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

Abstract

To enhance our understanding of the genetic basis of nitrogen use efficiency in maize (Zea mays), we have developed a quantitative genetic approach by associating metabolic functions and agronomic traits to DNA markers. In this study, leaves of vegetative recombinant inbred lines of maize, already assessed for their agronomic performance, were analyzed for physiological traits such as nitrate content, nitrate reductase (NR), and glutamine synthetase (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 correlated. These results suggest that increased productivity in maize genotypes was due to their ability to accumulate nitrate in their leaves during vegetative growth and to efficiently remobilize this stored nitrogen during grain filling. Quantitative trait loci (QTL) for various agronomic and physiological traits were searched for and located on the genetic map 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 candidate gene that can, at least partially, explain variations in yield or kernel weight. Because at this locus coincidences of QTLs for grain yield, GS, NR activity, and nitrate content were also observed, we hypothesize that leaf nitrate accumulation and the reactions catalyzed by NR and GS are coregulated and represent key elements controlling nitrogen use efficiency in maize.

Figure 1

Coincidences between QTLs for physiological traits and traits related to GY and its components. Location of the QTLs for physiological traits on the maize RFLP genetic map are indicated by oval symbols: blue for leaf NO₃⁻ content, green for leaf GS activity, and red for leaf NR activity. Locations of the QTLs for agronomic traits are indicated by vertical bars. Bars on the left side of the chromosomes are for plant grown under high nitrogen input (N⁺) and bars on the right side of the chromosomes are for plants grown under low nitrogen input (N⁻). Favorable allele from the parental line I_o is indicated by (+) and unfavorable allele by (−). The position of the loci for genes encoding enzymes involved in nitrogen assimilation is indicated in bold italics: AS1 and AS2 (Asn synthetase 1 and 2); Fd-GOGAT (Ferredoxin-dependent Glu synthase); GDH1 (GDH 1); gnl1 to 5 (Gln synthetase 1 to 5); NR (nitrate reductase); NiR (nitrite reductase); and NTR1 (high-affinity nitrate transporter). The position of the loci for genes encoding enzymes involved in carbon assimilation is indicated in small bold characters; see Causse et al. (1995). ADPG, ADP Glc pyrophosphorylase; INV, invertase; SPS, Suc phosphate synthase.