Bioprocessing engineering considerations in the production of secondary metabolites by plant cell suspension cultures

Plant cell cultivation technique serves as an alternative source of a variety of chemicals of diverse nature. However, major bottlenecks in large-scale cultivation of plant cells are low growth rate, shear sensitivity due to their fragile nature, adequate mixing, oxygen transfer and maintenance of aseptic conditions. To complicate the situation further, plant cell products are intracellular and concentrations are generally very low. These problems have been addressed to some extent by optimizing biological and/or technological parameters. Biological parameters include proper selection of medium composition with respect to its C and N source C/N ratio, phosphate ions, and precursor and plant growth regulators. Also light temperature and aeration can be altered to improve yield. The major technological parameter is mixing. Adequate mixing and homogenous conditions in reactor are particularly difficult to achieve due to shear sensitive nature of plant cells. The problem is further compounded when at high cell concentration the changes in fluid theology result in Non Newtonian behaviour leading to clumping of cells and enhanced sedimentation. This phenomenon restricts effective mass and heat transfer inside cell clumps and therefore has an unpredictive effect on growth and product formation. Plant cells therefore, require balanced aeration as high aeration may remove CO2 (possible nutrient) from culture broth. Another major challenge in growth of plant cells is the sticky nature of late exponential phase cells due to excretion of polysaccharides. This results in problems relating to nutrient and oxygen diffusion to cell. Optimum concentration of calcium ions &/or addition of pectinase/cellulase could be used to minimize clumping to the desirable level. There are several cases where plant cell culture has produced high amounts of secondary metabolites than the whole plant. One such example of our laboratory is Holarrhina antidysentrica, which produces antidysentric drug, conessine. Modification of MS medium by addition of growth factors, sucrose and optimal inoculum resulted in increased yield of the alkaloid. The same suspension culture was propagated in the bioreactor with precursor, cholesterol feeding, which led to 160 times higher productivity than that of the native plant. Salient features of this and other plant cell suspension systems will be discussed with respect to bioprocessing strategies for production of various phytochemicals. Efforts are being made to save valuable and rare plants of medicinal value from extinction. Another example within our laboratory is the development of plant cell culture of Podophyllum hexandrum which produces the precursor podophyllotoxin for anticancer drugs, etoposide and teniposide. MS and B5 media along with different growth factors have been studied to develop suspension culture in shake flask. B5 medium along with 2% coconut milk produced 4 g/L biomass (dry basis). Plackett Burman design yielded the sensitive environmental parameters as inoculum level, glucose concentration and pH for culture growth and podophyllotoxin production. The exact response equation by manipulating the above three parameters was found out by Central Composite Design. The response of cell culture in a bioreactor having special low shear Setric impeller will also be presented.