Polarity of spindle microtubules in Haemanthus endosperm.

Abstract

Structural polarities of mitotic spindle microtubules in the plant H. katherinae were studied by lyzing endosperm cells in solutions of neurotubulin under conditions that will decorate cellular microtubules with curved sheets of tubulin protofilaments. Microtubule polarity was observed at several positions in each cell by cutting serial thin sections perpendicular to the spindle axis. The majority of the microtubules present in a metaphase or anaphase half-spindle are oriented with their fast-growing or plus ends distal to the polar area. Near the polar ends of the spindle and up to .apprx. halfway between the kinetochores and the poles, the number of microtubules with opposite polarity is low: 8-20% in metaphase and 2-15% in anaphase cells. Direct examination of 10 kinetochore fibers shows that the majority of these microtubules, too, are oriented with their plus ends distal to the poles, as has been shown in animal cells. Sections from the region near the spindle equator reveal an increased fraction of microtubules with opposite polarity. Graphs of polarity vs. position along the spindle axis display a smooth transition from microtubules of 1 orientation near the 1st pole, through a region containing equal numbers of the 2 orientations, to a zone near the 2nd pole where the opposite polarity predominates. Apparently, the spindle of endosperm cells is constructed from 2 sets of microtubules with opposite polarity that interdigitate near the spindle equator. The length of the zone of interdigitation shortens from metaphase through telophase, consistent with a model that states that during anaphase, spindle elongation in Haemanthus, the interdigitating sets of microtubules are moved apart. No major changes in the distribution of microtubule polarity in the spindle interzone from anaphase to telophase occurred when cells are engaged in phragmoplast formation. The initiation and organization of new microtubules, thought to take place during phragmoplast assembly, must occur without significant alteration of the microtubule polarity distribution.