Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

Sugar release from the pedicel tissue of maize (Zea mays L.) kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of ¹⁴C-assimilate movement into treated versus intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 hours and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors, and solutes in order to characterize sugar unloading from this tissue.

Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 millimolar CaCl₂. Unloading was stimulated by 15 millimolar ethyleneglycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid which partially reversed the inhibitory effects of Ca²⁺. Based on these results, we suggest that passive efflux of sucrose occurs from the maize pedicel symplast followed by extracellular hydrolysis to hexoses.

     

Role of Exogenous Glutathione in Alleviating Abiotic Stress in Maize (Zea mays L.)

Journal of Plant Growth Regulation - The role of exogenous GSH in improving abiotic stress tolerance in maize was investigated in this study. GSH-treated plants showed significantly higher...     

Effect of Exogenous Abscisic Acid on Proline Dehydrogenase Activity in Maize (Zea mays L.)

Plant responses to drought stress include proline and abscisic acid (ABA) accumulation. Proline dehydrogenase (PDH) (EC 1.4.3) is the first enzyme in the proline oxidation pathway, and its activity has been shown to decline in response to water stress (PJ Rayapati, CR Stewart [1991] Plant Physiol 95: 787-791). In this investigation, we determined whether ABA treatment affects PDH activity in a manner similar to drought stress in maize (Zea mays L.) seedlings. Four exogenous ABA treatments (0, 11, 33, and 100 micromolar ABA) were applied to well-watered maize seedlings. Mitochondria were isolated and PDH was solubilized using Nonidet P-40. PDH activity was measured by the reduction of iodonitrotetrazolium violet under proline-dependent conditions. There was no effect of ABA on PDH activity at 33 and 100 micromolar ABA, but there was a 38% decline at 11 micromolar. This decline was less than the 69% reduction in activity under drought stress. Endogenous ABA determinations and plant growth rate showed that ABA entered the plant and was affecting metabolic processes. ABA treatments had a small effect on shoot and root proline concentration, whereas drought stress caused a 220% increase in root tissues. We conclude that ABA is not part of the pathway linking drought stress and decreased PDH activity.     

Electrotropism of Maize (Zea mays L.) Roots (Facts and Artifacts)

Intact and decapped primary roots of maize (Zea mays L.) were exposed to DC electric fields of 0.5 to 8.0 V/cm in low-salinity media to resolve conflicting results about the direction of electrotropism. In DC fields of 0.5 V/cm or 1.0 V/cm, intact roots always curved toward the cathode. In a field of 8.0 V/cm, intact roots curved toward the anode and stopped growth. Decapped roots also curved toward the anode both in weak and strong fields. The results indicate that growth toward the cathode is the true response of healthy roots.     

Fertility of Deep-frozen Maize (Zea mays L.) Pollen

On average, 50 per cent of maize pollen grains can be kept viableand almost 30 per cent remain fertile for up to a year whenthe water content on a fresh weight basis is reduced to about30 per cent of the original and the pollen is stored at –76or –196 °C. Over this period no significant differencewas found between storage at either temperature. Zea mays L., maize, pollen, fertility, viability     

Sources, Fluxes, and Sinks of Nitrogen during Early Reproductive Growth of Maize (Zea mays L.)

A study was designed to (a) identify sources and sinks of N in the maize (Zea mays L.) shoot, by estimating net N fluxes for each of seven parts of the shoot, (b) determine effects of N entering the plant upon fluxes of N absorbed before reproductive growth, and (c) determine the effects of the opaque-2 gene on N fluxes in the maize shoot during early reproductive growth. Plants of a maize hybrid (Pioneer 3369A) and its opaque-2 counterpart (Pioneer L3369) were grown in a greenhouse using nutrient solution/sand culture, with NO₃⁻ as the N source during the vegetative growth phase. Beginning at the time of pollination, the same nutrient regime was continued, except that some plants received no N, and others received 3.75 millimolar ¹⁵N as NO₃⁻-N.

Stalk and leaves were found to be primary N sources for the grain, while shank, husk, and cob acted first as N sinks, then as N sources during reproductive growth. Net fluxes of N for each plant part were estimated by calculating the first derivatives of regression equations used to fit data for N contents of each plant part as functions of time. All parts of the shoot were sinks for exogenous N (absorbed after pollination). Thirty-six days after pollination, the grain contained 60% endogenous N (absorbed before pollination) when 3.75 millimolar NO₃⁻-N was supplied after pollination. Rates of total N influx to the grain were identical whether or not N was supplied in the nutrient solution during reproductive growth. At 36 days after pollination, less N had accumulated in the grain of the opaque-2 genotype, but otherwise there were no differences in N contents or dry weights of the shoots due to the opaque-2 gene. Absence of N from the rooting medium significantly affected N fluxes throughout the shoot during reproductive growth, but there were no detectable effects of the opaque-2 gene on N fluxes in parts of the plant other than the grain.

