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.     

The nucleolus organizer region of maize (Zea mays L.)

Maize plants were produced partially triploid for the heterochromatic segment of the nucleolus organizer region (NOR) or partially triploid or tetraploid for the site giving rise to the secondary constriction of the NOR. These partially hyperploid plants were characterized in terms of chromosome and/or nucleolar constitution by light microscopy at pachytene, diakinesis, and quartet stages of microsporogenesis. DNA's of the various partially hyperploid plants and appropriate controls were extracted and hybridized with ³H-rRNA. The heterochromatic segment of the NOR was found to contain most of the rRNA cistrons, but has little or no interaction with the nucleolus. In contrast with the heterochromatic segment, the site giving rise to the secondary constriction contains few rRNA cistrons but is active in nucleolar formation as viewed at pachytene, diakinesis and quartet stages.     

Biochemical composition of maize (Zea mays L.) pollen

Summary Pollen grains containing either the Wx, wx, Su ₁, su ₁, Sh ₂ or sh ₂ alleles were stored at 0, 1, 2, 3, 4 and 5 days at 2 °C. After each storage period, a portion of pollen from each genotype was analyzed for free amino acid content. Over all genotypes, storage significantly altered the content of all 16 amino acids measured. With increasing storage, a relatively consistent increase in aspartic acid, isoleucine, leucine, phenylalanine, ethanolanine, aminobutyric acid, NH₃ and lysine was found. A relatively consistent decrease in glutamic acid, proline, glycine and alanine occurred with increasing storage. No consistent response to storage was obtained with threonine-serine, valine, histidine and the unknown. Apparently, storage or stage of viability loss has a pronounced effect on amino acid metabolism in maize pollen grains. The experiment was designed so that comparisons free of genetic background effects could be made between alleles at each locus. Significant allele X storage interactions at each locus were found as follows: at the waxy locus, aspartic acid, glycine, alanine and ethanolanine; at the sugary locus, aspartic acid, alanine, ethanolanine and aminobutyric acid; and at the shrunken locus, aspartic acid, alanine, valine, leucine and ethanolanine. Amino acid metabolism is apparently influenced by the action of the alleles at these loci. The differences between the loci in the amino acids affected indicate the different areas of amino acid metabolism are influenced by each locus.Journal Series Paper No. 4425, Florida Agricultural Experiment Station.     

Biochemical composition of maize (Zea mays L.) pollen

Summary Mean percentages of sugars, water-soluble polysaccharides, starch, total carbohydrates and lipids were 40.1, 7.4, 28.6, 76.1, and 1.8 respectively. Differences among the mutants were found only for water-soluble polysaccharides with both wx and sh ₂ decreasing the percentage a small but significant amount. In terms of the various carbohydrates measured, no correlation was found between the expression of these mutants in the pollen and the kernel.

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Zusammenfassung Die Untersuchung des Kohlenhydrat- und Lipid-Gehaltes im Pollen der Endosperm-Mutanten waxy, shrunken und sugary vom Mais ergab einen gemittelten Gehalt an Zucker von 40,1%, wasserlöslichen Polysacchariden 7,4%, Stärke 28,6, Gesamtkohlenhydraten 76,1% und Lipiden 1,8%.Unterschiede zwischen den Mutanten wurden lediglich hinsichtlich der wasserlöslichen Polysaccharide gefunden, und zwar bei wx und sh ₂, die einen signifikant niedrigeren Gehalt aufwiesen.Hinsichtlich der verschiedenen analysierten Kohlenhydrate wurde keine Korrelation zwischen der Expression der Mutanten im Pollen und in den Karyopsen gefunden.

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Herrn Professor Dr. J. Straub (Köln-Vogelsang) zu seinem 60. Geburtstag gewidmet.

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Journal Series Paper No. 3621, Florida Agricultural Experiment Station.     

Plot size for progeny selection in maize (Zea mays L.)

