In plants many aspects of cell shape regulation are controlled by actin-dependent processes. The ARP2/3 complex has been recognized as a regulator of actin organization. Mutations in genes encoding components of the ARP2/3 complex lead to cell shape defects in several cell types, including trichomes, epidermal pavement cells and hypocotyl cells. We show here that mutations in the GNARLED (GRL) gene cause a similar range of phenotypes. The GRL gene encodes the Arabidopsis homolog of NAP125, which in animals is known to act as one regulator of the ARP2/3-regulating complex WAVE-HSPC300. As an HSPC300 homolog is present in the Arabidopsis genome but no WAVE homolog has yet been found, the existence of a related regulation pathway was doubtful. Our finding that GRL encodes a putative regulator of the WAVE-HSPC300 complex, NAP125, combined with the phenotypic similarity between arp2/3 and grl mutants, provides evidence that the ARP2/3 complex is indeed regulated by the above mentioned pathway in Arabidopsis.