The role of ARPC4 in tip growth and alignment of the polar axis in filaments of Physcomitrella patens

When the actin related protein 2/3 (Arp2/3) complex member arpc4 was deleted in Physcomitrella patens (moss), the resulting null mutant (Delta arpc4) was viable and revealed no gross changes during morphogenesis of filaments into gametophores. However, we observed a striking reduction of filamentous tip growth, resulting in smaller, denser colonies. Although polar responses of Delta arpc4 filaments to unilateral white light were unaffected, these mutant filaments were defective in their response to polarized white light. These observations strongly suggest a specific role of the Arp2/3 complex as a downstream target for signals regulating oriented tip growth. Insertion of YFP-ARPC4 into Delta arpc4 rescued the mutant phenotypes and localized ARPC4 exclusively to the tip cell of filaments, the site of actin dynamics and polarized growth. The ability of Delta arpc4 to perform some but not all cellular responses will allow the study of its function in orientation of tip growth in response to directional cues (e.g. light) in a viable but mutated background.