Current Achievements and Future Challenges of Genotype-Dependent Somatic Embryogenesis Techniques in Hevea brasiliensis

The rubber tree (Hevea brasiliensis) is the most important commercial plant for producing natural rubber. Immature seed inner integument and anther-derived somatic embryogenesis techniques play a crucial role in the in vitro large-scale propagation and genetic transformation of the rubber tree. However, somatic embryogenesis is highly genotype-dependent, that is, only a limited number of H. brasiliensis genotypes, such as CATAS73397, CATAS917, and PB260, can be efficiently induced by somatic embryogenesis and used for large-scale propagation or transformation. The genotype dependence of the somatic embryogenesis technique is a conundrum for the application of Hevea biotechnology in most commercially important cultivars, such as Reken628 and CATAS879. Previous studies have shown that several somatic embryogenesis regulators can overcome genotype dependence and enhance the transformation and regeneration efficiency of recalcitrant plants and cultivars. In this review, we first describe the relevant successful applications of somatic embryogenesis technology in seedling production and genetic modification of H. brasiliensis. Second, we discuss the genotype dependence of somatic embryogenesis as the major challenge currently. Third, we summarize the recent significant advances in the understanding of the molecular mechanisms underlying somatic embryogenesis in other plants. Finally, we suggest a roadmap for using somatic embryogenesis regulatory genes to facilitate genotype-dependent somatic embryogenesis technology in H. brasiliensis.