Cell wall architecture of Physcomitrella patens is revealed by atomic force microscopy

The moss Physcomitrella patens (Hedw.) Bruch & Schimp. in B.S.G. serves as a nonvascular plant model system suitable for studying many plant developmental phenomena. The tip-growing filamentous protonemal stage of its life cycle exhibits polarized growth and various tropic responses. Conventional staining and light microscopy (LM) were used to provide the first direct evidence that protonemal cells of P. patens lack a cuticle. Atomic force microscopy (ATM) images reveal detailed surface structures identified by scanning electron microscopy (SEM). The cell wall ultrastructure is characterized by rounded protrusions that are uniformly distributed along each caulonemal filament, and longer fibrillar structures, which are disorganized at the apex, but become oriented in longitudinal arrays parallel to the growth axis in more proximal regions of caulonemal apical cells. The subapical cells are characterized by a polylamellated texture. There was no difference in gross surface ultrastructure between light-grown and dark-grown filaments, but the dimensions of the rounded protrusions at the apices of caulonemata cultured in the light and in darkness were significantly different. The convex and concave cell wall surfaces of a curved, gravitropically responding dark-grown caulonema appear structurally different. This investigation is the first to use AFM to probe the cell wall ultrastructure of a bryophyte. The data further elaborate a simple model of cell wall development in the caulonemata of P. patens that was proposed for other tip-growing filamentous plants.