Reorganization of microtubules in endosperm cells and cell fragments of the higher plant Haemanthus in vivo.

Abstract

Abstract. The reorganization of the microtubular meshwork was studied in intact Haemanthus endosperm cells and cell fragments (cytoplasts). This higher plant tissue is devoid of a known microtubule organizing organelle. Observations on living cells were correlated with microtubule arrangements visualized with the immunogold method. In small fragments, reorganization did not proceed. In medium and large sized fragments, microtubular converging centers formed first. Then these converging centers reorganized into either closed bushy microtubular spiral or chromosome-free cytoplasmic spindles/phragmoplasts. Therefore, the final shape of organized microtubular structures, including spindle shaped, was determined by the initial size of the cell fragments and could be achieved without chromosomes or centrioles. Converging centers elongate due to the formation of additional structures resembling microtubular fir trees. These structures were observed at the pole of the microtubular converging center in anucleate fragments, accessory phragmoplasts in nucleated cells, and in the polar region of the mitotic spindle during anaphase. Therefore, during anaphase pronounced assembly of new microtubules occurs at the polar region of acentriolar spindles. Moreover, statistical analysis demonstrated that during the first two-thirds of anaphase, when chromosomes move with an approximately constant speed, kinetochore fibers shorten, while the length of the kinetochore fiber complex remains constant due to the simultaneous elongation of their integral parts (microtubular fir trees). The half-spindle shortens only during the last one-third of anaphase. These data contradict the presently prevailing view that chromosome-to-pole movements in acentriolar spindles of higher plants are concurrent with the shortening of the half-spindle, the self-reorganizing property of higher plant microtubules (tubulin) in vivo. It may be specific for cells without centrosomes and may be superimposed also on other microtubulerelated processes. BSERVATIONS of microtubule reorganization in a newexperimental system, anucleate cell fragments in Haemanthus endosperm (7), have provided evidence for a previously unknown property of higher plant microtubules (MTs) t in vivo: the tendency of an initially irregular MT meshwork to reorganize itself until a very stable structure is formed. Such reorganization may be an intrinsic property of higher plant MTs, as it has not been observed in animal tissue culture (28). This is a sequential reorganization that follows always the same steps and is triggered merely by removing endosperm from the ovule. The final result may be the formation of chromosome-free spindles/phragmoplasts, or spirals that may close and form tings. Further observations of MT reorganization have revealed the presence of unexpected MT configurations, appearing under specific conditions, whose morphological structure often resembles that of fir trees (35). These MT converging centers, with the shape of J Abbreviations used in this paper. DIC, differen...

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