Abebe Menkir

The influence of drought stress on growth, yield and yield components of selected maize genotypes

The risk of drought is high in the Sudan savannah zone of West and Central Africa because rainfall in this area is unpredictable in quantity and distribution. Thus, improved maize genotypes tolerant to drought could stabilize maize grain yield in this zone, where recurrent drought threatens grain production. Six maize genotypes, two each of hybrids, open-pollinated varieties (OPVs) and landraces, were evaluated for tolerance to terminal water deficit before flowering. Water deficit significantly reduced growth, grain yield and yield components of the maize genotypes. Significant differences were observed among genotypes for all the traits measured. One hybrid, 9011-30, and two improved OPVs, STR-EV-IWD and IYFD-C0, that showed tolerance to water stress recorded higher grain yield, and accumulated and partitioned more assimilates to the grain than the drought-susceptible genotypes. Also the drought-tolerant genotypes, 9011-30, STR-EV-IWD and IYFD-C0 had more ears/plant and greater numbers of kernels/ear. These genotypes could serve as sources of drought tolerance for the development and improvement of new drought-tolerant maize genotypes.

Genetic analysis of resistance to gray leaf spot of midaltitude maize inbred lines

Gray leaf spot (GLS), caused by Cercospora zeae-maydis Tehon & E.Y. Daniels, poses a serious threat to maize (Zea mays L.) production in sub-Saharan Africa. The knowledge of inheritance of resistance to GLS in new inbred lines would be useful for efficient development of hybrids and synthetics. In this study, we determined (i) the mode of inheritance of resistance to GLS in midaltitude inbred lines, (ii) the effect of different doses of resistance to GLS in parents on the levels of resistance of their hybrids, and (iii) heterotic effects for GLS resistance. Ninety-six hybrids from 24 inbreds were produced using the Design II mating scheme. The parents and the hybrids were evaluated in separate trials in five environments in Nigeria. Both general (GCA) and specific (SCA) combining abilities were significant (P 70% of the variation for GLS scores, days to silking, plant height, ear height, ear aspect, and ear rot; 68% for grain yield; and 60% for plant aspect (visual phenotypic appeal) score. Predominantly, additive genetic effects influenced resistance to GLS and other traits in maize hybrids. Most of the crosses with one or more resistant parents produced resistant hybrids, whereas most crosses between susceptible lines generated susceptible hybrids. Prediction of GLS in hybrids using midparent values resulted in a R 2 value of 0.53 for GLS disease score recorded 38 d after midsilking (GLS Score2). Negative heterosis observed in 75 hybrids for GLS Score2 suggested that resistance to GLS could be improved in midaltitude hybrids.

Resistance to Aflatoxin Accumulation in Kernels of Maize Inbreds Selected for Ear Rot Resistance in West and Central Africa

Thirty-six inbred lines selected in West and Central Africa for moderate to high resistance to maize ear rot under conditions of severe natural infection were screened for resistance to aflatoxin contamination using the previously established kernel screening assay. Results showed that more than half the inbreds accumulated aflatoxins at levels as low as or lower than the resistant U.S. lines GT-MAS:gk or MI82. In 10 selected aflatoxin-resistant or aflatoxin-susceptible inbreds, Aspergillus flavus growth, which was quantified using an A. flavus transformant containing a GUS-beta-tubulin reporter gene construct, was, in general, positively related to aflatoxin accumulation. However, one aflatoxin-resistant inbred supported a relatively high level of fungal infection, whereas two susceptibles supported relatively low fungal infection. When kernels of the 10 tested lines were profiled for proteins using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, significant variations from protein profiles of U.S. lines were observed. Confirmation of resistance in promising African lines in field trials may significantly broaden the resistant germplasm base available for managing aflatoxin contamination through breeding approaches. Biochemical resistance markers different from those being identified and characterized in U.S. genotypes, such as ones inhibitory to aflatoxin biosynthesis rather than to fungal infection, may also be identified in African lines. These discoveries could significantly enhance the host resistance strategy of pyramiding different traits into agronomically useful maize germplasm to control aflatoxin contamination.

Genetic variation for grain mineral content in tropical-adapted maize inbred lines

Increasing the concentrations of Fe and Zn in staple food crops through breeding has been proposed as one strategy to minimize the adverse effects of widespread mineral deficiencies in humans. This approach requires the presence of adequate genetic differences in concentrations of grain minerals for improvement. Eight trials involving different sets of tropical maize inbred lines adapted to the lowlands and mid-altitudes were, therefore, evaluated for concentrations of grain Fe, Zn and other minerals in two locations. The combined analyses of variance showed significant variation in concentrations of grain minerals among inbred lines in each trial, which was always greater than the variation caused by locations and line×location interactions. The line×location interaction had no significant effect on concentrations of Fe, Zn, Cu, Mg and P in at least three trials of lowland inbred lines. The line×location interaction also did not significantly affect the concentrations of any minerals, except S, in at least three trials of mid-altitude inbred lines. The best-inbred lines identified from each trial had 32-78% more Fe and 14-180% more Zn than their trial average. The first two principal component axes, which accounted for 55-64% of the total variation in kernel mineral concentrations, stratified the inbred lines in each trial into four groups based on differences in their grain mineral compositions. None of the correlations of Fe and Zn with Mn, Cu, Ca, Mg, K, P and S was significant and negative in the various trials, while the correlations of Fe with Zn were positive and significant (r=0.55 to r=0.68, p<0.0001) in almost all the trials. These results suggest that a genetic potential exists for concurrent improvement of Fe and Zn without lowering the concentrations of other grain minerals in maize.

