Integrated physiological and transcriptomic analyses reveal the role of polyamines in blueberry fruit coloration

Polyamines (PAs) play important roles in regulating fruit ripening and coloration. This study investigated the effects of exogenous polyamines including spermidine (Spd), spermine (Spm) and putrescine (Put) and the polyamine synthesis inhibitor, methylglyoxal-bis(guanylhydrazone) (MGBG), on blueberry fruit ripening and coloration. We treated blueberry fruits at the green-stage and subsequently measured the levels of Spd, Spm, Put, abscisic acid (ABA), ethylene (ETH), anthocyanins, and the activities of enzymes associated with PA synthesis and metabolism. The results showed that SPD treatment significantly increased anthocyanin content and PA levels in blueberry fruits, promoting fruit coloration. To understand the molecular basis of fruit ripening and coloration, we further performed transcriptome sequencing on blueberry fruits treated with Spd and MGBG and analyzed differentially expressed genes (DEGs) involved in the anthocyanin and hormone biosynthesis pathways. The transcriptome data revealed significant up-regulation of several key genes which were involved in the anthocyanin biosynthesis and hormone synthesis. Additionally, heterologous over-expression of the blueberry spermidine synthase gene (VcSPDS) in tobacco significantly increased anthocyanin accumulation and upregulated the expression of both PA biosynthetic genes (VcSPDS, VcADC, and VcSPMS) and anthocyanin biosynthetic structural genes (VcPAL, VcCHS, VcCHI, VcF3H, VcF3'5'H, VcDFR, VcANS, and VcUFGT), preliminary providing molecular evidence that supports the role of Spd as a positive regulator of anthocyanin synthesis. These results suggest that PAs promote blueberry fruit coloration by affecting the anthocyanin synthesis pathway, providing a new molecular strategy for improving fruit quality.