Investigations of the turnover of the putative cellulose-synthesizing particle “rosettes” within the plasma membrane ofFunaria hygrometrica protonema cells

Summary In youngFunaria protonemata the influence of various inhibitors and treatments on cell elongation, fine-structure, and particle rosettes within the plasma membrane, putative parts of cellulose synthase complexes, was investigated. Cycloheximide (3×10^–5M) inhibited growth, reduced the number of rosettes and evened the gradient of rosette distribution at the beginning of treatment. The cell fine-structure was unaffected. Actinomycin D (10^–5M and 10^–4) caused an initial but transient decrease in rosette number. Alterations in cell elongation and fine-structure have not been observed. Application of 2.6-dichlorobenzonitrile (10^–5 M) for some minutes reduced the number of rosettes remarkably, while cell elongation seemed to be normal after the filaments had been transferred back to normal medium. An incubation of 2 h or longer stopped growth and caused cells to burst. The number of rosettes then rose to about 50% of the control values. When applied for 7 h biofluor (5×10^–4 M) promoted growth slightly, but generally it retarded it when used for a longer time. It did not markedly affect the number of rosettes. A short heat stock stopped elongation, caused the disappearance of rosettes and affected the structure of the mitochondria and of the Golgi apparatus. Plasmolysed cells did not grow and, initially, did not have rosettes. At reduced turgor, wider cells are formed. Freeze fracturing under UHV conditions and shadowing at very low specimen temperature revealed a small, central depression in the 8 nm rosette particles, suggesting that they are composed of subunits. Our results provide further evidence that the rosettes are parts of the cellulose synthase complexes. Their existence clearly depends on protein synthesis and on the constitution of the plasma membrane, but not on cellulose crystallization.