CsNAC8 positively regulates C6 aroma biosynthesis by activating key genes of lipoxygenase pathway in cucumber fruits under supplemental lighting

The freshness of cucumber fruits mainly results from the six-carbon compounds (C6 aroma), which exhibit a unique flavor. Actually, the aroma content is greatly affected by environmental factors. However, how the light affects C6 content in cucumber and its regulatory mechanism remains unknown. In this study, we found that C6 aroma content in cucumber fruits increased significantly under the supplemental LED lighting of a red-to-blue ratio of 3: 1. The expression of CsLOX01, the first key enzyme gene in the lipoxygenase pathway, increased significantly by 89.81∼99.22% and positively correlated with the C6 aroma content. C6 aroma content was significantly enhanced 2∼16 times in CsLOX01 overexpression cucumber lines. The transcription factors were screened basing on the co-expression network constructed by integrating the C6 aroma content and the transcriptomic data, and 13 candidate transcription factors were highly correlated with the content of C6 aroma, in which a NAC factor CsNAC8 increased by 37.56% under the supplemental LED lighting. CsNAC8 was found to bind directly to the CGTG motif of CsLOX01, 13-CsHPL and CsADH7 promoters, as identified through dual-luciferase, yeast one-hybrid and electrophoretic mobility shift assays. C6 aroma content increased significantly by 12.69∼51.26% and the expressions of CsLOX01, 13-CsHPL and CsADH7 were significantly enhanced in the CsNAC8 overexpression cucumber lines, as verified by the chromatin immunoprecipitation-qPCR. A model was established and elucidated that CsNAC8 positively regulated C6 aroma biosynthesis by activating key genes of the lipoxygenase pathway in cucumber fruits under supplemental lighting.