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 dist ribution of organelles in pollen tubes of Nicotiana tabacum L. Organel les were studied on living pollen tubes by means of fluorescence confo cal laser scanning microscopy and on cryo-fixed, freeze-substituted an d serially sectioned material by electron microscopy. In the tip a 300 nm to 400 nm thick wall was secreted that proximately gradually separ ated into a wall with an opaque inner side and a more translucent, lay ered outer side. Tubular endoplasmic reticulum was particularly abunda nt in the tip of the tube, surrounding the region where secretory vesi cles (SV) accumulated. Mitochondria were randomly distributed througho ut the cytoplasm, no accumulations were present. Dictyosomes, however, showed an increased abundance at 25-30 mu m behind the tip. The accum ulation of coated pits (CP) in a zone 6-15 mu m behind the tip identif ies this zone as the major site of endocytosis: 50% of all CP occur in this zone. Quantification of exoe- and endocytosis showed that only p art of the membrane material of the SV can be retrieved after exocytos is. The typical zonation in endocytotic activity may serve to maintain a difference in membrane protein composition between the tip and the tube.