The effects of lead,nickel, and strontium nitrates on cell division and elongation in maize roots

The effects of Pb, Sr, and Ni nitrates on the root growth, its cell division and elongation were studied. Two-day-old maize seedlings were incubated on the 35 μM Ni(NO₃)₂, 10 μM Pb(NO₃)₂, or 3 mM Sr(NO₃)₂ in the presence or absence of 3 mM Ca(NO₃)₂. Metal toxicity was evaluated after the inhibition of root growth for the first and second days of incubation in comparison with the roots kept on water or Ca(NO₃)₂ solution. The contents of metals were determined in the apical (the first centimeter from the tip) and basal (the third centimeter from the kernel) root parts by voltamperometry and atomic-absorption spectrophotometry. We measured the length of the meristem, the length of the fully elongated cells, counted the mitotic index (MI) in the meristem and the number of meristematic cells in the cortex row; we also calculated duration the cell cycle. In the absence of Ca(NO₃)₂, the metal content in the apical root region was higher than in basal one. In the presence of Ca(NO₃)₂, we observed reverse ratio most pronounced in the case of Pb and Sr. All metals tested markedly reduced MI in the cortex, which was determined by the increase in the cell cycle duration and accompanied by the meristem shortening. These metals affected differently cell division and elongation: Ni inhibited mainly cell division and to a lesser degree their elongation, whereas Sr and Pb affected both cell division and elongation; only Sr treatment resulted in the increased length of the fully elongated cells. In the presence of Ca, all studied growth indices changed less than in the absence of Ca, which was manifested in the less severe suppression of the root growth and was in agreement with the lower accumulation of the metals in the root tips. Possible causes for the heavy metal action on growth are discussed in connection with the specificity of their transport and accumulation.     

Development of the Casparian strip in primary roots of maize under salt stress

The Casparian strip in the endodermis of vascular plant roots appears to play an important role in preventing the influx of salts into the stele through the apoplast under salt stress. The effects of salinity on the development and morphology of the Casparian strip in primary roots of maize (Zea mays L.) were studied. Compared to the controls, the strip matured closer to the root tip with increase in the ambient concentration of NaCl. During growth in 200 mM NaCl, the number and the length of the endodermal cells in the region between the root tip and the lowest position of the endodermal strip decreased, as did the apparent rate of production of cells in single files of endodermal cells (the rate of cell formation being equal to the rate at which cells are lost from the meristem). The estimated time required for an individual cell to complete the formation of the strip after generation of the cell in the presence of 200 mM NaCl was not very different from that required in controls. Thus, salinity did not substantially affect the actual process of formation of the strip in individual cells. The radial width of the Casparian strip, a morphological parameter that should be related to the effectiveness of the strip as a barrier, increased in the presence of 200 mM NaCl. The mean width of the lignified region was 0.92 m in distilled water and 1.33 m in 200 mM NaCl at the lowest position of the strip. The mean width of the strip relative to that of the radial wall at this position was significantly greater after growth in the presence of 200 mM NaCl than in the controls, namely, 20.5% in distilled water and 33.9% in 200 mM NaCl. These observations suggest that the function of the strip is enhanced under salt stress.     

Inhibition of maize primary root elongation by spermidine: Effect on cell shape and mitotic index

Spermidine applied for 18 h to intact maize seedlings through their roots reduces root growth 70%, and the effect is reversible. Histological observations of longitudinal sections of 0.4-cm root apical segments from 2-day-old maize seedlings grown for 18 h in 0.5 m CaSO₄ solution with or without 1 mm spermidine contribute to the explanation of spermidine-dependent slow root growth. In the meristematic zone a strong reduction of the mitotic index and in the elongation zone an inhibition of cell elongation occur simultaneously. Cell shape analysis along the growth axis of the maize root apex expressed in terms of form factor (FCircle) values substantiates the dual effect of spermidine on mitotic activity and cell elongation.Abbreviations PA polyamine(s) - Spm spermine - Spd spermidine     

Cell wall proteome in the maize primary root elongation zone. II. Region-specific changes in water soluble and lightly ionically bound proteins under water deficit

