Cultured plant cells in suspension usually demonstrate an inverse relationship between growth and primary or secondary metabolism. Secondary metabolite formation usually does not reach optimum levels until the growth rate of the culture decreases substantially during the stationary phase. In other words, secondary metabolite generation could be maintained for extended periods if stationary phase cells are held in productive metabolic state. Large-scale production of secondary metabolites by plant cell culture is associated with problems such as lower growth rate, low product yield, genetic instability of cell lines, cell fragility, and intracellular accumulation of generated products. Some of these drawbacks may be minimized by immobilizing the cultured cells on an inert support system. To facilitate release of intracellular products, the cell membrane may be permeabilized using surface-active chemicals such as DMSO, phenetyl alcohol, chloroform, triton X-100. Alternatively, physical methods such as ultrasonication, electroporation, and ionophoretic release may also be used.

Such immobilized cells

• can be used as reusable biocatalysts for longer periods of up to 60 days
• are nondividing and hence, such stationary phase cells are in productive metabolic state for this entire duration
• are not subject to genetic changes and there is no cell division
• are protected from the shear stress of agitation due to stirring, rotation, tumbling, or shaking required for cells in suspension
• are not associated with viscosity, agitation, and aeration problems of suspension cultures
• show increased product accumulation and extended bio synthetic activity
• are sometimes induced to spontaneously release products normally stored within cells in suspension
• may effectively bring about biochemical conversions of precursors added to the extracellular medium
• reduce processing costs due to ease of product harvesting from the media

Cultured cells may be immobilized by several strategies such as:

• entrapment within preformed structures or inert support material such as alginate beads, polyurethane foam, fibre glass mats, polyester fiber, etc.
• enclosure within hollow fibre or flat semi-permeable membranes
• precipitation resulting in cell entrapment within a lattice of polymers such as agar, agarose, etc.
• ionic network formation with alginate, polyacrylamide, κ-carrageenan, and alginate in the form of calcium alginate is used extensively for plant cell entrapment by ion exchange reaction
• formation of photo cross linkable bonds with resins such as polyethylene glycol, polypropylene oligomers as pre polymers,
• microencapsulation

Surface immobilization on to a support matrix is ideal for secondary metabolite generation, as the cells are entrapped within it and then grow as a continuous tissue-like structure on its surface. The cells are cemented together by a mucilagenous material secreted by the entrapped cells. This natural tendency of plant cell to aggregate improves synthesis and accumulation of secondary metabolites.

A successfully immobilized plant cell system if viable for long periods is a highly cost-effective method of secondary metabolite generation especially if these are secreted extracellularly. This is because product harvest is much easier than from intact plant organs or from suspension cultures. Such immobilized systems may be cultivated in large reactors specifically designed for the process of large-scale production of plant secondary metabolites. Immobilized cells thus can also be used much similar to immobilized enzymes to facilitate bioconversion of compounds added to the medium

Some successful examples of plant cell immobilization are

• higher serpentine accumulation by Catharanthus roseus cells over suspension cultures;
• 50 times more capsaicin generated by Capsicum frutescens cells;
• 40% greater diosgenin production by immobilized D. deltoidea cells;
• double sanguinarine yield by Papaver somniferum cells;
• enzymatic conversion of β-methyl digitoxin to β -methyl digoxin by Digitalis lanata cells;
• biotransformation of codeinone to codeine by P. somniferum cells;
• Moringa cells immobilized with higher level of glucosinolates; and
• higher anthraquinone production by Morinda citrifolia cells.

Despite several successes in experimental trials on immobilized cell systems, the technique is still not a very feasible option for commercialization for harvesting secondary metabolites because of issues such as

• Secondary metabolite yield is enhanced only when its generation in media is independent of cell growth.
• Large-scale production is feasible only when the formed product is released into the medium.
• Cell membrane is to be permeabilized by chemical or physical methods, which may reduce cell viability.
• Product extracellular degradation is a possibility.
• Support matrix used to immobilize cells may pose an additional diffusion barrier for product release.
• Due to tendency of plant cells to aggregate, process engineering problems such as culture sedimentation, blockage of openings, etc., develop in large-scale reactors.