Normal Maize Endosperm Research Articles

Genetic diversity and heterotic orientation of South Africa maize inbred lines towards tropical and temperate testers

An efficient hybrid breeding program defines and utilizes a few heterotic groups. Objectives of this study were to determine genetic diversity and alignment of South Africa maize inbred lines collection towards tropical and temperate testers. Forty-four maize inbred lines were genotyped with 56,110 single nucleotide polymorphism (SNP) markers. A total of 40 lines were crossed to two tropical and two temperate inbred line testers. Due to seed shortages, testcross hybrids containing 14 and 18 lines were used in 2014/2015 and 2015/2016 seasons, respectively. Genotypes and specific combining ability (SCA) effects of hybrids were significantly different (P < 0.05) for grain yield. There was a weak correlation between molecular genetic distances and both grain yield mean performance and SCA effects of hybrids, indicating that productivity of maize inbred lines could not be reliably determined based on molecular genetic distances. The SCA data classified these maize inbred lines into three heterotic groups with respect to both tropical and temperate testers. The study also indicated high level of diversity among the maize inbred lines, which was shown by both the dendogram and molecular genetic distances. The SNP marker data classified the inbred lines into 11 clusters that could be simplified into three major groups of normal maize endosperm and two groups of quality protein maize endosperm types. However, the SNP data indicated that maize lines were more aligned towards the tropical than temperate inbred testers. This information would be useful for simplifying heterotic classification of the lines with profound implications for breeding progress.

Opaque16, a high lysine and tryptophan mutant, does not influence the key physico-biochemical characteristics in maize kernel.

The enhancement of lysine and tryptophan in maize is so far basedon opaque2(o2) mutant, that along with the endosperm-modifiersled to development of Quality Protein Maize[QPM]. Though many mutants improving the endospermic protein quality were discovered, they could not be successfully deployed. Recently discovered opaque16 (o16)mutant enhances the lysine and tryptophan content in maize endosperm. In the present study, the influence of o16 on the endosperm modification was analyzed in four F2 populations, two each segregating for o16 allele alone and in combination with o2. The recessive o16o16 seed endosperm was found to be vitreousphenotypically similar to wild-O16O16. The mutant did not influence the degree of kernel opaqueness in o2o2 genetic background as opaqueness in o2o2/O16O16 and o2o2/o16o16 was similar. Grain hardness of o16o16 was comparable with the normal and QPM maize. The pattern of microscopic organization of proteinaceous matrix and starch granules, and zein profiling of the storage protein in o16o16 were found to be similar with normal maize endosperm, but distinct from the o2o2-soft genotype. The pattern in o2o2/o16o16 was unique and different from o2o2 and o16o16 as well. Here we demonstrated the effects of o16 on physico-biochemical characteristics of endosperm and report of o16 possessing negligible influence on kernel modification and hardness, which holds a great significance in maize quality breeding programme.

Comparative analysis of physico-chemical properties and amino acids profile of three tropical maize hybrid cultivars in Nigeria

Purpose The purpose of this study was to compare the physico-chemical properties and amino acid profile of three maize hybrid cultivars grown in Nigeria. Design/methodology/approach Two normal maize endosperm varieties, yellow SUWAN-ISR (YNM) and white ART/98/SW05-OB-WC (WNM), and one yellow QPM variety, TZE-POP-DT-STR-QPM (YQPM), were selected for the study. Physico-chemical properties, physical tests, proximate composition analysis, functional properties and characteristics and amino acid profile tests were carried out on the grains using standard methods. Findings Protein was significantly higher (p &lt; 0.05) in YQPM (10.49 per cent) than in normal endosperm, YNM (8.83 per cent) and WNM (8.50 per cent). Amino acid profile of the grains revealed that total amino acid of YQPM (94.67 g/100 g of protein) and essential amino acid of YQPM (39.070) were the highest among the three, with highest significantly different value of tryptophan (0.388 g/100 g of protein) at p &lt; 0.05. The cooking quality of YQPM was found to be better than the other two, with highest hydration capacity and increase in volume after cooking (90.8 ± 0.01 g/1000 grains and 147.53 ± 0.02 per cent). Originality/value YQPM will be highly beneficial in the tropics, where maize is grown as the major staple food to reduce hunger and malnutrition because of its amino acid balance and its better cooking quality.

