Sterol Accumulation and Composition in Developing Zea mays L. Kernels

Kernels were collected from three maize (Zea mays L.) inbreds from 10 days after pollination until kernel maturity. Sitosterol, campesterol, and stigmasterol were the major sterols at all stages of kernel development. Cholesterol was less than 1% of the dry weight. The three major sterols accumulated during kernel development, but at a rate slower than dry weight. The ratio of the sterols did not vary greatly among the inbreds. At maturity, the three inbreds, Wf9, Oh43, and Ky226, had sterol levels of 325, 228, and 173 micrograms per kernel, respectively. Sitosterol accounted for 75 to 85% of the sterol. The relative amount of stigmasterol decreased during the linear phase of development, while sitosterol increased in the free fraction and campesterol increased in the steryl ester fraction.     

Abscisic Acid Biosynthesis in Isolated Embryos of Zea mays L

Previous labeling experiments with ¹⁸O₂ have supported the hypothesis that stress-induced abscisic acid (ABA) is synthesized through an indirect pathway involving an oxygenated carotenoid (xanthophyll) as a precursor. To investigate ABA formation under nonstress conditions, an ¹⁸O₂ labeling experiment was conducted with isolated embryos from in vitro grown maize (Zea mays L.) kernels. Of the ABA produced during the incubation in ¹⁸O₂, three-fourths contained a single ¹⁸O atom located in the carboxyl group. Approximately one-fourth of the ABA synthesized during the experiment contained two ¹⁸O atoms. These results suggest that ABA synthesized in maize embryos under nonstress conditions also proceeds via the indirect pathway, requiring a xanthophyll precursor. It was also found that the newly synthesized ABA was preferentially released into the surrounding medium.     

Biosynthesis of 1,4-benzoxazin-3-ones in Zea mays

¹⁴C- and ¹⁵N-anthranilic acid are incorporated into the 1,4-benzoxazin-3-ones in maize seedling leaves with low dilution of the isotope. o-Aminophenol and 3-hydroxyanthranilic acid are not incorporated and are probably not intermediates. The cyclic hydroxamic acid and lactam members of the 1,4-benzoxazin-3-one group of compounds are readily interconverted.     

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.     

Concentration and Metabolic Turnover of Indoles in Germinating Kernels of Zea mays L

The amounts and rates of metabolic turnover of the indolylic compounds in germinating kernels of sweet corn were determined. Knowledge of pool size and rate of pool turnover has permitted: (a) identification of indole-3-acetyl-myo-inositol as the major chemical form for transport of indole-3-acetic acid (IAA) from endosperm to shoot; (b) demonstration that the free IAA of the endosperm is turning over rapidly with a half-life of 3.2 hours; (c) identification of esters of IAA as the immediate precursors of IAA in the endosperm and shoot; (d) demonstration that neither tryptophan nor tryptamine is a major precursor of IAA for the seed or shoot; (e) identification of IAA-myo-inositol glycosides as precursors of IAA-myo-inositol.     

Niacin Biosynthesis in Seedlings of Zea mays

Evidence obtained from incubation of corn (Zea mays cv. Golden Bantam) seedlings in dl-[benzene ring-U-(14)C]tryptophan, l-[5-(3)H]tryptophan, l-[U-(14)C]aspartate and [U-(14)C]glycerol indicates that niacin is synthesized in these plants via oxidative degradation of tryptophan. Aspartate and glycerol do not appear to be precursors of niacin in corn seedlings.     

Occurrence and in Vivo Biosynthesis of Indole-3-Butyric Acid in Corn (Zea mays L.)

Indole-3-butyric acid (IBA) was identified as an endogenous compound in leaves and roots of maize (Zea mays L.) var Inrakorn by thin layer chromatography, high-performance liquid chromatography, and gas chromatography-mass spectrometry. Its presence was also confirmed in the variety Hazera 224. Indole-3-acetic acid (IAA) was metabolized to IBA in vivo by seedlings of the two maize varieties. The reaction product was identified by thin layer chromatography, high performance liquid chromatography, and gas chromatography-mass spectrometry after incubating the corn seedlings with [¹⁴C]IAA and [¹³C₆]IAA. The in vivo conversion of IAA to IBA and the characteristics of IBA formation in two different maize varieties of Zea mays L. (Hazera 224 and Inrakorn) were investigated. IBA-forming activity was examined in the roots, leaves, and coleoptiles of both maize varieties. Whereas in the variety Hazera 224, IBA was formed mostly in the leaves, in the variety Inrakorn, IBA synthesis was detected in the roots as well as in the leaves. A time course study of IBA formation showed that maximum activity was reached in Inrakorn after 1 hour and in Hazera after 2 hours. The pH optimum for the uptake of IAA was 6.0, and that for IBA formation was 7.0. The K_m value for IBA formation was 17 micromolar for Inrakorn and 25 micromolar for Hazera 224. The results are discussed with respect to the possible functions of IBA in the plant.     

