Abstract
Recessive mutations in Arabidopsis thaliana SPIRAL1 locus, SPR1, reduce anisotropic growth of endodermal and cortical cells in roots and etiolated hypocotyls, and induce a right handed helical growth in epidermal cells of these organs. It was proposed that a microtubule dependent process and SPR1 act antagonistically to control directional cell elongation. SPR1 encodes a novel protein of low molecular weight. We identified two genes, SPI1 and SPI2, encoding proteins that interact specifically with SPR1 in the yeast two-hybrid system assay. SPI1 and SPI2 define a new family of putative type Ib membrane proteins. We have isolated plants harboring T-DNA insertions in SPI1 and SPI2 genes. Characterization of spr1/spi mutants, together with the SPR1 and Myc-SPI tagged proteins association to the Arabidopsis microsomal protein fraction, strongly suggest that the putative SPR1/SPI membrane bound protein complex is involved in a cell expansion process that might be related with cortical microtubule arrays.
