MICROMANIPULATION OF STATOLITHS IN GRAVITY-SENSING CHARA RHIZOIDS BY OPTICAL TWEEZERS

Infrared laser traps (optical tweezers) were used to micromanipulate s tatoliths in gravity-sensing rhizoids of the green alga Chara vulgaris Vail. We were able to hold and move statoliths with high accuracy and to observe directly the effects of statolith position on cell growth in horizontally positioned rhizoids. The first step in gravitropism, n amely the physical action of gravity on statoliths, can be simulated b y optical tweezers. The direct laser microirradiation of the rhizoid a pex did not cause any visible damage to the cells. Through lateral pos itioning of statoliths a differential growth of the opposite flank of the cell wall could be induced, corresponding to bending growth in gra vitropism. The acropetal displacement of the statolith complex into th e extreme apex of the rhizoid caused a temporary decrease in cell grow th rate. The rhizoids regained normal growth after remigration of the statoliths to their initial position 10-30 mu m basal to the rhizoid a pex. During basipetal displacement of statoliths, cell growth continue d and the statoliths remigrated towards the rhizoid tip after release from the optical trap. The resistance to statolith displacement increa sed towards the nucleus. The basipetal displacement of the whole compl ex of statoliths for a long distance (> 100 mu m) caused an increase i n cell diameter and a subsequent regaining of normal growth after the statoliths reappeared in;the rhizoid apex. We conclude that the statol ith displacement interferes with the mechanism of tip growth, i.e. wit h the transport of Golgi vesicles, either directly by mechanically blo cking their flow and/or, indirectly, by disturbing the actomyosin syst em. In the presence of the actin inhibitor cytochalasin B the optical forces required for acropetal and basipetal displacement of statoliths were significantly reduced to a similar level. The lateral displaceme nt of statoliths was not changed by cytochalasin B. The results indica te: (i) the viscous resistance to optical displacement of statoliths d epends mainly on actin, (ii) the lateral displacement of statoliths is not impeded by actin filaments, (iii) the axially directed actin-medi ated forces against optical displacement of statoliths (for a distance of 10 mu m) are stronger in the basipetal than in the acropetal direc tion, (iv) the forces acting on single statoliths by axially oriented actin filaments are estimated to be in the range of 11-110 pN for acro petal and of 18-180 pN for basipetal statolith displacements.