Auxin plays an important role in many aspects of plant growth and development. Despite the importance of auxin, very little is known about how auxin is produced in plants since auxin biosynthesis in plants is extremely complicated. Characterizing the developmental defects caused by auxin deficiency can help us to improve our current understanding in the mechanisms of plant development. Under yuc4-/- background, a partial auxin deficient gene SYD was isolated through EMS forward screening. Disruption in the SYD gene causes defects that are more severe than in yuc4 mutants when comparing seedlings, leaves, and flowers. These findings suggest that SYD enhances the loss of function yuc4 and plays important roles throughout plant development and organogenesis. Furthermore, the genetic interactions of SYD and three other auxin genes NPY1, CUC1, and PIN1 were investigated and the results suggest that all these genes are likely to be involved in the same pathway