Hypocotyl elongation is regulated by supplemental blue and red light in cucumber seedling. 
Light is fundamental for plants in growth and development, with regulating by integration of photoreceptors, hormones, and transcription factors.  In this study, the cucumber seedlings were supplemented with different ratios of red and blue light (1R1B, 2R1B, 1R2B and weak daylight as control), with triggering change of phenotype, and gene expression profiles of CsPHYs and CsCRYs, CsGAs/BRs, and CsPIFs in hypocotyls.  The cucumber seedling growth was significantly improved by supplemental light quality as compared with CK, and the seedlings in 2R1B were the stoutest, with obviously shortening hypocotyls, and higher dry weight and seedlings index at two-leaf stage.  The gene expression of photoreceptor and hormone, including CsPHYA, CsPHYB, CsCRY1, CsGA20ox1, CsGA20ox2, CsGA3ox1, was significantly up-regulated in hypocotyl under different supplemental light conditions.  The cucumber seedlings silenced by pTRV2-PIF4 had an obvious shortened hypocotyl.  The expression level of CsCRY1, CsGA20ox1 and CsGA3ox1 was markedly down-regulation, whereas CsPHYA and CsPHYB expression increased observably and CsGA20ox2 expression was not dramatically difference in pTRV2-PIF4-infected seedlings.  Thus, cucumber seedlings hypocotyl elongation was regulated by different supplemental light through crosstalk of photoreceptor, GAs, PIFs, and increasing ratio of red light could promote suppression of hypocotyl elongation.