Turnover and distribution of root exudates of Zea mays

Decomposition and distribution of root exudates of Zea mays L. were studied by means of ¹⁴CO₂ pulse labeling of shoots on a loamy Haplic Luvisol. Plants were grown in two-compartment pots, where the lower part was separated from the roots by monofilament gauze. Root hairs, but not roots, penetrated through the gauze into the lower part of the soil. The root-free soil in the lower compartment was either sterilized with cycloheximide and streptomycin or remained non-sterile. In order to investigate exudate distribution, 3 days after the ¹⁴C labeling, the lower soil part was frozen and sliced into 15, one-mm thick layers using a microtome. Cumulative ¹⁴CO₂ efflux from the soil during the first 3 days after ¹⁴C pulse labeling did not change during plant growth and amounted to about 13–20% of the total recovered ¹⁴C (41–55% of the carbon translocated below ground). Nighttime rate of total CO₂ efflux was 1.5 times lower than during daytime because of tight coupling of exudation with photosynthesis intensity. The average CO₂ efflux from the soil with Zea mays was about 74 g C g^–1 day^–1 (22 g C m^–2 day^–1), although, the contribution of plant roots to the total CO₂ efflux from the soil was about 78%, and only 22% was respired from the soil organic matter. Zea mays transferred about 4 g m^–2 of carbon under ground during 26 days of growth. Three zones of exudate concentrations were identified from the distribution of the ¹⁴C-activity in rhizosphere profiles after two labeling periods: (1) 1–2 (3) mm (maximal concentration of exudates) 2) 3–5 mm (presence of exudates is caused by their diffusion from the zone 1); (3) 6–10 mm (very insignificant amounts of exudates diffused from the previous zones). At the distance further than 10 mm no exudates were found. The calculated coefficient of exudate diffusion in the soil was 1.9 × 10^–7 cm² s^–1.     

Mucilage strongly binds aluminum but does not prevent roots from aluminum injury in Zea mays

The possible role of mucilage in protecting roots from aluminum (Al) injury was investigated in Zea mays L. (cv. Golden Cross Bantam), focusing on binding of Al with mucilage and effects of mucilage on Al toxicity. Al was bound to mucilage after the treatment of roots with 10–50 μ M Al for 1 h and 30 μ M Al for 30, 60, 90 and 120 min. Using molecular sieve chromatography (Sephadex G-100), Al was co-eluted with a high molecular mass sugar and a low molecular mass sugar. The difficulty in desorbing Al from mucilage with organic acids confirmed the strong binding strength of Al by mucilage revealed by ²⁷Al nuclear magnetic resonance (NMR). Al could not be desorbed completely by succinic, malic, oxalic and citric acid at a molar ratio of 1:1. It could only be completely removed by oxalic acid at a molar ratio of 20:1 (oxalate:Al). Bioassay experiments showed that cell viability and callose formation were unaffected by Al bound to mucilage. However, mucilage deprived roots had only 0.21–0.59 nmol apex⁻¹ higher Al content than control roots after treatment with 30 μ M Al for 1, 1.5 and 2 h. Moreover, inhibition of root elongation by 5 μ M Al for 6, 12, 24 and 36 h was independent of the presence or absence of mucilage prior to the Al treatment. These results indicate that although mucilage affects the accumulation of Al by roots, it does not confer Al resistance to Z. mays root apices.     

Determination of stress responses induced by aluminum in maize (Zea mays)

To assess the alternative responses to aluminum toxicity, maize (Zea mays L. cv Karadeniz y?ld?z?) roots were exposed to different concentrations of AlCl3 (150, 300 and 450 μM). Aluminum reduced the root elongation by 39.6% in 150 μM, 44.1% in 300 μM, 50.1% in 450 μM AlCl3 after 96 h period. To correlate the root elongation with the alternative stress responses including aluminum accumulation, lipid peroxidation, mitotic abnormalities, reduction of starch content, intracellular Ca2+ accumulation, callose formation, lignin deposition and peroxidase activity, cytochemical and biochemical tests were performed. The results indicated that aluminum accumulation and lipid peroxidation were observed more densely on the root cap and the outer cortex cells. In addition to morphological deformations, cytochemical analysis displayed cellular deformations. Furthermore, mitotic abnormalities were observed such as c-mitosis, micronuclei, bi- and trinucleated cells in aluminum treated root tips. Aluminum treatment induced starch reduction, callose formation, lignin accumulation and intracellular Ca2+ increase. Moreover, the peroxidase activity increased significantly by 3, 4.4 and 7.7 times higher than in that of control after 96 h, respectively. In conclusion, aluminum is significantly stressful in maize culminating in morphological and cellular alterations.     

