BRICK1 Is Required for Apical Cell Growth in Filaments of the Moss Physcomitrella patens but Not for Gametophore Morphology

Pierre-François Perroud ¹ and Ralph S. Quatrano ¹^*

¹ Department of Biology, Washington University, St. Louis, Missouri 63130

^* To whom correspondence should be addressed. E-mail: rsq{at}wustl.edu.

When BRK1, a member of the Wave/SCAR complex, is deleted inPhyscomitrella patens ( ${Delta}$ brk1), we report a striking reductionof filament growth resulting in smaller and fewer cells withmisplaced cross walls compared with the normal protonemal cells.Using an inducible green fluorescent protein–talin todetect actin in living tissue, a characteristic broad accumulationof actin is observed at the tip of wild-type apical cells, whereasin ${Delta}$ brk1, smaller, more distinct foci of actin are present. Insertionof brk1-yfp into ${Delta}$ brk1 rescues the mutant phenotype and resultsin BRK1 being localized only in the tip of apical cells, theexclusive site of cell extension and division in the filament.Like BRK1, ARPC4 and At RABA4d are normally localized at thetip of apical cells and their localization is correlated withrapid tip growth in filaments. However, neither marker accumulatesin apical cells of ${Delta}$ brk1 filaments. Although the ${Delta}$ brk1 phenotypesin protonema are severe, the leafy shoots or gametophores arenormally shaped but stunted. These and other results suggestthat BRK1 functions directly or indirectly in the selectiveaccumulation/stabilization of actin and other proteins requiredfor polar cell growth of filaments but not for the basic structureof the gametophore.