The SPIRAL genes are required for directional central of cell elongation in Arabidopsis thaliana

Cells at the elongation zone expand longitudinally to form the straight cen tral axis of plant stems, hypocotyls and roots, and transverse cortical mic rotubule arrays are generally recognized to be important for the anisotropi c growth. Recessive mutations in either of two Arabidopsis thaliana SPIRAL loci, SPR1 or SPR2, reduce anisotropic growth of endodermal and cortical ce lls in roots and etiolated hypocotyls, and induce right-handed helical grow th in epidermal cell files of these organs, spr2 mutants additionally show right-handed twisting in petioles and petals. The spr1spr2 double mutant's phenotype is synergistic, suggesting that SPR1 and SPR2 act on a similar pr ocess but in separate pathways in controlling cell elongation. Interestingl y, addition of a low dose of either of the microtubule-interacting drugs pr opyzamide or taxol in the agar medium was found to reduce anisotropic expan sion of endodermal and cortical cells at the root elongation zone of wild-t ype seedlings, resulting in left-handed helical growth. In both spiral muta nts, exogenous application of these drugs reverted the direction of the epi dermal helix, in a dose-dependent manner, from right-handed to left-handed; propyzamide at 1 muM and taxol at 0.2-0.3 muM effectively suppressed the c ell elongation defects of spiral seedlings. The spr1 phenotype is more pron ounced at low temperatures and is nearly suppressed at high temperatures. C ortical microtubules in elongating epidermal cells of spr1 roots were arran ged in left-handed helical arrays, whereas the highly isotropic cortical ce lls of etiolated spr2 hypocotyls showed microtubule arrays with irregular o rientations. We propose that a microtubule-dependent process and SPR1/SPR2 act antagonistically to control directional cell elongation by preventing e longating cells from potential twisting. Our model may have implicit bearin g on the circumnutation mechanism.