Site-specific recombination in Zea mays

The elimination of marker genes after selection is recommended for the commercial use of genetically modified plants. We compared the applicability of the two site-specific recombination systems Cre/lox and Flp/FRT for marker gene elimination in maize plants. The selection marker gene pat surrounded by two identically directed lox or FRT sites was introduced into maize. Sexual crossing with plants harboring the corresponding constitutively expressed recombinase led to the precise and complete excision of the lox-flanked marker gene in the F₁ progeny, whereas Flp-mediated recombination of FRT sequences occurred rarely. Further examination of site-specific integration was done by biolistic bombardment of immature embryos harboring only one lox site with a lox.uidA sequence with results indicating directed integration.     

Cytokinins from Zea mays

A number of adenine derivatives with cytokinin activity were isolated from immature sweet corn (Zea mays) kernels. The following structures were assigned: 9-β-d-ribofuranosylzeatin, 9-β-d-ribofuranosylzeatin 5′-monophosphate, 6-(1-carboxy-2-hydroxypropylamino)-9-ribofuranosylpurine, 6-(2,3,4-trihydroxy-3-methylbutylamino)purine, 2-hydroxy-6-(4-hydroxy-3-methylbut-trans-2-enylamino)purine, 6-(3,4-dihydroxy-3-methylbutylamino)purine, a 9-glycoside of zeatin(identity of sugar moiety not established), and 6-(1,2-dicarboxyethylamino)-9-β-d-ribofuranosylpurine.     

Characterization and distribution of invertase activity in developing maize (Zea mays) kernels

Invertase ( β -fructofuranoside fructohydrolase, EC 3.2.1.26) activity in developing maize ( Zea mays L. inbred W64A) was separated into soluble and particulate forms. The particulate form was solubilized by treatment with 1 M NaCl or with other salts. However, CaCl₂ inhibited invertase activity, and neither detergents nor 0.5 M methyl mannoside were effective in solubilizing the invertase activity. The soluble and particulate invertases were both glycoproteins, both had pH optima of 5.0 and K_m values for sucrose of 2.83 and 1.84 m M , respectively. The apparent molecular weight of salt-solubilized invertase was 40 kDa. Gel filtration of the soluble invertase showed multiple peaks with apparent molecular weights ranging from 750 kDa to over 9 000 kDa. Histochemical staining of cell wall preparations for invertase activity suggested that the particulate invertase is associated with the cell wall. Also, nearly all the invertase activity was localized in the basal endosperm and pedicel tissues, which are sites of sugar transport. No invertase activity was found in the upper endosperm, the embryo or in the placento-chalazal tissue. In contrast, sucrose synthase (EC 2.4.1.13) activity was found primarily in the embryo and the upper endosperm, which are areas of active biosynthesis of storage compounds.     

Occurrence of sorbitol in Zea mays

The identity of sorbitol (d-glucitol) from maize seeds was confirmed by GC/MS of the TMSi-ether and by co-chromatography with authentic sorbitol. Sorbitol was found in seeds and silks but not in pollen or leaves. Both endosperm and embryo contained sorbitol, but endosperm accounted for most of the sorbitol recovered from intact seeds.     

Gibberellins and geotropism in Zea mays coleoptiles

Summary Diffusible gibberellins were obtainable in agar from excised 4 mm tips of etiolated coleoptiles of Zea mays. Placing the tips in a horizontal position increased the total yield of gibberellins by approximately five times. With horizontal tips, the ratio of gibberellin activity recovered from lower and upper halves, expressed as a percentage of total yield, was 80.76:19.24 (lower:upper).     

Interchromosomal recombination in Zea mays.

A new allele of the 27-kD zein locus in maize has been generated by interchromosomal recombination between chromosomes of two different inbred lines. A continuous patch of at least 11,817 bp of inbred W64A, containing the previously characterized Ra allele of the 27-kD zein gene, has been inserted into the genome of A188 by a single crossover. While both junction sequences are conserved, sequences of the two homologs between these junctions differ considerably. W64A contains the 7313-bp-long retrotransposon, Zeon-1. A188 contains a second copy of the 27-kD zein gene and a 2-kb repetitive element. Therefore, recombination results in a 7.3-kb insertion and a 14-kb deletion compared to the original S+A188 allele. If nonpairing sequences are looped out, 206 single base changes, frequently clustered, are present. The structure of this allele may explain how a recently discovered example of somatic recombination occurred in an A188/W64A hybrid. This would indicate that despite these sequence differences, pairing between these alleles could occur early during plant development. Therefore, such a somatically derived chimeric chromosome can also be heritable and give rise to new alleles.     

Flavonoids from Zea mays pollen

The flavonol glycosides of quercetin, isorhamnetin and kaempferol were isolated from Zea mays pollen. The most prominent flavonols were diglycosides of quercetin and isorhamnetin. Flavonol 3-O-glucosides of quercetin, isorhamnetin and kaempferol, and triglucosides of quercetin and isorhamnetin, were minor components. The flavonoid pattern of maize pollen is characterized by the accumulation of quercetin and isorhamnetin diglycosides and by the absence of flavones, which are common in other maize tissues.     

