Towards a better understanding of the genetic and physiological basis for nitrogen use efficiency in maize

To enhance our understanding of the genetic basis of nitrogen use efficienc y in maize (Zea mays), we have developed a quantitative genetic approach by associating metabolic functions and agronomic traits to DNA markers. In th is study, leaves of vegetative recombinant inbred lines of maize, already a ssessed for their agronomic performance, were analyzed for physiological tr aits such as nitrate content, nitrate reductase (NR), and glutamine synthet ase (GS) activities. A significant genotypic variation was found for these traits and a positive correlation was observed between nitrate content, GS activity and yield, and its components. NR activity, on the other hand, was negatively con elated. These results suggest that increased productivity i n maize genotypes was due to their ability to accumulate nitrate in their l eaves during vegetative growth and to efficiently remobilize this stared ni trogen during grain filling, Quantitative trait loci (QTL) for various agro nomic and physiological traits were searched for and located on the genetic may of maize. Coincidences of QTL for yield and its components with genes encoding cytosolic GS and the corresponding enzyme activity were detected. In particular, it appears that the GS locus on chromosome 5 is a good candi date gene that can, at least partially, cr;plain variations in yield or ker nel weight. Because at this locus coincidences of QTLs for grain yield, GS, NR activity, and nitrate content were also observed, we hypothesize that l eaf nitrate accumulation and the reactions catalyzed by NR and GS are coreg ulated and represent key elements controlling nitrogen use efficiency in ma ize.