Comparative Metabolomic and Transcriptomic Analyses Reveal the Causes behind the Unique Aroma Formation of Mulberries (<em>Morus macroura</em> L. cv. ‘BaiChang’)

Mulberries is a type of fruit tree that is rich in nutrients and possesses both medicinal and dietary properties, deeply loved by consumers. The fragrance is a significant factor in the sensory quality of mulberries, playing a crucial role in consumer purchasing behavior. 'Baichang' mulberry is a unique variety of mulberry that emits a strong, creamy-like fragrance, which has enormous potential market value. However, the key metabolites and mechanisms responsible for the formation of this fragrance are still unclear. In this study, we utilized GC-MS technology and RNA-seq to examine the Volatile Organic Compounds (VOCs) and gene expression in 'Baichang' (Morus macroura) mulberries and 'Hongchang' (Morus laevigata) mulberries, the latter of which do not have a distinct fragrance. The results showed that 661 volatile metabolites were detected in both mulberry varieties. Most of these were found in higher relative amounts in the 'Baichang' mulberry variety, with 47 types being present only in the 'Baichang' mulberries, suggesting a rich composition of VOCs in the 'Baichang' mulberry. 312 metabolites were identified as differential metabolites; combining the sensory annotation information and relative content of VOCs, 70 candidate VOCs were annotated as related to the formation of 'sweet, waxy, coconut, and oily' sensations. They could potentially be the key metabolites contributing to the unique aroma of 'Baichang' mulberries. The relative odor activity value (rOAV) analysis revealed that 2(3H)-Furanone, 5-hexyldihydro- was the key VOC contributing to the formation of the special aroma of 'Baichang' mulberries. By RNA-seq, 19 key structural genes related to ester synthesis were identified, including AAT , ACX , ALDH, CYP, EHL, FAD, HPL, and LOX. The high expression of these structural genes in the "Baichang" mulberry fruit is the main reason for its unique creamy aroma. This study provides insights into the aroma differences among different kinds of mulberrie.