The
ARP2/3 complex, a highly conserved nucleator of
F-actin polymerization, and its activator, the
SCAR complex, have been shown to play important roles in leaf epidermal cell morphogenesis in Arabidopsis. However, the intracellular site(s) and function(s) of
SCAR and
ARP2/3 complex-dependent actin polymerization in plant cells remain unclear. We demonstrate that putative
SCAR complex subunits BRK1 and
SCAR1 are localized to the plasma membrane at sites of cell growth and wall deposition in expanding cells of leaves and roots. BRK1 localization is
SCAR-dependent, providing further evidence of an association between these
proteins in vivo. Consistent with plasma membrane localization of
SCAR complex subunits, cortical
F-actin accumulation in root tip cells is reduced in brk1 mutants. Moreover, mutations disrupting the
SCAR or
ARP2/3 complex reduce the growth rate of roots and their ability to penetrate semi-solid medium, suggesting reduced rigidity. Cell walls of mutant roots exhibit abnormal structure and composition at intercellular junctions where BRK1 and
SCAR1 are enriched in the adjacent plasma membrane. Taken together, our results suggest that
SCAR and
ARP2/3 complex-dependent actin polymerization promotes processes at the plasma membrane that are important for normal growth and wall assembly.