Isolation and Expression of Transcription Factors Involved in Somatic Embryo Development by Transcriptome Analysis of Embryogenic Callus of Thuja koraiensis

Thuja koraiensis Nakai (Cupressaceae) is an endangered and ecologically important conifer endemic to Korea. Previously, we established a protocol for micropropagation in T. koraiensis, which involved somatic embryogenesis from embryogenic callus of T. koraiensis. However, the molecular mechanisms underlying somatic embryogenesis remain unclear. Herein, we performed transcriptomic analysis to identify somatic embryogenesis-related genes of T. koraiensis via Illumina RNA sequencing. We conducted de novo transcriptome assembly using a Trinity assembler, which produced 274,077 transcript contigs clustered into 205,843 transcripts (unigenes), with an average length of 825 base pairs. Of all the unigenes, 14.69%, 18.62%, and 7.4% had homologs in the Gene Ontology, NCBI Non-redundant Protein, and NCBI Nucleotide databases, respectively. Among these mRNA sequences, expression of putative embryogenesis-associated transcription factors, namely BABYBOOM (BBM), WUSCHEL-RELATED HOMEOBOX (WOX), and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK), was analyzed during somatic embryogenesis. RT-PCR analysis revealed that TkBBM, TkWOX, and TkSERK were highly expressed in embryogenic callus and seedling roots, whereas their expression was reduced in seedling leaves. Our findings provide new insights into the roles of BBM, WOX, and SERK in somatic embryogenesis. Our results may serve as a reference for comparative transcriptome analysis in related species and further aid functional genomics studies in conifers.