The work a is a cooperation between scientists from the University of Lorraine, Nancy, France and the University of Stavanger. Mathematical modeling has become an important tool in trying to understand the behavior and regulatory properties of complex biological systems. In this seminar Ruoff and colleagues point towards aspects how secondary metabolites are unregulated when nitrogen levels are low.
Secondary metabolic compounds are crucial for defending plants against environmental assaults at less optimum growth conditions. For example, at low nitrate conditions part of the primary carbon flux is diverted into secondary metabolic pathways, which produce phenolic compounds for defense.
The mechanisms how primary and secondary metabolic pathways are connected and the factors that mediate the partitioning of compounds between them are still poorly understood. Based on the observation that sucrose is under homeostatic control a model how secondary metabolites are upregulated under low nitrate/nitrogen conditions is proposed.
At sufficiently high nitrate levels sucrose homeostasis is maintained due to a nitrate-dependent stimulation of the C-flux from sucrose into the primary C-pathway leading to optimum growth. At low and insufficient nitrate concentrations, the nitrate-activated primary C-flux decreases accordingly. In order to avoid a buildup of sucrose at low nitrate conditions by photosynthesis due to the diminished demand for sucrose by the primary C-flux, excess of sucrose is diverted into the secondary phenolic pathway by a sucrose-dependent negative feedback regulation.
The model can account for the accumulation of starch during the light phase of plant growth and the sucrose remobilization by starch degradation during the night. An integrative light sensing mechanism for variable light-dark regimes is suggested, which can explain the complete starch consumption during the night for each light-dark regime. Furthermore, the model can also account for the increased accumulation of starch under nitrate limitation.