Luis Narro

Low-P tolerance by maize (Zea mays L.) genotypes: Significance of root growth, and organic acids and acid phosphatase root exudation

We investigated some mechanisms, which allow maize genotypes to adapt to soils which are low in available P. Dry matter production, root/shoot-ratio, root length and root exudation of organic acids and acid phosphatase were investigated in four maize genotypes grown under P-deficient and P-sufficient conditions in sterile hydroponic culture. A low-P tolerant, an acid-tolerant and a low-P susceptible genotype of maize were compared with a Swiss commercial cultivar. The study found increased root development and increased exudation of acid phosphatase under P-deficient conditions in all maize genotypes, except for the Swiss cultivar. Effects on root formation and acid phosphatase were greater for the low-P tolerant than for the low-P susceptible, and the acid soil tolerant genotypes. Organic acid contents in root tissues were increased under P deficiency and related to increased PEPC activity. However, the increase in contents was associated with an increase in exudation for the low-P tolerant genotype only. The low-P susceptible genotype was characterized by high organic acid content in roots and low organic acid exudation. The organic acids content in the phloem exudates of shoots was related to root exudation under different P supply, to the difference between lines in organic acids root content, but not to the low-P tolerance or susceptibility of maize genotypes.

Additive, dominant, and epistatic effects for maize grain yield in acid and non-acid soils

Abstract Acid soils severely reduce maize (Zea mays L.) yield in the tropics. Breeding for tolerance to soil acidity provides a permanent, environmentally friendly, and inexpensive solution to the problem. This study was carried out to determine the relative importance of additive, dominant, and epistatic effects on maize grain-yields in different tropical genotypes. Divergent selection in three populations (SA4, SA5, and SA7) provided inbred lines tolerant or sensitive to acid soils. The tolerant and sensitive lines from each population were used to obtain the F1, F2, F3, back-crosses, second back-crosses, and selfed back-cross generations. In addition, the tolerant lines from SA4 and SA5 were crossed with a sensitive line from the Tuxpeño Sequía population, from which the same generations were also derived. All generations from each of the five sets of crosses were evaluated in three acid-soil environments and one non-acid-soil environment. A generation-mean analysis was performed on each set for yield. The sequential sum of squares associated with additive, dominance, and digenic epistatic effects were used to estimate the relative importance of each genetic effect. Epistasis was not important in any set in the non-acid-soil environment, with dominance accounting for 80.76% of the total variation among generation means across sets. In acid-soil environments, epistasis was more important. The relative importance of digenic epistasis was greater in those evaluations with large experimental errors. The tolerant line from population SA5 was prone to severe root lodging, suggesting a very poor root system. Apparently, the tolerance to soil acidity in this line is not associated with a large root system.

Genome-wide association mapping reveals novel sources of resistance to northern corn leaf blight in maize

BackgroundNorthern corn leaf blight (NCLB) caused by Exserohilum turcicum is a destructive disease in maize. Using host resistance to minimize the detrimental effects of NCLB on maize productivity is the most cost-effective and appealing disease management strategy. However, this requires the identification and use of stable resistance genes that are effective across different environments.ResultsWe evaluated a diverse maize population comprised of 999 inbred lines across different environments for resistance to NCLB. To identify genomic regions associated with NCLB resistance in maize, a genome-wide association analysis was conducted using 56,110 single-nucleotide polymorphism markers. Single-marker and haplotype-based associations, as well as Anderson-Darling tests, identified alleles significantly associated with NCLB resistance. The single-marker and haplotype-based association mappings identified twelve and ten loci (genes), respectively, that were significantly associated with resistance to NCLB. Additionally, by dividing the population into three subgroups and performing Anderson-Darling tests, eighty one genes were detected, and twelve of them were related to plant defense. Identical defense genes were identified using the three analyses.ConclusionAn association panel including 999 diverse lines was evaluated for resistance to NCLB in multiple environments, and a large number of resistant lines were identified and can be used as reliable resistance resource in maize breeding program. Genome-wide association study reveals that NCLB resistance is a complex trait which is under the control of many minor genes with relatively low effects. Pyramiding these genes in the same background is likely to result in stable resistance to NCLB.