Changes in Endogenous Cytokinin Levels in Kernels of Zea mays L. during Imbibition and Germination

In order to assess the potential role of endogenous cytokininsin the germination of Zea mays L. caryopses, cytokinin activitywas determined in mature kernels and in kernels 1, 2, and 3d after imbibition. Cytokinin activity was also recorded inthe endosperm and embryo tissue of mature kernels and of kernels3 d after imbibition to establish the localization and changesin the levels of endogenous cytokinins during imbibition andearly germination. Using chromatographic, chemical, and enzymic techniques, compoundswith chromatographic properties resembling those of zeatin,zeatin riboside, zeatin glucoside, zeatin riboside glucoside,and their respective dihydro derivatives were detected in extractsfrom mature maize kernels. From analyses of the endosperm andembryo tissue at the two stages, it appears that the cytokininglucosides present in the endosperm are transported to the embryonicaxis for utilization in growth and development of the seedling.This concept is supported by the fact that the levels of ß-glucosidaseactivity detected were highest in the embryos, particularlyin the radicles, 3 d after imbibition.     

Biosynthesis of Cytokinin in Germinating Seeds of Zea mays

The embryos of germinating Zea mays seed were supplied with[¹⁴C]-adenine Following incubation, the tissue was extractedand extensively purified by non-exchange chromatography andthin layer chromatography. Radioactivity was found to be incorporatedinto zeatin nucleotide indicating that the embryo in the germinatingseed is capable of cytokinin biosynthesis. Key words: Zea mays cytokinin, zeatin nucleotide, biosynthesis, seed     

Biosynthesis of Indole-3-Acetic Acid from L-Tryptophan in Coleoptile Tips of Maize (Zea mays L.)

Coleoptile tips (about 2.5 mm in length) were excised from 3-day-olddark-adapted maize (Zea mays L.) seedlings and incubated indarkness in potassium phosphate buffer that contained ¹⁴C-L-tryptophan(Trp). Subsequent analysis by gas chromatography-mass spectrometryindicated that a significant portion of endogenous indole-3-aceticacid (IAA) had been labeled with ¹⁴C. About 8% of the IAA thatdiffused from the tissue into the medium during incubation from0.5 to 2 h was labeled, and 12% of the IAA extracted from thetissue after a 2-h incubation was labeled. On the other hand,30% of the Trp extracted from the tissue after a 2-h incubationwas ¹⁴C-Trp, which was more than those determined for IAA. Sincethe experiments were carried out under the non-steady-stateconditions in which the tissue content of ¹⁴C-Trp increasedwith time, and since the extracted Trp included the ¹⁴C-Trpin the apoplastic space, it seemed that synthesis de novo fromTrp was the major means by which IAA was produced in the coleoptiletip. The conversion of Trp to IAA was not detected in sub-apicalsegments (5–7.5 mm from the top) that were incubated similarly,an indication that synthesis of IAA occurs specifically in thetip region. When intact seedlings were irradiated with a pulseof red light 2 h before excision of tips and the applicationof ¹⁴C-Trp, the amounts of extractable and diffusible IAA werereduced by 40–60% without a change in the rate of ¹⁴Cincorporation. This result indicated that the production ofIAA from Trp is controlled by a red-light signal. (Received May 15, 1995; Accepted September 1, 1995)     

The Effect of Morphactin on the Kinetics of Indol-3yl-acetic Acid-2-14C Transport in Zea mays L. Coleoptile Segments

This paper elucidates the effects of chloroflurenol (morphactin,IT 3456) treatment on the kinetics of ¹⁴C-IAA transport throughZea mays L. (cv. Orla-266) coleoptile segments. Maximum inhibitionof transport was achieved when chloroflurenol remained in contactwith the tissue segments for at least 20 min or more. The treatment,without materially affecting the ¹⁴C-IAA-transport polarity,or uptake, significantly reduced the velocity from 20.5 mm h^–1to 8.79 mm h^–1 and also the intensity from 919 (ct/min)h^–1 to 413 (ct/min) h^–1.     

Lateral Root Development in Attached and Excised Roots of Zea mays L. Cultivated in the Presence or Absence of Indol-3-yl Acetic Acid

The initiation of lateral root primordia and their subsequentemergence as secondary roots have been examined in attachedand excised roots of Zea mays grown in the presence or absenceof indol-3-yl acetic acid (IAA). Exposure to IAA enhanced anlageinception in both batches of roots. In the attached roots, theIAA-induced stimulation of primordium initiation was followedby a similar increase in lateral emergence. IAA treatment, however,had no effect on the number of laterals produced, per centimetreof root, in the excised primaries. Thus, exposure to IAA didnot directly enhance lateral emergence in the attached rootsnor did it stimulate such emergence in the excised ones. Nocorrelation was found between proliferative activity in themeristem at the apex of the primary or the rate of root elongationon the one hand, and either the number of primordia initiated,or the number of laterals produced, per centimetre of primary,on the other. Zea mays, maize, root, primordium, lateral, indol-3-yl acetic acid, meristematic activity     

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.     

