Transcriptomics integrated with targeted metabolomics reveals endogenous hormone changes in tuberous root expansion of Pueraria

Pueraria is a widely cultivated medicinal and edible homologous plant in Asia, and its tuberous roots are commonly used in the food, nutraceutical, and pharmaceutical industries. "Gange No. 5" is a local variety of Pueraria montana var. thomsonii (Bentham) M.R. Almeida (PMT) in Jiangxi Province, China. After optimizing its cultivation technique, we shortened the cultivation cycle of this variety from two years to one year, suggesting that the regulatory mechanism of the endogenous hormone system during tuberous root expansion may have changed significantly. In this study, we focused on the molecular mechanisms of endogenous hormones in promoting tuberous root expansion during one-year cultivation of "Gange No. 5". The mid-late expansion period (S4) is critical for the rapid swelling of "Gange No. 5" tuberous roots during annual cultivation. At S4, the number of cells increased dramatically and their volume enlarged rapidly in the tuberous roots, the fresh weight of a single root quickly increased, and the contents of multiple nutrients (total protein, total phenol, isoflavones) and medicinal components (puerarin, puerarin apigenin, and soy sapogenin) were at their peak values. We compared the transcriptomes and metabolomes of S1 (the pre-expansion period), S4, and S6 (the final expansion period), and screened 42 differentially accumulated hormone metabolites and 1,402 differentially expressed genes (DEGs) associated with hormone biosynthesis, metabolism, and signaling. Most Auxin, cytokinins (CKs), jasmonic acids (JAs), salicylic acid (SA), melatonin (MLT), and ethylene (ETH), reached their maximum levels at S1 and then gradually decreased; however, abscisic acid (ABA) appeared in S6, indicating that most of the endogenous hormones may play a key role in regulating the initiation of tuberous root expansion, while ABA mainly promotes tuberous root maturation. Notably, multiple key genes of the 'Tryptophan metabolism' pathway (ko00380) were significantly differentially expressed, and COBRA1, COBRA2, YUCCA10, IAA13, IAA16, IAA20, IAA27, VAN3, ACAA2, and ARF were also identified to be significantly correlated with the expansion of "Gange No. 5" tuberous roots. Our study has revealed how endogenous hormone regulation affects the expansion of "Gange No. 5" tuberous roots. These findings offer a theoretical foundation for improving the yield of PMT tuberous roots.