Summary Six progeny trials that included 147 half-sib progenies of maize (Zea mays L.) population ESALQ PB-5 were conducted for the purpose of studying plot size and its consequences in recurrent selection programs. The progenies were evaluated in three 7x7 duplicate simple lattice experiments using one-row plots of 5 m². At harvest each plot was partitioned into five sub-plots (sampling units), and data was collected from each sampling unit. At the same time and place the same progenies were evaluated in three 7x7 duplicate simple lattice experiments using 1-m² (linear row with 5 plants) plots. Data were collected for plant and ear height, ear diameter, total ear weight, and total grain yield. The data were combined by using adjacent sampling units, and the analyses were performed by considered five plot sizes in addition to those of the independent trials with 1-m² plots. The experiments with 1-m² plots were less efficient in discriminating for yield traits among progenies than those with 5-m² plots. The combination of plot size and number of progenies evaluated indicated that an optimum plot size for yield was between 3 and 4 m², or 15–20 plants per plot. With such sizes the expected gain was maximized for the four replications used in this study. If the total area covered by each progeny is constant, the maximum gain from selection, however, is attained by decreasing plot size and increasing the number of replications. The minimum size of plots is, however, limited by practical or theoretical criteria. Plot size affected the estimates of additive genetic variance, coefficient of heritability, and genetic coefficient of variation for all of the traits. No practical limitation was observed for conducting experiments with 1-m² plot.     

Hydroxylation reactions in roots of maize (Zea mays L.)

The authors succeeded in demonstrating accumulation of tyrosine and p-coumaric acid in three day-old roots of maize (Zea mays L.) fed with L-phenylalanine and cinnamic acid. Phenylpyruvic acid applied under the same conditions gave rise to phenylalanine, indicating the presence of the corresponding transferase activity. Even simultaneous application of inhibitors of transaminase activity—hydroxylamine and isonicotinic acid hydrazide—did not result in the formation of p-hydroxyphenylpyruvic acid.     

Hydroxylation reactions in roots of maize (Zea mays L.)

The authors succeeded in demonstrating accumulation of tyrosine and p-coumaric acid in three day-old roots of maize (Zea mays L.) fed with L-phenylalanine and einnamie acid. Phenylpyruvic acid applied under the same conditions gave rise to phenylalanine, indicating the presence of the corresponding transferase activity. Even simultaneous application of inhibitors of transaminase activity — hydroxylamine and isonicotinic acid hydrazide — did not result in the formation of p-hydroxyphenylpyruvic acid.     

Biochemical composition of maize (Zea mays L.) pollen

Summary Proline was the most abundant amino acid with a mean value of 186.28 moles/mg dry pollen. The other amino acids tested were below 33 moles/mg dry pollen. The mutant wx significantly increased aspartic acid, valine, histidine and an unknown but significantly decreased aminobutyric acid. The mutant sh ₂ significantly increased glutamic acid, proline, lysine, histidine and an unknown but significantly decreased aspartic acid and aminobutyric acid. The effect of su ₁ was altered by the genetic background involved. In one genetic background, su ₁ did not significantly increase any amino acid but significantly decreased alanine and aminobutyric acid. However, in a distinctly different background, su ₁ significantly increased aminobutyric acid but significantly decreased aspartic acid and glutamic acid. Apparently the genetic background is capable of producing major shifts in the amino acid pattern in addition to the action of these mutants.The fatty acids, palmitic and linolenic were the most common with percentages of 54.1 and 34.4 respectively. The mutants tested did not affect the fatty acid distribution.

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Zusammenfassung Prolin war die am reichlichsten vorkommende Aminosäure mit einem mittleren Gehalt von 186,28 Mikromol per mg trockenen Pollen. Die anderen Aminosäuren erreichten weniger als 33 Mikrogramm per mg trockenen Pollen.Die Mutante wx zeigte einen signifikant erhöhten Gehalt an Asparaginsäure, Valin, Histidin, sowie einer nicht identifizierten Komponente, während der Gehalt an -Aminobuttersäure signifikant erniedrigt war. Die Mutante sh ₂ ist gekennzeichnet durch einen signifikant erhöhten Gehalt an Glutaminsäure, Prolin, Lysin, Histidin, sowie einer unbekannten Fraktion; der Gehalt an Asparaginsäure und -Aminobuttersäure war dagegen signifikant erniedrigt. Die Wirkung des mutierten Gens su ₁ wurde durch das übrige Genom, in dem es sich befand, geändert. In dem einen genetischen Milieu verursachte su ₁ keine signifikante Erhöhung des Gehaltes irgend einer Aminosäure, während der Gehalt an Alanin und -Aminobuttersäure signifikant erniedrigt war. In einem anderen genetischen Milieu jedoch zeigte su ₁ eine signifikante Erhöhung der -Aminobuttersäure; Asparaginsäure und Glutaminsäure waren signifikant erniedrigt.Offensichtlich ist das übrige Genom zusätzlich zu der Wirkung der genannten Mutanten in der Lage, wesentliche Verschiebungen im Verteilungsmuster der Aminosäuren zu verursachen.Von den Fettsäuren wurden am häufigsten Palmitin- und Linolen-Säure mit einem Gehalt von 54,1 bzw. 34,4% gefunden. Die untersuchten Endosperm-Mutanten zeigten keinen Einfluß auf die Fettsäureverteilung im Pollen.

