Dominant complementation of biological pathways in maize hybrid lines is associated with heterosis.

MAIN CONCLUSION: Allele-specific expressed genes (ASEGs) are widespread in maize hybrid lines and play important roles of complementation of biological pathways in heterosis. Heterosis (hybrid vigor) is an important phenomenon with both theoretical and practical value. However, our understanding of the genetic and molecular mechanisms behind heterosis is still limited. Here, we analyzed a comprehensive dataset of maize (Zea mays L.), including RNA-seq data from three hybrid-parent triplets (HPTs) and acetylated protein data from one HPT. The gene expression patterns exhibited extensive variation between the hybrids and their parents, and a substantial number of allele-specific expressed genes (ASEGs) were identified in the hybrids. Notably, ASEGs from different HPTs were significantly enriched in various conserved pathways. The parental alleles of ASEGs with fewer deleterious single-nucleotide polymorphisms were more likely to be expressed in hybrid lines than other parental alleles. ASEGs were mainly enriched in the functional gene ontology terms protein biosynthesis, photosynthesis, and metabolism. In addition, the ASEGs across the three HPTs were involved in key photosynthetic pathways and might enhance the photosynthetic efficiency of the hybrids. These findings suggest that ASEGs involved in complementary biological pathways in maize hybrids contribute to heterosis, shedding new light on the molecular mechanism of heterosis.

Effect of starch properties and zein content of commercial maize hybrids on kinetics of starch digestibility in an in vitro poultry model.

BACKGROUND: The kinetics of starch digestion is a key determinant of poultry performance. Research so far has shown that starch digestibility kinetics depends on the molecular structure of starch but also on the properties of the complex matrix in which starch granules are embedded in most feedstuffs. However, the manner in which genotype differences in the same plant affect starch digestibility kinetics has not yet been addressed. The present study explored the extent to which the starch digestibility rate in commercial high-yielding maize hybrids depended on amylose / amylopectin content, starch granule size and shape, and zein in total starch (TS) content. RESULTS: Hybrids differed in all the traits examined, giving the following ranges: amylose content, 165-207 g kg-1 DM; zein in TS content, 70-89 g kg-1 DM; starch granule equivalent diameter, 11.5-12.3 µm, and in vitro starch digestion rate, 1.22-1.44 h-1 . The starch digestion rate correlated negatively with zein in TS content (r = -0.36) and positively with equivalent diameter (r = 0.45). The negative correlation between starch digestion rate and zein in TS suggests that some zein remained after grinding and pepsin incubation and acted as a barrier to amylolytic enzymes. CONCLUSIONS: When starch granules are embedded in a complex protein matrix, zein limits their accessibility to enzymes and affects the starch digestibility rate. Surprisingly, our results suggest that when enzymes reach starch granules, they digest a greater proportion of the starch when the granules are larger. © 2019 Society of Chemical Industry.

Maize ARGOS1 (ZAR1) transgenic alleles increase hybrid maize yield.

Crop improvement for yield and drought tolerance is challenging due to the complex genetic nature of these traits and environmental dependencies. This study reports that transgenic over-expression of Zea mays AR GOS1 (ZAR1) enhanced maize organ growth, grain yield, and drought-stress tolerance. The ZAR1 transgene exhibited environmental interactions, with yield increase under Temperate Dry and yield reduction under Temperate Humid or High Latitude environments. Native ZAR1 allele variation associated with drought-stress tolerance. Two founder alleles identified in the mid-maturity germplasm of North America now predominate in Pioneer's modern breeding programme, and have distinct proteins, promoters and expression patterns. These two major alleles show heterotic group partitioning, with one predominant in Pioneer's female and the other in the male heterotic groups, respectively. These two alleles also associate with favourable crop performance when heterozygous. Allele-specific transgene testing showed that, of the two alleles discussed here, each allele differed in their impact on yield and environmental interactions. Moreover, when transgenically stacked together the allelic pair showed yield and environmental performance advantages over either single allele, resembling heterosis effects. This work demonstrates differences in transgenic efficacy of native alleles and the differences reflect their association with hybrid breeding performance.

Haploid identification in maize.