Previous work on the adaptation of maize (Zea mays) primary roots to water deficit showed that cell elongation is maintained preferentially toward the apex, and that this response involves modification of cell wall extension properties. To gain a comprehensive understanding of how cell wall protein (CWP) composition changes in association with the differential growth responses to water deficit in different regions of the elongation zone, a proteomics approach was used to examine water soluble and loosely ionically bound CWPs. The results revealed major and predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. In total, 152 water deficit-responsive proteins were identified and categorized into five groups based on their potential function in the cell wall: reactive oxygen species (ROS) metabolism, defense and detoxification, hydrolases, carbohydrate metabolism, and other/unknown. The results indicate that stress-induced changes in CWPs involve multiple processes that are likely to regulate the response of cell elongation. In particular, the changes in protein abundance related to ROS metabolism predicted an increase in apoplastic ROS production in the apical region of the elongation zone of water-stressed roots. This was verified by quantification of hydrogen peroxide content in extracted apoplastic fluid and by in situ imaging of apoplastic ROS levels. This response could contribute directly to the enhancement of wall loosening in this region. This large-scale proteomic analysis provides novel insights into the complexity of mechanisms that regulate root growth under water deficit conditions and highlights the spatial differences in CWP composition in the root elongation zone.     

The distribution of plasmodesmata in the root tip of maize

Summary The distribution of plasmodesmata in different regions of the root apex of Zea mays has been analysed from electron micrographs. There are many more plasmodesmata traversing transverse walls than across longitudinal walls in all the regions studied. When the number of plasmodesmata per unit cell volume is calculated, cells in non-dividing tissue have a considerably lower value than cells in dividing tissue. Evidence for the transport of materials between cells via plasmodesmata is summarised. If it is accepted that plasmodesmata do act as channels for intercellular communication then we believe that their pattern of distribution may be a contributory factor to the process of cell differentiation.     

Differential expression profiles of growth-related genes in the elongation zone of maize primary roots

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. Rates of root segmental elongation change from accelerating to decelerating as cell development proceeds from newly formed to fully elongated status. One of the primary variables regulating these changes in elongation rates is the extensibility of the elongating cell walls. To help decipher the complex molecular mechanisms involved in spatially variable root growth, we performed a gene identification study along primary root tips of maize (Zea mays) seedlings using suppression subtractive hybridization (SSH) and candidate gene approaches. Using SSH we isolated 150 non-redundant cDNA clones representing root growth-related genes (RGGs) that were preferentially expressed in the elongation zone. Differential expression patterns were revealed by Northern blot analysis for 41 of the identified genes and several candidate genes. Many of the genes have not been previously reported to be involved in root growth processes in maize. Genes were classified into groups based on the predicted function of the encoded proteins: cell wall metabolism, cytoskeleton, general metabolism, signaling and unknown. In-situ hybridization performed for two selected genes, confirmed the spatial distribution of expression shown by Northern blots and revealed subtle differences in tissue localization. Interestingly, spatial profiles of expression for some cell wall related genes appeared to correlate with the profile of accelerating root elongation and changed appropriately under growth-inhibitory water deficit.     

Activation of cell division in the quiescent center of excised maize root tip

The phenomenon of activation of cell proliferation in the quiescent center of excised maize roots is described. The root tips were grown on wet filter paper in Petri dishes. This phenomenon was observed in 8 to 14 maize cultivars and was absent in excised Arabidopsis root tips. The distribution of mitoses in meristems greatly varied in roots of individual seedlings from the same seed lot and seedlings of different cultivars. Meristem opening was observed after the removal of small root tips not longer than 3 mm and intact seminal roots. Sucrose (2%) and 10(-6)-10(-8) M indole-3-acetic acid did not prevent meristem opening. These findings indicate that the state of quiescent center is maintained by a system of intercellular and interorgan relations, which are to be clarified.     

Water transport in maize roots under the influence of mercuric chloride and water stress: a role of water channels

The influence of inhibitor of water channels, HgCl₂, on water diffusion in maize (Zea mays L.) seedling roots was investigated with the pulsed nuclear magnetic resonance (NMR) method. Blocking of water channels decreased the water permeability of cell membranes by 1.5 – 2 times. This effect of HgCl₂ was exhibited only in the roots of seedlings grown in a nutrient solution containing Ca²⁺ and was reversed with Hg-scavenging agent β-mercaptoethanol. Subsequent incubation of Ca²⁺-deficient roots in the nutrient solution with Ca²⁺ recovered the sensitivity to HgCl₂. The water stress decreased water diffusion rates similarly to HgCl₂ and the effects of water stress and HgCl₂ were not additive. The obtained data demonstrate the possibilities of the pulsed NMR method for study of the transmembrane water exchange in vivo in connection with water channel functioning.     