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Journal Series Paper No. 3468, Florida Agricultural Experiment Station.     

Thiols in cadmium- and copper-treated maize (Zea mays L.)

Maize plants (Zea mays L. cv. Honeycomb F-1) were grown on quartz sand containing amounts of Cd or Cu which resulted in comparable internal contents in the roots. Fresh and dry weights and the content of Cd or Cu were measured in roots and shoots after eight weeks. In addition, cysteine, γ-glutamylcysteine (γEC), glutathione (GSH) and the thiols in heavy-metal-binding peptides (HMBPs) were determined in the roots. At low internal contents, Cd and Cu inhibited root growth to the same extent. Inhibition by Cu was enhanced, however, at high internal contents, indicating that Cu was more toxic than Cd. Separation of extracts from roots of Cd- and Cutreated plants on a Sephadex G-50 column resulted in HMBP complexes with relative molecular masses (M_rs) of 6200 and 7300, respectively. Separation of these HMBP-complexes using HPLC resulted in a distinct pattern of thiol compounds for each heavy metal. The accumulation of HMBPs was linearly dependent on the content of Cd at all values examined. In Cu-treated roots, HMBP accumulation was linearly dependent on the internal Cu content only up to 7.1 μmol·g^?1 dry weight. At internal contents which caused an enhanced inhibition of root growth, no further significant increase in the HMBP content was detected. At these internal Cu contents an increased transport of Cu to the shoot was measured. This result indicates that HMBPs are involved in reducing heavy-metal transport from roots to shoots.     

Biochemical characterization of maize (Zea mays L.) for salt tolerance

The inherent differences for salt tolerance in two maize cultivars (Agatti-2002 and Sahiwal-2002) were evaluated in pot experiments. Plants were grown in half-strength of Hoagland nutrient solution added with 0, 80, 100, 120, 140 and 160 mM of NaCl. Salt stress markedly reduced the shoot and root lengths and fresh and dry masses. Reduction in growth attributes was more pronounced in cv. Agatti-2002 than cv. Sahiwal-2002. Both maize cultivars exhibited significant perturbations in important biochemical attributes being employed for screening the crops for salt tolerance. Cultivar Sahiwal-2002 was found salt tolerant as compared to cv. Agatti-2002 because it exhibited lower levels of H₂O₂, malondialdehyde (MDA) and higher activities of antioxidant enzymes. In addition, cultivar Sahiwal-2002 exhibited less salt-induced degradation of photosynthetic pigments, lower levels of toxic Na⁺ and Cl^?  and higher endogenous levels of K⁺ and K⁺/Na⁺ ratio. The results indicate that salt stress induced a marked increase in MDA, H₂O₂, relative membrane permeability, total soluble proteins and activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase andascorbate peroxidase). Moreover, increase in endogenous levels of Na⁺ and Cl^?  and decrease in K⁺ and K⁺/Na⁺ ratio and photosynthetic pigments were recorded in plants grown under salinity regimes.     

Induced cytomictic diversity in maize (Zea mays L.) inbred

Mutation breeding has been used for improving oligogenic and polygenic characters, disease resistance and quantitative characters including yielding ability. The cytological stability of maize inbred lines is an important consideration in view of their extensive use in genetics and plant breeding research. Investigation in Zea mays L. confirms that the migration of chromosomes is a real event that cannot be misunderstood as an artifact produced by fixation or mechanical injuries. During present investigation, we found that out of six inbred lines of Zea mays L. viz. CM-135, CM-136, CM-137, CM-138, CM-142 and CM-213 at various treatment doses of gamma irradiations viz. 200, 400 and 600 Gy, some of the plants of inbred line CM- 138 at 200 Gy dose displayed characteristic cytoplasmic connections during all the stages of meiosis. Four plants from this treatment set were found to be engaged in a rare phenomenon reported as "Cytomixis". It elucidates that in inbred of Zea mays L., induced cytomixis through gamma rays treatment may be considered to be a possible source of production of aneuploid and polyploid gametes. This phenomenon may have several applications in Zea mays L. improvement in the sense of diversity and ever yield potential.     

