Active Anthocyanin Degradation in Solanum macranthum Flowers Involves Both Peroxidase and β-Glucosidase Enzymes

Anthocyanin coloration of fruit, foliage, and flowers is dependent both on pigment synthesis and degradation. Our previous comprehensive study on in planta anthocyanin degradation was conducted on the purple Brunfelsia calycina flowers, whitening due to a one-step process, involving a single vacuolar peroxidase. Here, we reveal a novel two-step in planta degradation process in the purple Solanum macranthum flowers, as they whiten. This process involves both vacuolar β-glucosidases and peroxidases, similar to the in vitro processes described in fruit juices, with β-glucosidase enzymes stripping the pigments from their sugar moieties, followed by enzymes oxidizing the aglycones. We show that the activities of both β-glucosidase and peroxidase are crucial for the in planta degradation to occur in S. macranthum flowers. A specific vacuolar β-glucosidase (SmBGL7) and two peroxidase isozymes (SmPrx01, SmPrx02) increased in their activity parallel to the degradation process. One vacuolar β-glucosidase gene and two peroxidase genes are induced in the flower tissue just prior to the onset of anthocyanin degradation, with MWs related to those found for the corresponding isozymes of all three enzymes. SmPrx01 has an identical active site proximal heme-ligand signature sequence to the B. calycina degrading peroxidase gene, BcPrx01, and binds both malvidin (the main aglycone in B. calycina) and petunidin (the main aglycone in S. macranthum) equally. The second peroxidase, unique to S. macranthum, SmPrx02, has a stronger binding to petunidin than to malvidin, suggesting potential variability and synergistic involvement of peroxidases in anthocyanin degradation.