Positional Cloning And Functional Study Of The Stemless Dwarf1 Gene In Rice

Dwarfism is a major phenotypic trait in rice breeding.Rice dwarfism is caused not only by defects in gibberellin and brassinosteroid biosynthesis or signaling,but also defects in cell division and expansion.Kinesins are members of a superfamily of ATP-driven microtubule-based motor proteins with important functions in diverse cellular processes,including cell division.Cell division in higher plants is a fundamental process associated with four distinct microtubule(MT)arrays:the interphase cortical array,the preprophase band,the spindle and the phragmoplast.The molecular and cellular mechanisms regulating organization of these MT arrays in rice(Oryza sativa)are not well understood.In this study,we characterized the rice stemless and severely dwarfed 1(std1)mutant,which has a severely dwarfed phenotype associated with abnormal cell shapes and disorganized cell arrangements.Molecular analysis showed that STD1 encodes a kinesin-related protein and is mainly expressed in actively dividing tissues.STD1 is a homo log of Arabidopsis thaliana PAKRP2,and Physcomitrella patens KINID1a and KINID1b,which are phragmoplast-specific kinesins.However,we found that STD1 is not only associated with the phragmoplast but also colocalizes with the preprophase band.In the std1 mutant,substitution of a conserved valine(Val-40)for glutamic acid in the STD1 motor domain significantly reduced its MT-stimulated ATPase activity,leading to the formation of wider preprophase bands,abnormal phragmoplast organization,and incomplete cell plate formation.In the std1 mutant,cell-cycle progression arrests in S and G2/M phases and there were large and irregularly swollen cells with multinuclear.Thus,STD1 plays a critical role in the organization of the preprophase bands and the phragmoplast,and in the formation of the cell plate.