Restoration of movement and apical growth in the angiosperm pollen tube following cytochalasin-induced paralysis

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Abstract
Cytochalasin D (CD) at 5 pg ml-1arrested growth and vectorial movement in pollen tubes ofNarcissus pseudonarcissusandEndymion nonscriptusand caused the mainly longitudinally oriented actin fibrils in the vegetative cells to coalesce and form massive, more randomly oriented, cables. As extension growth was arrested, the tubes formed apical bulbs and abnormal wall thickenings. During recovery from a 10 min treatment period inE. nonscriptus, an essentially normal fibril system was reconstituted by partial dissociation of the thick cables formed during the exposure to CD. As this progressed movement was restored in the vegetative cells. Some 80 % of the blocked tubes initiated new growing points, either by producing randomly oriented swellings in sites where the wall was thinner, or by erosion and penetration of thicker zones. Contrary to expectation, the sites of the prospective growing points were not indicated in advance by any special disposition of the actin cytoskeleton. With the transition to cylindrical growth in the secondary tubes the standard stratification of the tube wall reappeared, with outer pectocellulosic and inner callosic layers. Normal movement pathways were established concomitantly, together with the apical zonation of organelles and other cytoplasmic inclusions characteristic of the extending tube. CD-treatment brought about rapid contraction of the vegetative nuclei with the loss of the elastic extensions of the nuclear envelopes. The extended form was resumed as the actin cytoskeleton was restored during recovery, and vegetative nuclei and generative cells moved into the secondary tubes where they continued to track the apex as in the normal tube.