Cellular Dimorphism in the Maize Root Cortex: Involvement of Microtubules,Ethylene and Gibberellin in the Differentiation of Cellular Behaviour in Postmitotic Growth Zones

Detailed morphometric analysis of cell shapes and an immunofluorescent study of microtubules were carried out on primary roots of Zea mays L. Two types of cells were found to be formed within the postmitotic isodiametric growth (PIG) region of the root cortex that were differentially responsive to low level of exogenous ethylene. The innermost and central cell rows of the cortex were sensitive to ethylene treatment and showed a disturbed distribution of cortical microtubules (CMTs) as well as changed polarity of cell growth, whereas the 2–3 outermost cell rows were less sensitive in this respect. This suggests that post-mitotic cells of the inner cortex are specific targets for ethylene action. These properties of the inner cortex are compatible with its cells being involved in the formation of aerenchyma; they may also favour root growth in compacted soil. By contrast, the specific properties of the outer cortex indicate that this tissue domain is necessary for the gaseous impermeability and the mechanical strengthening of subjacent aerenchymatous cortex, especially in the mature region of the root. Ethylene affected neither the pattern of cortical cell expansion in the meristem nor the position of the PIG region with respect to the root tip. This contrasts with gibberellin-deficiency which affected these parameters in both parts of the cortex. These observations indicate a fundamental difference between the role of these two phytohormones in the morphogenesis and development of maize roots.     

Contribution of root cap mucilage and presence of an intact root cap in maize (Zea mays) to the reduction of soil mechanical impedance

BACKGROUND AND AIMS: The impedance to root growth imposed by soil can be decreased by both mucilage secretion and the sloughing of border cells from the root cap. The aim of this study is to quantify the contribution of these two factors for maize root growth in compact soil. METHODS: These effects were evaluated by assessing growth after removing both mucilage (treatment I -- intact) and the root cap (treatment D -- decapped) from the root tip, and then by adding back 2 micro L of mucilage to both intact (treatment IM -- intact plus mucilage) and decapped (treatment DM -- decapped plus mucilage) roots. Roots were grown in either loose (0.9 Mg m(-3)) or compact (1.5 Mg m(-3)) loamy sand soils. Also examined were the effects of decapping on root penetration resistance at three soil bulk densities (1.3, 1.4 and 1.5 Mg m(-3)). KEY RESULTS: In treatment I, mucilage was visible 12 h after transplanting to the compact soil. The decapping and mucilage treatments affected neither the root elongation nor the root widening rates when the plants were grown in loose soil for 12 h. Root growth pressures of seminal axes in D, DM, I and IM treatments were 0.328, 0.288, 0.272 and 0.222 MPa, respectively, when the roots were grown in compact soil (1.5 Mg m(-3) density; 1.59 MPa penetrometer resistance). CONCLUSIONS: The contributions of mucilage and presence of the intact root cap without mucilage to the lubricating effect of root cap (percentage decrease in root penetration resistance caused by decapping) were 43 % and 58 %, respectively. The lubricating effect of the root cap was about 30 % and unaffected by the degree of soil compaction (for penetrometer resistances of 0.52, 1.20 and 1.59 MPa).     

Signal transmission during gravitropic curvature of primary roots of Zea mays

Primary roots of Zea mays cv. Ageotropic are nonresponsive to gravity and elongate approximately 0.80 mm h^?1. Applying mucilage-like material (K-Y Jelly) to the terminal 1.5 cm of these roots induces graviresponsiveness and slow elongation 28% (i.e. from 0.80 to 0.58mm h^?1). Applying mucilage-like material to one side of the terminal 1.5 cm of the root induces curvature toward the mucilage, irrespective of the root's orientation to gravity. Applying a 2-mm-wideband of mucilage-like material to a root's circumference 8 to 10 mm behind the root cap neither induces gravicurvature nor affects elongation significantly. Similarly, applying mucilage-like material to only the root cap does not significantly affect elongation or graviresponsiveness. Gravicurvature of mutant roots occurs only when mucilage-like material is applied to the root/root-cap junction. Reversing the caps of wild-type and mutant roots produces gravitropic responses characteristic of the root cap rather than the host root. These results are consistent with the suggestion that gravitropic effectors are growth inhibitors that move apoplastically through mucilage between the root cap and root.     

