Ion Transport to the Xylem in Aerenchymatous Roots of Zea mays L.

Aerenchyma was induced in the developing nodal (adventitious)roots of maize (Zea mays L. cv. LG11) in solution culture bytreatment with an atmosphere of 5% (v/v) oxygen (oxygen partialpressure 5.0 k Pa). Ion uptake by aerenchymatous and ordinary(non-aerenchymatous) roots was then compared to determine theeffect of degeneration of much of the cortex in aerenchymatousroots on radial ion movement to the xylem. Rates of uptake andtransport of phosphate, potassium and chloride were followedby labelling 4–12mm length segments of intact roots usingradioactive tracers. In some experiments, enhanced rates ofphosphate transport were induced by means of a divided rootsystem and nutrient deprivation. For all three nutrient ions,and over a range of concentration for phosphate (10–100mmol m^–3 and potassium (0·25–5·0 molm^–3 and with enhanced rates of transport, aerenchymatousroots were at least as effective as ordinary roots (per mm³of root) and sometimes more effective (per mm root length).These findings are discussed in relation to hypotheses concerningthe pathways for radial ion movement across the cortex in ordinaryand aerenchymatous roots. Correspondence to: Long Ashton Research Station, Long Ashton,Bristol BS18 9AF, U.K. Key words: Aerenchyma, ion transport, mineral nutrition, oxygen deficiency, radio-isotopic tracers, roots, structure, Zea mays     

Possible Linkage between NADH-Oxidation and Proton Secretion in Zea mays L. Roots

A possible involvement of two different systems in proton translocationand the correlation of this factor to growth rates were measuredsimultaneously by means of a pH stat and an optical system.Ferricyanide, which can accept electrons at the plasmalemma,led to an immediate increase of net H⁺ -efflux but also decreasedroot growth rate. The reduced form, ferrocyanide, inhibitednet H⁺ -effluxwithout changing the growth rate. Thus, corn rootgrowth was not determined by proton secretion exclusively. Vanadatestrongly inhibited net H⁺ -efflux by the roots but did not preventthe stimulating effect of fcrricyanide. Moreover, the extentof enhancement of net H⁺ -effluxby ferricyanide was exactlythe same in vanadate pretreated as in untreated roots. Alcoholswere used to try to increase the intracellular NADH level throughthe action of the cytoplasmic alcohol dehydrogenase presentin the roots and coleoptiles. Alcohols, known to be substratesfor alcohol dehydrogenase such as propan- 1-ol, ethanol andbutan-l -ol increased net H⁺ -effluximmediately but methanoland secondary alcohols which are not substrates had no effecton proton secretion. The K_m values of alcohol dehydrogenasefor the alcohols correspond only partly to their effects onproton secretion. However, the specificlty observed suggeststhat increased H⁺ -efflux arose from reduction of endogenousNAD by ADH and consequent increased membrane NADH-oxidasc activitytrans locating protons and electrons out of the cells. Decreased oxygen concentrations slowed proton secretion at valuesfar higher than are necessary to saturate cytochrome c oxidase.The results of these experiments suggest two distinct systemscontributing to proton efflux. Key words: ADH, proton transport, redox chain     

Gravitropism of Primary Roots of Zea mays L. at Different Displacement Angles

Primary roots of Zea mays were oriented at various angles fromthe vertical ranging from 99° to 1° and their subsequentbending analysed from filmed records. The maximum rate of bendingand the time before bending commenced both varied two-fold,but showed no correlation with the initial angle of tip displacement.Roots orientated to small initial angles (< 40°) oftenovershot the vertical and proceeded to oscillate around thisorientation, whereas roots oriented to large initial angles(> 60°) often failed to achieve the vertical. Roots inthis latter group resumed bending after an indeterminate time,or did so immediately after a second displacement of their tip,showing that they were not intrinsically unable to bend. Theapparently spontaneous resumption of bending after a temporaryplagiogravitropic phase is suggested as being due to noise inthe graviperception system in the root cap. The tips of rootsgrowing vertically downwards showed oscillatory bending movementsup to 10° either side of vertical. This angle correspondsto the minimum angle of displacement which induces gravitropicbending. Only when roots were oriented 10-20° from verticaldid they begin unequivocally to show a gravitropism since atsuch angles the deflection of their tips exceeded that due totheir natural oscillation.Copyright 1993, 1999 Academic Press Gravitropism, roots, Zea mays     

The Diversity of Archaea and Bacteria in Association with the Roots of Zea mays L.

