Control of cortical microtubule arrays by SPIRAL and LEFTY genes

Abstract

After screening mutagenized Arabidopsis seedlings for directional helical growth mutants, we identified 4 right-handed SPIRAL loci and 5 left-handed LEFTY loci. In spiral mutants, epidermal cell files of axial organs grow twisted forming right-handed helix, and the ground tissues expand laterally; lefty mutants show opposite left-handed helical growth. The growth difference in the longitudinal direction between epidermis and ground tissues may be compensated by skewing the epidermal cell files.
It is generally assumed that cellulose microfibrils aligning parallel to the transversely oriented cortical microtubules (cMT) specify plant cell elongation in the longitudinal direction. cMT arrays in the rapidly elongating epidermis are arranged in left-handed helix in spiral1, whereas the arrays are in right-handed helix in lefty mutants. lefty mutants are caused by dominant negative amino acid substitutions in alpha or beta tubulins, and SPIRAL1 and SPIRAL2 are distinct members of plant-specific protein families which localize at cMT in vivo.