Bioreactors for Coffee Mass Propagation by Somatic Embryogenesis

Coffee somatic embryogenesis in liquid medium is a powerful alternative to other vegetative propagation techniques for mass propagation of selected Coffea canephora (Robusta) clones and F1 Coffea arabica hybrids. This review presents the different types of bioreactors used for coffee somatic embryogenesis by Nestlé R&D Centre-Tours and by other scientific teams. Mechanically agitated bioreactors were used for the production of torpedo-shaped embryos. Critical parameters are the inoculation density (0.5 to 1.0 g FW L), medium renewing and the initial oxygen transfer rate (KLa: 5 h). In this system, Robusta embryo concentrations range between 200,000 to 400,000 L within 2 months. Maturation from the torpedo to the cotyledonary-stage embryos was achieved in various temporary immersion bioreactors (TIB): in 1-L RITA system (up to 1,000 cotyledonary embryos per system), in 10-L glass bottles (up to 20,000) and in 10-L flexible disposable bags. The latter one, the so-called “Box-in-Bags”, insures a higher light transmittance to the biomass due to its horizontal design. At the end of the maturation phase, the somatic embryos are green and able to photosynthesize: these pregerminated embryos can be directly transplanted to the greenhouse to get fully germinated plantlets. More recently, a temporary root immersion bioreactor (TRI) has been described for the growth of individualized Arabusta plantlets in photoautautrophic conditions, i.e. in sugar-free medium with enriched CO2 and high light intensity. The pros and cons of these different bioreactors will be discussed considering how they can be integrated in a mass propagation process. We present a “state of the art” by describing a pilot scale process for the production of pregerminated Robusta embryos and some examples of diffusion of coffee selected genotypes. These last years, two major trends have been developed for industrial micropropagation: i) bulk-cultivation of small propagules in photomixotrophic conditions (with sugar) followed by their selection and transfer to the greenhouse for their conversion to plant, ii) production of singulated and fully developed plantlets in the laboratory under photoautotrophic conditions. Next development in coffee mass propagation by somatic embryogenesis will probably originate from the combination of these two approaches. The usage of the “micro-environment” method, combined with media releasing CO2, is well adapted for the ex vitro germination of coffee embryos. Particularly, this method can be a relevant alternative to the conventional one, consisting on insufflating CO2 in the culture rooms or in the greenhouses. _____________________________________________________________________________________________________________