BT1, a possible adenylate translocator, is developmentally expressed in maize endosperm but not detected in starchy tissues from several other species

Brittle 1 protein (BT1), the major protein in amyloplast envelope membranes of wild‐type maize endosperm, is required for normal levels of starch accumulation in maize kernels. To gain information about the Bt1 locus in control of starch synthesis in plants, we examined Bt1 expression in kernels from different ages and from different genotypes of maize as well as in starchy tissues from different species. Both BT1 protein and Bt1 transcripts were detected in kernels from about 10 days post‐pollination (DPP) and increased up to 14 DPP. BT1 protein level was similar in kernels after 14 DPP. Furthermore, membranes from amyloplasts containing larger starch granules contained more BT1 protein than those containing smaller starch granules from the same age of kernels. These results indicate that Bt1 gene expression was developmentally regulated and that BT1 protein level was correlated with starch accumulation in normal maize endosperm. Unexpectedly, except for a weak band at a higher molecular mass in microsomal proteins extracted from spring barley grain which reacted with maize BT1 antibody, no BT1 protein was observed in starchy tissues from several other species, including grains of sorghum, wheat, winter barley and oats, potato tubers, pea roots and cauliflower buds. Thus either the BT1 antibody raised against a fusion protein including the 56 amino acid residues in the carboxyl terminal sequence of maize endosperm BT1 protein does not recognize BT1 homologues in amyloplasts from other tissues or species or amyloplasts from different tissues or species have alternative membrane transporters.

Regulatory mechanisms involved in the biosynthesis of starch

Abstract

Purification and characterization of proteolytic enzymes from normal and opaque-2Zea mays L. developing endosperms

Purification and characterization of proteases from developing normal maize endosperm and high lysine opaque-2 maize endosperm have been carried out with a view to understand their role in storage protein modification. At day 15, normal maize endosperm had two types of proteolytic enzymes, namely, protease I and protease II, while at day 25 protease n disappeared and in place protease III appeared. However, in opaque-2 maize endosperm at both the stages only one type of enzyme (protease I) was present. These proteases had many properties in common-optimum pH and temperature were respectively, 5.7and 40°C; their activity was inhibited to the extent of 75 –93 % by p-chloromercuribenzoate; trypsin inhibitor inhibited the activity more at early stages of endosperm development; all proteases cleaved synthetic substrates p-tosyl-L-arginine methylesler and N-benzoyl-L-tyrosine ethyl ester and poly-L-glutamic acid. TheKm values of day 15 and 25 normal maize endosperm proteases ranged from 2.73–3.30, while for opaque-2 maize endosperm protease I it was 3.33 mg azocasein per ml assay medium. These enzymes, however, differed with respect to proteolytic activity towards poly-L-lysine. Only normal maize endosperm protease III at day 25 followed by protease II at day 15 showed high activity towards this homopolypeptide suggesting thereby their role in determining the quality of normal maize endosperm protein.

Degradation in vitro of [3H]-Lysine and [14C]-Leucine Labelled Protein by Proteases from Developing Normal and Opaque-2 Zea mays Endosperm1)

Summary During later stages of kernel development total lysine content per endosperm in normal and opaque-2 maize endosperms decreased in spite of net increase in total protein. The proteolytic enzymes increased during development until 25 d after pollination and later declined in both normal and opaque-2 endosperms. Release of lysine in vitro from [3H]-lysine and [14C]-Leucine labelled normal maize endosperm proteins by proteases from developing normal maize endosperms was higher than in opaque-2 endosperms towards later stages of maturity. Present study suggests that lysine-liberating proteases present in endosperms also contribute to decrease in lysine and consequently the quality of protein.