Doubled haploid (DH) line production through in vivo maternal haploid induction is widely adopted in maize breeding programs. The established protocol for DH production includes four steps namely in vivo maternal haploid induction, haploid identification, genome doubling of haploid, and self-fertilization of doubled haploids. Since modern haploid inducers still produce relatively small portion of haploids among undesirable hybrid kernels, haploid identification is typically laborious, costly, and time-consuming, making this step the second foremost in the DH technique. This manuscript reviews numerous methods for haploid identification from different approaches including the innate differences in haploids and diploids, biomarkers integrated in haploid inducers, and automated seed sorting. The phenotypic differentiation, genetic basis, advantages, and limitations of each biomarker system are highlighted. Several approaches of automated seed sorting from different research groups are also discussed regarding the platform or instrument used, sorting time, accuracy, advantages, limitations, and challenges before they go through commercialization. The past haploid selection was focusing on finding the distinguishable marker systems with the key to effectiveness. The current haploid selection is adopting multiple reliable biomarker systems with the key to efficiency while seeking the possibility for automation. Fully automated high-throughput haploid sorting would be promising in near future with the key to robustness with retaining the feasible level of accuracy. The system that can meet between three major constraints (time, workforce, and budget) and the sorting scale would be the best option.

Agronomic performance and yield stability of extra-early maturing maize hybrids in multiple environments in the Sahel.

Frequent occurrence of drought, heat, low soil fertility and Striga infestation are the main stress factors reducing maize yield in the Sahel. Adoption of stable multiple stress tolerant maize cultivars in the region is crucial for achieving food security. However, selection of a stable high yielding cultivar is complicated by genotype × environment interaction (GEI) due to differential responses to growing conditions. Eleven extra-early maturing multiple-stress tolerant maize hybrids and two checks arranged in a randomized complete block design was evaluated across nine locations for two years in Mali and Niger. The objectives of this study were to identify (i) stable and high-yielding maize hybrids, and (ii) suitable test locations for selecting promising extra-early maize hybrids. GGE biplot was used for graphical analysis. Significant genotype, location and GEI effects were detected for grain yield and number of ears per plant. EEWQH-13 produced the highest grain yield (3860 kg ha-1) while EEYQH-1 had the poorest yield (2663 kg ha-1) with trial mean of 3395 kg ha-1 for all hybrids. GGE biplot explained 69.6 % of the total variation in grain yield among the hybrids. The polygon view identified EEWQH-13 as the best hybrid across six of the nine test locations. EEPVAH-58 was identified as the most stable high yielding hybrid across the nine test locations followed by EEWQH-16 and EEWQH-13. The nine locations were clustered under two mega-environments (ME1, ME2). Among the nine test locations, Tara and Aderaoua clustered in ME1 were the most suitable ones for selecting promising extra-early maize hybrids for wider adaptation. The three hybrids, EEPVAH-58, EEWQH-16, and EEWQH-13, identified in this study could be recommended for on-farm evaluation to confirm the consistency of their yield performance for possible release and commercialization in Mali and Niger.

Multi-trait selection for mean performance and stability of maize hybrids in mega-environments delineated using envirotyping techniques.

Under global climate changes, understanding climate variables that are most associated with environmental kinships can contribute to improving the success of hybrid selection, mainly in environments with high climate variations. The main goal of this study is to integrate envirotyping techniques and multi-trait selection for mean performance and the stability of maize genotypes growing in the Huanghuaihai plain in China. A panel of 26 maize hybrids growing in 10 locations in two crop seasons was evaluated for 9 traits. Considering 20 years of climate information and 19 environmental covariables, we identified four mega-environments (ME) in the Huanghuaihai plain which grouped locations that share similar long-term weather patterns. All the studied traits were significantly affected by the genotype × mega-environment × year interaction, suggesting that evaluating maize stability using single-year, multi-environment trials may provide misleading recommendations. Counterintuitively, the highest yields were not observed in the locations with higher accumulated rainfall, leading to the hypothesis that lower vapor pressure deficit, minimum temperatures, and high relative humidity are climate variables that -under no water restriction- reduce plant transpiration and consequently the yield. Utilizing the multi-trait mean performance and stability index (MTMPS) prominent hybrids with satisfactory mean performance and stability across cultivation years were identified. G23 and G25 were selected within three out of the four mega-environments, being considered the most stable and widely adapted hybrids from the panel. The G5 showed satisfactory yield and stability across contrasting years in the drier, warmer, and with higher vapor pressure deficit mega-environment, which included locations in the Hubei province. Overall, this study opens the door to a more systematic and dynamic characterization of the environment to better understand the genotype-by-environment interaction in multi-environment trials.

