Figure 1.

Experimental design to investigate C and N interactions. (a) Hypothetical models to explain regulation by C and N metabolites. The four possible models of gene expression response to N and C treatments are illustrated. Model 1 (N independent of C) represents genes that are regulated by N in a manner that is independent of the amount of C present. Model 2 (C independent of N) is equivalent to model 1 but for C. Model 3 represents different types of interactions between C and N. Model 4 represents regulation by the ratio of C/N. In this case, neither C nor N can affect gene expression. Regulation according to all models could be positive or negative, but only positive examples are depicted. (b) Systematic experimental space to investigate C and N interactions. To investigate gene responses to C and N, we used experiments where plants were exposed to C, N or C+N. The graphs summarize the experiments carried out. Each point in the graphs corresponds to one experiment. The x-axis indicates the concentration of nitrate used (nitrogen source) in the experiment. The y-axis indicates the concentration of sucrose used (carbon source) in the experiment. For example, points on the x-axis correspond to experiments in which plants were treated with nitrate in the absence of sucrose.