This work demonstrated that melatonin increases continuously in seeds, particularly seed coats, during berry ripening. Exogenous melatonin treatments significantly increased the proanthocyanidin (PA) content, partially through ethylene signaling, in seed coats. VvMYB14 expression exhibited patterns similar to melatonin accumulation over time, which was largely induced by melatonin treatment in seed coats during berry ripening. Additionally, VvMYB14 bound to the MBS element of the VvMYBPA1 promoter to activate expression. VvMYB14 overexpression largely upregulated expression of VvMYBPA1, VvMYBPA2 and VvLAR1 and increased the PA content in grape seed-derived calli. Similar increases in AtTT2 and AtBAN expression and PA content were found in VvMYB14-overexpressing Arabidopsis seeds. It was also observed that VvMYB14 overexpression increased ethylene production and thereby induced expression of VvERF104, which bound to the ERF element of the VvMYBPA2 promoter and activated its expression. Additionally, VvERF104 suppression reduced the VvMYB14 overexpression-induced increases in expression of VvMYBPA2 and VvLAR1 and PA content. Further experiments revealed that melatonin-induced increases in the expression of VvMYBPA1, VvMYBPA2, VvERF104 and VvLAR1 and PA accumulation were significantly reduced in VvMYB14-suppressing grape calli and leaves. Collectively, VvMYB14 mediates melatonin-induced PA biosynthesis by directly transactivating VvMYBPA1 expression and indirectly upregulating VvMYBPA2 expression via VvERF104.

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