Reversible protein phosphorylation regulates the dynamic organization of the pollen tube cytoskeleton: effects of calyculin A and okadaic acid

We investigated the cytoskeleton of Lilium longiflorum pollen tubes and examined the effects of the type 2A protein phosphatase (PP2A) inhibitors calyculin A and okadaic acid. An improved method for actin visualization, the simultaneous fixation and staining with rhodamine-labelled phalloidin during microscopical observation, revealed abundant actin filaments of no preferential orientation in the apical clear zone. Microtubules, visualized by indirect immunofluorescence, were mostly absent from the apices of straight-growing pollen tubes but present in those with irregular shape. Double labelling showed that both actin bundles and microtubules had a similar longitudinal or slightly helical orientation in the pollen tube shaft. In the presence of 30 nM calyculin A or okadaic acid, pollen tubes grew very slowly, branched frequently, and contained isolated, randomly oriented, curved actin bundles and microtubules. Treating pollen tubes with calyculin A or okadaic acid after germination arrested growth immediately, reversibly altered the alignment of actin bundles from axial to transverse, and disassembled microtubules. The changes in actin organization caused by the PP2A inhibitors were similar to those observed upon overexpression of AtRop1 (Y. Fu, G. Wu, Z. Yang, Journal of Cell Biology 152: 1019-1032, 2001), suggesting that hyperphosphorylation interferes with the signalling pathway of small GTPases. The effects of the PP2A inhibitors could be ameliorated with nanomolar concentrations of latrunculin B.