Submitted to: Molecular Plant
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2008
Publication Date: 12/15/2008
Citation: Dyachok, J., Shao, M.R., Vaughn, K.C., Bowling, A.J., Facette, M., Djakovic, S., Clark, L., Smith, L.G. 2008. Plasma membrane-associated SCAR complex subunits promote cortical F-actin accumulation and normal growth characteristics in Arabidopsis roots. Molecular Plant, Volume 1, Number 6, pp. 990-1006.
Interpretive Summary: The biochemical changes that enable formation of cell walls in plants are highly complex and are slowly being unraveled used the tools of modern cell biology. Scientists in the ARS Southern Weed Science Research unit collaborated with cell biologists from U.C. San Diego and the Carnegie Institute to assess the role of expression of various genes encoding for a protein complex called SCAR and the promotion of polymerization of actin in the roots of the plant Arabidopsis. Using genetic mutants, electron microscopy, immuno-localization and protein expression techniques this research demonstrated how these proteins promote processes in membranes leading to the assembly of cell walls in in plant roots. This pioneering work lends to our knowledge on how plant cell walls expand during cell growth.
Technical Abstract: 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 complex 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 rapid cell growth and wall deposition in expanding cells to leaves and roots. BRK1 and SCAR1 co-immunoprecipitate and BRK1 localization to the plasma membrane is SCAR-dependent, providing further evidence of an association between these two proteins in vivo. Mutations disrupting the SCAR complex or the ARP2/3 complex reduce cortical F-actin polymerization in root tip cells and reduced root growth rate. Moreover, these mutations reduce the ability of roots to penetrate agar, suggesting reduced cell wall rigidity and thus altered wall composition. Cell walls of mutant roots exhibit abnormal structure and composition at intercellular junctions where BRK1 and SCAR1 are enriched in the adjacent plasma membrane, and contrain reduced amounts of pectin. Taken together, our results suggest that SCAR complex- and ARP2/3 complex-dependent actin polymerization promotes processes at the plasma membrane that are important for normal root growth and wall assembly.