The diversity of bacteria and archaea associating on the surface and interior of maize roots (Zea mays L.) was investigated. A bacterial 16S rDNA primer was designed to amplify bacterial sequences directly from maize roots by PCR to the exclusion of eukaryotic and chloroplast DNA. The mitochondrial sequence from maize was easily separated from the PCR-amplified bacterial sequences by size fractionation. The culturable component of the bacterial community was also assessed, reflecting a community composition different from that of the clone library. The phylogenetic overlap between organisms obtained by cultivation and those identified by direct PCR amplification of 16S rDNA was 48%. Only 4 bacterial divisions were found in the culture collection, which represented 27 phylotypes, whereas 6 divisions were identified in the clonal analysis, comprising 74 phylotypes, including a member of the OP10 candidate division originally described as a novel division level lineage in a Yellowstone hot spring. The predominant group in the culture collection was the actinobacteria and within the clone library, the a-proteobacteria predominated. The population of maize-associated proteobacteria resembled the proteobacterial population of a typical soil community within which resided a subset of specific plant-associated bacteria, such as Rhizobium- and Herbaspirillum-related phylotypes. The representation of phylotypes within other divisions (OP10 and Acidobacterium) suggests that maize roots support a distinct bacterial community. The diversity within the archaeal domain was low. Of the 50 clones screened, 6 unique sequence types were identified, and 5 of these were highly related to each other (sharing 98% sequence identity). The archaeal sequences clustered with good bootstrap support near Marine group I (crenarchaea) and with Marine group II (euryarchaea) uncultured archaea. The results suggest that maize supports a diverse root-associated microbial community composed of species that for the first time have been described as inhabitants of a plant-root environment.     

Cytochemical Localization of Calcium in Cap Cells of Primary Roots of Zea mays L.

The distribution of calcium (Ca) in caps of vertically- andhorizontally-oriented roots of Zea mays was monitored to determineits possible role in root graviresponsiveness. A modificationof the antimonate precipitation procedure was used to localizeCa in situ. In vertically-oriented roots, the presumed graviperceptive(i.e., columella) cells were characterized by minimal and symmetricstaining of the plasmalemma and mitochondria. No precipitatewas present in plasmodesmata or cell walls. Within 5 min afterhorizontal reorientation, staining was associated with the portionof the cell wall adjacent to the distal end of the cell. Thisasymmetric staining persisted throughout the onset of gravicurvature.No staining of lateral cell walls of columella cells was observedat any stage of gravicurvature, suggesting that a lateral flowof Ca through the columella tissue of horizontally-orientedroots does not occur. The outermost peripheral cells of rootsoriented horizontally and vertically secrete Ca through plasmodesmata-likestructures in their cell walls. These results are discussedrelative to proposed roles of root-cap Ca in root gravicurvature. Key words: Antimonate, calcium, columella cell, peripheral cell, root gravitropism, Zea mays L.     

Collection of Gravitropic Effectors from Mucilage of Electrotropically-stimulated Roots of Zea mays L.

We placed agar blocks adjacent to tips of electrotropicallystimulated primary roots of Zea mays. Blocks placed adjacentto the anode-side of the roots for 3 h induced significant curvaturewhen subsequently placed asymmetrically on tips of vertically-orientedroots. Curvature was always toward the side of the root ontowhich the agar block was placed. Agar blocks not contactingroots and blocks placed adjacent to the cathode-side of electrotropicallystimulated roots did not induce significant curvature when placedasymmetrically on tips of vertically-oriented roots. Atomicabsorption spectrophotometry indicated that blocks adjacentto the anode-side of electrotropically-stimulated roots containedsignificantly more calcium than (1) blocks not contacting roots,and (2) blocks contacting the cathode-side of roots. These resultsdemonstrate the presence of a gradient of endogenous Ca in mucilageof electrotropically-stimulated roots (i.e. roots undergoinggravitropic-like curvature). Zea mays, corn, mucilage, root gravitropism, electrotropism     

The Nuclear Endoreduplication Cycle in Metaxylem Cells of Primary Roots of Zea mays L.

