Putrescine Treatment Delayed the Softening of Postharvest Blueberry Fruit by Inhibiting the Expression of Cell Wall Metabolism Key Gene VcPG1

The postharvest shelf life of blueberries is very short at room temperature owing to softening, which reduces their edible value. Putrescine (Put) plays an important role in maintaining the firmness and prolonging the storage time of fruits. Therefore, we investigated the relationship between Put and the cell wall metabolism and their roles in the postharvest softening of blueberry. Harvested blueberry fruit was immersed in 1 mM Put aqueous solution for 10 min. After treatment, the blueberries were stored at 20 ± 0.5 °C and 80% relative humidity for 10 days. The results show that Put delayed the softening of the blueberries. Compared to the control, the blueberry fruit treated with Put showed higher levels of firmness and protopectin. Moreover, the activity and expression levels of the cell wall metabolism enzymes were markedly inhibited by the Put treatment, including polygalacturonase (PG), β−galactosylase (β−Gal), and β−glucosidase (β−Glu). The Put treatment promoted the expression of the Put synthesis gene VcODC and inhibited the expression of the Put metabolism gene VcSPDS. Further tests showed that the fruit firmness decreased significantly after the overexpression of VcPG1, which verified that VcPG1 is a key gene for fruit softening. The key transcription factors of fruit softening were preliminarily predicted and the expressions were analyzed, laying a foundation for the subsequent study of transcriptional regulation. These results indicate that Put delays the softening of postharvest blueberry by restraining the cell wall metabolism and maintaining the fruit firmness.