     

Endopolyploidy in Diploid and Tetraploid Maize (Zea mays L.)

Nuclei from different tissues such as stem, mesocotyl, nodalroot and root tip of diploid and tetraploid maize were isolated,stained with propidium iodide and passed through an EPICS-751flow-cytometer cell sorter. Variations in flow histograms wereobserved in different tissues. Stem tissues of both the diploidand tetraploid had two peaks representing G1 and G2 somaticnuclei. The remaining tissues in both the diploids and tetraploidsexhibited three peaks. The first peak observed in these tissuesrepresents G1 somatic nuclei of the lowest ploidy level. Thesecond peak represent G2 somatic nuclei of the lowest ploidylevel+G1 somatic nuclei of the next ploidy level. The thirdpeak represents G2 of the higher ploidy level+G1 somatic nucleiof the next higher ploidy level. Statistically significant differenceswere observed between the diploid and tetraploid maize tissueswith respect to nuclei distribution in the higher ploidy levelpeaks implying variation in the degree of endopolyploidy inthe diploid and tetraploid maize. The results of this studysuggest that the amount of endopolyploid observed in maize genotypeshas an effect on their overall agronomic performance under thefield conditions.Copyright 1993, 1999 Academic Press Zea mays L., maize, endopolyploidy, diploid, tetraploid, flow cytometry     

Purification and Partial Characterization of Maize (Zea mays L.) beta-Glucosidase

Maize (Zea mays L.) β-glucosidase (β-d-glucoside glucohydrolase, EC 3.2.1.21) was extracted from the coleoptiles of 5- to 6-day-old maize seedlings with 50 millimolar sodium acetate, pH 5.0. The pH of the extract was adjusted to 4.6, and most of the contaminating proteins were cryoprecipitated at 0°C for 24 hours. The pH 4.6 supernatant from cryoprecipitation was further fractionated by chromatography on an Accell CM column using a 4.8 to 6.8 pH gradient of 50 millimolar sodium acetate, which yielded the enzyme in two homogeneous, chromatographically different fractions. Purified enzyme was characterized with respect to subunit molecular weight, isoelectric point, amino acid composition, NH₂-terminal amino acid sequence, pH and temperature optima, thermostability, and activity and stability in the presence of selected reducing agents, metal ions, and alkylating agents. The purified enzyme has an estimated subunit molecular mass of 60 kilodaltons, isoelectric point at pH 5.2, and pH and temperature optima at 5.8 and 50°C, respectively. The amino acid composition data indicate that the enzyme is rich in Glx and Asx, the sum of which approaches 25%. The sequence of the first 20 amino acids in the N-terminal region was H₂N-Ser-Ala-Arg-Val-Gly-Ser-Gln-Asn-Gly-Val-Gln-Met-Leu-Ser-Pro-(Ser?) -Glu-Ile-Pro-Gln, and it shows no significant similarity to other proteins with known sequence. The enzyme is extremely stable at 0 to 4°C up to 1 year but loses activity completely at and above 55°C in 10 minutes. Likewise, the enzyme is stable in the presence of or after treatment with 500 millimolar 2-mercaptoethanol, and it is totally inactivated at 2000 millimolar 2-mercaptoethanol. Such metal ions as Hg²⁺ and Ag⁺ reversibly inhibit the enzyme at micromolar concentrations, and inhibition could be completely overcome by adding 2-mercaptoethanol at molar excess of the inhibitory metal ion. The alkylating agents iodoacetic acid and iodoacetamide irreversibly inactivate the enzyme and such inactivation is accelerated in the presence of urea.     

Nonadditive protein accumulation patterns in Maize (Zea mays L.) hybrids during embryo development

Heterosis describes the superior performance of heterozygous F(1)-hybrid plants compared to their homozygous parental inbred lines. In the present study, heterosis was detected for length, weight, and the time point of seminal root primordia initiation in maize (Zea mays L.) embryos of the reciprocal F(1)-hybrids UH005xUH250 and UH250xUH005. A two-dimensional gel electrophoresis (2-DE) proteome survey of the most abundant proteins of the reciprocal hybrids and their parental inbred lines 25 and 35 days after pollination revealed that 141 of 597 detected proteins (24%) exhibited nonadditive accumulation in at least one hybrid. Approximately 44% of all nonadditively accumulated proteins displayed an expression pattern that was not distinguishable from the low parent value. Electrospray ionization-tandem mass spectrometry (ESI-MS/MS) analyses and subsequent functional classification of the 141 proteins revealed that development, protein metabolism, redox-regulation, glycolysis, and amino acid metabolism were the most prominent functional classes among nonadditively accumulated proteins. In 35-day-old embryos of the hybrid UH250xUH005, a significant up-regulation of enzymes related to glucose metabolism which often exceeded the best parent values was observed. A comparison of nonadditive protein accumulation between rice and maize embryo data sets revealed a significant overlap of nonadditively accumulated proteins suggesting conserved organ- or tissue-specific regulatory mechanisms in monocots related to heterosis.     

