Abstract: The shapes of organs in plants are determined by defined cell divisions and cell elongations. To understand how organ shape is genetically controlled, we used ethyl methanesulfonate (EMS) mutagenesis to identify a mutant with slender leaves, slender leaves and cells (slc). Cytological studies showed that the mutation affected cell division, as well as cell shape and number. The interdigitation in pavement cells in slc was greatly reduced, in parallel with the formation of gigantic cells with 1-D cell elongation. Of note, the mean cell size in slc was no larger than that in the wild type, which suggests that expansion is at the cost of reduced cell size in other cells. The function of SLC may be to coordinate cell division and expansion to allow proper interdigitation among cells. Map-based cloning revealed that SLC is allelic to AGO1, a key component in post-transcriptional gene silencing. The mutation led to a change from a highly conserved histidine to tyrosine. Our results suggest that AGO1-mediated gene silencing is critical for determining the shapes of cells and subsequent organs.