Root elongation, water stress, and mechanical impedance: a review of limiting stresses and beneficial root tip traits

Root elongation in drying soil is generally limited by a combination of mechanical impedance and water stress. Relationships between root elongation rate, water stress (matric potential), and mechanical impedance (penetration resistance) are reviewed, detailing the interactions between these closely related stresses. Root elongation is typically halved in repacked soils with penetrometer resistances >0.8-2?MPa, in the absence of water stress. Root elongation is halved by matric potentials drier than about -0.5?MPa in the absence of mechanical impedance. The likelihood of each stress limiting root elongation is discussed in relation to the soil strength characteristics of arable soils. A survey of 19 soils, with textures ranging from loamy sand to silty clay loam, found that ～10% of penetration resistances were >2?MPa at a matric potential of -10?kPa, rising to nearly 50% >2?MPa at - 200?kPa. This suggests that mechanical impedance is often a major limitation to root elongation in these soils even under moderately wet conditions, and is important to consider in breeding programmes for drought-resistant crops. Root tip traits that may improve root penetration are considered with respect to overcoming the external (soil) and internal (cell wall) pressures resisting elongation. The potential role of root hairs in mechanically anchoring root tips is considered theoretically, and is judged particularly relevant to roots growing in biopores or from a loose seed bed into a compacted layer of soil.     

Differential expression of cell wall related genes in the elongation zone of rice roots under water deficit

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. It has been known that rice seminal root elongation could be enhanced by water stress. In the present study, 17 cell-wall related genes were identified by cDNA-amplified fragment length polymorphism (cDNA-AFLP) technique. Five genes encoded cell-wall loosening enzymes and six genes were involved in the lignin biosynthesis. The six other genes were related to the metabolism of polysaccharide and protein matrices in cell wall. Northern blot analysis confirmed that they were differentially expressed in the elongation zone of rice seminal roots under water stress, and none of them was root-specific. The results indicated that the activity of cell-wall loosening enzymes was enhanced in the early stage (within 16 h), and some cell wall matrices were synthesized rapidly in the middle stages (from 16 to 48 h), while lignin biosynthesis was enhanced in the middle and late stages of water stress (from 48 to 72 h). Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 3, pp. 437–443. The text was submitted by the authors in English.     

Effect of inhibition of abscisic Acid accumulation on the spatial distribution of elongation in the primary root and mesocotyl of maize at low water potentials

Previous work showed that accumulation of endogenous abscisic acid (ABA) acts both to maintain primary root growth and inhibit shoot growth in maize seedlings at low water potentials (ψ_w) (IN Saab, RE Sharp, J Pritchard, GS Voetberg [1990] Plant Physiol 93: 1329-1336). In this study, we have characterized the growth responses of the primary root and mesocotyl of maize (Zea mays L. cv FR27 × FRMo 17) to manipulation of ABA levels at low ψ_w with a high degree of spatial resolution to provide the basis for studies of the mechanism(s) of ABA action. In seedlings growing at low ψ_w and treated with fluridone to inhibit carotenoid (and ABA) biosynthesis, ABA levels were decreased in all locations of the root and mesocotyl growing zones compared with untreated seedlings growing at the same ψ_w. In the root, low ψ_w (−1.6 megapascals) caused a shortening of the growing zone, as reported previously. The fluridone treatment was associated with severe inhibition of root elongation rate, which resulted from further shortening of the growing zone. In the mesocotyl, low ψ_w (−0.3 megapascal) also resulted in a shortened growing zone. In contrast with the primary root, however, fluridone treatment prevented most of the inhibition of elongation and the shortening of the growing zone. Final cell length measurements indicated that the responses of both root and mesocotyl elongation to ABA manipulation at low ψ_w involve large effects on cell expansion. Measurements of the relative changes in root and shoot water contents and dry weights after transplanting to a ψ_w of −0.3 megapascal showed that the maintenance of shoot elongation in fluridone-treated seedlings was not attributable to increased water or seed-reserve availability resulting from inhibition of root growth. The results suggest a developmental gradient in tissue responsiveness to endogenous ABA in both the root and mesocotyl growing zones. In the root, the capacity for ABA to protect cell expansion at low ψ_w appears to decrease with increasing distance from the apex. In the mesocotyl, in contrast, the accumulation of ABA at low ψ_w appears to become increasingly inhibitory to expansion as cells are displaced away from the meristematic region.     