Grain Yield and Heterosis of Maize Hybrids under Nematode Infested and Nematicide Treated Conditions.

Plant-parasitic nematodes are present on maize but resistant genotypes have not been identified in Uganda. This study was aimed at determining the level of nematode resistance among F(1) hybrids, and to estimate grain yield, heterosis and yield losses associated with maize hybrids under nematode infestation. The 30 F(1) hybrids and two local checks were evaluated in a split plot design with nematode treatment (nematode infested versus nematicide treated) as the whole plot factor, and the hybrids as subplot factors arranged in an 8 x 4 alpha-lattice design. The experiment was conducted simultaneously at three sites. The hybrids were also evaluated in a split plot design under greenhouse conditions at IITA-Namulonge. Results revealed 24 P. zeae susceptible hybrids compared to only six P. zeae resistant hybrids. Grain yield across sites was higher by about 400 kg ha(-1) under nematicide treatment than under nematode infestation. The nematode tolerant/resistant hybrids exhibited yields ranging from 5.0 to 8.4 t ha(-1) compared to 5.0 t ha(-1) obtained from the best check. Grain yield loss was up to 28% among susceptible hybrids, indicating substantial economic yield losses due to nematodes. Under field conditions, desired heterosis was recorded on 18 hybrids for P. zeae, and on three hybrids for Meloidogyne spp. Under nematode infestation, only 16 hybrids had higher relative yield compared to the mean of both checks, the best check and the trial mean, whereas it was 20 hybrids under nematicide treated plots. Overall, most outstanding hybrids under nematode infestation were CML395/MP709, CML312/5057, CML312/CML206, CML312/CML444, CML395/CML312 and CML312/CML395. Therefore, grain yield loss due to nematodes is existent but can be significantly reduced by growing nematode resistant hybrids.

Assessment of Functional and Pasting Properties of Fresh Orange Maize Hybrids and Open-Pollinated Varieties as Influenced by Genotype, Harvesting Time, and Growing Location.

The study evaluates the effects of genotype, maturity, and growing location on the functional and pasting properties of freshly harvested orange maize hybrids and open-pollinated varieties (OPVs). Eight fresh orange maize hybrid and eight fresh maize OPV, including the control, were harvested at three stages: 20, 27, and 34 days after planting (DAP). The freshly harvested maize samples were lyophilized and characterized for the pasting and functional properties using standard laboratory methods. The peak viscosity, final viscosity, and swelling power of the OPVs increased between 20 and 27 DAP. Additionally, the water absorption capacity increased between 20 and 27 DAP for the maize hybrids, with a decreasing trend between 27 and 34 DAP. However, genotypes 2, from the orange maize hybrid, and 5, amongst the OPV, were outstanding with the highest peak viscosities, indicating good final product quality. The combined ANOVA for the fresh orange maize hybrid and OPV showed a highly significant effect (p ≤ 0.01 and p ≤ 0.001) for the maturity and location on the pasting and functional properties except for the pasting temperature, final viscosity, and pasting time which showed no significant effect. In contrast, the location by genotypes by maturity interactions had no significant effects on the pasting and functional properties of the fresh maize hybrid and the orange OPV except only for the setback, which was highly significant at p ≤ 0.001. Nutritionists, food scientists, and maize breeders could use the information from this study to select the best maize genotypes at the appropriate harvesting period suitable for the production of the preferred maize-based products of consumers.

Characterization of Maize Hybrids (Zea mays L.) for Detecting Salt Tolerance Based on Morpho-Physiological Characteristics, Ion Accumulation and Genetic Variability at Early Vegetative Stage.

