Investigation of the roles of Kinesin-5 motor protein family members in Arabidopsis thaliana

Eukaryotic cells contain a cytoskeleton that performs a range of essential functions, including intracellular transport, nuclear division and shaping the cell. There are various kinesin motor proteins involved in organizing the microtubule cytoskeleton during interphase and mitosis. The plant Arabidopsis thaliana has the largest number of kinesins amongst all sequenced organisms. Many of those kinesins have not been studied. Members of the Kinesin-5 family cross link microtubules and are crucial for separation of mitotic spindle poles. There are 4 members of this family in Arabidopsis: Kinesin-5 A, B, C and D, but only Kinesin-5 C has been studied previously. Each animal species only has either one or two of Kinesin-5 members, which have been well studied and can be used as a reference for other Kinesin-5 proteins. There is evidence that some functions are conserved between Kingdoms; however the multiple family members in Arabidopsis suggests that Kinesin-5 motors play some roles unique to plants. We have characterized T-DNA insertion lines of Arabidopsis Kinesin-5 A, B and D genes, two of which (A and B) are knockout mutants. Our analysis of growth rates, cortical microtubules, mitotic indexes, mitotic arrays, trichome morphology and pavement cell morphology indicates that Kinesin-5 A and Kinesin-5 B perform some different, and relatively minor functions in Arabidopsis growth and development. Kinesin-5 A appears to stabilize microtubule bundles in leaf epidermal pavement cells. Kinesin-5 B affects root growth rate, possibly through the organization of cortical microtubules in the root. Kinesin-5 A and B also play roles in mitosis. We propose that the third Kinesin-5 member, Kinesin-5 D, is an essential protein and for that reason we were unable to recover any homozygous mutants