Somatic embryogenesis (SE) is not only the most effective method among various strategies for the asexual propagation of forest trees but also a basis for genetic improvement. However, some bottlenecks, such as the recalcitrance of initiation, the maintenance of embryogenic potential during proliferation and the low efficiency of maturation as well as high rate of abnormal embryo development remain unresolved. These bottlenecks refer to complex mechanisms, including transcriptional regulatory networks, epigenetic modifications and physiological conditions. In recent years, several small molecules utilized in animal stem cell research have exhibited positive effects on plant regeneration, including conifer species, which offers a potential novel approach to overcome the challenges associated with SE in conifers. In this review, we summarize the small molecules used in conifers, including redox substances, epigenetic regulatory inhibitors and other metabolism-related molecules, which overcome these difficulties without the use of genetic engineering. Moreover, this approach also has the advantages of dynamic reversibility, simple operation, and simultaneous regulation of multiple targets, which might be one of the best choices for optimizing plant regeneration systems including SE.

Liquidambar species are rich sources of essential oils, with applications in cosmetics, perfumes, and pharmaceuticals. Traditionally, the industrial utilization of Liquidambar essential oil has focused on extraction from the resin, while the full potential of their leaf-derived essential oils remains underexplored. This study systematically compares the essential oil composition, sensory characteristics, and aroma profiles of leaves from various Liquidambar species, including hybrid sweetgum, using headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GC-MS). A precise internal standard method was conducted to identify progenies exhibiting heterosis in volatile content from a hybrid sweetgum population of 110 full-sib progenies. A total of 44 volatile compounds were identified, with monoterpenes (α-pinene, β-pinene, d-limonene) dominating Liquidambar styraciflua, Liquidambar formosana and hybrid sweetgum, while Liquidambar orientalis exhibited a unique d-Camphene-rich profile. Hybrid sweetgum progenies exhibiting heterosis in essential oil content were found, with several progenies significantly exceeding the average level. Sensory analysis, combined with odor activity value (OAV) calculations, revealed species-specific aroma profiles: L. styraciflua and hybrid sweetgum were characterized by citrus/herbaceous notes (high d-limonene OAV = 11.07), whereas L. orientalis exhibited pungent camphoraceous tones (d-camphene OAV = 15.9). Multivariate analyses identified eight key volatile compounds with high variable influence on projection (VIP > 1). This study highlights the hybrid sweetgum population as a promising genetic resource for high essential oil yield, with seven elite progenies showing industrial potential. The distinct volatile profiles of Liquidambar species, particularly the d-camphene dominance in L. orientalis, underscore genus-wide metabolic diversity.

Embryos propagated from vegetative organs can maintain the excellent characteristics of the ortet tree and can make full use of the advantages of somatic embryogenesis technology in the large-scale clonal propagation of forest trees. However, in forest trees, a major obstacle to reproducing seedlings through somatic embryogenesis is the challenge of inducing somatic embryos using vegetative organs as explants. In this study, we have successfully developed a procedure to induce somatic embryogenesis (SE) in adult hybrid sweetgum trees for the first time. Leaves, petioles, and stem segments isolated from test-tube seedlings of three genotypes of hybrid sweetgum trees were used as explants to induce SE. The induction of SE was significantly influenced by genotype, explant type, and medium composition. The highest induction and proliferation efficiencies were achieved using a modified Blaydes’ medium supplemented with 1.0 mg/L 2,4-D and 0.5 mg/L 6-BA. Mature somatic embryos were obtained in media without plant growth regulators (PGRs). Among the three genotypes, only FX-12 failed to induce somatic embryos in all the explants. Petiole explants of FX-2 yielded 22 somatic embryos per gram. In FX-54, somatic embryos were induced from both leaf and petiole explants. The PGR concentration in the germination medium significantly affected the efficiency of somatic embryo germination, with the best germination results observed in modified Blaydes’ medium containing 0.5 mg/L 6-BA. This procedure resulted in over 60% of somatic embryos developing normally into plantlets. This study develops an SE system using vegetative organs as explants for the first time, providing technical support for large-scale asexual propagation and molecular breeding in hybrid sweetgum.

High-quality genetic maps are effective tools for elucidating the genetic mechanisms of complex quantitative traits and facilitating marker-assisted breeding. Species within the genus Liquidambar (commonly called sweetgum), particularly Liquidambar styraciflua and Liquidambar formosana, are significant forest resources worldwide. These sweetgum trees have been extensively utilized in medical and cosmetic applications for centuries as they contain large amounts of valuable secondary metabolites. Among these, shikimic acid is a notable metabolite with significant pharmaceutical applications. Despite advances in conventional breeding and propagation techniques for sweetgum, the genetic basis and regulatory mechanisms of valuable traits remain largely unexplored. In this study, we constructed the first high-density genetic map for sweetgum using whole-genome resequencing (WGR) of 220 progeny individuals derived from a cross of L. styraciflua × L. formosana. The genetic map spanned a total distance of 1428.51 centimorgans (cM) with an average inter-marker distance of 0.33 cM, incorporating 4268 bin markers across 16 linkage groups. To identify the genetic loci controlling the shikimic acid content, quantitative trait locus (QTL) mapping was carried out based on the genetic map. Two QTLs located on linkage group (LG) 12 were detected, encompassing a total of 213 genes within the QTL interval. Some of these genes are closely related to secondary metabolism in plants, including YUCCA and DXS genes. This study presents the first high-quality genetic map of sweetgum and provides a preliminary QTL analysis for shikimic acid content. Our findings establish a foundational framework for the genetic improvement of sweetgum through marker-assisted breeding and offer valuable insights for further research in sweetgum genetics.