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     

Glyceraldehyde-3-Phosphate Dehydrogenase in Greening Zea mays L. Leaves

Nicotinamide adenine dinucleotide phosphate (NADP)-dependent glyceraldehyde-3-phosphate dehydrogenase (GPDH) (EC 1.2.1.13), a chloroplast enzyme, had low activity in etioplasts of maize leaves. A light dependent increase of enzyme activity of 7-day-old etiolated seedlings showed a lag period of about 2.5 hours followed by a rapid increase in activity during the next 10 hours. The chlorophyll content followed a similar pattern of increasing concentration, but its formation was not directly related to NADP-GPDH formation. The specific activity of NADP-GPDH was lowest in the morphologically youngest tissue near the base of the lamina. The increase in NADP-GPDH was inhibited by cycloheximide but not by chloramphenicol. This indicates that at least some of the enzyme polypeptides are synthesized by 80S ribosomes in the cytoplasm, transported into chloroplasts and become active in chloroplasts. In etiolated maize shoots subjected to a combination of both 3-(p-chlorophenyl)-1,1-dimethylurea, monuron at 7 x 10(-5)m and far red light treatment for 15 hours, the NADP-GPDH activity increased 42% over the dark control compared to 70% increase for the light control. It is concluded that NADPH is not absolutely required for the activation of NADP-GPDH in maize leaves under physiological conditions.     

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     

Expression of the Acc1 Gene-Encoded Acetyl-Coenzyme A Carboxylase in Developing Maize (Zea mays L.) Kernels

A mutation (Acc1-S2) in the structural gene for maize (Zea mays L.) acetyl-coenzyme A carboxylase (ACCase) that significantly reduces sethoxydim inhibition of leaf ACCase activity was used to investigate the gene-enzyme relationship regulating ACCase activity during oil deposition in developing kernels. Mutant embryo and endosperm ACCase activities were more than 600-fold less sensitive to sethoxydim inhibition than ACCase in wild-type kernel tissues. Moreover, in vitro cultured mutant kernels developed normally in the presence of sethoxydim concentrations that inhibited wild-type kernel development. The results indicate that the Acc1-encoded ACCase accounts for the majority of ACCase activity in developing maize kernels, suggesting that Acc1-encoded ACCase functions not only during membrane biogenesis in leaves but is also the predominant form of ACCase involved in storage lipid biosynthesis in maize embryos.     

Epistasis in maize (Zea mays L.)

Summary Three flint and three dent maize (Zea mays L.) inbred lines, their possible F₁ crosses, F₂ and backcross progenies, and all possible three-way crosses were evaluated in a three-year experiment for yield, ear moisture, and plant height. The purpose was to estimate genetic parameters in European breeding materials from (i) generation means analysis, (ii) diallel analysis of generation means, and (iii) analysis of F₁ and three-way cross hybrids. Method (i) was based on the F-metric model and methods (ii) and (iii) on the Eberhart-Gardner (1966) genetic model; both models extended for heterotic maternal effects.Differences among generation means for yield and plant height were mainly attributable to dominance effects. Epistatic effects were significantly different from zero in a few crosses and considerably reduced heterosis in both traits. Additive x additive and domiance x dominance effects for yield were consistently positive and negative, respectively. Significant maternal effects were established to the advantage of generations with a heterozygous seed parent. In the diallel analysis, mean squares for dominance effects were greater than for additive effects for yield and plant height but smaller for ear moisture. Though significant for yield and plant height, epistatic variation was small compared to additive and dominance variation. Estimates of additive x additive epistasis for yield were significantly negative in 11 of 15 crosses, suggesting that advantageous gene combinations in the lines had been disrupted by recombination in the segregating generations. The analysis of hybrids supported the above findings regarding the analysis of variance. However, the estimates of additive x additive epistasis for yield were considerably smaller and only minimally correlated with those from the diallel analysis. Use of noninbred materials as opposed to materials with different levels of inbreeding is considered the main reason for the discrepancies in the results.     

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     

Movement of C-Labeled Assimilates into Kernels of Zea mays L: I. Pattern and Rate of Sugar Movement

Carbon-14, photosynthetically fixed in leaves of Zea mays L. and translocated to developing kernels, passed through specialized basal endosperm cells prior to movement into the starchy endosperm and embryo. Radioactivity migrated in the endosperm at a maximum rate of 2.7 millimeters per hour, and there was no difference in the rate of movement in kernels treated 14 to 30 days after pollination.