reduction in ethylene concentration or inhibition of ethylene action. The results found in this study may be used to improve the rooting efficiency of other apple cultivars and rootstocks, and possibly of other plant species. Received: 2 March 1997 / Revision received: 1 July 1997 / Accepted: 18 July 1997     

Tillage and N fertilization effects on maize root growth and root:shoot ratio

Two methods for estimating the size of the maize (Zea mays l.) root system from soil cores taken in the field were compared. The spatially weighed block method of estimation accounted for variation in root density by using 18 samples per plant which varied in distance from plant and soil depth. This method was compared to an estimation which averaged all of the 18 samples together. Both methods gave surprisingly similar estimates for total root growth. Increased root growth in the surface soil layers, due to tillage and N fertilization, did not impact on the estimation of total root growth. Total root length remained unchanged or increased with N fertilization, while root weight remained the same or decreased. Root mass per length decreased with N fertilization. The estimated size of the root system was used to calculate root:shoot weight ratios. The largest root:shoot ratio was found in the vegetative stage and decreased throughout the rest of the season. In this field experiment, the estimated size of the root system at 8 weeks after planting was not significantly different from the size at silking or harvest. Nitrogen fertilization significantly decreased the root:shoot weight ratio. However, tillage did not significantly change the ratio.