Chlorophyll Synthesis from Protochlorophyll(ide) in Chill-Stressed Maize (Zea mays L.)

Illumination of aetiolated maize at temperatures lower than20 °C results in negligible accumulation of chlorophyll.Illumination of leaf tissue, previously incubated in 10 molm^–3 ALA in darkness, shows only a slight conversion ofprotochlorophyll(ide) to chlorophyll a and b at temperaturesless than 20 °C. A refined procedure for measuring photosynthesisby photo-acoustic spectroscopy in leaves that differ in chlorophyllcontent is presented. Studies of photosynthesis in aetiolatedseedlings illuminated at different temperatures by photo-acousticspectroscopy suggests that impairment of the chlorophyll pathwayis paralleled by an aberrant development of the thylakoid membrane. Key words: Protochlorophyll(ide), temperature, photo-acoustic spectroscopy, membrane biogenesis     

Acetolactate Synthase Activity in Developing Maize (Zea mays L.) Kernels

Acetolactate synthase (EC 4.1.3.18) activity was examined in maize (Zea mays L.) endosperm and embryos as a function of kernel development. When assayed using unpurified homogenates, embryo acetolactate synthase activity appeared less sensitive to inhibition by leucine + valine and by the imidazolinone herbicide imazapyr than endosperm acetolactate synthase activity. Evidence is presented to show that pyruvate decarboxylase contributes to apparent acetolactate synthase activity in crude embryo extracts and a modification of the acetolactate synthase assay is proposed to correct for the presence of pyruvate decarboxylase in unpurified plant homogenates. Endosperm acetolactate synthase activity increased rapidly during early kernel development, reaching a maximum of 3 micromoles acetoin per hour per endosperm at 25 days after pollination. In contrast, embryo activity was low in young kernels and steadily increased throughout development to a maximum activity of 0.24 micromole per hour per embryo by 45 days after pollination. The sensitivity of both endosperm and embryo acetolactate synthase activities to feedback inhibition by leucine + valine did not change during kernel development. The results are compared to those found for other enzymes of nitrogen metabolism and discussed with respect to the potential roles of the embryo and endosperm in providing amino acids for storage protein synthesis.     

Further Studies on Mosaic Disease of Maize (Zea mays L.)

Schizaphis graminum (Rondani) is proved to be an additional vector of maize mosaic virus (MMV). The pH range for the infectivity of the virus in extracted juice is found to be from 4.4 to 9.0, the optimum being 5.6 to 7.2. Effect of certain chemicals on the virusin vitro has also been studied. Cross protection between MMV and Sugar-cane mosaic virus (SMV) indicated positive results. It has been concluded on the basis of similar physical properties, tolerance towards certain chemicals, host range, symptomatology, aphid vectors and positive immunological tests, that MMV and SMV are related viruses.     

Development of Endopeptidase Activities in Maize (Zea mays L.) Endosperms

An activity stain was used after native polyacrylamide gel electrophoresis, and at least 17 different endopeptidase activities were detected in maize (Zea mays L.) endosperm extracts prepared during the first 6 d after imbibition. The enzymes detected were classified into four groups based on their time of appearance and on their mobility in polyacrylamide gels. The first group, which included two enzymes present in dry endosperms, disappeared soon after imbibition. The second group, comprising five activity bands, appeared during the first 2 to 3 d after imbibition and then disappeared. The third set of enzymes increased continuously throughout the experimental period. The fourth group appeared after d 3 and remained at a constant level after that time. The endopeptidase activities were characterized by the effect of specific inhibitors on their activities. The two enzymes of the first group are metalloendopeptidases based on their sensitivity to ethylenediaminetetracetate (EDTA). Enzymes of the second, third, and fourth groups are sulfhydryl-endopeptidases as judged by their sensitivity to antipain, chymostatin, leupeptin, and E-64 and by their requirement for 2-mercaptoethanol. Pepstatin, phenylmethylsulfonyl fluoride, or EDTA had no effect on these enzymes. Many of the second, third, and fourth group enzymes cleaved [alpha]-zein-rich proteins as well as such easily obtained proteins as gelatin (used in our standard assay) and hemoglobin. The second group had a high affinity for [gamma]-zein, whereas none of the bands in the fourth group of enzymes cleaved this type of zein. The two metalloenzymes of the first group cleaved neither [alpha]- nor [gamma]-zeins.     