The importance of anatomical structural study of roots for the understanding of physiological processes

Abstract

Besides being species‐specific, the inner structure of the root is influenced by the place and time of origin during the growth period. From the root tip up to the base of a particular root, the zones of cell division, cell elongation, formation of root hairs and root branching can be distinguished. The root tip that is covered by a root cap and mucilage is protected against evaporation and water contact. From the end of the lateral parts of the root cap, the cells become exposed to the surrounding environment. The cells can elongate by water uptake or can shrink by water loss. All processes of geotropic growth take place there. In this study, some differences are illustrated using Zea mays plants. Radicle and roots emerging from several nodes of the shoot as well as lateral roots are compared. The distances from the tip and from the base of the root are also very important for characterization of particular root functions. Distinctive features such as root diameter, size of the stele and of the cortex, ratio of cortex to stele, number and width of the xylem vessels, size of cells, special thickenings and stage of lignification as well as symptoms of maturation are observed.     

A role for gibberellic acid in orienting microtubules and regulating cell growth polarity in the maize root cortex

The role of gibberellins and cortical microtubules in determining the polarity of cell growth in the root cortex of maize (Zea mays L.) was examined. Inhibition of gibberellin biosynthesis, either naturally through mutation (d5 mutant) or by means of chemicals such as 2S,3S paclobutrazol, caused thickening of root apices and increased their starch content. Immunofluorescence microscopy of cortical microtubules, coupled with a comparison of cell widhts, lengths and shapes, indicated that the meristem and immediate post-mitotic zone were the targets of gibberellin deficiency. Cortical cells in these regions were impaired in their ability to develop highly ordered transversal arrays of cortical microtubules. Consequently, the cells became wider and shorter. Application of gibberellic acid re-established the arrangements of cortical microtubules and the polarity of cell growth characteristic for roots having normal levels of gibberellins, it also decreased the starch content. These results indicate that gibberellins are morphogenetically active substances, not only in shoots but also in roots of maize.Abbreviations CMT cortical microtubule - GA gibberellin - GA₃ gibberellic acid - MT microtubule - PIG postmitotic isodiametric growth The authors acknowledge the support to F.B. from the Royal Society (London UK). We also thank Dr. J. Lenton (University of Bristol, Long Ashton Research Station) who kindly supplied us with 2S,3S paclobutrazol and grains of the GA-deficient d5 mutant of maize.     

Gravity-Induced Polar Transport of Calcium across Root Tips of Maize

Calcium movement across primary roots of maize (Zea mays, L.) was determined by application of ⁴⁵Ca²⁺ to one side of the root and collection of radioactivity in an agar receiver block on the opposite side. Ca movement across the root tip was found to be at least 20 times greater than movement across the elongation zone. The rapid movement of Ca across the tip was severely inhibited in roots from which the root cap had been removed. Ca movement across the tip was also strongly retarded in roots pretreated with 2,4-dinitrophenol or potassium cyanide. Orientation of roots horizontally had no effect on Ca movement across the elongation zone but caused a strong asymmetry in the pattern of Ca movement across the tip. In gravistimulated roots, the movement of Ca from top to bottom increased while movement from bottom to top decreased. The data indicate that gravistimulation induces polar movement of Ca toward the lower side of the root cap. An earlier report (Lee, Mulkey, Evans 1983 Science 220: 1375-1376) from this laboratory showed that artificial establishment of calcium gradients at the root tip can cause gravitropic-like curvature. Together, the two studies indicate that Ca plays a key role in linking gravistimulation to the gravitropic growth response in roots.     

The Possible Involvement of Root-cap Mucilage in Gravitropism and Calcium Movement Across Root Tips of Allium cepa L

Roots of Allium cepa L. grown in aerated water elongate rapidly,but are not graviresponsive. These roots (1) possess extensivecolumella tissues comprised of cells containing numerous sedimentedamyloplasts, (2) lack mucilage on their tips, and (3) are characterizedby a weakly polar movement of calcium (Ca) across their tips.Placing roots in humid air correlates positively with the (1)onset of gravicurvature, (2) appearance of mucilage on tipsof the roots, and (3) onset of the ability to transport Ca polarlyto the lower side of the root tip. Gravicurvature of roots previouslysubmerged in aerated water is more rapid when roots are orientedvertically for 1–2 h in humid air prior to being orientedhorizontally. The more rapid gravicurvature of these roots correlatespositively with the accumulation of mucilage at the tips ofroots during the time the roots are oriented vertically. Therefore,the onset of gravicurvature and the ability of roots to transportCa to the lower sides of their tips correlate positively withthe presence of mucilage at their tips. These results suggestthat mucilage may be important for the transport of Ca acrossroot caps. Allium cepa, root gravitropism, root mucilage, calcium, onion     

