Natural Variation in the AAT1 Promoter Is Responsible for the Disparity in Ester Aroma Between Actinidia chinensis and Actinidia eriantha

To understand ester compound biosynthesis in kiwifruit, two Actinidia species with distinct characteristics were compared. The firmness of Donghong (Actinidia chinensis) and Ganlv-1 (Actinidia eriantha) fruits gradually decreased as the fruit ripened after harvest, whereas the total soluble solids increased continuously, reaching a peak on the 8th day. The Ganlv-1 fruit ester content was significantly lower than in the Donghong fruits at the optimal eating stage, and the alcohol acyltransferase (AAT) activity in the Ganlv-1 fruits was significantly lower than in the Donghong fruits. The gene expression levels of AAT1 and AAT17 in the Donghong fruits were significantly higher than in the Ganlv-1 fruits, with a particularly remarkable difference observed for AAT1, which exhibited a 36-fold higher expression in the Donghong fruits when compared with the fully ripened Ganlv-1 fruits. A transient overexpression of the AAT1 gene in the kiwifruit led to a significant increase in ester content. Interestingly, a natural variation was observed in the promoter sequence of AAT1 between the Donghong and Ganlv-1 cultivars. Furthermore, separate analyses of the respective promoter activities revealed significantly higher activity levels in the Donghong fruits than in the Ganlv-1 fruits. In conclusion, a natural variation in the AAT1 promoter is primarily responsible for the disparity in AAT1 gene expression between the Donghong and Ganlv-1 fruits, resulting in a divergent accumulation of ester aroma compounds during the postharvest ripening stages.