We have developed an approach combining physiology and quantitative genetics to enhance our understanding of nitrogen (N) metabolism during kernel germination. The physiological study highlighted the central role of glutamine (Gln) synthetase (GS) and Gln synthesis during this developmental process because a concomitant increase of both the enzyme activity and the amino acid content was observed. This result suggests that Gln is acting either as a sink for ammonium released during both storage protein degradation and amino acid deamination or as a source for amino acid de novo synthesis by transamination. In the two parental lines used for the quantitative genetics approach, we found that the increase in Gln occurred earlier in Io compared with F 2 , a result consistent with its faster germinating capacity. The genetic study was carried out on 140 F6 recombinant inbred lines derived from the cross between F 2 and Io. Quantitative trait locus mapping identified three quantitative trait loci (QTLs) related to germination trait (T50, time at which 50% of the kernels germinated) that explain 18.2% of the phenotypic variance; three QTLs related to a trait linked to germination performance, kernel size/weight (thousand kernels weight), that explain 17% of the phenotypic variance; two QTLs related to GS activity at early stages of germination that explain 17.7% of the phenotypic variance; and one QTL related to GS activity at late stages of germination that explains 7.3% of the phenotypic variance. Coincidences of QTL for germination efficiency and its components with genes encoding cytosolic GS (GS1) and the corresponding enzyme activity were detected, confirming the important role of the enzyme during the germination process. A triple colocalization on chromosome 4 between gln3 (a structural gene encoding GS1) and a QTL for GS activity and T50 was found; whereas on chromosome 5, a QTL for GS activity and thousand kernels weight colocalized with gln4, another structural gene encoding GS1. This observation suggests that for each gene, the corresponding enzyme activity is of major importance for germination efficiency either through the size of the grain or through its faster germinating capacity. Consistent with the possible nonoverlapping function of the two GS1 genes, we found that in the parental line Io, the expression of Gln3 was transiently enhanced during the first hours of germination, whereas that of gln4 was constitutive.Seeds are complex structures consisting anatomically and genetically of three parts: the embryo, the endosperm, and the seed coat. During their development, seeds are supplied by the mother plant with assimilates. From a physiological point of view, seeds behave as sink organs accumulating C and N metabolites stored as starch, proteins, amino acids, or lipids. C and N metabolism in developing seeds has been extensively studied in various monocot species, i.e. maize (Zea mays) and wheat (Triticum aestivum), as well as dicot species, i.e. pea (Pisum sativum), soybean (Glycine max), and alfalfa ...

To determine (1) the correlation among imbibition time at which 50% of maize seeds germinate (T50), 1000-kernel weight (TKW) and glutamine synthetase [glutamate-ammonia ligase] (GS) activity and (2) the quantitative trait locus (QTL) for the traits, the following investigations were carried out: determination of the germination properties of two maize parental genotypes, Io and F2, at 21 and 25°C; the physiological characterization of nitrogen metabolism during maize kernel germination; the genetic analysis and quantitative trait loci mapping of nitrogen metabolism in relation to maize kernel germination; and the genetic analysis of maize kernel germination in relation to its size/weight. Germination was studied as a complex developmental trait in parallel with a physiological trait, activity of GS, a major enzyme of amino acid metabolism during kernel germination. The approach allowed the identification of GS as a candidate gene in the control of germination, which was strongly supported by the co-localization of QTLs of germination (T50) and GS activity at gln3 locus on chromosome 4 and the transient expression of gln3 during the germination process.