Root cap removal increases root penetration resistance in maize (Zea mays L)

The root cap assists the passage of the root through soil by means of its slimy mucilage secretion and by the sloughing of its outer cells. The root penetration resistance of decapped primary roots of maize (Zea mays L. cv. Mephisto) was compared with that of intact roots in loose (dry bulk density 1.0 g cm-3; penetration resistance 0.06 MPa) and compact soil (1.4 g cm-3; penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the impedance to root growth. Root elongation rate and diameter were the same for decapped and intact roots when the plants were grown in loose soil. In compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots, whilst the diameter was 30% larger. Root penetration resistances of intact and decapped seminal axis were 0.31 and 0.52 MPa, respectively, when the roots were grown in compacted soil. These results indicated that the presence of a root cap alleviates much of the mechanical impedance to root penetration, and enables roots to grow faster in compacted soils.     

硝态氮(NO-3)对水稻侧根生长及其氮吸收的影响

侧根是植物吸收利用土壤养分的重要器官,其生长发育受内部遗传因子和外部环境矿质养分的影响.通过琼脂分层培养发现:局部供应NO-3可以诱导水稻( Oryza sativa L.)主根或不定根上侧根的生长.为研究旱种条件下NO-3对水稻侧根发育及其N吸收的影响,设置了3个蛭石培养实验:分根处理、全株缺N、全株供N处理.分根处理(一半根系供应3 mmol/L KNO3,另一半根系供应3 mmol/L KCl)结果表明:局部供应NO-3 能够促进水稻侧根生长.而在全株处理下,N饥饿诱导了侧根的伸长.水稻根系对NO-3的这两种反应都存在着显著的基因型差异.同时对地上部N浓度、可溶性总糖含量及N含量分析表明,这些生理指标在分根处理与全株加N处理中的差异均不显著,表明分根处理也能基本满足植株正常生长对N的需求.在分根处理中,水稻的N含量与分根处理中供N一侧的平均侧根长度存在显著正相关,这表明在养分不均一的介质中,侧根长度对水稻N素吸收具有十分重要的作用.而在N素充足的条件下,两者之间的相关性并不显著,这暗示在养分充足的环境下,侧根长度可能并不是决定根系吸收N素的主要因素.     

Root elongation against a constant force: experiment with a computerized feedback-controlled device

Axial force was applied to the root tip of corn (Zea mays L. cv. Merit) seedlings using a computerized, feedback-controlled mechanical device. The system's feedback capability allowed continuous control of a constant tip load, and the attached displacement transducer provided the time course of root elongation. Loads up to 7.5 g decreased the root elongation rate by 0.13 mm h-1 g-1, but loads 7.5 to 17.5 g decreased the growth rate by only 0.04 mm h-1 g-1. Loads higher than 18 g stopped root elongation completely. Measurement of the cross-sectional areas of the root tips indicated that the 18 g load had applied about 0.98 MPa of axial pressure to the root, thereby exceeding the root's ability to respond with increased turgor pressure. Recorded time-lapse images of loaded roots showed that radial thickening (swelling) occurred behind the root cap, whose cross-sectional area increased with tip load.     

边缘细胞对荞麦根尖铝毒的防护效应和对细胞壁多糖的影响

以2个荞麦(Fygopyrum esculentum Moench)基因型‘江西荞麦’(耐性)和‘内蒙荞麦’(敏感)为材料,采用悬空培养(保持边缘细胞附着于根尖和去除根尖边缘细胞),研究边缘细胞对根尖铝毒的防护效应以及对细胞壁多糖组分的影响。结果表明,铝毒抑制荞麦根系伸长,导致根尖Al积累。去除边缘细胞的根伸长抑制率和根尖Al含量高于保留边缘细胞的根。去除边缘细胞使江西荞麦和内蒙荞麦根尖的酸性磷酸酶(APA)活性显著升高,前者在铝毒下增幅更大。同时,铝毒胁迫下去除边缘细胞的根尖果胶甲酯酶(PME)活性和细胞壁果胶、半纤维素1、半纤维素2含量显著高于保留边缘细胞的酶活性和细胞壁多糖含量。表明边缘细胞对荞麦根尖的防护效应,与其阻止Al的吸收,降低根尖细胞壁多糖含量及提高酸性磷酸酶活性有关,以此缓解Al对根伸长的抑制。     