The nuclear DNA content of metaxylem cells in roots of Zea mayscv. Golden Bantam reaches 16C or 32C by successive rounds ofDNA endoreduplication. Each phase of endoreduplication (endo-S)is separated by a non-DNA synthetic phase (endo-G). These phasesseem to occur in zones at fixed distances from the root tip.The duration of the phases in two of the endoreduplication cycles(4C–8C, 8C–16C) has been estimated in two ways.The first makes use of the rate of movement of cells throughthe positions along the root where the different phases of thecycle are occurring, the second uses labelling with methyl-[³H]thymidineand autoradiography. Both methods indicate that the endo-S phaseswhich cause the nuclear DNA content to rise from 4C to 8C andfrom 8C to 16C last 8–10 h, and that the intervening endo-Gphase lasts 8–12 h. DNA endoreduplication keeps pace withthe increase of nuclear volume; cell volume increases at a morerapid rate, however. Comparison of the endoreduplication cyclein the metaxylem with the mitotic cycle in the adjoining filesof parenchyma cells shows that the mitotic cells complete theircycle more slowly. DNA synthesis, endoreduplication cycle, mitotic cycle, root apex, Zea mays     

Mechanisms of Inhibition of Water Movement in Anaerobically Treated Roots of Zea mays L.

Changes in water flux (Jv) across detopped, 7-d-old, maize rootswere characterized during the initial 24 h of being made anoxicby exposure to an anaerobic nutrient solution. Suction (50 kPa)was applied to the xylem and samples of the xylem sap were collectedat intervals and the osmolality and ionic content were measured. Values of Jv through anoxic roots fell below those of aerobiccontrols 1 h after the equilibrium oxygen partial pressure inthe bathing medium dropped below 20 kPa (air = 20.6 kPa). Thereduction in Jv was due primarily to a nullification of thediurnal rhythm in hydraulic conductivity (Lp) that was measuredin aerobic roots. However, about one-quarter of the reductionin Jv could be accounted for by a smaller osmotic componentof the driving force () on water movement. The significance of changes in Jv in anoxic roots is discussedin terms of the reliability of estimates of Lp, the reflectioncoefficient () and . Key words: Anaerobiosis, hydraulic conductivity, osmotic potential, water     

Somatic embryogenesis in Zea mays L.

Summary Combining ability studies with respect to such green fodder quality characteristics as oxalic acid, calcium, sodium, potassium and green fodder yield were carried out in a 12 × 12 diallel cross set in pearl millet (Pennisetum typhoides (Burm) S. & H.). With regard to differential expression of gene effects, studies for quality traits were carried out in different seasons and on different plant parts. The relative proportions of general and specific combining variances indicated the preponderance of non-additive genetic variance. Parents possessing desirable fodder quality characteristics were identified on the basis of combining ability and per se performance, and selection criterion for crosses was discussed. It was recommended that leaf portion should be biochemically analysed and manipulated in an environment when the genes are expressed.Part of the Ph. D. dissertation submitted to the Punjab Agricultural University by the senior author in partial fulfilment of the requirements for the degree     

Leaf vasculature in Zea mays L.

The vascular system of the Zea mays L. leaf consists of longitudinal strands interconnected by transverse bundles. In any given transverse section the longitudinal strands may be divided into three types of bundle according to size and structure: small, intermediate, large. Virtually all of the longitudinal strands intergrade structurally however, from one bundle type to another as they descend the leaf. For example, all of the strands having large-bundle anatomy appear distally as small bundles, which intergrade into intermediates and then large bundles as they descend the leaf. Only the large bundles and the intermediates that arise midway between them extend basipetally into the sheath and stem. Most of the remaining longitudinal strands of the blade do not enter the sheath but fuse with other strands above and in the region of the blade joint. Despite the marked decrease in number of longitudinal bundles at the base of the blade, both the total and mean cross-sectional areas of sieve tubes and tracheary elements increase as the bundles continuing into the sheath increase in size. Linear relationships exist between leaf width and total bundle number, and between cross-sectional area of vascular bundles and both total and mean cross-sectional areas of sieve tubes and tracheary elements.     

