Transcriptomic profiling reveals histone acetylation-regulated genes involved in somatic embryogenesis in Arabidopsis thaliana
Somatic embryogenesis (SE) exemplifies the unique developmental plasticity of plant cells. The regulatory processes, including epigenetic modifications controlling embryogenic reprogramming of cell transcriptome, have just started to be revealed. To identify the genes of histone acetylation-regulated expression in SE, we analyzed global transcriptomes of Arabidopsis explants undergoing embryogenic induction in response to treatment with histone deacetylase inhibitor, trichostatin A (TSA). The TSA-induced and auxin (2,4-dichlorophenoxyacetic acid; 2,4-D)-induced transcriptomes were compared. RNA-seq results revealed the similarities of the TSA- and auxin-induced transcriptomic responses that involve extensive deregulation, mostly repression, of the majority of genes. Within the differentially expressed genes (DEGs), we identified the master regulators (transcription factors TF) of SE, genes involved in biosynthesis, signaling, and polar transport of auxin and NITRILASE-encoding genes of the function in indole-3-acetic acid (IAA) biosynthesis. TSA-upregulated TF genes of essential functions in auxin-induced SE, included LEC1/LEC2, FUS3, AGL15, MYB118, PHB, PHV, PLTs, and WUS/WOXs. The TSA-induced transcriptome revealed also extensive upregulation of stress-related genes, including those related to stress hormone biosynthesis. In line with transcriptomic data, TSA-induced explants accumulated salicylic acid (SA) and abscisic acid (ABA), suggesting the role of histone acetylation (Hac) in regulating stress hormone-related responses during SE induction. Since mostly the adaxial side of cotyledon explant contributes to SE induction, we also identified organ polarity-related genes responding to TSA treatment, including AIL7/PLT7, RGE1, LBD18, 40, HB32, CBF1, and ULT2. Analysis of the relevant mutants supported the role of polarity-related genes in SE induction. The study results provide a step forward in deciphering the epigenetic network controlling embryogenic transition in somatic cells of plants. Overall design: The immature zygotic embryos of Arabidopsis at the cotyledonary stage of development were cultured on two alternative SE-induction media, ET and EA, supplemented with TSA (1 µM) and auxin (5 µM 2,4-D), respectively. The explants that were cultured on a non-embryogenic medium (E0) developed into seedlings. The EA-, ET- and E0-cultured explant tissues were sampled for RNA-seq analysis at two culture points representing the early (5th day) and the advanced (10th day) stage of SE induction.  In total, six experimental combinations in three replicates were used in RNA-seq analysis.