(175bg) Metabolic Systems Biology of Poplar, a Biofuel Crop

The optimization of upstream processing in biofuel production, particularly from biofuel-producing trees such as poplar, is critical for scaling up to commercial levels. Advancements in systems and synthetic biology could be crucial in transforming upstream processing. Systems biology tools such as flux balance analysis (FBA) and flux variability analysis (FVA), which use stoichiometry to predict the range of possible metabolic states, can rapidly evaluate biomass yield and biofuel production. Thus, these tools can accelerate the selection of environmental conditions and genetic modifications for optimal growth and productivity. This talk will explore the application of these analyses to understand nitrogen flow, storage and cycling in the biofuel crop poplar.

Poplar and other perennial trees undergo an annual cycle of leaf senescence in fall and regrowth in spring. During this cycle, trees orchestrate major changes in their metabolic landscape to conserve nitrogen, which is a scarce but crucial nutrient used in the production of biomass. Field observations have highlighted glutamine (Gln) as the predominant or sole nitrogen carrier between tissues during these metabolic changes. However, past experimental studies had not been able to pinpoint the metabolic benefit of using Gln as a nitrogen carrier, and it is currently puzzling why Gln almost exclusively plays this role. Toward a fundamental understanding of this problem, we report the application of FBA on a two-tissue metabolic model of poplar. Previously, we reported FBA simulations in which different amino acids (AAs) function as sole nitrogen carriers from the leaf tissue to the bark tissue. A breakdown of the costs and benefits associated with these scenarios showed that Gln offers the lowest cost and thus, the highest benefit, as a nitrogen carrier.

In this work, we extended this inquiry by performing FVA and Pareto analysis to evaluate trade-offs in metabolic resource allocation during nitrogen transport. This analysis revealed that if used as the sole nitrogen carrier, Gln can maintain the minimally required flux through the photosynthetic enzyme RuBisCO. Further analysis showed a Pareto relationship between RuBisCO flux and photon flux during leaf regrowth in spring. This implies an intricate balance between carbon and energy use during nitrogen transport. Exploring further, we constructed a model to simulate concurrent nitrogen transport by two amino acids. While maximizing leaf biomass maximization and concurrently minimizing RuBisCO flux, this model predicted that amino acid combinations involving Gln, asparagine, arginine, and glycine yielded the highest leaf biomass. However, Gln demonstrated

This talk will cover these insights as well as model predictions along the same lines on nitrogen mobilization during leaf senescence in fall. These findings highlight computational flux analyses as foundational tools for rational design in metabolic engineering and increased biofuel synthesis.