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.

Anther-derived somatic embryogenesis is a valuable approach in rubber tree (Hevea brasiliensis) breeding; however, its effectiveness is highly influenced by the developmental stage of the microspores. The present investigation focused on male flower buds of the cultivar Reyan 73397 at successive developmental stages to examine the relationship between visible bud characteristics and internal microspore development, assess how microspore developmental stage affects callus induction and somatic embryo formation, and identify the stage with the greatest embryogenic potential. Cytological observations distinguished six well-defined phases of microspore development, spanning from microspore mother cells to fully mature pollen grains, each reliably linked to particular bud diameters, coloration, and anther morphology. Anthers corresponding to each developmental phase were cultured in vitro, and their ability to initiate callus and produce somatic embryos was systematically evaluated. Anthers containing uninucleate microspores exhibited the highest rates of both callus formation and somatic embryogenesis, with the early-uninucleate stage showing the strongest response. This stage consistently matched flower buds measuring 1.42–1.57 mm in transverse diameter and displaying a green to yellowish-green appearance. In contrast, anthers collected at the microspore mother cell and tetrad stages did not produce embryogenic responses. Histological evidence has indicated that both callus and somatic embryos originate from diploid somatic tissues of the anther wall, particularly connective parenchyma cells, rather than from microspores themselves. Based on these findings, a rapid, non-destructive selection method integrating bud diameter, bud color, and sieve-based size separation was developed to identify responsive explants efficiently. Overall, this study defines the optimal developmental window for anther culture in rubber trees, verifies the somatic origin of embryogenic tissues, and provides a practical morphological and cytological basis for improving anther culture efficiency in rubber tree breeding programs.

Propagation of rubber tree (Hevea brasiliensis) via secondary somatic embryogenesis (SSEis) is a reliable method. However, its efficiency is relatively low. The aim of this study was to understand more about the factors related to SSEis in rubber trees, trying to improve the efficiency of somatic embryo (SE) yield. Our study showed that the orientations of explants, i.e., the fragments of primary SE (PSE), on the medium affected secondary SE (SSE) yield significantly. Among five experimental tests, the highest yield was 2.6 ± 0.9 secondary somatic embryos (SSEs) per explant, which was achieved by orienting the abaxial side of the explant in contact with the medium and then the adaxial side after a period of culture time. Based on histological evidence, SSEis was induced from the epidermal cells and adjacent cells on the adaxial side of the explants. A remarkable difference in embryogenic capacity difference existed among individual PSE. The concentrations of soluble proteins, starch, soluble sugars, and the superoxide dismutase activity (SOD) levels in the explants were measured during a 25-day long SSEis induction treatment and compared between explants of high and low embryogenic capacity. This study proves that the explant orientation toward the culture medium plays a crucial role in SSEis, while the concentration changes of these biochemical compounds correlate to morphological changes in the explants during induction, as do the changes in SOD activity. Furthermore, the trend of the dynamic changes in the explants reflected a process of de-differentiation and re-differentiation, which started from mature SE tissues during SSE induction.