Genetic and Genomic Toolbox of Zea mays

Maize has a long history of genetic and genomic tool development and is considered one of the most accessible higher plant systems. With a fully sequenced genome, a suite of cytogenetic tools, methods for both forward and reverse genetics, and characterized phenotype markers, maize is amenable to studying questions beyond plant biology. Major discoveries in the areas of transposons, imprinting, and chromosome biology came from work in maize. Moving forward in the post-genomic era, this classic model system will continue to be at the forefront of basic biological study. In this review, we outline the basics of working with maize and describe its rich genetic toolbox.     

Subcellular distribution of alanine aminotransferase activity in maize (Zea mays L.) leaves

The intracellular distribution of alanine aminotransferase (AlaAT, EC 2.6.1.2) activity with L-alanine and 2-oxoglutarate as a substrates in maize whole leaf extract and bundle sheath cells was studied. After isolation of the mitochondrial-peroxisomal fraction, mitochondria and peroxisomes were separated by centrifugation on a linear 40–52 % (w/w) sucrose gradient. L-Alanine-2-oxoglutarate transaminating activity of whole leaf extract showed two peaks: first distinctly higher associated with mitochondria and second lower with peroxisomes. In bundle sheath cells only one peak of this activity was found. It corresponded to the mitochondrial region of the gradient. It is proposed that mitochondrial L-alanine — 2-oxoglutarate activity was brought about by AlaAT. Glycine aminotransferase (EC 2.6.1.4) could be responsible for the same activity in peroxisomes. This work was supported by the State Committee for Scientific Research, a grant No. 5PO6A00510     

Glucolipids of Zea mays and Pisum sativum

The glucolipids formed upon feeding (U-¹⁴C)glucose to embryos of Zea mays were partially characterized with respect to: (a) metabolic turnover, (b) acid lability, (c) phosphorus content, (d) chromatographic properties, and (e) hydrolysis products. The chloroform-methanol-soluble assimilated radioactivity was examined specifically for occurrence of a glycosylated prenol phosphate. With the extraction conditions used, no evidence was found for formation of a glucosylated prenol phosphate. Several, as yet unidentified, acid-labile glucolipids undergoing metabolic turnover were observed. Four diglycerides were characterized as hydrolysis products of a fraction that contained ¹⁴C-glucose and phosphorus, and was subject to metabolic turnover. Examination of the 1-butanol-soluble glucolipids from pea (Pisum sativum) seedlings also demonstrated anionic glucolipids, evidencing metabolic turnover but none with the properties of glucosylated prenol phosphate.     

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.     

Sucrose-udp glucosyltransferase of Zea mays endosperm

The synthetic and degradative activities toward sucrose of maize (Zea mays L.) endosperm sucrose-UDP glucosyltransferase preparations behave differently in several respects. Mg²⁺ or Ca²⁺ stimulate the synthetic activity but inhibit the degradative activity. Nueleotides have no effect on the synthetic activity but inhibit the degradative activity. The two activities have different pH optima, and ATP inhibits the degradative activity across the pH range tested. However, both activities exhibit identical patterns of heat inactivation, and various purification procedures employed have failed to separate these two activities. The K_m values at pH 6.5 (degradation) and pH 8 (synthesis) are sucrose, 40 mM; UDP, 0.14 mM; ADP, 1,25 mM; UDPglucose, 1. 14 rnM; and fructose, 2.08 mM. In the developing endosperm, sucrose-6-P synthetase activity is only ca 1 % of the synthetic activity of sucrose-UDP glucosyltransferase.     

Chilling stress and oxygen metabolizing enzymes in Zea mays and Zea diploperennis*

Abstract. The activities of five active-oxygen scavenging enzymes were compared for cold-lability and three were compared for chilling induction in two Zea genotypes of contrasting susceptibility to photoinhibition during chilling. Activities of superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (GTR, EC 1.6.4.2) in leaf extracts from plants grown without chilling stress were assayed at 19°C and 5°C. Enzymes from the chilling-susceptible Z. Mays cv. LG11 had lower specific activities at 5°C than did enzymes from the chilling-tolerant Z. diploperennis, except for MDHAR where no significant differences were observed. The activities of SOD and APX from Z. diploperennis were double those of Z. mays at both assay temperatures. Monodehydroa-scrobate reductase and glutathione reductase activities in both species were reduced by 63–78% at a 5°C assay temperature. The dehydroascorbate reductase (DHAR) showed the greatest low-temperature lability losing 96% (Z. diploperennis) and 100% (Z. mays) of its activity at 5°C. To examine possible chilling-induced changes in levels of enzyme activity, plants of both Zea genotypes were transferred to growth chambers at 10°C at moderate light intensities. Glutathione reductase activity was found to increase within 24h in Z. diploperennis, but it decreased slightly in Z. mays. MDHAR activity decreased by 50% in Z. diploperennis but showed only a transient increase in activity in Z. mays.