Plant Molecular Farming - Integration and Exploitation of Side Streams to Achieve Sustainable Biomanufacturing

Abstract

Plants have unique advantages over other systems such as mammalian cells for the production of valuable small molecules and proteins. The benefits cited most often include safety due to the absence of replicating human pathogens, simplicity because sterility is not required during production, scalability due to the potential for open-field cultivation with transgenic plants, and the speed of transient expression potentially providing gram quantities of product in less than 4 weeks. Initially there were also significant drawbacks, such as the need to clarify feed streams with a high particle burden and the large quantities of host cell proteins, but efficient clarification is now readily achieved. Several additional advantages have also emerged reflecting the fact that plants are essentially biodegradable, single-use bioreactors. This article will focus on the exploitation of this concept for the production of biopharmaceutical proteins, thus improving overall process economics. Specifically, we will discuss the single-use properties of plants, the sustainability of the production platform, and the commercial potential of different biomass side streams. We find that incorporating these side streams through rational process integration has the potential to more than double the revenue that can currently be achieved using plant-based production systems.

Keywords: biomass conversion; biopharmaceuticals; biorefinery; molecular farming; plant secondary metabolites; process sustainability.

FIGURE 1

Overview of plant-based expression systems. (A) Advantages and challenges typically encountered during plant molecular farming. This study expands the previously reported aspects (dark green and orange) with new potential benefits (light green). (B) Approvals of recombinant-protein-based biopharmaceuticals. Starting in 1985, the approved drugs expressed in microorganisms (prokaryotic and eukaryotic), mammalian cells, or plant cells are plotted in 5-year intervals based on data published up to 2014 (Walsh, 2014). The forecast up to 2035 is based on reported product pipelines (e.g., Gleba et al., 2014).

FIGURE 2

Comparison of waste streams generated in a mammalian cell culture-based process relying on single-use equipment and a plant-based counterpart. SUB, single-use bioreactor.

FIGURE 3

Potential product side streams in plant molecular farming. Most often the primary product constitutes less than 1% of the biomass produced during upstream processing. Opportunities arise for additional revenue when biomass side streams, especially proteins and small molecules, are included in the valorization, which broadens the diversity of the value chain. An integration with existing facilities, e.g., biogas plants, may be required for a cost-effective processing of side products with low margin.

FIGURE 4

Flow scheme of a partially integrated plant-based production process. The primary process (black) produces a high value pharmaceutical product, here, a mAb. Several side streams (bold) can be used to extract additional products. For example, a re-extraction of the residual biomass (orange) with an organic solvent can yield different small molecule products like rutin. Furthermore, co-expressed technical enzymes or diagnostic agents like DsRed can be isolated from liquid process wastes like the flow-through of a chromatography step (red). Each of the process branches stretch through one or several of the three typical process phases, i.e., plant cultivation (green background), extraction, and clarification (yellow background) and purification (blue background). CHA, ceramic hydroxyl apatite; IMAC, immobilized metal-ion affinity chromatography; RP, reversed phase; UF/DF, ultrafiltration/diafiltration.