The polarity of gravitropism in the moss Physcomitrella patens is reversed during mitosis and after growth on a clinostat

We report two situations in which the polarity of gravitropism of single protonemal cells of the moss Physcomitrella patens is reversed. Dark‐grown protonemata of wild‐type P. patens grow negatively gravitropically. Time‐lapse video‐microscopy reveals that a temporary reversal of growth polarity occurs during mitotic division which is independent of the cells’ growth rate. A transitory reversal of growth direction is also observed when the unidirectional gravitropic stimulus is interrupted by a period of growth on a clinostat. A third situation, in which a mutant class responds by growing positively gravitropically, has been described previously (Jenkins, Courtice & Cove, 1986). These observations are discussed in terms of possible mechanisms for cell morphogenesis and tropic growth.