Quality protein maize (QPM) varieties have been produced by the introduction of opaque-2 modifier genes. Two QPM varieties, BR451 and BR473, a wild type and an opaque-2 variety, have been used to study key enzymes controlling lysine metabolism in the endosperm during development. Aspartate kinase and homoserine dehydrogenase enzymes, which are involved in lysine and threonine biosynthesis, respectively, exhibited identical activity patterns during endosperm development, with a maximum specific activity at 16 days after pollination. The QPM varieties exhibited higher levels of aspartate kinase activity in the endosperm, suggesting an increased rate of lysine biosynthesis when compared to the opaque-2 and wild-type genotypes. Similar results were observed for the lysine ketoglutarate reductase and saccharopine dehydrogenase enzymes, which form a single bifunctional polypetide involved in endosperm lysine degradation. Both enzyme activities were strongly reduced in the opaque-2 maize variety when compared to the wild-type maize, whereas the QPM varieties exhibited even lower levels of lysine ketoglutarate reductase-saccharopine dehydrogenase activities when compared to the opaque-2 variety. The developmental pattern of enzyme activity showed a different profile when compared to the enzymes involved in lysine biosynthesis, with activity being detected only 12-16 days after pollination (DAP) and maximum activities approximately 24 DAP. These results also suggest that the modifier genes have intensified the effect of the opaque-2 mutation on lysine ketoglutarate reductase-saccharopine dehydrogenase. These alterations lead to an increase in soluble lysine in the endosperm of the QPM varieties when compared to the opaque-2 and wild type.

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Phosphorus (P) deficiency is a major limiting factor for crop production in several countries. A better understanding of the genetic components of P use efficiency (PUE) is required to improve crop performance in low‐P soils. To date, no QTLs (quantitative trait loci) were mapped for PUE using grain yield and other late phenotypic data in tropical conditions. Thus, we evaluated the genetic architecture of PUE in tropical maize (Zea mays L.) using multiple interval mapping for design III in a population of 140 RILs (recombinant inbred lines) backcrossed with both parental lines. The parental lines contrasted for yield and for PUE, a phenotypic index that was further decomposed into P acquisition efficiency (PAE) and P utilization efficiency. Our results showed that dominance effects were more important than additive effects for explaining the variations in PUE and its components. Approximately 80% of the QTLs mapped for PAE co‐localized with those for PUE, indicating that the efficiency in acquiring P is the main determinant of PUE in tropical maize. Also, QTLs for PUE and PAE were located near to candidate genes previously associated with root development. Thus, we present important information to guide breeding strategies for the development of maize cultivars more adapted to P deficiency.

The maize (Zea mays L.) populations Iowa Stiff Stalk Synthetic (BSSS) and Iowa Corn Borer Synthetic No. 1 (BSCB1) have undergone reciprocal recurrent selection (RRS) since their establishment in 1949. This study focused on molecular genetic variation of the progenitor inbred lines used to synthesize BSSS and BSCB1 as well as elite inbred lines derived from different cycles of selection. Our objectives were to investigate changes in allele frequencies and genetic diversity from progenitors to derived lines and evaluate trends in genetic diversity among elite lines derived from early and advanced selection cycles. Genotypic data for 105 restriction fragment length polymorphism (RFLP) loci were collected from four groups: 16 progenitors and 18 elite lines derived from BSSS, 12 progenitors and 7 elite lines derived from BSCB1. Each progenitor group had a broad genetic base but both were genetically similar. The groups of derived lines diverged substantially from each other. A larger Roger's distance was found between the groups of lines derived from advanced cycles than between the groups of lines derived from Cycle 0. Allelic variation within each group of lines, however, decreased just slightly with the elite lines capturing almost 75 and 67% of the allelic variation present in the progenitor lines of BSSS and BSCB1, respectively. The results of this study confirm the long‐term potential of this RRS program and the importance of the choice of broadly based progenitor materials.