Resistance to Striga hermonthica in a maize inbred line derived from Zea diploperennis.

Breeding for resistance to Striga in maize (Zea mays), with paucity of donor source and known mechanisms of resistance, has been challenging. Here, post-attachment development of S. hermonthica was monitored on two maize inbreds selected for field resistance and susceptibility reactions to Striga at the International Institute of Tropical Agriculture. Haustorial invasion of the parasite into roots of these inbreds was examined histologically. Morphological differences were observed between roots of the susceptible and the resistant inbreds. The resistant maize had fewer Striga attachments, delayed parasitic development and higher mortality of attached parasites compared with the susceptible inbred. Striga on the susceptible inbred usually penetrated the xylem and showed substantial internal haustorial development. Haustorial ingress on the resistant inbred was often stopped at the endodermis. Parasites able to reach resistant host xylem vessels showed diminished haustorial development relative to those invading susceptible roots. These results suggest that the resistant inbred expresses a developmental barrier and incompatible response against Striga parasitism.

Genotypic differences in concentration and bioavailability of kernel iron in tropical maize varieties grown under field conditions

Abstract Iron deficiency is estimated to affect over one‐half the world population. Improving the nutritional quality of staple food crops through breeding for high bioavailable iron represents a sustainable and cost effective approach to alleviating iron malnutrition. Forty‐nine late maturing tropical elite maize varieties were grown in a lattice design with two replications in three locations representing three agroecologies in West and Central Africa to identify varieties with high levels of kernel‐Fe. Bioavailable iron was assessed for some varieties selected for high Fe concentration in kernel and improved agronomic traits using an in vitro digestion/Caco‐2 cell model. Significant differences in kernel‐Fe and ‐zinc concentration were observed among varieties (P < 0.001). Kernel‐Fe levels ranged from 16.8 to 24.4 mg kg−1, while kernel‐Zn levels ranged from 16.5 to 24.6 mg kg−1. Environment did not have a significant effect on kernel‐iron and ‐zinc levels, but genotype by environment (G × E) interaction was highly significant. The genetic component accounted for 12% of the total variation in kernel‐Fe and 29% for kernel‐Zn levels. Kernel‐Fe was positively correlated with kernel‐Zn (R 2 = 0.51, P < 0.0001). Significant differences in iron bioavailability were detected among selected Fe‐rich varieties grown at one location. Mean bioavailable Fe ranged between 30% below to 88% above the reference control variety. The results indicate that genetic differences exist in kernel‐Fe and ‐Zn concentrations and Fe bioavailability. These differences may be useful in biofortification intervention programs, but additional research is needed to determine the efficacy of iron‐rich maize varieties in alleviating iron deficiency in humans. #This research was supported by USDA-ARS/USAID and IITA.

Environmental stability of iron and zinc concentrations in grain of elite early-maturing tropical maize genotypes grown under field conditions

SUMMARY Assessment of the stability of micronutrients is important in breeding for the enhanced nutritional quality of staple food crops as a means to alleviate malnutrition. Twenty early-maturing elite tropical maize (Zea mays L.) genotypes were evaluated over 2 years at three locations representing three distinct agroecologies in West and Central Africa (WCA). The objectives were to analyse the pattern of genotyperenvironment interactions (GEI) and environmental stability of iron and zinc con- centrations in grain using the Additive Main Effects and Multiplicative Interaction (AMMI) stat- istical model. Results indicated that the effects of genotypes, environments and GEI were significant (P<0 . 05) for both micronutrients. The effect of GEI was about double the contribution of the genotypes for grain iron and more than double the effect of genotypes for grain zinc. Partitioning of GEI indicated that varietyrlocation was the dominant source of a significant amount of GEI for both micronutrients. Scores of the first two interaction principal component axes (IPCA1 and IPCA2) from the AMMI were significant and accounted for 0 . 68-0 . 75 of the pattern of GEI for both micronutrients. About half of the genotypes evaluated were stable for grain iron and zinc concen- tration over the set of environments. The AMMI model identified ACR98TZEMSR-W as the most stable genotype for grain iron and MAKA-SRBC5 was the most stable for grain zinc. However, the yellow genotype, AK94-DMR-ESR-Y was the most promising, with high and moderately stable concentrations of iron and zinc in the grain. Because it is yellow, with beta-carotene content and high concentrations of iron and zinc in the grain, it might significantly contribute to an improved intake of these micronutrients in populations who rely on maize for a major portion of their daily diet.

