Transcriptome and miRNAome analysis reveals expression profiles of platycodin biosynthesis-related genes and their...

Platycodon grandiflorus is a widely used medicinal and edible species in Asia. With ongoing global climate warming, the rising incidence of extreme heat episodes has emerged as a major constraint on its productivity. To dissect the molecular regulatory network governing the response of P. grandiflorus to high-temperature stress, this study measured physiological indices and performed transcriptome sequencing (RNA-seq) on seedlings treated at 40 °C for 0, 1, 6, 12, 24, and 48 h. This was combined with microRNA (miRNA) sequencing data from the 0 h and 48 h time points for an integrated multi-omics analysis. Under high-temperature (HT) stress, the activities of superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) were markedly elevated, whereas malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) levels exhibited a transient increase followed by a decline. A comparison between the control and five HT treatment groups identified 1019 common significantly differentially expressed genes. miRNA sequencing identified 62 HT-responsive differentially expressed miRNAs targeting 157 predicted genes, and the integrated analysis further screened 32 differentially expressed target genes. Based on this, the study focused on elucidating the expression profiles of structural enzyme genes (BASs, CYPs, UGTs, BGLUs) in the platycodin biosynthesis pathway. It was found that upstream genes involved in terpenoid backbone synthesis were generally down-regulated, whereas downstream genes related to oxidative modification and glycosylation dynamics exhibited differential response patterns. Integrated analysis further revealed that ent-kaurene synthase (KS), the major rate-limiting enzyme in the gibberellin (GA) biosynthesis pathway, is potentially co-targeted by pgy-miR408-3p-3 and pgy-miR408d at the post-transcriptional level and exhibits significantly up-regulated expression. Conversely, BGLU11, a β-glucosidase gene involved in saponin deglycosylation, was negatively regulated by pgy-miR395b-3 and exhibited significantly down-regulated expression. These results suggest that under HT stress, P. grandiflorus employs a complex miRNA-mRNA regulatory network to mediate a potential redistribution of terpenoid metabolic flux between the saponin and gibberellin pathways, while dynamically modifying saponin structures to maintain metabolic homeostasis. This study constructs a transcriptional and post-transcriptional regulatory framework for the response of P. grandiflorus to HT stress, providing key genetic resources and molecular clues for in-depth analysis of its thermotolerance mechanisms and for stress-resistance breeding.