Abstract
The Arabidopsis SPIRAL1 (SPR1) gene encodes a novel plant-specific protein. spr1 mutants show right-handed twisting in the epidermis of roots and etiolated hypocotyls presumably due to impaired organization of cortical microtubules (cMTs). Both SPR1::GFP and SPR1::His fusion proteins were able to compliment spr1 mutants, and the GFP fluorescence co-localized with cMTs. Bacterially expressed SPR1::His, however, failed to bind taxol-stabilized tubulin polymers, suggesting indirect interaction between SPR1 and cMTs. The Arabidopsis genome contains five SPIRAL1-LIKE (SP1L) genes. While double mutants of spr1 and some sp1ls showed novel twisting phenotype in the inflorescence, CaMV35S::SP1L fusion genes were able to rescue the spr1 defects. SPR1 and SP1Ls share high identity in both N- and C-terminal regions. GFP fusion experiments indicated that these regions alone are sufficient for targeting SPR1 to cMTs. These results suggest that the conserved N- and C-regions of SPR1 and SP1Ls indirectly act on cMTs through as yet unidentified molecule(s).