A USA-Africa collaborative strategy for identifying, characterizing, and developing maize germplasm with resistance to aflatoxin contamination

Aflatoxin contamination of maize by Aspergillus flavus poses serious potential economic losses in the US and health hazards to humans, particularly in West Africa. The Southern Regional Research Center of the United States Department of Agriculture, Agricultural Research Service (USDA-ARS-SRRC) and the International Institute of Tropical Agriculture (IITA) initiated a collaborative breeding project to develop maize germplasm with resistance to aflatoxin accumulation. Resistant genotypes from the US and selected inbred lines from IITA were used to generate backcrosses with 75% US germplasm and F1 crosses with 50% IITA and 50% US germplasm. A total of 65 S4 lines were developed from the backcross populations and 144 S4 lines were derived from the F1 crosses. These lines were separated into groups and screened in SRRC laboratory using a kernel-screening assay. Significant differences in aflatoxin production were detected among the lines within each group. Several promising S4 lines with aflatoxin values significantly lower than their respective US resistant recurrent parent or their elite tropical inbred parent were selected for resistance-confirmation tests. We found pairs of S4 lines with 75–94% common genetic backgrounds differing significantly in aflatoxin accumulation. These pairs of lines are currently being used for proteome analysis to identify resistance-associated proteins and the corresponding genes underlying resistance to aflatoxin accumulation. Following confirmation tests in the laboratory, lines with consistently low aflatoxin levels will be inoculated with A. flavus in the field in Nigeria to identify lines resistant to strains specific to both US and West Africa. Maize inbred lines with desirable agronomic traits and low levels of aflatoxin in the field would be released as sources of genes for resistance to aflatoxin production.

Marker-trait association analysis of functional gene markers for provitamin A levels across diverse tropical yellow maize inbred lines.

BackgroundBiofortification of staple crops is a cost effective and sustainable approach that can help combat vitamin A and other micronutrient deficiencies in developing countries. PCR -based DNA markers distinguishing alleles of three key genes of maize endosperm carotenoid biosynthesis (PSY1, lcyE and crtRB1) have been developed to facilitate maize provitamin A biofortification via marker assisted selection. Previous studies of these functional DNA markers revealed inconsistent effects. The germplasm previously employed for discovering and validating these functional markers was mainly of temperate origin containing low frequencies of the favourable allele of the most significant polymorphism, crtRB1-5′TE. Here, we investigate the vitamin A biofortification potential of these DNA markers in a germplasm panel of diverse tropical yellow maize inbred lines, with mixed genetic backgrounds of temperate and tropical germplasm to identify the most effective diagnostic markers for vitamin A biofortification.ResultsThe functional DNA markers crtRB1-5′TE and crtRB1-3′TE were consistently and strongly associated with provitamin A content across the tropical maize inbred lines tested. The alleles detected by these two functional markers were in high linkage disequilibrium (R2 = 0.75) and occurred in relatively high frequency (18%). Genotypes combining the favourable alleles at the two loci (N = 20) displayed a 3.22 fold average increase in β-carotene content compared to those genotypes lacking the favourable alleles (N = 106). The PSY1 markers were monomorphic across all of the inbred lines. The functional DNA markers for lcyE were associated with lutein, and with the ratio of carotenoids in the alpha and beta branches, but not with provitamin A levels. However, the combined effects of the two genes were stronger than their individual effects on all carotenoids.ConclusionsTropical maize inbred lines harbouring the favourable alleles of the crtRB1-5′TE and 3′TE functional markers produce higher levels of provitamin A. Such maize lines can be used as donor parents to speed up the development of provitamin A biofortified tropical maize varieties adapted to growing conditions and consumer preferences, providing a route towards mitigation of vitamin A malnutrition in Sub-Saharan Africa.

Discovery and characterization of proteins associated with aflatoxin-resistance: evaluating their potential as breeding markers.

Host resistance has become a viable approach to eliminating aflatoxin contamination of maize since the discovery of several maize lines with natural resistance. However, to derive commercial benefit from this resistance and develop lines that can aid growers, markers need to be identified to facilitate the transfer of resistance into commercially useful genetic backgrounds without transfer of unwanted traits. To accomplish this, research efforts have focused on the identification of kernel resistance-associated proteins (RAPs) including the employment of comparative proteomics to investigate closely-related maize lines that vary in aflatoxin accumulation. RAPs have been identified and several further characterized through physiological and biochemical investigations to determine their causal role in resistance and, therefore, their suitability as breeding markers. Three RAPs, a 14 kDa trypsin inhibitor, pathogenesis-related protein 10 and glyoxalase I are being investigated using RNAi gene silencing and plant transformation. Several resistant lines have been subjected to QTL mapping to identify loci associated with the aflatoxin-resistance phenotype. Results of proteome and characterization studies are discussed.