Early morphogenetic patterns of protonemata and gametophores of Physcomitrium patens (Hedw.) Mitt.

In axenic culture, protonemata of Physcomitrium (formerly Physcomitrella) patens (Hedw.) Mitt. are comprised of chloronemata and caulonemata that can be distinguished morpho-structurally. On solid nutrient agar medium, a protonemal inoculum proliferates, initially generating primary chloronemata, and expands radially across the surface of the substratum forming approximately round colonies. The expansion of the colonies derives mainly from centrifugal elongation of newly formed caulonemata, which arise by differentiation of primary chloronemal apical cells in the central region. Most commonly, oblique cross walls separate cells of the uniseriate caulonemal filaments. Each oblique wall organises a smaller and a larger angle with the outer wall of a tubular caulonemal cell. The smaller angle may lie in four different positions, vertically upward or downward, and laterally, to the right or left, parallel to the surface of the solid medium. Chloronemal filaments, which contain cells separated by transverse cross walls, develop from side branch initials (SBIs) at or near the smaller angle at the distal end of caulonemal cells. Like the smaller angle of the oblique cross walls in caulonemata, these secondary chloronemata also show four different orientations, vertically, upward or downward, and laterally, right or left. A strong inverse correlation exists between the placement of SBIs and secondary chloronemata and the direction of caulonemal tip growth. If the caulonemal tips curve downward into the medium, chloronemata are situated vertically on the upper side of caulonemata; similarly, if they turn upward, chloronemata are situated on the lower side. A similar correlation exists when caulonemal tips curve to the right or left on the surface of solid medium, in which case laterally directed chloronemata grow in the opposite direction, i.e. on the outside of the curve. The procumbent caulonemata with their associated protruding secondary chloronemal branches are reminiscent of the heterotrichous branching habit of filaments of some green algae. Gametophore buds develop from a minority of caulonemal SBIs and also occasionally from chloronemata; a stalk cell, which resembles a chloronemal cell, supports each bud. Buds develop into leafy gametophores that are arranged in a fairy ring. We discuss the relative merits of several models that might explain the temporally and spatially regulated clustering of buds resulting in the formation of a fairy ring.