RESUMEN
To ensure an even segregation of chromosomes during somatic cell division, eukaryotes rely on mitotic spindles. Here, we measured prime characteristics of the Arabidopsis mitotic spindle and built a three-dimensional dynamic model using Cytosim. We identified the cell-cycle regulator CYCLIN-DEPENDENT KINASE B1 (CDKB1) together with its cyclin partner CYCB3;1 as key regulators of spindle morphology in Arabidopsis. We found that the augmin component ENDOSPERM DEFECTIVE1 (EDE1) is a substrate of the CDKB1;1-CYCB3;1 complex. A non-phosphorylatable mutant rescue of ede1 resembled the spindle phenotypes of cycb3;1 and cdkb1 mutants and the protein associated less efficiently with spindle microtubules. Accordingly, reducing the level of augmin in simulations recapitulated the phenotypes observed in the mutants. Our findings emphasize the importance of cell-cycle-dependent phospho-control of the mitotic spindle in plant cells and support the validity of our model as a framework for the exploration of mechanisms controlling the organization of the eukaryotic spindle.
RESUMEN
Plant species collected from various climatic zones and stressed in vitro at various temperatures reveal changes in cellular ultrastructure which are in accordance with the climate at their sampling sites. This observation initiated the investigation to establish if stress at different temperatures may cause diverse extents of changes in the ultrastructure of microalgal strains originating from different geographic zones. The study revealed that the six Cosmarium strains demonstrated ultrastructural characteristics that were consistent with their source location under optimal, low and high temperature conditions, pointing to their preference to specific climatic niches. Interestingly, chloroplasts of all of the Cosmarium strains correspond to a sun-adapted type, which is concomitant with earlier statements that these strains are rendered as high-light adapted algae. The Cosmarium strains developed multiple ultrastructural responses which enabled them to cope with excessive temperatures, occasionally occurring in desmid natural habitats. The appearance of cubic membranes and increased number of plastoglobules may represent the first line in protection against high-temperature stress, which is accompanied by the alteration of protein synthesis and the appearance of stress granules in order to preserve cell homeostasis. However, the prolonged warm- or cold-temperature stress obviously initiated the programmed cell death, as concluded from the appearance of several ultrastructural features observed in all of the Cosmarium strains. The fair acclimation possibilities and the ability to undergo programmed cell death in order to save the population, certainly favor the cosmopolitan distribution of the genus Cosmarium.