Regulation of starch biosynthesis in normal and opaque-2 maize during endosperm development

The absolute activities of sucrose-UDP glucosyltransferase, glucose-6-phosphate ketoisomerase and soluble and bound ADPG-starch glucosyltransferase have been studied in normal and Opaque-2 maize endosperms during development. In general, the activities of these enzymes except sucrose-UDP glucosyltransferase were higher up to 20 days post-pollination and lower at the 30 day stage in Opaque-2 than in normal maize endosperms. However, sucrose-UDP glucosyltransferase activity was higher in normal maize endosperm up to the 20 day stage while it was lower at subsequent stages than in Opaque-2. It is suggested that the lower level of these enzymes, except sucrose-UDP glucosyltransferase, might be responsible for the reduced accumulation of starch in Opaque-2 endosperm during later stages of endosperm development.

Metabolism of Acetate-[2-14C] in Hard Endosperm Opaque-2 Maize

Summary Acetate-[2-14C] metabolism by developing normal, modified opaque and opaque-2 maize endosperm showed considerable differences in incorporation of label into protein and free amino acids, organic acids and sugar. Protein synthesizing efficiency in modified opaque and normal maize endosperm was higher as compared to opaque-2 at 20 and 25 days post-pollination while at 30 day stage, it was similar. Considerably higher label appeared in organic acids fraction in modified opaque maize endosperm at 30 day stage as compared to normal and opaque-2 maize endosperm. Greater label appeared in free amino acids in opaque-2 maize and protein amino acids in normal endosperm during early stage of maturity. Differences in the proportion of label in different organic acids were also evident. The specific activity in protein lysine was the highest in opaque-2, intermediate in modified opaque and the least in normal maize endosperm at early stage. Distribution of label and specific activity of other amino acids also differed during development. Based on incorporation of label from acetate it is concluded that modified opaque endosperm differ from that of normal maize endosperm in their metabolism.

Effects of floury-2 locus on zein accumulation and RNA metabolism during maize endosperm development.

Zein accumulation patterns during mutant and normal maize endosperm development were determined. Accompanying an increase in the number of floury-2 alleles present in the endosperm was a well-defined stepwise depression in zein accumulation. Analysis of the zein accumulated in endosperms containing zero, one, two, and three doses of the floury-2 allele by sodium dodecylsulfate--polyacrylamide gel electrophoresis revealed a proportionate reduction in the two major zein components, Z1 and Z2. In contrast, the relative proportions of the minor zein bands were altered. Membrane-bound polysomes isolated from kernels of floury-2 and normal maize were predominantly large size classes. The presence of increasing numbers of the floury-2 allele in the endosperm decreased recovery of membrane-bound polysomal material in a stepwise fashion. However, major alterations in polysome size-class distributions were not observed. The reduction in membrane-bound polysome material correlated linearly with reductions in in vitro zein synthesis and in vivo zein accumulation.

Characterization of polysomes and incorporation in vitro of leucine and lysine in normal and opaque-2 zea mays endosperm during development

Polysome preparations obtained from opaque-2 and normal maize endosperms during development did not show any significant difference in sedimentation coefficient or nucleotide composition. The pattern of incorporation in vitro of lysine and leucine, however, differed quite distinctly in these two preparations. During early stages of maturity the polysomes from opaque-2 incorporated substantially more lysine and less leucine as compared with those from normal maize. Although the trend was reversed at 25 days post-pollination, this did not result in any significant zein accumulation since very little total protein was synthesized after that stage in opaque-2 maize endosperm. It is, therefore, suggested that the opaque-2 gene exerts a regulatory control on mRNA synthesis, required for zein formation at early stages of maturation.