Integrated Transcriptome and Metabolome to Elucidate the Mechanism of Aluminum-Induced Blue-Turning of Hydrangea Sepals

Abstract

Hydrangea macrophylla is an ornamental plant with varied calyx colors. Interestingly, from red, to purple, to blue, the colors of all Hydrangea macrophylla are formed by unique delphinidin-3-O-glucoside and aluminum ions (Al3+) and 5-O-p-coumaroylquinic acid. The sepals of ‘Blue Mama’ changed from pink to blue, and the contents of delphinidin-3-O-glucoside and aluminum ions increased under 3 g/L aluminum sulfate treatment. However, the mechanism of the effect of aluminum ions on the synthesis and metabolism of anthocyanins in Hydrangea macrophylla is still unclear. In this project, transcriptome sequencing and anthocyanin metabolome analysis were performed on the sepals of ‘Blue Mama’ during flower development at the bud stage (S1), discoloration stage (S2) and full-bloom stage (S3) under aluminum treatment. It was found that delphinidin, delphinidin-3-O-glucoside and delphinidin-3-O-galactoside were the main differential metabolites. The structural genes CHS, F3H, ANS, DFR and BZI in the anthocyanin synthesis pathway were up-regulated with the deepening in sepal color. There was no significant difference between the aluminum treatment and the non-aluminum treatment groups. However, seven transcription factors were up-regulated and expressed to regulate anthocyanin synthesis genes CHS, F3H, BZI and 4CL, promoting the sepals to turn blue. The KEGG enrichment pathway analysis of differentially expressed genes showed that the glutathione metabolism and the ABC transporter pathway were closely related to anthocyanin synthesis and aluminum-ion transport. GST (Hma1.2p1_0158F.1_g069560.gene) may be involved in the vacuolar transport of anthocyanins. The expression of anthocyanin transporter genes ABCC1 (Hma1.2p1_0021F.1_g014400.gene), ABCC2 (Hma1.2p1_0491F.1_g164450.gene) and aluminum transporter gene ALS3 (Hma1.2p1_0111F.1_g053440.gene) were significantly up-regulated in the aluminum treatment group, which may be an important reason for promoting the transport of anthocyanin and aluminum ions to vacuoles and making the sepals blue. These results preliminarily clarified the mechanism of aluminum ion in the synthesis and transport of anthocyanin in Hydrangea macrophylla, laying a foundation for the further study of the formation mechanism of ‘blue complex’ in Hydrangea macrophylla.