In multicellular organisms, the balance be-tween cell division and differentiation determines organ size, and represents a central unknown in devel-opmental biology. In Arabidopsis roots, this balance is mediated between cytokinin and auxin through a reg-ulatory circuit converging on the IAA3/SHORT HYPO-COTYL 2 (SHY2) gene. Here, we show that crosstalk between brassinosteroids (BRs) and auxin occurs in the vascular transition zone to promote root meristem de-velopment. We found that BR increases root meristem size by up-regulating expression of the PINFORMED 7 (PIN7) gene and down-regulating expression of the SHY2 gene. In addition, BES1 could directly bind to the pro-moter regions of both PIN7 and SHY2, indicating that PIN7 and SHY2 mediate the BR-induced growth of the root meristem by serving as direct targets of BES1. Moreover, the PIN7 overexpression and loss-of-function SHY2 mutant were sensitive to the effects of BR and could partially suppress the short-root phenotypes associated with deficient BR signaling. Interestingly, BRs could inhibit the accumulation of SHY2 protein in response to cytokinin. Taken together, these findings suggest that a complex equilibrium model exists in which regulatory interactions among BRs, auxin, and cytokinin regulate optimal root growth.