The curvature ofPhyscomitrium patenscaulonemal filaments in relation to their capacity for phototropic and thigmotropic responses and nutation

Abstract

AbstractLight-grown, whole gametophytic colonies ofPhyscomitrium patensexhibit a spiral morphology resulting from the strongly co-ordinated curvature of the population of peripheral caulonemal filaments. The direction of curvature is predominantly clockwise when cultures are illuminated from above and anticlockwise when illuminated from below. Transferring cultures from top to bottom illumination provided a means of estimating the cell cycle time for caulonemal apical cells. InPhyscomitrium, side branch initials emerge from caulonemal subapical cells on the outside of the curve. By contrast, the curvature of the caulonemata ofFunaria hygrometricais predominantly anticlockwise when colonies are illuminated from above and clockwise when illuminated from below. InFunaria, side branch initials emerge from subapical caulonemal cells on the inside of the curve. We have discounted a role for gravity in these phenomena and discuss possible mechanistic explanations in terms of aberrant nutation and phototropic or thigmotropic responses on the slippery, solid agar medium. We describe the first known case of thigmotropism of protonemata of a bryophyte. Thigmotropism occurs in response to crowding of protonemal filaments in a thin layer of agar medium and is restricted to caulonemata.