A POSITIVELY GRAVITROPIC MUTANT MIRRORS THE WILD-TYPE PROTONEMAL RESPONSE IN THE MOSS CERATODON PURPUREUS

Wild-type Ceratodon purpureus (Hedw.) Brid. protonemata grow up in the dark by negative gravitropism. When upright wild-type protonemata are reoriented 90 degrees, they temporarily grow down soon after reorient ation (''initial reversal'') and also prior to cytokinesis (''mitotic reversal''). A positively gravitropic mutant designated wrong-way resp onse (wwr-1) has been isolated by screening ultraviolet light-mutageni zed Ceratodon protonemata. Protonemata of wwr-1 reoriented from the ve rtical to the horizontal grow down with kinetics comparable to those o f the wild-type. Protonemata of wwr-1 also show initial and mitotic re versals where they temporarily grow up. Thus, the direction of gravitr opism, initial reversal, and mitotic reversal are coordinated though e ach are opposite in wwr-1 compared to the wild-type. Normal plastid zo nation is still maintained in dark-grown wwr-1 apical cells, but the p lastids are more numerous and plastid sedimentation is more pronounced . In addition, wwr-1 apical cells are wider and the tips greener than in the wild-type. These data suggest that a functional WWR gene produc t is not necessary for the establishment of some gravitropic polarity, for gravitropism, or for the coordination of the reversals. Thus, the WWR protein may normally transduce information about cell orientation .