Reduction of ATP ase activity in the rice kinesin protein Stemless Dwarf 1 inhibits cell division and organ development

Summary Several kinesins, the ATP ‐driven microtubule ( MT )‐based motor proteins, have been reported to be involved in many basic processes of plant development; however, little is known about the biological relevance of their ATP ase activity. Here, we characterized the Oryza sativa (rice) stemless dwarf 1 ( std1 ) mutant, showing a severely dwarfed phenotype, with no differentiation of the node and internode structure, abnormal cell shapes, a shortened leaf division zone and a reduced cell division rate. Further analysis revealed that a substantial subset of cells was arrested in the S and G2/M phases, and multinucleate cells were present in the std1 mutant. Map‐based cloning revealed that STD 1 encodes a phragmoplast‐associated kinesin‐related protein, a homolog of the Arabidopsis thaliana PAKRP 2, and is mainly expressed in the actively dividing tissues. The STD 1 protein is localized specifically to the phragmoplast midzone during telophase and cytokinesis. In the std1 mutant, the substitution of Val‐40‐Glu in the motor domain of STD 1 significantly reduced its MT ‐dependent ATP ase activity. Accordingly, the lateral expansion of phragmoplast, a key step in cell plate formation, was arrested during cytokinesis. Therefore, these results indicate that the MT ‐dependent ATP ase activity is indispensible for STD 1 in regulating normal cell division and organ development.