Dedicated to Professor Walter Gustav Url on the occasion of his 70th birthdaySummary. In most plant cells, transfer to hypertonic solutions causes osmotic loss of water from the vacuole and detachment of the living protoplast from the cell wall (plasmolysis). This process is reversible and after removal of the plasmolytie solution, protoplasts can re-expand to their original size (deplasmolysis). We have investigated this phenomenon with special reference to cytoskeletal elements in onion inner epidermal cells. The main processes of plasmolysis seem to be membrane dependent because destabilization of cytoskeletal elements had only minor effects on plasmolysis speed and form. In most cells, the array of cortical microtubules is similar to that found in nonplasmolyzed states except that longitudinal patterns seen in some control cells were never observed in plasmolyzed protoplasts of onion inner epidermis. As soon as deplasmolysis starts, cortical microtubules become disrupted and only slowly regenerate to form an oblique array, similar to most nontreated cells. Actin microfilaments responded rapidly to the plasmolysis-induced deformation of the protoplast and adapted to its new form without marked changes in organization and structure. Both actin microfilaments and microtubules can be present in Hechtian strands, which, in plasmolyzed cells, connect the cell wall to the protoplast. Anticytoskeletal drugs did not affect the formation of Hechtian strands.

SummaryA high resolution ultraviolet (UV) bright-®eld microscope was used to analyse the formation of Hechtian strands and the Hechtian reticulation that remain attached to the cell wall after plasmolysis and deplasmolysis of onion inner epidermal cells. In real time video images, UV microscopy allowed a detailed investigation of the dynamic behaviour of the plasma membrane during the processes of osmotic water loss and uptake. Furthermore, the role of cytoskeletal elements as possible linkers of the plasma membrane to the cell wall was probed by application of cytoskeletal drugs during plasmolysis. Microtubules were depolymerized in oryzalin, and latrunculin B was used to destabilize actin micro®laments. The results showed no visible changes in the formation of the Hechtian reticulation or strands. Plasmolysis forms appeared to be normal, indicating stong membrane-to-wall attachments independent of cytoskeletal elements. During re-expansion of the protoplast in deplasmolysis, the plasma membrane incorporated Hechtian strands and subprotoplasts, fused with the Hechtian reticulation and ®nally realigned at the cell wall.

Self-incompatibility (SI) in Papaver rhoeas triggers a ligand-mediated signal transduction cascade, resulting in the inhibition of incompatible pollen tube growth. Using a cytomechanical approach we have demonstrated that dramatic changes to the mechanical properties of incompatible pollen tubes are stimulated by SI induction. Microindentation revealed that SI resulted in a reduction of cellular stiffness and an increase in cytoplasmic viscosity. Whereas the former cellular response is likely to be the result of a drop in cellular turgor, we hypothesize that the latter is caused by as yet unidentified cross-linking events. F-actin rearrangements, a characteristic phenomenon for SI challenge in Papaver, displayed a spatiotemporal gradient along the pollen tube; this suggests that signal propagation occurs in a basipetal direction. However, unexpectedly, local application of SI inducing S-protein did not reveal any evidence for localized signal perception in the apical or subapical regions of the pollen tube. To our knowledge this represents the first mechanospatial approach to study signal propagation and cellular responses in a well-characterized plant cell system. Our data provide the first evidence for mechanical changes induced in the cytoplasm of a plant cell stimulated by a defined ligand.