ClPIF3-ClHY5 module regulates ClPSY1 to promote watermelon fruit lycopene accumulation in advance under supplementary red lighting
Lycopene content is one of important factor for determining watermelon fruit quality. In this study, a small-type watermelon was grown in a greenhouse with supplementary red lighting 10 h per day after pollination 10 days. The results showed that supplementary red lighting promoted the lycopene accumulation earlier in watermelon flesh than the control. qRT-PCR analysis showed that among the lycopene metabolism pathway genes, ClPSY1 (phytoene synthase) expression increased significantly. Moreover, we identified PHYTOCHROME INTERACTING FACTORS 3 (ClPIF3) and bZIP transcription factor ELONGATED HYPOCOTYL 5 (ClHY5) in watermelon flesh, and red light has opposing effects on ClHY5 and ClPIF3 expression levels. The interaction experiments showed that ClHY5, a potent ClPIF3 antagonist, regulated ClPSY1 expression by directly targeting a common promoter cis-element (G-box). Collectively, our findings unraveled that ClHY5 and ClPIF3 form a dynamic activation-suppression transcriptional module responsive to red light cues to regulate watermelon lycopene accumulation.