Both chloronemal and caulonemal cells expand by tip growth in the moss Physcomitrella patens

Tip growth is a mode of cell expansion in which all growth is restricted to a small area that forms a tip in an elongating cell. In green plants, tip growth has been shown to occur in root hairs, pollen tubes, rhizoids, and caulonema. Each of these cell types has a longitudinally elongated shape, longitudinally oriented microtubules and actin microfilaments, and a characteristic cytoplasmic organization at the growing tip which is required for growth. Chloronema are elongated cylindrical shaped cells that form during the development of the moss protonema. Since there are no published reports on the precise mode of chloronema elongation and conflicting interpretations of its cytology, the mechanism of cell growth has remained unclear. To determine if chloronema elongate by tip or diffuse growth, time-lapse light microscopy was employed to follow the movement of fluorescent microspheres attached to the surface of growing cells. It is shown here that chloronemal cells elongate by a form of tip growth. However, the slower growth of chloronema compared with caulonema is probably the result of differences in cytological organization of the growing tip.