Transport and Metabolism of Kinetin-8-14C in Zea mays L. Roots

The movement of kinetin has been investigated in roots of Zeaseedlings, grown in the dark, using kinetin-8-¹⁴C. Segments (10 and 11 mm) with and without an apex were used andthe following results obtained: The transport of kinetin is polarized basipetally. Kinetin-8-¹⁴C is transported in roots without vascular tissue,but the amount transported is always lower than in roots withvascular tissue. Thin-layer chromatography showed that the molecule of kinetinis metabolized in the root segments, adenine being one of theproducts.     

Internode length in Zea mays L.

[¹³C, ³H]Gibberellin A₂₀ (GA₂₀) has been fed to seedlings of normal (tall) and dwarf-5 and dwarf-1 mutants of maize (Zea mays L.). The metabolites from these feeds were identified by combined gas chromatography-mass spectrometry. [¹³C, ³H]Gibberellin A₂₀ was metabolized to [¹³C, ³H]GA₂₉-catabolite and [¹³C, ³H]GA₁ by the normal, and to [¹³C, ³H]GA₂₉ and [¹³C, ³H]GA₁ by the dwarf-5 mutant. In the dwarf-1 mutant, [¹³C, ³H]GA₂₀ was metabolized to [¹³C, ³H]GA₂₉ and [¹³C, ³H]GA₂₉-catabolite; no evidence was found for the metabolism of [¹³C, ³H]GA₂₀ to [¹³C, ³H]GA₁. [¹³C, ³H]Gibberellin A₈ was not found in any of the feeds. In all feeds no dilution of ¹³C in recovered [¹³C, ³H]GA₂₀ was observed. Also in the dwarf-5 mutant, the [¹³C]label in the metabolites was apparently undiluted by endogenous [¹³C]GAs. However, dilution of the [¹³C]label in metabolites from [¹³C, ³H]GA₂₀ was observed in normal and dwarf-1 seedlings. The results from the feeding studies provide evidence that the dwarf-1 mutation of maize blocks the conversion of GA₂₀ to GA₁.Abbreviations GA_n gibberellin A_n - GC-MS combined gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography - RP reverse phase     

A Morphometric Analysis of the Ultrastructure of Columella Statocytes in Primary Roots of Zea mays L.

A morphometric analysis of the ultrastructure of columella statocytesin primary roots of Zea mays was performed to determine theprecise location of cellular organelles in graviperceptive cells.Vacuoles occupy the largest volume in the cell (11.4 per centof the protoplasm). The nucleus (9.51 per cent), amyloplasts(7.57 per cent), mitochondria (3.42 per cent), spherosomes (2.13per cent) and dictyosomes (0.55 per cent) occupy progressivelysmaller volumes of the statocytes. All organelles are distributedasymmetrically within the cell. Amyloplasts, spherosomes anddictyosomes are found in greatest numbers (and relative volumes)in the lower (i.e. ‘bottom’) third of the cell.The largest numbers and relative volumes of mitochondria arein the lower and middle thirds of the cell. Nuclei tend to befound in the middle third of the statocytes. Only the hyaloplasmis concentrated in the upper (i.e. ‘top’) thirdof Z. mays statocytes. When the sedimentation of amyloplasts(and the resulting exclusion of other organelles from the lowerthird of the cell) is corrected for, all cellular constituentsremain asymmetrically distributed within the cell. Therefore,the sedimentation of amyloplasts alone is not responsible forthe differential distribution of other cellular organelles inZ. mays statocytes. The quantitative ultrastructure of Z. maysstatocytes is discussed relative to the graviperceptive functionof these cells. Zea mays, corn, maize, root cap, stereology, columella, statocytes, graviperception, ultrastructure     

