Integrated metabolome and transcriptome analysis reveals candidate genes involved in metabolism of terpenoids and phthalides in celery seeds

Abstract

Celery (Apium graveolens L.) seeds have a unique aroma and can be used in the pharmaceutical industry. Terpenoids are the main volatile organic compounds (VOCs) of celery seed essential oil, and phthalides are the source of its aroma. However, thus far, no studies have been conducted on the metabolism of terpenoids and phthalides in celery. In this study, 195 VOCs were detected in celery seeds by using headspace injection combined with gas chromatography–mass spectrometry (GC–MS). The content of 85 VOCs, including 20 terpenoids and 6 phthalides, was found to be significantly different during the five stages of celery seed development. Based on the pattern of changes in the concentrations of terpenoids and phthalides, an integrated transcriptome and metabolome analysis of three developmental celery seed stages was performed. The results showed that 11 differentially expressed structural genes involved in the metabolism of terpenoids have a positive correlation coefficient with 16 terpenoids, while 103 differentially expressed transcription factor genes are correlated with these 11 differentially expressed genes (DEGs). Among these differentially expressed transcription factor genes, two basic helix-loop-helix (bHLH) genes are similar with Arabidopsis thaliana bHLH gene (MYC2), which has already known to be involved in the regulation of terpenoid metabolism. The results of the integrated analysis also revealed that 88 differentially expressed transcription factor genes are related to 6 phthalides, and one myeloblastosis (MYB) gene is similar with petunia MYB gene (ODORANT1), which is involved in the regulation of the metabolism of volatile benzene compounds in petunia (Pharbitis nil) flowers. Therefore, this study has laid a foundation for further research related to the identification of key genes that regulate the metabolism of terpenoids and phthalides in celery. Moreover, these results can contribute significantly to the improvement in the quality of celery seed essential oil.