Another Arm-repeat protein identified in pollen, TIO, affects the cell division process. Formation of the male gametes (pollen) requires two consecutive mitotic divisions of haploid meiocyte cells. The first (asymmetric) division forms a small generative cell and a large vegetative cell. A second, symmetrical division of the generative cell makes two sperm cells that migrate inside the vegetative cell, forming a tricellular pollen grain (McCormick 2004). A screen for pollen cell division mutants identified the two-in-one (tio) mutant, which has aberrant cytokinesis (daughter cell separation) at the end of the first cell division. The cell plate (new cell membrane and cell wall) deposited between the vegetative and generative cell is incomplete, and the second mitotic division does not occur (Oh et al. 2005). TIO functions throughout the plant, as seedlings with reduced TIO function have a severe growth defect: the meristems cannot complete cell division and large, multi-nucleate cells form, indicating incomplete cytokinesis.
The TIO protein is a kinase related to human and Drosophila FUSED, which transduces developmental signals. Arabidopsis TIO contains four Arm repeats (Fig. 1), as do its orthologues in rice and the protozoan Leishma-nia. Human FUSED contains HEAT repeats, while Drosophila FUSED has no Arm/HEAT domain (Oh et al. 2005). TIO protein localises specifically to the midline of the phragmoplast, an actin- and microtubule-containing ring that determines the deposition of new cell plates during cytokinesis, allowing the formation of two independent daughter cells. These data combined with the incomplete cell plate phenotype of the tio mutant suggest that TIO is involved in the expansion of the phragmoplast as cytokinesis progresses. Interestingly, Drosophila FUSED associates with the microtubule cytoskeleton via a kinesin (Stegman et al. 2000), and the same may be true of TIO in Ara-bidopsis, despite its divergent function in a plant-specific cell division process (Fig. 2).
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