Actin polymerization is essential for pollen tube growth

Actin microfilaments, which are prominent in pollen tubes, have been implic ated in the growth process; however, their mechanism of action is not well understood. In the present work we have used profilin and DNAse I injection s, as well as latrunculin B and cytochalasin D treatments, under quantitati vely controlled conditions, to perturb actin microfilament structure and as sembly in an attempt to answer this question. We found that a similar to 50 % increase in the total profilin pool was necessary to half-maximally inhib it pollen tube growth, whereas a similar to 100% increase was necessary for half-maximal inhibition of cytoplasmic streaming. DNAse I showed a similar inhibitory activity but with a threefold more pronounced effect on growth than streaming. Latrunculin B, at only 1-4 nM in the growth medium, has a s imilar proportion of inhibition of growth over streaming to that of profili n. The fact that tip growth is more sensitive than streaming to the inhibit ory substances and that there is no correlation between streaming and growt h rates suggests that tip growth requires actin assembly in a process indep endent of cytoplasmic streaming.