Comparative transcriptome analysis highlights the hormone effects on somatic embryogenesis in Catalpa bungei

The major pathways and key events related to somatic embryo development in Catalpa bungei were illustrated by deep analysis of DEGs and quantification of hormone contents. Catalpa bungei C.A. Meyer is a valuable timber species, known as "The king of wood" in China. Due to the low propagation rate, somatic embryogenesis-based rapid propagation can regenerate a large number of new plants in a very short period of time and thus has great commercial value for this timber species. However, the mechanisms of somatic embryogenesis in C. bungei remain largely unclear so far. In our previous study, we established the vegetative propagation system in C. bungei using immature zygotic embryo as explants. Here, we further compared the transcriptional profiles and hormones contents between the embryogenic callus (EC) and non-embryogenic callus (NEC). RNA-seq analysis showed a total assembly of 73038 unigenes, and identified 12310 differentially expressed genes (DEGs) between EC and NEC. Also, six DEGs were chosen to verify the authenticity of the transcriptome sequencing results by qRT-PCR. Moreover, by using LC-MS approaches, we quantified various plant hormone contents and found that auxin and ABA were dramatically higher in EC than those in NEC. Accordingly, DEGs were enriched in plant hormone signaling pathways. Taken together, we highlight the hormone effects on somatic embryogenesis in a tree species, C. bungei. The use of certain genes as markers of embryogenesis induction in C. bungei regeneration process will provide new tools to pre-screen genotypes or tissue culture hormone combinations suitable for somatic embryo production. Our results provide theoretical references for the somatic embryogenesis mechanism and experimental bases for breeding and rapid propagation of C. bungei.