A Whole-Plant Metabolic Model of Maize

Zea mays L., commonly known as maize or corn, is an essential food and energy crop. The whole-plant metabolic reconstruction links the root, stalk, leaf, kernel, and tassel tissues using the phloem as the main form of metabolite transport among the tissues. Tissue specific gene-protein-reaction (T-GPR) relationships were defined for each reaction. Whole-plant transcriptomic data and the T-GPR relationships were used to determine the set of reactions known to be present within each tissue. This set not including duplicate reactions from compartmentalization for the root, shoot, leaf, seed, and tassel consisted of 1919, 1851, 1982, 1938, and 1951 reactions, respectively. A tissue-specific biomass equation was defined based on the known metabolic function of each tissue. The flow through the biomass reaction of each tissue was restored by preferentially adding reactions known to occur in maize with no tissue localization information over reactions corresponding to low expression in the focus tissue. The whole-plant model was used to analyze the flow of metabolites among the tissues and divided to analyze specific tissues. The activity of approximately 40 enzymes was measured within the leaf for 20 different maize lines representative of American and European plant diversity, including an ancestral teosinte line. These 40 enzymes, mainly involved in primary carbon and nitrogen assimilation, were mapped to their associated reactions and the flux ranges were restricted accordingly. By comparing the non-overlapping flux ranges in the leaf tissue at maximum biomass, we have determined the change in metabolism in a pair-wise manner among the 20 lines. The B73 reference line is the most disparate, sharing on average only 69.2% of active metabolic reaction ranges with the other 19 strains. The remaining 19 strains share an average of between 85.2% and 89.2% of their active metabolism in terms of reaction flux ranges. The whole-plant genome-scale model is required to analyze the flow of nitrogen from the plant root to the other tissues, suggest genetic interventions to alleviate bottlenecks in metabolism, and study the effect of nitrogen on sugar storage in the seed.