ACTIN AND THE ELONGATION OF PLANT-CELLS

We have investigated in parallel the effects of different types of inhibitors on elongation of oat coleoptile cells in IAA and on the integrity of the longitudinally oriented actin-containing microfilaments present in control cells as detected by rhodamine phalloidin (RP) staining. Where growth was 50% inhibited by cytochalasin D (CD), we observed extensive to complete breakdown of the microfilaments (MFs) with the appearance of new RP staining in a few nuclei and markedly along the cross walls. When the CD-treated coleoptiles were held at 4-degrees-C the nuclei were uniformly strongly stained and cross wall staining was not seen, suggesting that translocation to the nuclei may be an intermediate step in final disposition of the actin. The divalent ions calcium and magnesium both inhibited growth in a dose dependent way. with calcium giving 50% inhibition at 65 mM and magnesium at 25 mM. KCl was not inhibitory and did not reverse the inhibition by divalent ions even at 250 mM. At 50% inhibition by either ion, the long MFs in many cells were replaced either by short fragmented MFs and small brightly staining granules (calcium) or by short usually twisted MFs and large, less intensely staining masses (magnesium). Iodoacetate at 2 mM inhibited growth almost completely and resulted in short. fragmented, twisted or curled MFs in most of the cells. Abscisic acid also caused replacement of some MFs with faintly fluorescent bodies somewhat larger than those in CaCl2; occasionally granules similar to those in CaCl2 were also seen. Only mannitol and galactose, which inhibit growth by their osmotic effect, did not cause breakup of the MFs; indeed the MFs in mannitol appeared if anything wider and thicker. The results show that under the influence of three types of growth inhibitors the actin-containing MFs in the cells are broken down to different extents resulting in new structures. The results support the idea that the integrity of the MF bundles is linked, perhaps causally, to the elongation of the Avena cells.