Received September 5, 2009
Returned for revision December 19, 2009
Accepted January 13, 2010
The 
-Tubulin Complex Protein GCP4 Is Required for Organizing Functional Microtubule Arrays in Arabidopsis thaliana
Zhaosheng Kong 1, Takashi Hotta 1, Yuh-Ru Julie Lee 1, Tetsuya Horio 2, and Bo Liu 3*
1 Department of Plant Biology, University of California, Davis, California 95616
2 Department of Molecular Biosciences, University of Kansas, Lawrence, Kansas 66045
3 Department of Plant Biology, University of California, Davis, California 95616
* To whom correspondence should be addressed. E-mail: bliu{at}ucdavis.edu..
Microtubule (MT) nucleation and organization depend on the evolutionarily conserved protein 
-tubulin, which forms a complex with GCP2-GCP6 (GCP for -Tubulin Complex Protein). To date, it is still unclear how GCP4-GCP6 (the non-core GCPs) may be involved in acentrosomal MT nucleation in plant cells. We found that GCP4 was associated with -tubulin in vivo in Arabidopsis thaliana. When GCP4 expression was repressed by an artificial microRNA, transgenic plants exhibited phenotypes of dwarfism and reduced organ size. In mitotic cells, it was observed that the -tubulin signal associated with the mitotic spindle, and the phragmoplast was depleted when GCP4 was downregulated. Consequently, MTs failed to converge at unified spindle poles, and the bipolar phragmoplast MT array frequently had discrete bundles with extended minus ends, resulting in failed cytokinesis as reflected by cell wall stubs in leaf epidermal cells. In addition, cortical MTs in swollen guard cells and pavement cells of the leaf epidermis became hyperparallel and bundled, which was likely caused by frequent MT nucleation with shallow angles on the wall of extant MTs. Therefore, our results support the notion that GCP4 is an indispensable component for the function of -tubulin in MT nucleation and organization in plant cells.