A strategy for assembling the maize (Zea mays L.) genome

Because the bulk of the maize (Zea mays L.) genome consists of repetitive sequences, sequencing efforts are being targeted to its 'gene-rich' fraction. Traditional assembly programs are inadequate for this approach because they are optimized for a uniform sampling of the genome and inherently lack the ability to differentiate highly similar paralogs. RESULTS: We report the development of bioinformatics tools for the accurate assembly of the maize genome. This software, which is based on innovative parallel algorithms to ensure scalability, assembled 730,974 genomic survey sequences fragments in 4 h using 64 Pentium III 1.26 GHz processors of a commodity cluster. Algorithmic innovations are used to reduce the number of pairwise alignments significantly without sacrificing quality. Clone pair information was used to estimate the error rate for improved differentiation of polymorphisms versus sequencing errors. The assembly was also used to evaluate the effectiveness of various filtering strategies and thereby provide information that can be used to focus subsequent sequencing efforts.     

Ultrastructural studies on pollen embryogenesis in maize (Zea mays L.)

Maize anthers have been induced on modified N6 medium to produce embryoids. Different stages from the cultures were sampled and prepared for microscopical examination. The microspores at the onset of culture were in an early developmental stage, with the nucleus and numerous organelles centred in the middle, surrounded by many small vacuoles with a lipid content. The binuclear pollen grains contained small vesicles and much starch. The partially condensed vegetative nucleus indicated participation of the vegetative component in the formation of multicellular pollen grains (MPGs). Several MPGs have been observed which differed in morphology. We suggest, on the basis of these ultrastructural observations, that in maize mainly the vegetative cell contributes to the MPG which further develops directly into embryoids.     

Microsatellite megatracts in the maize (Zea mays L.) genome.

Long tracts (megatracts) of (CAG)n, (TAG)n, and (GAA)n microsatellite sequences capable of forming composite DNA segments were found in the maize (Zea mays L.) genome. Some of the (CAG)n and (TAG)n megatracts were organized in clusters of up to 1 Mb on several chromosomes, as detected by fluorescence in situ hybridization (FISH), as well as on extended DNA fibers. Extensive polymorphism was found among different maize inbred lines with respect to the number and size of microsatellite megatract clusters on the A chromosomes. Polymorphism was also common among B chromosomes of different nuclei in the inbred line Zapalote Chico. Different retrotransposable elements were often inserted into the microsatellite tracts. Size variation in some (TAG)n and (GAA)n megatracts was observed in consecutive generations among siblings of the inbred lines, indicating that these loci are highly unstable and predisposed to dynamic mutations similar to those described in mammalian systems.     

Cytoplasmic effects on quantitative characters in maize (Zea mays L.)

Summary The investigations were carried out with ten genetically diverse maize varieties and all possible crosses between them, including reciprocals, at two agro-climatically different locations in Punjab (India). The materials were studied in a split-split plot design with plant population level of 59200, 74000 and 98700 plants per hectare. The variances due to reciprocal cross effects were significant for plant height, ear height and ear girth in the pooled analysis and the effects were quite consistent over plant population levels and locations. The data on days to silk were recorded only at one location and highly significant reciprocal effects were observed. The cytoplasmic effects, however, did not depend on the cytoplasm alone but also on the interaction of genotype with cytoplasm. The reciprocal effects were more distinct in early × late combinations of varieties than in early × early or late × late combinations. Early parents when used as female tended to reduce plant and ear height and days to silk, indicating a common developmental pathway for these three traits. Since the cytoplasmic effects for these characters were not associated with significant effects on yield, the results can be of practical significance. The cytoplasmic effects for days to silk were maintained even in the F₂ and back-crosses. These findings encourage the use of particular cytoplasm in developing early maturing varieties.     

Proteome profile of the developing maize (Zea mays L.) rachis

In this study, we performed the first high‐throughput proteomic analysis of developing rachis (cob) from maize genotype Mp313E. Using two proteomic approaches, 2‐DE and 2‐D LC, we identified 967 proteins. A 2‐D proteome reference map was established. Functional classification of identified proteins revealed that proteins involved in various cellular metabolisms, response to stimulus and transport, were the most abundant.     

Plantlet regeneration from glume calli of maize (Zea mays L.)

Summary Totipotent callus cultures were established from anther-free glumes of Sweet corn, Seed corn, DHM 103 and DHM 101 on MS medium supplemented with 1–2 mg/l 2,4-D. The callusing response of the glumes was tested on six different media. Glumes at the uninucleate stage of pollen development callused with a high frequency compared to other stages. Organogenesis was observed in 40% of the cultures on media devoid of hormones. A total of 76 plantlets were regenerated on medium with 0.5–1.0 mg/l of both IAA and kinetin. Cytological observations in root tips indicated a diploid chromosome number (2n=20).