A Phenomenological Model of Aggregation Dynamics in in Vitro Plant Cell Culture Systems

In vitro plant cell culture systems represent a renewable alternative to supply valuable natural products, which may often be in limited supply due to production constraints in both natural harvest and chemical synthesis. However, low and variable product yields have prevented widespread commercialization of this technology. One unique characteristic of plant cell cultures is the tendency of cells to form aggregates, as they remain connected after cell division via a shared cell wall. These aggregates can reach up to several mm in diameter, resulting in the formation of intra-aggregate nutrient gradients which affect cellular metabolism. We study Taxus spp. for production of the anti-cancer agent paclitaxel and have demonstrated that aggregate size affects secondary metabolite production. A more rigorous understanding of aggregation dynamics could be used as the basis for cell culture process optimization to increase the production rates of valuable secondary metabolites such as paclitaxel.

We have developed a population balance equation model which can be used to relate aggregation dynamics to total culture parameters such as growth, substrate consumption, and product accumulation. We have also developed a method, using the electrical sensing zone technique (Coulter counter), to measure the aggregate size distribution and are able to track the evolution of this distribution over the batch period and over multiple batch cycles. The aggregate size distribution data can be directly correlated with the model output to both calibrate parameters and validate model predictions. The model can be used to guide strategies focused on manipulating the aggregate size profile to increase productivity and represents an important advance in revealing the underlying causes of variability that have thus far hindered the widespread use of plant cell culture technology.