Programmed Changes in Form during Moss Development.

Picture an organism that begins development as a germinated spore to give rise to a filament via tip growth. Within days of germination, and with the involvement of only a few cells and cell types, the pattern of growth changes dramatically from two dimensional to three dimensional as a leafy structure is produced on the filament. With this simple picture in mind, we can begin to think about vegetative development in moss. In this review, we introduce the reader to the processes involved in moss development, speculate on some of the underlying mechanisms, outline some of the advantages of using moss development to understand elements of general eukaryotic development, and identify areas that require additional research and clarification. We focus our discussion on the development of Funaria hygrometrica grown in culture, with additional information from studies using Physcomitrella patens. The very early stages of moss development are characterized by cellular differentiation during filament growth. Spore germination leads to the formation of a filament that is made up of a tip cell (an apical cell) and a linear array of subapical cells that are produced by successive divisions of the tip cell. These subapical cells have walls that are perpendicular to the filament axis and filled with large round chloroplasts, hence their name, chloronema (Figure 1A). As is characteristic of tip-growing organisms, the subapical chloronema cells, which are -1 O0 pm in length, do not grow. The tip cell continues to elongate, reaches a maximum length of -250 pm, and divides every 1 O to 12 hr to extend the filament. Chloronema filament growth continues until, in response to increases in light and auxin, the appearance of the chloronema tip cell begins to change, ultimately giving rise to the second filament cell type, the caulonema. Fully developed caulonema tip cells differ from chloronema tip cells in severa1 important respects. They grow much longer (up to 400 pm), divide more often (every 5 to 6 hr), and possess fewer, smaller, and more elliptical chloroplasts. During the transition from chloronema to caulonema, the newly formed filament cells appear transitional, but after 5 to 6 days, the caulonema cells are long (-250 pm), nearly clear, and have cross walls that are oblique to the filament axis (Figure 1 B). Once caulonema cell formation has begun, a new axis of developmental polarity is set up as an initial cell is formed.