Chiral events in developing gametophores of Physcomitrella patens and other moss species are driven by an unknown, universal direction‐sensing mechanism

Premise of the Study We used the model species Physcomitrella patens to examine chirality in moss gametophores. Chirality is manifested in the direction of consecutive apical cell divisions, cell plate configurations, and deviations of leaf connecting lines from the vertical course. However, the frequencies of chiral configurations of all these processes as well as their mutual dependence—especially in the case of gametophore branching—are not known. Other moss species were checked to determine the universality of our findings. Methods The gametophore structure of Physcomitrella patens grown in the laboratory under controlled conditions was investigated using light microscopy and compared with that of other moss species collected from their natural stands. Key Results In all investigated moss species, the tetrahedral apical cell exhibits either clockwise or counterclockwise consecutive divisions, and selection of this directionality in the primary axis is random. It is, however, related to cell plate configuration. If the plate is skewed, leaf‐producing segments arising from the apical cell cleavage exhibit circumferential rotation. Three parallel lines connecting the leaves deviate from a vertical course, but always in the same direction as that of leaf initiation; thus, the angular distance between consecutive leaves increases to >120°. Lateral branches are exclusively antidromous. Conclusions Gametophore chiral configuration appears to be useful in resolving problems of moss modular growth and branching. Morphological and anatomical evidence strongly suggests that an unknown direction‐sensing mechanism controls the development of moss axial organs. We propose that leaves are responsible for a horizontal gradient of sugar signals that develops along the gametophore circumference, thus influencing branching‐unit chirality.