During leaf development, the formation of leaf adaxial-abaxial polarity at the primordium stage is crucial for subsequent leaf expansion. However, little is known about the genetic control from polarity establishment to blade outgrowth. The leaf margin, comprising elongated margin cells and hydathodes, is thought to affect leaf expansion. Here, we show that mutants with defective leaf polarity or with loss of function in the multiple auxin-biosynthetic YUCCA (YUC) genes exhibited a similar abnormal leaf margin and less-expanded leaves. Leaf margins of these mutants contained fewer hydathodes and an increased number of cell patches in which the patterns of epidermal cells resembled those of hydathodes. The previously characterized leaf-abaxialized asymmetric leaves2 (as2) revoluta (rev) and leaf-adaxialized kanadi1 (kan1) kan2 double mutants both produce finger-shaped, hydathode-like protrusions on adaxial and abaxial leaf surfaces, respectively. YUCs are required for formation of the protrusions, as those produced by as2 rev and kan1 kan2 were absent in the yuc1 yuc2 yuc4 triple mutant background. Expressions of YUC1, YUC2, and YUC4 were spatially regulated in the leaf, being associated with hydathodes in wild-type leaves and protrusions on as2 rev and kan1 kan2 leaves. In addition, inhibition of auxin transport by treatment of seedlings with N-(1-naphtyl) phtalamic acid or disruption of the auxin gradient by transforming plants with the 35S:YUC1 construct also blocked leaf margin development. Collectively, our data show that expressions of YUCs in the leaf respond to the adaxial-abaxial juxtaposition, and that the activities of auxin mediate leaf margin development, which subsequently promotes blade outgrowth.