QTLs for the elongation of axile and lateral roots of maize in response to low water potential

Changes in root architecture and the maintenance of root growth in drying soil are key traits for the adaptation of maize (Zea mays L.) to drought environments. The goal of this study was to map quantitative trait loci (QTLs) for root growth and its response to dehydration in a population of 208 recombinant inbred lines from the International Maize and Wheat Improvement Center (CIMMYT). The parents, Ac7643 and Ac7729/TZSRW, are known to be drought-tolerant and drought-sensitive, respectively. Roots were grown in pouches under well-watered conditions or at low water potential induced by the osmolyte polyethylene glycol (PEG 8000). Axile root length (L _Ax) increased linearly, while lateral root length (L _Lat) increased exponentially over time. Thirteen QTLs were identified for six seedling traits: elongation rates of axile roots (ER_Ax), the rate constant of lateral root elongation (k _Lat), the final respective lengths (L _Ax and L _Lat), and the ratios k _Lat/ER_Ax and L _Lat/L _Ax. While QTLs for lateral root traits were constitutively expressed, most QTLs for axile root traits responded to water stress. For axile roots, common QTLs existed for ER_Ax and L _Ax. Quantitative trait loci for the elongation rates of axile roots responded more clearly to water stress compared to root length. Two major QTLs were detected: a QTL for general vigor in bin 2.02, affecting most of the traits, and a QTL for the constitutive increase in k _Lat and k _Lat/ER_Ax in bins 6.04–6.05. The latter co-located with a major QTL for the anthesis-silking interval (ASI) reported in published field experiments, suggesting an involvement of root morphology in drought tolerance. Rapid seedling tests are feasible for elucidating the genetic response of root growth to low water potential. Some loci may even have pleiotropic effects on yield-related traits under drought stress.     

The effects of soluble-Al on root growth and radicle elongation

Summary In order to determine the effects of concentration on plant growth, aluminium (Al) was extracted (10^–3M CaCl₂) from 4 acid brown hill soils which had been treated with superphosphate at rates equivalent to 0 to 300 kg P ha^–1. The soils ranged in pH (CaCl₂) from 3.5 to 4.9, and Al concentration from 0 to 0.6 mM. The effects of Al on ryegrass growth in the 4 soils in a glasshouse was compared with its effect on radicle elongation of seeds germinated in contact with CaCl₂ extracts from the same soils.Ryegrass root growth in the glasshouse, and radicle elongation in the bioassay test were both unaffected by Al concentrations below 0.1 mM. Root growth was substantially reduced when Al concentration exceeded 0.1 mM and above 0.2 mM growth was almost completely inhibited. Radicle elongation rate was also reduced when the concentration of Al was greater than 0.2 mM agreeing well with the observation from the pot experiment.It is concluded that because of its speed and convenience the bioassay method offers a useful method of establishing critical levels of Al for crop plants.     

Influence of inorganic nitrogen and pH on the elongation of maize seminal roots

BACKGROUND AND AIMS: Root absorption and assimilation of inorganic nitrogen usually alters rhizosphere pH, but the immediate influence of such pH changes on root elongation as well as that of exogenous inorganic nitrogen itself has been uncertain. METHODS: A differential extensiometer that monitored on a real-time, continuous basis root elongation in an intact 3-d-old maize plant was developed. Treatments included root media at pH 6.5 or 5.6 that lacked nitrogen and ones at pH 6.5 that contained 100 mmol m(-3) NH(4)(+) or NO(3)(-). KEY RESULTS: Acidifying the root medium from pH 6.5 to 5.6 nearly doubled the elasticity of the seminal root, but slightly decreased its elongation. Plasticity of the root apex was not detectable in all treatments. The presence of ammonium or nitrate in the medium stimulated elongation by 29 % or 14 %, respectively. Addition of an osmoticum to the medium had no effect on root elongation in the absence of inorganic nitrogen, but diminished the stimulation of elongation in the presence of ammonium and nitrate. This indicates that these ions or their by-products serve partially as osmolytes. CONCLUSIONS: In nutrient solution, root elongation of a maize seedling--even one with ample nitrogen reserves--depended most strongly on exogenous inorganic nitrogen, and less so, if at all, on either the pH of the bulk nutrient solution or the mechanical properties of cell walls.     

The longevity and activity of the primary root of maize

The longevity of the main root cylinder and the laterals of the primary root of maize plants was determined under controlled greenhouse conditions by means of nuclear staining with acridine orange. The cortex of the main root was found to be alive for the whole life-span of the plant, whereas the life-span of the root hairs was only 2 to 3 days as evidenced by electronmicroscopical examination of cell integrity. The onset of senescence of laterals was observed at the older part of the main root at the 6-leaf stage of the plant. Senescence of 1st and 2nd order laterals commenced near the root tip a few days after their protrusion and advanced towards the basal region of the root. In any root segment death of the cortex cells preceeded that of the stele. At the late grain filling stage all laterals along the main root exhibited advanced senescence, but stainable nuclei were seen in the root tissues of the basal part of 1^st order laterals (both cortex and stele) as well as of the 2^nd order laterals which emerged from that root segment. The pattern of the dying of the root tissue is discussed with regard to the P-nutrition of the shoot system by the primary root.     

