Genome-wide identification and functional validation of SmMYB108 target genes associated with saponins biosynthesis in Sapindus mukorossi

Sapindus mukorossi is an important woody industrial crop valued for its abundant triterpenoid saponins, yet the transcriptional regulation of saponins biosynthesis remains poorly understood. Here, DNA affinity purification sequencing (DAP-seq) was used to characterize the genome-wide binding landscape of the R2R3-MYB transcription factor SmMYB108 and to identify its downstream regulatory targets. DAP-seq identified 28,453 reproducible binding peaks, with strong enrichment in promoter-associated regions. Motif analysis revealed a conserved MYB-like cis-element, and integrated peak annotation and motif screening yielded 1796 high-confidence candidate targets enriched in terpenoid metabolism and transcriptional regulation. Yeast one-hybrid and electrophoretic mobility shift assays demonstrated that SmMYB108 directly bound the promoters of SmIDI1 and SmAGL104 . Dual-luciferase assays in Nicotiana benthamian a showed that SmMYB108 repressed SmIDI1 promoter activity but activated SmAGL104 promoter activity, indicating target-dependent transcriptional regulation. Transient overexpression further caused opposite expression changes in the endogenous NtIDI1 and NtAGL104 homologs, supporting the in planta regulatory effects of SmMYB108. Moreover, transient heterologous overexpression of SmMYB108 significantly increased total triterpenoid accumulation in N. benthamiana leaves, suggesting that SmMYB108 can influence triterpenoid-related metabolic output in a heterologous transient system. These results establish SmIDI1 and SmAGL104 as direct downstream targets of SmMYB108 and demonstrate that SmMYB108 functions as an upstream regulator in the saponins-associated transcriptional network of S. mukorossi . This study provides mechanistic insight into saponins trait regulation and offers candidate targets for molecular improvement of this industrial tree species.