BRICK1 Is Required for Apical Cell Growth in Filaments of the MossPhyscomitrella patensbut Not for Gametophore Morphology

When BRK1, a member of the Wave/SCAR complex, is deleted in Physcomitrella patens (Deltabrk1), we report a striking reduction of filament growth resulting in smaller and fewer cells with misplaced cross walls compared with the normal protonemal cells. Using an inducible green fluorescent protein-talin to detect actin in living tissue, a characteristic broad accumulation of actin is observed at the tip of wild-type apical cells, whereas in Deltabrk1, smaller, more distinct foci of actin are present. Insertion of brk1-yfp into Deltabrk1 rescues the mutant phenotype and results in BRK1 being localized only in the tip of apical cells, the exclusive site of cell extension and division in the filament. Like BRK1, ARPC4 and At RABA4d are normally localized at the tip of apical cells and their localization is correlated with rapid tip growth in filaments. However, neither marker accumulates in apical cells of Deltabrk1 filaments. Although the Deltabrk1 phenotypes in protonema are severe, the leafy shoots or gametophores are normally shaped but stunted. These and other results suggest that BRK1 functions directly or indirectly in the selective accumulation/stabilization of actin and other proteins required for polar cell growth of filaments but not for the basic structure of the gametophore.