Changes of cytosolic Ca2+ fluorescence intensity and plasma membrane calcium channels of maize root tip cells under osmotic stress

The changes of cytosolic Ca²⁺ fluorescence intensity and the activities of calcium channel of primary maize root tip cells induced by PEG6000 or abscisic acid(ABA) were studied by both confocal techniques and the whole-cell patch clamping in this study. The Ca²⁺ fluorescence intensity increased while treated with PEG or ABA within 10 min, illuminating that Ca²⁺ participated in the process of ABA signal transduction. For further proving the mechanism and origin of cytosolic Ca²⁺ increase induced by PEG treatments, N,N,N′,N′-tetraacetic acid (EGTA), Verapamil (VP) and Trifluoperazine (TFP) were added to the PEG solution in the experiments separately. The results showed that Ca²⁺ fluorescence intensity induced by PEG was suppressed by both EGTA and VP obviously in the root tip cells. The Ca²⁺ fluorescence intensity of plants changed after the addition of CaM inhibitor TFP while subjected to osmotic stress, which seemed to show that CaM participated in the process of signal transduction of osmotic stress too. The mechanism about it is unknown today. Further, a hyperpolarization-activated calcium permeable channel was recorded in plasma membrane of maize root tip cells. The Ca²⁺ current (I_Ca) intensity increased remarkably after PEG treatment, and the open voltage of the calcium conductance increased. Similar changes could be observed after ABA treatment, but the channel opened earlier and the current intensity was stronger than that of PEG treatment. The activation of calcium channel initiated by PEG strongly was inhibited by EGTA, VP or TFP respectively. The results revealed that Ca²⁺ participated in the signals transduction process of osmotic stress, and the cytosolic free Ca²⁺ increase by osmotic stress mainly came from the extracellular, and some came from the release of cytoplasmic calcium pool.     

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum: The role of cell turgor in long-distance water transport

Summary Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the cell turgor and the osmotic pressure of leaf cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 1 decrease in the cell turgor pressure of the mesophyll cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the cell turgor pressure (ratio of about 1 1). The cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and cell turgor pressure. From the xylem, cell turgor, and cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero turgor pressure.     

The fine structure of the cells that perceive gravity in the root tip of maize

Summary Within the root cap, in maize, the cells believed to be responsible for the perception all possess large well-developed amyloplasts. They also have normal mitochondria and Golgi bodies, normal rough-surfaced ER with a very striking pattern of distribution, few free ribosomes, walls with an abnormal reticulate encrusting material, irregularly distributed plasmodesmata and an as yet unidentified fine quadruple membranous system. All of these features are discussed in relation to the role of the cells in perception.     

DCPTA对盐胁迫下玉米苗期根系生长、渗透调节及膜透性的影响

黑龙江省西部地区盐碱土面积约有66.7×104hm2,制约着黑龙江省粮食生产水平的进一步提高。叔胺类活性物质2-(3,4-二氯苯氧基)-乙基-二乙胺(DCPTA)具有提高作物抗逆性作用,为探讨DCPTA提高玉米耐盐性机制,采用营养液水培试验,在前期试验基础之上,选取"先玉335"(耐盐性强)和"丰禾1号"(耐盐性弱)2个品种,研究DCPTA(15mg·L-1)对Na Cl胁迫下(150 mmol·L-1)玉米幼苗根系生长、渗透调节及质膜透性的影响。结果表明:DCPTA能增加盐胁迫下幼苗的根长、根表面积、根体积、根鲜重和根干重,缓解盐胁迫对根系生长的抑制;DCPTA维持了根系水分平衡,提高了盐胁迫下幼苗根系相对含水量,"先玉335"和"丰禾1号"分别提高3.6%和6.4%;DCPTA通过提高根系可溶性糖含量和可溶性蛋白含量,降低幼苗根系脯氨酸含量,增强了盐胁迫下玉米幼苗根系的渗透调节能力,并通过降低根系丙二醛(MDA)含量和相对电导率,保护了根系质膜结构和功能;与盐胁迫相比,"先玉335"丙二醛含量和相对电导率分别降低21.6%和24.2%,"丰禾1号"分别降低28.1%和30.4%;DCPTA缓解盐胁迫对根系伤害的效果表现为"丰禾1号""先玉335"。