Polyamine Content and Action in Roots of Zea mays L. in Relation to Aerenchyma Development

Roots of Zea mays L. developed more aerenchyma (cortical gas-filledspace) when partially deficient in oxygen (3 kPa) than whensupplied with air (20·8 kPa oxygen) in association withfaster production of ethylene (ethene). The possibility wastested that the additional ethylene production resulted fromdecreases in spermidine (spd) and spermine (spm) which share,with ethylene, a common precursor, S-adenosylmethionine. However,no decreases in spd and spm were seen in root tissue up to 4d-old. Removing oxygen completely also had little effect onspd and spm, but strongly suppressed both ethylene productionand aerenchyma formation. Partial oxygen shortage (3 kPa) increased the concentrationof putrescine (put), the precursor of spd and spm. This increasewas not a response to the extra ethylene formed by such rootssince ethylene treatment did no reproduce the effect. Applicationof inhibitors of put biosynthesis, difluoromethylarginine anddifluoromethylornithine, led to increased aerenchyma formation.Exogenous put inhibited the development of aerenchyma whilestimulating rather than inhibiting ethylene production, whentested in either air or 3 kPa oxygen. Thus, put appears to limitaerenchyma formation by suppressing ethylene action rather thanits production.Copyright 1993, 1999 Academic Press Ethylene, ethene, roots, aerenchyma, polyamines, oxygen shortage, anaerobiosis, environmental stress, Zea mays     

干旱胁迫对玉米幼苗根系中NO信号转导的影响

以玉米(Zea mays L.)幼苗根系为材料,研究其生长发育对内外源NO的感应及在干旱条件下内外源NO对其生长发育的影响.结果表明:干旱抑制根系NO释放,NO的产生可能不依赖于NO合酶系统,而依赖于NADPH硝酸还原酶的酶促反应;用活性氧清除剂NAC、NAC与干旱复合处理均能促使内源NO的产生和释放:其根系生物量的分析表明内源NO的释放明显抑制根系的生长发育.     

Ionic Current Changes Associated with the Gravity-Induced Bending Response in Roots of Zea mays L

A vibrating probe was used to measure the changes in ionic currents around gravistimulated roots of Zea mays L. in an effort to determine whether these currents are involved in stimulus transduction from the root cap to the elongation zone. We did not observe a migration of the previously reported auxin-insensitive current efflux associated with gravity sensing (T. Björkman, A.C. Leopold [1987] Plant Physiol 84:841-846) back from the root cap. Instead, beginning 10 to 15 min after gravistimulation, an asymmetry in current developed simultaneously along the root around the meristem and apical regions of the elongation zone. This asymmetry comprised a proton efflux from the upper surface, which was superimposed on the symmetrical pattern around the vertical root. The gravity-induced proton efflux was inhibited by the application of the auxin transport inhibitor, 2,3,5-triiodobenzoic acid, whereas the calcium channel blocker, lanthanum, had little effect. Because the onset of the gravity-induced current asymmetry coincided both spatially and temporally with the onset of the differential growth response, we suggest that this current efflux may result from auxin-requiring acid-growth phenomena in the upper root tissue. The implications of this simultaneous onset of both proton efflux and elongation for theories about gravity stimulus transduction are discussed.     

Hypoxic Induction of Anoxia Tolerance in Roots of Adh1 Null Zea mays L

Seedlings of alcohol dehydrogenase 1 null mutants (Adh1-) of Zea mays L., which fail to synthesize alcohol dehydrogenase 1 (ADH1) isozymes, were hypoxically acclimated by 18 h of exposure to an atmosphere of 4% (v/v) O2 in N2 at 25[deg]C. Their ability to tolerate subsequent anoxia by exposure to anaerobic (O2-free) conditions was compared with that of unacclimated seedlings that were transferred immediately from an atmosphere of 40% (v/v) O2 to anaerobic conditions. Only 10% of the root tips of unacclimated seminal roots survived 6 h of anoxia, whereas 70% of the hypoxically acclimated root tips were viable at 24 h. During anoxia, acclimated root tips had enhanced ADH activity compared with unacclimated root tips, through induction of Adh2. Despite this, enzyme activity was still only about 5% that of acclimated, wild-type root tips and about half that of unacclimated, wild-type root tips. During anoxia, acclimated Adh1- root tips showed a higher rate of anaerobic respiration and ethanol production, greater concentrations of ATP and total adenylates, and a greater adenylate energy charge compared with unacclimated root tips. These results suggest that although enhanced ADH activity may have raised fermentation rates in acclimated Adh1- tissues and thereby contributed to energy metabolism and viability, the high levels of ADH activity inducible in acclimated, wild-type maize root tips appear to be in excess of that required to increase rates of fermentation.     

