Plants have evolved an ability to adjust their morphology to the challenge imposed by a wide range of fluctuating environments. Competition for light among nearby plants induces shade avoidance responses including the elongation of hypocotyl and petiole, leaf hyponasty, and changes in leaf shape. These developmental adaptations are closely related to changes in light quality, especially the red to far-red ratio, and auxin levels. Despite recent progress in understanding shade avoidance responses, molecular mechanisms that mediate light and auxin signaling in adult plants are not well known. Here this study presents that ASYMMETRIC LEAVES 1 (AS1) conveys the light quality information for leaf hyponasty through the promotion of auxin transport. The AS1 overexpression plant shows increased leaf elevation angles throughout the day compared to wild-type plants, and the increased leaf angles in the AS1 overexpression plants are caused by the upward bending of petioles. In addition, transcriptome analysis identifies that most of the differentially expressed genes (DEGs) in the two different types of AS1 overexpression plants such as AUX/IAAs, SUARs, PINs are related to the auxin responses. Furthermore, the AS1-induced leaf hyponasty is dramatically reduced by low concentration of the polar auxin transporter inhibitor NPA, indicating that the function of AS1 plays a crucial role in the local distribution of auxin levels in the leaf hyponastic response. In the AS1 overexpression plants, the amount of auxin accumulated relatively more in petioles than in leaf blades. Morevoer, light quality regulation of AS1 protein stability, which highly accumulates in the nucleus under far-red conditions, supports the notion that AS1 may mediate far-red induced the leaf hyponastic response. All together, these findings suggest that AS1 function contributes to phenotypic plasticity that facilitates plants to adapt to variable environmental challenges by linking light and auxin signaling.