Specific cell types are generated partly by differential growth (when certain cell regions rapidly expand). In animal cells, this requires the actin cytoskeleton. Mathur et al. now report, on p. 3137, that an actin-based mechanism might also regulate differential growth in plants, with their discovery that a mutation in the plant orthologue of ARPC5 causes random cell expansion and aberrant cell shape in Arabidopsis CROOKEDmutants. ARPC5 is a subunit of the ARP2/3 complex – an actin polymerization modulator in many organisms that generates fine F-actin arrays. By studying polarized cells in mutant and normal plants, the authors found that localized cell expansion occurs only where fine F-actin is maintained,implicating F-actin density as a likely determinant of cell shape. Importantly, the rescue of CROOKED cells with human ARPC5 provides the first evidence that this complex is functionally conserved in higher plants.