Plant regeneration by somatic embryogenesis in Eucalyptus spp.: current status and future perspectives

Abstract

Forest tree improvement programs benefit from the emergence of new biotechnological strategies that complement plant developmental biology and discovery of genes associated with complex multigenic traits. Recently, significant progress has been made in the area of plant regeneration via somatic embryogenesis (SE) for economically important tree species (e.g. pines). These advances have opened up new scenarios for deployment of new high-performance clonally replicated planting stock to forest plantations and may also be a valuable tool for the development of efficient gene transfer techniques. Although high rates of plant propagation from axillary shoot proliferation can be achieved easily in many Eucalyptus species, even higher multiplication rates through SE have been recorded in other tree species. If the clonal propagation of Eucalyptus through SE proves to be an effective propagation method, it has the potential to meet the increasing industrial demands for high-quality uniform materials and to rapidly capture the benefits of breeding programs. Since 2002 a reproducible protocol for SE induction from mature zygotic embryos of E. globulus has been available. However, for SE to be useful in E. globulus improvement programs, the frequency of SE initiation, maturation, germination and acclimatisation needs to be improved and controlled. If this technology could be extended to elite germplasm, it would become an economically feasible tool for large-scale production and delivery of improved planting stock. This is one of the greatest current challenges in Eucalyptus tissue culture. In this review we update the most important aspects of SE in Eucalyptus with particular emphasis on E. globulus. We highlight both genetic control and the influence of different environmental factors in the SE process (e.g. medium composition, antioxidants, light and plant growth regulators), from induction to plant acclimatisation in both primary and secondary SE.