QUANTITATIVE-ANALYSIS OF THE DISTRIBUTION OF ORGANELLES IN TOBACCO POLLEN TUBES - IMPLICATIONS FOR EXOCYTOSIS AND ENDOCYTOSIS

The present study provides the first quantitative analysis on the distribution of organelles in pollen tubes of Nicotiana tabacum L. Organelles were studied on living pollen tubes by means of fluorescence confocal laser scanning microscopy and on cryo-fixed, freeze-substituted and serially sectioned material by electron microscopy. In the tip a 300 nm to 400 nm thick wall was secreted that proximately gradually separated into a wall with an opaque inner side and a more translucent, layered outer side. Tubular endoplasmic reticulum was particularly abundant in the tip of the tube, surrounding the region where secretory vesicles (SV) accumulated. Mitochondria were randomly distributed throughout the cytoplasm, no accumulations were present. Dictyosomes, however, showed an increased abundance at 25-30 mu m behind the tip. The accumulation of coated pits (CP) in a zone 6-15 mu m behind the tip identifies this zone as the major site of endocytosis: 50% of all CP occur in this zone. Quantification of exoe- and endocytosis showed that only part of the membrane material of the SV can be retrieved after exocytosis. The typical zonation in endocytotic activity may serve to maintain a difference in membrane protein composition between the tip and the tube.