Small molecules, enormous functions: potential approach for overcoming bottlenecks in embryogenic tissue induction and maintenance in conifers.

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.