Identification of the key genes contributing to the LOX-HPL volatile aldehyde biosynthesis pathway in jujube fruit

Jujube (Ziziphus jujuba Mill.) is a traditional popular fruit widely grown in China. The volatiles in jujube determine its unique flavor and the high fruit quality required by consumers. However, the biosynthesis of volatiles in jujube were remain unknown. By using gas chromatography-mass spectrometry, there were 46 volatile compounds were identified and determined from three representative jujube fruit types at six developmental stages, including the dry-used (Z. jujuba cv. 'Junzao'), the fresh-used (Z. jujuba cv. 'Dongzao'), and wild jujube (Z. jujuba var. spinosa Hu. cv. 'Qingjiansuanzao'). The aldehydes were identified as major volatile contributors to flavor, of which (E)-2-hexenal was the primary volatile in jujube fruit. Then LOX and HPL gene family were identified in jujube, which were involved in aldehyde biosynthesis through the lipoxygenase-hydroperoxide lyase (LOX-HPL) pathway. Gene expression analysis suggested that ZjLOX3, ZjLOX4, and ZjHPL1 were highly correlated with the accumulation of (E)-2-hexenal, and their proteins were localized to the nucleus and cytoplasm. Transient over-expression of ZjLOX3, ZjLOX4, and ZjHPL1 in jujube fruit significantly enhanced the accumulation of (E)-2-hexenal. Our study provides valuable information on the major volatiles and their biosynthesis in different types of jujube fruit. These results will help determine flavor improvements for future breeding.