Biochemical and genetic analyses of N metabolism in maize testcross seedlings: 2. Roots

Abstract

Intracellular factors differentially affected enzyme activities of N assimilation in the roots of maize testcrosses where alanine aminotransferase and glutamate synthase were the main enzymes regulating the levels of glutamate. N is a key macronutrient for plant growth and development. Breeding maize with improved efficiency in N use could help reduce environmental contamination as well as increase profitability for the farmers. Quantitative trait loci (QTL) mapping of traits related to N metabolism in the root tissue was undertaken in a maize testcross mapping population grown in hydroponic cultures. N concentration was negatively correlated with root and total dry mass. Neither the enzyme activities nor metabolites were appreciably correlated between the root and leaf tissues. Repeatability measures for most of the enzymes were lower than for dry mass. Weak negative correlations between most of the enzymes and dry mass resulted likely from dilution and suggested the presence of excess of enzyme activities for maximal biomass production. Glutamate synthase and alanine aminotransferase each explained more variation in glutamate concentration than either aspartate aminotransferase or asparagine synthetase whereas glutamine synthetase was inconsequential. Twenty-six QTL were identified across all traits. QTL models explained 7-43% of the variance with no significant epistasis between the QTL. Thirteen candidate genes were identified underlying QTL within 1-LOD confidence intervals. All the candidate genes were located in trans configuration, unlinked or even on different chromosomes, relative to the known genomic positions of the corresponding structural genes. Our results have implications in improving NUE in maize and other crop plants.

Fig. 1

Enzymes and proteins involved in N-acquisition and assimilation in C4 plants (created with Adobe Illustrator CS2). AlaAT and GOGAT each explained more variation in glutamate than AspAT and ASN. AlaAT alanine aminotransferase, ASN asparagine synthase, AspAT aspartate aminotransferase, GDH glutamate dehydrogenase, GOGAT glutamate synthase, GS glutamine synthetase, NR nitrate reductase, NiR nitrite reductase, PEPC phosphoenolpyruvate carboxylase, PPDK pyruvate orthophosphate dikinase (adapted from Trucillo Silva et al. 2017)

Fig. 2

Relationship of root dry mass to root N concentration (created with Adobe Illustrator CS2). The R² values for root N versus  shoot dry mass and root N versus total dry mass were 0.35 and 0.39, respectively; *** indicates statistically significant at P < 0.001

Fig. 3

Shoot to root ratios for various enzymes and metabolites in maize IBMsyn10-DH test crosses (created with Adobe Illustrator CS2). The ratio for shoot dry mass to root dry mass was 6.3

Fig. 4

Pearson correlation coefficients for enzyme activities and metabolite concentrations between root and shoot tissues in the IBMSyn10-DH testcross population (created with Adobe Illustrator CS2). *, **, *** indicate statistically significant at P < 0.05, 0.01, and 0.001, respectively

Fig. 5

Repeatability values for traits measured on root and shoot tissues in the maize IBMSyn10-DH TC population (created with Adobe Illustrator CS2). AlaAT alanine aminotransferase, ASN asparagine synthase, AspAT aspartate synthase, GOGAT glutamine oxoglutarate aminotransferase or glutamate synthase, GS glutamine synthetase, NiR nitrite reductase, NR nitrate reductase, PEPC phosphoenol pyruvate carboxylase, Glu glutamate, DM dry matter. For the shoot tissue, enzyme activities were measured on the youngest, fully expanded leaf at V4 stage (Trucillo Silva et al. 2017)

Fig. 6

Direct and indirect relationship between glutamate and enzymes directly involved in its formation and utilization (created with Adobe Illustrator CS2). See pathway in Fig. 1. On the right is the table showing partial R² for each of the enzymes. Double arrows describe the correlation coefficients between various pairs of enzymes. Path coefficients from each of the enzymes are shown as lines with single arrows. Unexplained path coefficient, which is the square root of the unexplained variation (1 − R²), is shown separately. *, **, *** indicate statistically significant at P < 0.05, 0.01, and 0.001, respectively

Fig. 7

Genetic map and distribution of QTL associated with N metabolism related enzymes and metabolites measured on root tissue in the maize IBMSyn10-DH TC population. Created with MapChart 2.2 (Voorrips 2002). QTL positions are shown at left of chromosomes (in cM) and the lengths of  QTL bars are determined by 2-LOD confidence intervals. Only selected markers are displayed in the figure to the right of chromosomes. QTL associated with the enzyme activities are in blue, while QTL associated with metabolites are in red (color figure online)