Implications of Soil-Acidity Tolerant Maize Cultivars to Increase Production in Developing Countries

About 3950 million ha (30%) of the total ice-free land area in the world is under acid soils. Acid soils are characterized by low fertility caused by high levels of Al, Mn, and Fe, and deficiencies of P, Ca, Mg, K, S, and Zn. Of these, Al toxicity and P deficiency seem to be the most important causes for low maize yields (about 400 kg/ha for land race cultivars) in these soils. Maize is a staple for millions of people in developing countries where imports are growing up by 1.5 million tons (7%) per year. There are at least two alternatives to increasing maize production in acid soils. The first is to use amendments (lime, gypsum) to correct soil acidity. This is expensive and not available for small farmers. Another disadvantage is that only the upper 30 cm of soil is corrected making maize roots to be concentrated in that layer and not growing beyond. The second approach is to develop tolerant cultivars. This solution is relatively inexpensive, environmentally clean, permanent, and energy conserving. International Center for the Improvement of Wheat and Maize (CIMMYT), in collaboration with several National Agriculture Research NARS all over the world, has been developing soil-acidity tolerant maize cultivars to increase maize production. For this purpose, acid soil tolerant maize populations were formed and recurrent selection was used to improve these populations for grain yield under both acid and normal soils. Cultivars developed from these populations show a consistent increase in grain yield in both acid and non-acidic soils. Under acid soils, the average grain yield of genotypes used to form the base population in 1977, was below 0.4 t/ha. The average grain yield of acid soil tolerant open pollinated varieties (OPVs) developed in 1993 and evaluated across 13 acid soil environments was 3.2 t/ha, while that of non conventional hybrids developed in 1995 and evaluated across six acid soil environments was 3.84 t/ha. In acid soils, high parent heterosis of up to 42.5% has been observed in inter-variety crosses and up to 261% in single crosses. Also, although superiority of hybrids compared to OPVs has been reported, OPVs will continue to be more important than hybrids for the acid soils over the next years, due to the poor economic conditions of farmers cultivating them. From our agronomic research, response to P of Sikuani, an acid soil tolerant cultivar, is higher than that of Tuxpeno (a susceptible cultivar) both in acidic and non-acidic soils. Grain yield of maize was higher using amendments with Ca, Mg, and S than with Ca and Mg only, although there were no differences among methods of application of amendments. Studies of microelements in acid soils showed a highly significant response to Zn. Planting maize in association with pastures in savannas could be a profitable alternative for farmers allowing them to raise more animals/ha with an associated gain of weight/animal. We are also looking for morphological and biochemical traits that may improve grain yield. Molecular and physiological studies are being continued to improve the efficiency of our breeding program. These activities are being developed in collaboration with prominent universities and research institutions worldwide.

Genetic distance and hybrid value in tropical maize under P stress and non stress conditions in acid soils

An emphasis in maize breeding for areas with acid soils is the development of varieties with tolerance to P-deficiency plus high yield potential in acidic as well as normal soils. This study was carried out to assess the (i) genetic diversity within a set of tropical inbred lines developed from acid soil-tolerant populations; (ii) F1 yield performance, mid-parent heterosis (MPH), high-parent heterosis (HPH), and specific combining ability (SCA) in a diallel set of crosses under P stress (low P) and non-stress (high P) conditions; and (iii) the effect of P stress on the relationship between genetic distance (GD) and hybrid performance. Using field evaluation and molecular marker studies, the results show that these germplasm from the South American maize breeding program of CIMMYT for improving tolerance to acid soils had only a moderate level of genetic diversity. The utility of GD as a predictor of hybrid value is best up to a certain threshold, as correlations with GD became inconsistent when the inbred parents were greatly divergent. There was no correlation between GD and F1 grain yield, MPH, HPH and SCA when the GD was >0.77. The high correlation of GD with F1 grain yield and with SCA in specific subsets of crosses having a narrower range of GD shows that GD can be put to practical use in predicting hybrid performance. The highest correlation between GD and SCA, seen in the subset of crosses between lines within a cluster, was reasonably stable even when the environment had a severe effect on yield.

Genetic effects for maize traits in acid and non-acid soils

Breeding programs for acid-soil tolerance are desirable as a relatively inexpensive and permanent way for increasing maize (Zea mays L.) yield on these soils. Our objective was to compare the genetic effects controlling the expression of maize traits in acid and non-acid soils. Seven related and one unrelated inbred lines, with different levels of tolerance to acid soil, and their F 1 ,F 2 ,B C 1 , and BC 2 generations were evaluated in four acid and two non-acid soils. Estimates of additive, dominance, and epistatic effects were computed for grain yield, plant height, days to mid-silk, and prolificacy, using the generation means analysis procedure. For all traits the major part of the variation was accounted for by additive and dominance effects, with dominance effects being more important than additive and epistatic effects for both acid and non-acid soils. Epistatic effects were significant for some crosses only, being more pronounced for plant height than for the other traits. Furthermore, epistatic effects were randomly distributed among the crosses and were not related to the grain yield of the single-crosses (F1’s) and to the genetic relationships of the inbreds in either type of soil. The results suggest that similar pooled gene effects control the expression of the traits assessed in both acid and non-acid soils.