The AcbHLH153-AcBBX19 Module Modulates Kiwifruit Ester Biosynthesis via Repression of <i>AcAAT1</i>

Aroma biosynthesis in postharvest kiwifruit is of great significance for fruit quality; yet, the underlying regulatory mechanisms remain unclear. This study investigated the dynamic changes and regulatory mechanisms governing ester aroma biosynthesis in postharvest kiwifruit. Ethylene accelerated fruit softening, sugar core formation, soluble solid accumulation, ethylene production, and ester biosynthesis, whereas 1-methylcyclopropene exerted the opposite effects. <i>AcAAT1</i> was identified as a key acyltransferase gene responsible for ester biosynthesis. Functional analyses demonstrated that overexpression of <i>AcAAT1</i> increased ester content, while its silencing reduced the ester compounds. Furthermore, transcription factor AcBBX19 was identified as a direct transcriptional repressor of <i>AcAAT1</i>. Ethylene signaling directly suppressed <i>AcBBX19</i> expression. Protein interaction assays demonstrated that AcBBX19 physically interacts with AcbHLH153. Although AcbHLH153 cannot directly bind to the <i>AcAAT1</i> promoter, it enhances the repressive activity of AcBBX19. Transient overexpression and silencing experiments confirmed that both proteins collaboratively inhibit <i>AcAAT1</i> expression and ester biosynthesis. Collectively, our findings establish that ethylene promotes kiwifruit ester biosynthesis by suppressing the AcbHLH153-AcBBX19 transcriptional module, thereby alleviating the repression of the key gene <i>AcAAT1</i>. This study provides novel insights into the transcriptional regulatory network governing aroma formation in postharvest fruit.