DISPLACEMENT AND RETURN MOVEMENT OF CHLOROPLASTS IN THE MARINE DINOPHYTE PYROCYSTIS-NOCTILUCA - EXPERIMENTS WITH OPTICAL TWEEZERS

Infrared laser traps (optical tweezers) were used to study laser-induc ed organelle movements in the marine alga Pyrocystis noctiluca (Dinoph yta). These cells are highly suitable for optical micromanipulation du e to their large size and extensive vacuole. Experiments were done wit h plastids held by optical tweezers and moved from the nuclear area in to the vacuole. The subsequent retraction movement was analysed for sp eed. The displaced organelles remained connected to their original pos ition by a thin cytoplasmic strand, often less than 1 mu m in diameter . When the organelles were released they rapidly returned at an initia l rate of 81.7 +/- 7.8 mu m . s(-1) (overall displacement 50 mu m, mea sured distance 20 mu m, 25 degrees C +/- 1 degrees C, number of cells 22), slowing down with progressive retraction of the connecting strand . The return movement was reduced to 4.2 +/- 0.2 mu m s(-1) (n = 10) w hen the organelles were displaced and held for 1 min. Displacement to a longer distance increased the rate of return movement. A change from a high to a low environmental temperature significantly reduced movem ent from 94.5 +/- 9.0 mu m . s(-1) (30 degrees C +/- 1 degrees C, n = 22) to 34.5 +/- 2.7 mu m s(-1) (5 degrees C +/- 1 degrees C, n = 22). Nocodazole and N-ethylmaleimide (NEM), inhibitors of microtubules and acto-myosin, respectively, did not affect the retraction of the connec ting strand, but at high concentrations of NEM it became increasingly difficult to move organelles away from the nuclear area. We suggest th at the return movement of organelles within laser-induced artificial s trands mainly depends on the viscoelastic properties of the tonoplast. The quantification of these properties by optical tweezers allows det ermination of reactions of plant cells to temperature changes.