The root meristem of Arabidopsis thaliana is protected by the root cap of which the size is tightly regulated by the balance between the formative cell divisions and the dispersal of the outermost cells. We isolated an enhancer-tagged dominant mutant displaying the short and twisted root by the overexpression of ZINC-FINGER OF ARABIDOPSIS THALIANA1 (ZAT1) encoding an EAR motif-containing zinc-finger protein. The growth inhibition activity of ZAT1 was shared by ZAT4 and ZAT9, the ZAT1 homologues. The ZAT1 promoter was specifically active in the outermost cells of the root cap in which ZAT1-GFP was localized when expressed by the ZAT1 promoter. The outermost cell-specific expression pattern of ZAT1 did not altered in the sombrero (smb) or smb bearskin1 (brn1) brn2 accumulating additional root cap layers. In contrast, ZAT4-GFP and ZAT9-GFP fusion proteins were distributed to the inner root cap cells in addition to the outermost cells where ZAT4 and ZAT9 promoters were active. Overexpression of ZAT1 induced the ectopic expression of PUTATIVE ASPARTIC PROTEASE3 involving in the programmed cell death. The EAR-motif was essential for the growth inhibition activity of ZAT1. These results suggest that the three related ZATs might regulate the maturation of the outermost cells of root cap.