Effects of Moniliformin on Mitosis in Maize (Zea mays L.)

Maize (Zea mays L. ‘Norfolk White’) roots were treatedwith solutions of moniliformin (a metabolite of Fusarium moniliformeSheldon) at 0.0001 M and 0.001 M for 8, 24, and 48 h. Only aslight inhibition of division was noted in root tips treatedwith the lower concentration. The higher concentration causeda disruption of the spindle apparatus and consequent C-mitosis,and metaphase accumulation. (Received February 14, 1984; Accepted May 18, 1984)     

不同基因型玉米间作的群体质量

采用大田试验,研究了不同基因型玉米间作的群体质量特征.结果表明,HF9‖XD20间作,植株中部叶片平均叶龄延长,而对下部和上部叶片影响不大;ZD958‖LD981间作,ZD958植株下部叶片平均叶龄延长,而中、上部叶片则缩短,LD981植株下、中、上部叶片平均叶龄均有所延长.吐丝前,群体叶面积指数(LAI)单间作无明显差异,吐丝后,HF9和LD981的LAI分别大于和显著大于单作群体,而ZD958和XD20则分别小于和显著小于单作群体.紧凑型品种和半紧凑型品种间作增加了群体透光率,吐丝后10d,4个品种棒三叶叶色值(SPADR)均有所增加,并且除ZD958外,其余3个品种棒三叶净光合速率均有所增加,其中LD981增加显著.间作对吐丝以前的群体干物质积累量影响不大,吐丝后,半紧凑型品种(HF9和LD981)的干物质积累量增加,其中LD981增加显著,而紧凑型品种(XD20和ZD958)的干物质积累量减少,其中ZD958显著减少;间作还提高了收获指数,并且两种间作群体的土地当量比(LER)均大于1.结果提示,紧凑型与半紧凑型玉米品种的间作可以提高群体质量,延长叶片功能期,提高光合效率,增加籽粒产量.     

Light-dependent Proton Excretion of Wheat (Triticum aestivum L.) and Maize (Zea mays L.) Roots

We investigated the change of root net proton excretion of seedlings of Triticum aestivum L. and Zea mays L. with daily variation of illumination using a multi-channel pH-stat system. We found an increase of net proton excretion during darkness and a drop after the beginning of illumination. Inhibition of carotenoid biosynthesis by norflurazone and photooxidation of chlorophylls did not change the periodicity or its induction. The induction of diurnal periodicity was possible with blue, green and red light. After induction the oscillation of net proton excretion continued for at least two cycles under constant light. We conclude that net H⁺ excretion of wheat and maize roots may be regulated by an endogenous clock or by a signal from the leaves. The nature of such a hypothetical signal remains unknown.     

Concurrent Measurements of Oxygen and Carbon Dioxide Exchange during Lightflecks in Maize (Zea mays L.)

Leaves of maize (Zea mays L.) were enclosed in a temperature-controlled cuvette under 35 Pa (350 [mu]bars) CO2 and 0.2 kPa (0.2%)O2 and exposed to short periods (1-30 s) of illumination (light-flecks). The rate and total amount of CO2 assimilated and O2 evolved were measured. The O2 evolution rate was taken as an indicator of the rate of photosynthetic noncyclic electron transport (NCET). In this C4 species, the response of electron transport during the lightflecks qualitatively mimicked that of C3 species previously tested, whereas the response of CO2 assimilation differed. Under short-duration lightflecks at high photon flux density (PFD), the mean rate of O2 evolution was greater than the steady-state rate of O2 evolution under the same PFD due to a burst of O2 evolution at the beginning of the lightfleck. This O2 burst was taken as indicating a high level of NCET involved in the buildup of assimilatory charge via ATP, NADPH, and reduced or phosphorylated metabolites. However, as lightfleck duration decreased, the amount of CO2 assimilated per unit time of the lightfleck (the mean rate of CO2 assimilation) decreased. There was also a burst of CO2 from the leaf at the beginning of low-PFD lightflecks that further reduced the assimilation during these lightflecks. The results are discussed in terms of the buildup of assimilatory charge through the synthesis of high-energy metabolites specific to C4 metabolism. It is speculated that the inefficiency of carbon uptake during brief light transients in the C4 species, relative to C3 species, is due to the futile synthesis of C4 cycle intermediates.