Comprehensive Identification and Abscisic Acid-Responsive Expression Profiling of NAC Transcription Factor in Triterpenoid Saponin in Hedera helix

Triterpenoid saponins are important secondary metabolites in plants. Abscisic acid (ABA), as one of the indispensable regulatory hormones in plants, promotes the accumulation of bioactive components in various plants, including triterpenoid saponins; however, its induced mechanism in <i>Hedera helix</i> remains unclear. In this study, the treatment of <i>H. helix</i> leaves with 100 μM ABA led to the identification of 7108 differentially expressed genes (DEGs) within 6 h post-treatment through transcriptomic and bioinformatic analysis. Enrichment analyses of GO terms and KEGG pathways indicated significant enrichment of DEGs in terpenoid backbone biosynthesis pathways. Analysis of DEGs revealed the NAC transcription factor, which is crucial for plant growth regulation, stress response, and secondary metabolite biosynthesis. A total of 182 HhNACs were identified at the genome-wide level, named <i>HhNAC1</i> to <i>HhNAC182</i> according to their chromosomal positions. Numerous ABA-responsive cis-regulatory elements (CREs) were presented at upstream promoters of <i>HhNAC1</i> to <i>HhNAC182</i>. They demonstrated diversified tissue-specific expression profiling among stems, roots, and leaves of <i>H. helix</i>. Notably, <i>HhNAC93</i> was predominantly expressed in <i>H. helix</i> leaves. Correlation analysis unveiled a markedly positive relationship among ABA-induced <i>HhNAC93</i> expression, triterpenoid saponin accumulation, and the expression of essential saponin biosynthetic genes. HhNAC93 likely functions as a candidate regulator in triterpenoid saponin biosynthesis. These findings provide crucial evidence for further exploring the biological role of HhNAC transcription factor in <i>H. helix</i>.