ACTIN MICROFILAMENTS DO NOT FORM A DENSE MESHWORK IN LILIUM-LONGIFLORUM POLLEN-TUBE TIPS

Controversy over whether the apical region of a growing pollen tube co ntains a dense array of actin microfilaments (MFs) was the impetus for the present study. Microinjection of small amounts of fluorescently l abeled phalloidin allowed the observation of MF bundles in Lilium long iflorum pollen tubes that were growing and functioning normally. The r esults show that while the pollen tube contains numerous MF bundles ar ranged axially, the apical region is essentially devoid of them. The M F bundles could be seen shifting and changing in distribution as the c ells grew, but they always remained out of the apical regions. Perturb ation of normal growth and function by caffeine causes a change in the MF distribution, which returns to normal upon removal of caffeine fro m the growth medium. The lack of MFs in the apex is confirmed by caref ul immunogold electron microscopic analysis of thin sections of rapidl y frozen and freeze-substituted pollen tubes, in which very fine MF bu ndles could be seen somewhat closer to the tip than is discernible wit h fluorescence microscopy. Still, these are very few in number and are basically absent from the very tip. Thus a reassessment of current as sumptions about the distribution of actin in the pollen tube apical re gion is required.