硝态氮（NO3^—）对水稻侧根生长及其氮吸收的影响

侧根是植物吸收利用土壤养分的重要器官 ,其生长发育受内部遗传因子和外部环境矿质养分的影响。通过琼脂分层培养发现 :局部供应NO-3 可以诱导水稻 (OryzasativaL .)主根或不定根上侧根的生长。为研究旱种条件下NO-3 对水稻侧根发育及其N吸收的影响 ,设置了 3个蛭石培养实验 :分根处理、全株缺N、全株供N处理。分根处理 (一半根系供应 3mmol/LKNO3,另一半根系供应 3mmol/LKCl)结果表明 :局部供应NO-3 能够促进水稻侧根生长。而在全株处理下 ,N饥饿诱导了侧根的伸长。水稻根系对NO-3 的这两种反应都存在着显著的基因型差异。同时对地上部N浓度、可溶性总糖含量及N含量分析表明 ,这些生理指标在分根处理与全株加N处理中的差异均不显著 ,表明分根处理也能基本满足植株正常生长对N的需求。在分根处理中 ,水稻的N含量与分根处理中供N一侧的平均侧根长度存在显著正相关 ,这表明在养分不均一的介质中 ,侧根长度对水稻N素吸收具有十分重要的作用。而在N素充足的条件下 ,两者之间的相关性并不显著 ,这暗示在养分充足的环境下 ,侧根长度可能并不是决定根系吸收N素的主要因素     

Extracellular proteins in pea root tip and border cell exudates

Newly generated plant tissue is inherently sensitive to infection. Yet, when pea (Pisum sativum) roots are inoculated with the pea pathogen, Nectria haematococca, most newly generated root tips remain uninfected even though most roots develop lesions just behind the tip in the region of elongation. The resistance mechanism is unknown but is correlated spatially with the presence of border cells on the cap periphery. Previously, an array of >100 extracellular proteins was found to be released while border cell separation proceeds. Here we report that protein secretion from pea root caps is induced in correlation with border cell separation. When this root cap secretome was proteolytically degraded during inoculation of pea roots with N. haematococca, the percentage of infected root tips increased from 4% +/- 3% to 100%. In control experiments, protease treatment of conidia or roots had no effect on growth and development of the fungus or the plant. A complex of >100 extracellular proteins was confirmed, by multidimensional protein identification technology, to comprise the root cap secretome. In addition to defense-related and signaling enzymes known to be present in the plant apoplast were ribosomal proteins, 14-3-3 proteins, and others typically associated with intracellular localization but recently shown to be extracellular components of microbial biofilms. We conclude that the root cap, long known to release a high molecular weight polysaccharide mucilage and thousands of living cells into the incipient rhizosphere, also secretes a complex mixture of proteins that appear to function in protection of the root tip from infection.     

多胺生物合成抑制剂D-精氨酸对拟南芥幼苗根系生长的影晌

为探讨多胺生物合成抑制剂D-精氨酸（D-arginine,D-Arg）对拟南芥根系生长的影响,首先用腐胺（0．1mmol‘L-1）和D—Arg（1．0mmol·L-1）处理种子萌发后生长2d的拟南芥幼苗。腐胺（0．1mmol·L-1）显著促进主根伸长,D-Arg（1．0mmol-L-1）显著抑制主根伸长,并对主根根尖的细胞形态有明显影响。为了进一步了解D—Arg影响拟南芥主根生长的机理,采用浓度梯度D．Arg处理幼苗根系。实验结果表明,随着D-Arg浓度增加（0．2～1．0mmol·L-1）,拟南芥幼苗主根生长受抑制的程度越严重。微分干涉观察主根根尖发现,外源施加D—Arg,引起拟南芥主根根尖分生区的细胞数目减少,使拟南芥幼苗表现出主根的伸长生长变缓。当分生区数目较少时,出现主根几乎不再仲长的现象。由此推测,多胺生物合成抑制剂D-Arg对拟南芥幼苗根生长的抑制作用机制,是D-Arg影响了其根尖分生区的细胞分裂活动,使分生区细胞数目减少,从而引起分生区长度减小,最终导致拟南芥主根仲长生长受到抑制。     

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.     

