Bioprocessing Engineering Considerations in the Production of Secondary Metabolites by Plant Cell Suspension Cultures

Abstract

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 rheology 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.