Abstract
Callerya speciosa (Champ. ex Benth.) Schot is a traditional Chinese medicine characterized by tuberous roots as the main organ of isoflavonoid accumulation. Root thickening and isoflavonoid accumulation are two major factors for yield and quality of C. speciosa. However, the underlying mechanisms of root thickening and isoflavonoid biosynthesis have not yet been elucidated. Here, integrated morphological, hormonal and transcriptomic analyses of C. speciosa tuberous roots at four different ages (6, 12, 18, 30 months after germination) were performed. The growth cycle of C. speciosa could be divided into three stages: initiation, rapid-thickening and stable-thickening stage, which cued by the activity of vascular cambia. Endogenous changes in phytohormones were associated with developmental changes during root thickening. Jasmonic acid might be linked to the initial development of tuberous roots. Abscisic acid seemed to be essential for tuber maturation, whereas IAA, cis-zeatin and gibberellin 3 were considered essential for rapid thickening of tuberous roots. A total of 4337 differentially expressed genes (DEGs) were identified during root thickening, including 15 DEGs participated in isoflavonoid biosynthesis, and 153 DEGs involved in starch/sucrose metabolism, hormonal signaling, transcriptional regulation and cell wall metabolism. A hypothetical model of genetic regulation associated with root thickening and isoflavonoid biosynthesis in C. speciosa is proposed, which will help in understanding the underlying mechanisms of tuberous root formation and isoflavonoid biosynthesis.
Highlights
Abbreviations AP2/ERF APETALA2/ethylene-responsive factor BGLU Beta-glucosidase bHLH Basic helix loop-helix transcription factor CK Cytokinin GBSS Granule-bound starch synthase genes HI4′OMT Isoflavone 4′-O-methyltransferase IAA Indole-3-acetic acid KEGG Kyoto encyclopedia of genes and genomes phenylalanine ammonialyase (PAL) Phenylalanine ammonialyase
Our results showed that the expression of some genes (e.g., HIDM-3, isoflavone synthase (IFS), I3′H, vestitone reductase (VR), and HI4′OMT) was at lower levels at 12 months after germination (MAG) in the present study (Fig. 6), which allowed us to speculate that the tailoring processes, including hydroxylation, methylation, and glycosylation, might be suppressed at the early-rapid thickening stage of C. speciosa
Integrated morphological, hormonal and transcriptomic analyses were performed in the present study, and multiple tuberous root development and isoflavonoid biosynthesis associated events, including cambium development, endogenous hormones change and corresponding candidate genes were revealed
Summary
Abbreviations AP2/ERF APETALA2/ethylene-responsive factor BGLU Beta-glucosidase bHLH Basic helix loop-helix transcription factor CK Cytokinin GBSS Granule-bound starch synthase genes HI4′OMT Isoflavone 4′-O-methyltransferase IAA Indole-3-acetic acid KEGG Kyoto encyclopedia of genes and genomes PAL Phenylalanine ammonialyase. Transcriptome techniques have contributed to our understanding of these genes involved in regulation tuberous root formation. In medicinal plant Rehmannia glutinosa, transcriptome analysis indicated that 6032 DEGs related to hormone signaling, signal transduction and light signaling were identified during tuberous root formation[5]. Some of these genes were involved in plant hormone signal transduction, with GID1-like GA receptor (StGID1) being up-regulated[12]. Several transcriptome analyses revealed that a large number of genes were highly regulated during the storage root formation, which participated in starch and lignin synthesis, cell division, and expansion[10,13]. A comprehensive understanding of the genes involved in isoflavonoid biosynthesis during the development of tuberous roots in C. speciosa will help in revealing the mechanism of isoflavonoid biosynthesis
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