Transcriptomic and Physiological Analysis Reveal Phytohormone and Phenylpropanoid Biosynthesis in Root of Cynanchum Auriculatum

Baishouwu ( Cynanchum auriculatum ), a medicinal and food dual-use plant, has been cultivated for centuries and is favored by consumers. C. auriculatum tuberous roots contain enormous amounts of flavonoids, lignin, and other nutrients. However, the developmental characteristics and phenylpropanoid metabolic mechanism in C. auriculatum have not been clarified. Here, the phenotypes of C. auriculatum tuberous roots at three stages of developmental were observed, compared with root forming stage (S1), there were significant morphological differences at root expanding stage (S2) and harvest stage (S3). Through next-generation sequencing (NGS) on Illumina HiSeq2500 platform, nine transcriptomic libraries were constructed for transcriptomic analysis. A total of 28,926 DEGs were identified during the development of C. auriculatum tuberous root, and many DEGs were enriched in GO terms/ KEGG pathways of ‘phytohormone signal transduction’ and ‘phenylpropanoid biosynthesis’. The analysis of phytohormone content and gene expression revealed that auxin and cytokinin acted in concert to regulate C. auriculatum tuberous root development, while the biosynthesis and signaling of ethylene were inhibited. Phloroglucinol staining results showed that lignified cells were mainly distributed in the central xylem at S1, followed by ring-like structure formation at S2, and finally formed the connecting rays between the xylem and the phloem at S3. Lignin content increased at S2 and then decreased at S3, and the expression of lignin synthesis genes also presented a similar trend. Total flavonoids content showed a gradually increasing trend, and the expression of flavonoid synthesis genes was also gradually up-regulated. The flavonoid synthesis pathway was enhanced by reducing the activity of key enzymes in lignin synthesis in C. auriculatum . This study provided a basis for the developmental mechanism of C. auriculatum and the further utilization of C. auriculatum tuberous roots.