Molecular Plant Advance Access originally published online on October 8, 2008
Molecular Plant 2008 1(6):990-1006; doi:10.1093/mp/ssn059
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Plasma Membrane-Associated SCAR Complex Subunits Promote Cortical F-Actin Accumulation and Normal Growth Characteristics in Arabidopsis Roots
a Section of Cell and Developmental Biology, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0116, USA
b Southern Weed Science Research Unit, USDA–ARS, 141 Experiment Station Road, Stoneville, MS 38776, USA
c Carnegie Institution, Department of Plant Biology, 260 Panama Street, Stanford, CA 94305, USA
d Present address: Division of Plant Biology, The Samuel Roberts Noble Foundation, 2510 Sam Noble Pkwy, Ardmore, OK 73401, USA
e Present address: Section of Neurobiology, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0116, USA
1 To whom correspondence should be addressed. E-mail lgsmith{at}ucsd.edu, fax 858-534-7108, tel. 858-822-2531.
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.
Key Words: cell expansion • cell morphogenesis • cytoskeleton • root biology • Arabidopsis