Transcriptome sequencing and identification of key callus browning-related genes from petiole callus of tree peony (Paeonia suffruticosa cv. Kao) cultured on media with three browning inhibitors

Tree peony (Paeonia suffruticosa Andrews) has ornamental, oil, and medicinal values, and demand in the markets for uniform tree peony seedlings is increasing. Micropropagation could quickly propagate uniform seedlings. However, the heavy browning phenomenon hinders large-scale development of uniform tree peony seedlings. In this paper, we measured the total phenolic compounds content, and sequenced the transcriptomes of tree peony `Kao' petiole calluses cultured on media with three browning antagonist treatments and fresh petioles to identify the key genes involved in callus browning. Polyvinylpyrrolidone (PVP) treatment can reduce production of phenolic compounds and promote callus regeneration. A total of 218,957 unigenes were obtained from fresh petiole and three kinds of browning petiole calluses by transcriptome sequencing. The average sequence length of unigenes was 446 bp with an N50 of 493 bp. Functional annotation analysis revealed that 43,428, 45,357, 31,194, 30,019, and 21,357 unigenes were annotated using the NCBI-NR database, Swiss-Prot, KOG, GO, and KEGG, respectively. In total, 33 differentially expressed genes (DEGs) were identified as potentially associated with callus browning. Among these DEGs, 12 genes were predicted to participate in phenolic compounds biosynthesis, three genes were predicted to be involved in phenolic compounds oxidation, and six genes were predicted to participate in callus regeneration. Moreover, six transcription factors were observed to be differentially expressed in the fresh petiole and three treated petioles in tree peony. This study comprehensively identifies browning-related gene resources and will possibly help in deciphering the molecular mechanisms of callus browning of tree peony in the future.