Root cap influences root colonisation by Pseudomonas fluorescens SBW25 on maize

We investigated the influence of root border cells on the colonisation of seedling Zea mays roots by Pseudomonas fluorescens SBW25 in sandy loam soil packed at two dry bulk densities. Numbers of colony forming units (CFU) were counted on sequential sections of root for intact and decapped inoculated roots grown in loose (1.0 mg m(-3)) and compacted (1.3 mg m(-3)) soil. After two days of root growth, the numbers of P. fluorescens (CFU cm(-1)) were highest on the section of root just below the seed with progressively fewer bacteria near the tip, irrespective of density. The decapped roots had significantly more colonies of P. fluorescens at the tip compared with the intact roots: approximately 100-fold more in the loose and 30-fold more in the compact soil. In addition, confocal images of the root tips grown in agar showed that P. fluorescens could only be detected on the tips of the decapped roots. These results indicated that border cells, and their associated mucilage, prevented complete colonization of the root tip by the biocontrol agent P. fluorescens, possibly by acting as a disposable surface or sheath around the cap.     

Root extracellular traps versus neutrophil extracellular traps in host defence,a case of functional convergence?

The root cap releases cells that produce massive amounts of mucilage containing polysaccharides, proteoglycans, extracellular DNA (exDNA) and a variety of antimicrobial compounds. The released cells – known as border cells or border‐like cells – and mucilage secretions form networks that are defined as root extracellular traps (RETs). RETs are important players in root immunity. In animals, phagocytes are some of the most abundant white blood cells in circulation and are very important for immunity. These cells combat pathogens through multiple defence mechanisms, including the release of exDNA‐containing extracellular traps (ETs). Traps of neutrophil origin are abbreviated herein as NETs. Similar to phagocytes, plant root cap‐originating cells actively contribute to frontline defence against pathogens. RETs and NETs are thus components of the plant and animal immune systems, respectively, that exhibit similar compositional and functional properties. Herein, we describe and discuss the formation, molecular composition and functional similarities of these similar but different extracellular traps.     

Primary Root Growth and the Pattern of Root Apical Meristem Organization are Coupled

Root apical meristems (RAMs) in dicotyledonous plants have two organizational schemes; closed (with highly organized tiers) and open (tiers lacking or disorganized). These schemes are commonly believed to remain unchanged during the growth of the root axis. Individual roots are commonly thought to have indeterminate growth. We challenge these two generalizations through the study of five species with closed apical organization: Clarkia unguiculata L., Oxalis corniculata L., Dianthus caryophyllus L., Blumenbachia hieronymi Urb., and Salvia farinaceae Benth. cv. Strata. These roots have phased growth patterns where early growth is followed by deceleration, after which the initial cells stop dividing, elongation ceases, and the root reaches its determinate length. At or before reaching determinacy, the root apical meristem stops maintaining its closed organization and becomes less organized. These observations will be placed in context with observations from the literature to suggest two new generalizations, namely, that apical organization does change over the growth phases of roots, and that roots are determinate.     

Possible Involvement of Cytokinin in Nitrate-mediated Root Growth in Maize

Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solutions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO ₃ ⁻ supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimulatory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at concentration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO ₃ ⁻ concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone.     

Primary and Lateral Root Development of Dark- and Light-Grown Cotton Seedlings under Salinity Stress*

Primary root growth dynamics and lateral root development of dark- and light grown cotton seedlings (Gossypium hirsutum L., cv. Acala SJ-2) were studied under control and salinity stress conditions. The seedlings were grown by two methods: A) in paper-lined, vermiculite-filled beakers with the plants growing between the paper and the glass wall (Gladish and Rost, 1993), and B) in hydroponics after germination and initial growth in germination paper rolls saturated with the treatment solutions (Kent and Läuchli, 1985). After germination, daily primary root elongation rate gradually incrased to a maximum, then gradually declined to close to zero for dark-grown seedlings, or to sustained rates of about 10 mm per day for light-grown control plants. Salinity stress delayed primary root growth and reduced peak elongation rates, without changing the general primary root growth pattern. These results suggest that salinity changed the time-scale, but did not modify the normal developmental sequence. Lateral root growth was more inhibited by salinity than primary root growth. In addition, elongation of lateral roots was more inhibited by salinity than their initiation and emergence. Light exposure of the shoot favored both sustained primary root growth from 7 days after planting, and lateral root emergence and growth. Salinity effects were more severe on seedlings germinated and grown in hydroponics (method B) than on vermiculite-grown plants (method A). These results emphasize the importance of growing conditions for the NaCl-induced effects on cotton root development. In addition, the differential effects of salinity on primary and lateral roots became evident, pointing to diverse control mechanisms for the development of these root types.