Increasing soil salinity due to global warming severely restricts crop growth and yield. To select and recommend salt-tolerant cultivars, extensive genotypic screening and examination of plants' morpho-physiological responses to salt stress are required. In this study, 18 prescreened maize hybrid cultivars were examined at the early growth stage under a hydroponic system using multivariate analysis to demonstrate the genotypic and phenotypic variations of the selected cultivars under salt stress. The seedlings of all maize cultivars were evaluated with two salt levels: control (without NaCl) and salt stress (12 dS m-1 simulated with NaCl) for 28 d. A total of 18 morpho-physiological and ion accumulation traits were dissected using multivariate analysis, and salt tolerance index (STI) values of the examined traits were evaluated for grouping of cultivars into salt-tolerant and -sensitive groups. Salt stress significantly declined all measured traits except root-shoot ratio (RSR), while the cultivars responded differently. The cultivars were grouped into three clusters and the cultivars in Cluster-1 such as Prabhat, UniGreen NK41, Bisco 51, UniGreen UB100, Bharati 981 and Star Beej 7Star exhibited salt tolerance to a greater extent, accounting for higher STI in comparison to other cultivars grouped in Cluster-2 and Cluster-3. The high heritability (h2bs, >60%) and genetic advance (GAM, >20%) were recorded in 13 measured traits, indicating considerable genetic variations present in these traits. Therefore, using multivariate analysis based on the measured traits, six hybrid maize cultivars were selected as salt-tolerant and some traits such as Total Fresh Weight (TFW), Total Dry Weight (TDW), Total Na+, Total K+ contents and K+-Na+ Ratio could be effectively used for the selection criteria evaluating salt-tolerant maize genotypes at the early seedling stage.

Enhancing ß-Carotene Concentration in Parental Lines of CO6 Maize Hybrid Through Marker-Assisted Backcross Breeding (MABB).

Vitamin A deficiency (VAD) is a global health problem; many people around the world, especially children and pregnant women, are VAD deficient or insufficient. Maize is known as an important source of provitamin A for humans. Hence, enhancement of provitamin A carotenoids (pVAC) in maize varieties through breeding or biofortification is a good option for alleviating VAD in developing countries, especially India. So far, numerous maize hybrids have been developed in India. Among them, CO6, derived from UMI1200 × UMI1230, is a popular maize hybrid and adapted to different agro-climatic zones of India, especially Tamil Nadu, a southern state of India. However, CO6 is deficient for pVAC carotenoid ß-carotene. Thus, the objectives of this study were to increase the ß-carotene concentration in UMI1200 and UMI1230 and generate the ß-carotene enriched hybrids through marker-assisted backcross breeding (MABB). For this purpose, the maize genotype HP467-15 was used as the donor for transferring the ß-carotene gene, crtRB1, into UMI1200 and UMI1230. In the MABB scheme, we used one gene-specific marker (crtRB1 3'TE) and 214 simples sequence repeat (SSR) markers for foreground and background selection, respectively. As a result, six improved lines with recurrent parent genome recovery (RPGR) ranging from 90.24 to 92.42%, along with good agronomic performance, were generated. The ß-carotene concentration of the improved lines ranged from 7.056 to 9.232 µg/g. Furthermore, five hybrid combinations were generated using improved lines and evaluated in a comparative yield trial (CYT) and multi-location trials (MLT) along with the original hybrid CO6 and commercial hybrids. It was revealed that ACM-M13-002 was a superior hybrid with a 7.3-fold increase in ß-carotene concentration and with a comparable yield to CO6. In summary, the improved maize inbreds can be used as possible donors for the development of ß-carotene-rich cultivars in maize breeding programs and the ß-carotene enriched hybrid developed in this study will hold great promise for food and nutritional security.

The Effect of Maize Hybrid on Biology and Life Table Parameters of the Trogoderma granarium (Coleoptera: Dermestidae).

The khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), is a polyphagous pest that infests many stored grains and products. The effect of nine maize hybrids including 704, AR 89, AS 71, AS 77, BC 678, KSC 703, PL 472, SC 704, and Simax was studied on biology and life table parameters of T. granarium at controlled conditions (33 ± 1 °C, 65 ± 5% RH, and a photoperiod of 14:10 [L:D] h). According to the results of this study, the immature period was the longest on BC 678 (56.79 ± SE: 1.51 d) and the shortest on PL 472 (39.90 ± 0.48 d). The highest values of fecundity and fertility were observed on PL 472 (67.95 ± 1.23 eggs and 74.81 ± 0.68%, respectively) and the lowest values were on BC 678 (40.00 ± 1.63 and 48.24 ± 1.35, respectively). The net reproductive rate (R0) of T. granarium ranged from 10.40 ± 0.11 offsprings on BC 678 to 30.43 ± 0.20 offsprings on KSC 703. The highest intrinsic rate of increase (rm) was for T. granarium reared on KSC 703 (0.0773 ± 0.0001 d-1), and the lowest was on BC 678 (0.0390 ± 0.0002 d-1). According to the obtained results, BC 678 is an unfavorable hybrid for population increase of T. granarium, which can be recommended to be grown in regions where the damage of T. granarium is considerable to minimize maize infestations by this pest.