Dynamics of microtubules in higher plant mitosis: Functional kinetochore fiber in Haemanthus endosperm

Abstract

The majority of conceptual interpretations of the molecular mechanism of chromosome transport is based on the centriolar spindle. The prevailing interpretations have been modified and altered frequently and, in spite of considerable progress recently, the molecular mechanism remains elusive. The reasons for this slow progress are multiple. We believe that, underestimated and trivial as it seems, one of the major problems is the emphasis on the centriolar spindle where the presence of the aster obstructs the organization of the spindle pole and clouds the essential problem of the anchorage of the kinetochore fiber. Therefore, we have concentrated deliberately on higher plant endosperm of Haemanthus, which has the largest and clearest acentriolar mitotic spindles known. A large number of dividing cells on each preparation (averaging about 500 with a maximum of about 1800), combined with natural variation and experimental lowering of the available tubulin, have allowed us to characterize division in cells, which we believe have the threshold number of microtubules required for a functional spindle. Our conclusions on how mitosis works are very different from any current interpretation.Endosperm cells of Haemanthus katherinae Bak. were used as material. Microtubule distribution was analyzed in immunogold stained cells fixed about 45 minutes after preparation. Therefore all cells damaged during and shortly after preparation were excluded because they do not immuno-stain. Over 95% of the remaining cells complete mitosis. Cells with average microtubule numbers (see Figs in ) were compared with cells containing very low numbers of microtubules, occasionally found in controls (Figs. 1-4) and after experimental depletion. Microtubule depletion occurs in about 30% of the cells after routine growing of plants 10°C - 12°C for 10-14 days and is retained for 2-3 days at room temperatures (21°C - 24°C). Observations are based on several hundreds of cells, comprising multiple examples of transitions between various degrees of microtubule depletion.