NEGATIVE GRAVITROPISM IN CHARA-PROTONEMATA - A MODEL INTEGRATING THE OPPOSITE GRAVITROPIC RESPONSES OF PROTONEMATA AND RHIZOIDS

The unicellular protonema of Chara fragilis Desv. was investigated in order to establish a reaction chain for negative gravitropism in tip-g rowing cells. The time course of gravitropic bending after stimulation at angles of 45 degrees or 90 degrees showed three distinct phases of graviresponse. During the first hour after onset of stimulation a str ong upward shift of the tip took place. This initial response was foll owed by an interval of almost straight growth. Complete reorientation was achieved in a third phase with very low bending rates. Gravitropic reorientation could be completely abolished by basipetal centrifugati on of the cells, which lastingly removed conspicuous dark organelles f rom the protonema tip, thus identifying them as statoliths. Within min utes after onset of gravistimulation most or all statoliths were trans ported acropetally from their resting position 20-100 mu m from the ce ll apex to the lower side of the apical dome. This transport is actin- dependent since it could be inhibited with cytochalasin B. Within minu tes after arrival of the statoliths, the apical dome flattened on its lower side and bulged on the upper one. After this massive initial res ponse the statoliths remained firmly sedimented, but the distance betw een this sedimented complex and the cell vertex increased from 7 mu m to 22 mu m during the first hour of stimulation and bending rates shar ply declined. From this it is concluded that only statoliths inside th e apical dome convey information about the spatial orientation of the cell in the gravitropic reaction chain. After inversion of the protone ma the statoliths transiently arranged into a disk-shaped complex abou t 8 mu m above the vertex. When this statolith complex tilted towards one side of the apical dome, growth was shifted in the opposite direct ion and bending started. It is argued that the statoliths intruding in to the apical dome may displace a growth-organizing structure from its symmetrical position in the apex and may thus cause bending by bulgin g. In the positively gravitropic Chara rhizoids only a more stable anc horage of the growth-organizing structure is required. As a consequenc e, sedimented statoliths cannot dislocate this structure from the vert ex. Instead they obstruct a symmetrical distribution of cell-wall-form ing vesicles around the structure and thus cause bending by bowing.