Maize source leaf adaptation to nitrogen deficiency affects not only nitrogen and carbon metabolism but also control of phosphate homeostasis

Abstract

Crop plant development is strongly dependent on the availability of nitrogen (N) in the soil and the efficiency of N utilization for biomass production and yield. However, knowledge about molecular responses to N deprivation derives mainly from the study of model species. In this article, the metabolic adaptation of source leaves to low N was analyzed in maize (Zea mays) seedlings by parallel measurements of transcriptome and metabolome profiling. Inbred lines A188 and B73 were cultivated under sufficient (15 mM) or limiting (0.15 mM) nitrate supply for up to 30 d. Limited availability of N caused strong shifts in the metabolite profile of leaves. The transcriptome was less affected by the N stress but showed strong genotype- and age-dependent patterns. N starvation initiated the selective down-regulation of processes involved in nitrate reduction and amino acid assimilation; ammonium assimilation-related transcripts, on the other hand, were not influenced. Carbon assimilation-related transcripts were characterized by high transcriptional coordination and general down-regulation under low-N conditions. N deprivation caused a slight accumulation of starch but also directed increased amounts of carbohydrates into the cell wall and secondary metabolites. The decrease in N availability also resulted in accumulation of phosphate and strong down-regulation of genes usually involved in phosphate starvation response, underlining the great importance of phosphate homeostasis control under stress conditions.

Figure 1.

Effective PSII quantum yield of maize leaves at 28 d after germination. Plants were cultivated under control (high-N) or low-N conditions. The asterisk indicates significant differences in comparison with L6 from high-N-grown plants. Fluorescence could not be measured in L7 from low-N-treated plants. nd, Not determined. All data points represent average from four plants ± sd.

Figure 2.

PCA. A, Calculated from 41,780 transcripts (PC2 versus PC3). B, Calculated from 531 metabolites peaks (PC1 versus PC2).

Figure 3.

Transcripts differentially regulated by low- versus high-N conditions. Transcripts are grouped according to participation in main biological processes.

Figure 4.

Metabolites significantly different in leaves under low- versus high-N conditions. Metabolites are grouped according main compound classes.

Figure 5.

Dendrogram of selected transcript modules, metabolites, and phenotypic features calculated by the WGCNA analysis tool. The small graphs show the average pattern from features on the two main branches of the dendrogram.

Figure 6.

Transcriptional changes of primary C and N metabolism under low N. From left to right, the four columns represent changes measured in A188 at 20 d, in B73 at 20 d, in A188 at 30 d, and in B73 at 30 d. Heat maps show log2 fold changes in low- versus high-N conditions. Thin frame, Pathway preferentially in mesophyll; double frame, pathway in both cell types; thick frame, pathway preferentially in bundle sheath.

Figure 7.

Amino acid metabolism under low-N conditions. In the pathways, changes measured for metabolite concentrations are given as heat maps of log2 fold changes in low- versus high-N-treated maize source leaves, and changes of transcripts of the participating enzymes are given on the right. A, Glu, Gln, Asp, and Asn metabolism. B, Ala metabolism. C, Ser, Gly, and Cys metabolism. D, Arg and Pro metabolism. E, Thr and Lys metabolism. F, Tyr, Phe, and Trp metabolism. G, Val, Leu, and Ile metabolism.

Figure 8.

Changes in metabolite profile under low N. From left to right, the four columns represent changes measured in A188 at 20 d, in B73 at 20 d, in A188 at 30 d, and in B73 at 30 d. Heat maps show log2 fold changes in low- versus high-N conditions.