Functions as ATP-binding component of the Arp2/3 complex which is involved in regulation of actin polymerization and together with an activating nucleation-promoting factor (NPF) mediates the formation of branched actin networks. Seems to contact the pointed end of the daughter actin filament (By similarity). Arp2/3 complex plays a critical role in the control of cell morphogenesis via the modulation of cell polarity development. Involved in the control of cell morphogenesis in leaf epidermal pavement cells, root hairs, hypocotyls epidermal cells and trichomes, especially during rapid cell expansion. Regulates the directionality of cell expansion by regulating the actin organization, and thus the microtubules distribution and the fusion of small vacuoles. Component of the Arp2/3 complex composed of ARP2, ARP3, ARPC1/p41-ARC, ARPC2/p34-ARC, ARPC3/p21-ARC, ARPC4/p20-ARC and ARPC5/p16-ARC. Interacts directly with ARP3/DIS1 and ABI1. Expressed at low levels in roots, seedlings, stems, leaves, flowers, pollen, siliques and at a higher level in inflorescences. Specifically localized in cells adjacent to mature xylem and in developing xylem vessels. Repressed by light. Distorted trichomes and altered epidermal cell types. 'Wurm' means 'worm' in German. Plants impaired in WURM display worm-shaped trichomes. Belongs to the actin family. ARP2 subfamily.