Somatic embryogenesis and virus elimination in Citrus

Citrus represents one of the most important fruit crops worldwide, in terms of economic value and human nutrition. Among fruit tree species, Citrus is the most affected by graft-transmissible pathogens, mainly represented by virus and virus-like agents. These pathogens cannot be directly controlled by chemical treatments, thus prevention by the use of 'healthy' propagating material becomes essential. Somatic embryogenesis, which is a recently developed technique for Citrus sanitation, has proved to be highly effective in the complete elimination of several citrus virus and virus-like diseases. Therefore, this technique can successfully replace the shoot-tipgrafting sanitation method for the production of healthy citrus mother trees of numerous genotypes used for propagation. Moreover, as the international demand for Citrus is high and the risk of introducing/spreading harmful infectious agents is considerable, in vitro conservation of plant material regenerated by somatic embryogenesis is also very promising for the safe international exchange of citrus healthy germplasm. The regeneration of somatic embryos in Citrus has been obtained from different explant types. Since many Citrus species are polyembryonic, and apomictic embryos are generated in vivo from the nucellar cells, several protocols for the regeneration of somatic embryos in Citrus have been developed using this tissue. However, this system provides poor results with many important monoembryonic genotypes. Several approaches have been tested to regulate somatic embryogenesis in Citrus including modification of plant growth regulators and other additives of culture media, explant type and pre-treatment of explants. To detect the genetic fidelity of regenerated plants, the flow cytometric analyses and several DNA markers have been used. This chapter will describe the wide range of approaches which are currently used to regulate somatic embryogenesis and sanitary aspects in Citrus including the role of plant growth regulators. Moreover, analysis of genetic stability of regenerants is also discussed.