Active Glucose Transport and Proton Pumping in Tonoplast Membrane of Zea mays L. Coleoptiles Are Inhibited by Anti-H-ATPase Antibodies

A tonoplast enriched fraction was obtained from Zea mays L. coleoptiles by isopycnic centrifugation of microsomal membranes in a sucrose step gradient. At the 18/26% interface chloride-stimulated and nitrate-inhibited proton pumping activity coincided with a Mg²⁺-ATP dependent accumulation of 3-O-methyl-d-glucose (OMG) as determined by a membrane filtration technique using ¹⁴C-labeled substrate. OMG transport showed an apparently saturable component with a K_m of 110 micromolar, and was completely inhibited by 10 micromolar carbonyl cyanide m-chlorophenylhydrazone. Polyclonal antibodies against solubilized native tonoplast H⁺-ATPase and its 62 and 72 kilodalton subunits were assayed for their ability to inhibit proton pumping and OMG accumulation. Antibodies against both the native enzyme and the putative catalytic subunit (72 kilodalton) strongly inhibited proton pumping and OMG transport whereas antibodies against the 62 kilodalton subunit had only a slight effect on both processes.     

Calcium-induced sperm fusion in Zea mays L.

 In this study, we examined morphological changes of isolated maize (Zea mays L.) sperm cells in the presence of Brewbaker and Kwack salts (BKS) or the individual components of BKS using light, transmission electron and scanning electron microscopy. Freshly isolated sperms are 7.5 μm in diameter. Treatment with BKS for 5 h resulted in large cells with a diameter up to 41 μm. Staining of sperm nuclei with 4′, 6-diamidino-2-phenylindole (DAPI) revealed two or more nuclei in a single cell, suggesting that BKS induces cell fusion. Treatment with each BKS component showed that cell fusion occurs only in the presence of calcium nitrate. Use of several calcium salts showed the same results, suggesting that the calcium ion, alone, is responsible for the observed cell fusion. Further studies were conducted to examine the relationship between calcium distribution and sperm location in pollen tubes using chlorotetracycline and DAPI. Growing maize pollen tubes exhibited a high membrane calcium region within 20–50 μm from the tip. The Sperms are found no closer than 90 μm to the tip of the tube, suggesting that sperms are located in a low calcium region prior to being released to the degenerating synergid. Received: 12 August 1996 / Revision accepted: 6 December 1996     

Metabolic Adaption in Mature Roots of Salt-stressed Zea mays

Respiratory gas exchange and incorporation of ¹⁴C-leucine intoprotein were studied in proximal root segments from 25-day-oldmaize plants grown for the last ten days in 50 mM Na₂SO₄. ¹⁴C-leucineincorporation, and oxygen uptake in the presence of glucose,were as large in Na₂SO₄-grown tissues tested under saline conditionsas in tissues exposed to non-saline solutions throughout Thisadaptation was attributed to an increased metabolic capacityof Na₂SO₄-treated tissues, because these tissues, when returnedto non-saline solutions, evolved oxygen and incorporated ¹⁴C-leucinefaster than tissues exposed continuously to non-saline solutions. These changes are interpreted as a ‘compensation’for the inhibitory effects found when non-adapted tissues wereexposed to 50 mM Na₂SO₄. Moreover, we have related them to ultrastructuralchanges observed previously in xylem parenchyma cells of thesetissues, and to the possible involvement of these xylem parenchymacells in the re-absorption of sodium from the ascending xylemfluid Zea mays L., maize, salt-stress, respiration, protein synthesis