The SPIRAL genes are required for directional control of cell elongation in Aarabidopsis thaliana.

Cells at the elongation zone expand longitudinally to form the straight central axis of plant stems, hypocotyls and roots, and transverse cortical microtubule arrays are generally recognized to be important for the anisotropic growth. Recessive mutations in either of two Arabidopsis thaliana SPIRAL loci, SPR1 or SPR2, reduce anisotropic growth of endodermal and cortical cells in roots and etiolated hypocotyls, and induce right-handed helical growth 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 process but in separate pathways in controlling cell elongation. Interestingly, addition of a low dose of either of the microtubule-interacting drugs propyzamide or taxol in the agar medium was found to reduce anisotropic expansion of endodermal and cortical cells at the root elongation zone of wild-type seedlings, resulting in left-handed helical growth. In both spiral mutants, exogenous application of these drugs reverted the direction of the epidermal helix, in a dose-dependent manner, from right-handed to left-handed; propyzamide at 1 microM and taxol at 0.2-0.3 microM effectively suppressed the cell elongation defects of spiral seedlings. The spr1 phenotype is more pronounced at low temperatures and is nearly suppressed at high temperatures. Cortical microtubules in elongating epidermal cells of spr1 roots were arranged in left-handed helical arrays, whereas the highly isotropic cortical cells of etiolated spr1 hypocotyls showed microtubule arrays with irregular orientations. We propose that a microtubule-dependent process and SPR1/SPR2 act antagonistically to control directional cell elongation by preventing elongating cells from potential twisting. Our model may have implicit bearing on the circumnutation mechanism.

N(omega)-nitro-L-arginine methyl ester protects retinal neurons against N-methyl-D-aspartate-induced neurotoxicity in vivo.

We investigated whether the inhibition of nitric oxide (NO) synthesis with N(omega)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of NO synthase, affects N-methyl-D-aspartate (NMDA)-induced neurotoxicity in the rat retina in vivo. A single intravitreal injection of NMDA damaged the ganglion cell layer and the inner plexiform layer without affecting the other retinal layers 7 days after injection. Intravitreal injection of (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5, 10-imine hydrogen maleate (MK-801) with NMDA significantly reduced NMDA-induced degeneration of the retina. NMDA-induced degeneration was also prevented by intravitreal injection of L-NAME but not of D-NAME. The protective effect of L-NAME was antagonized by L-arginine. These results suggest that NO plays an important role in NMDA-induced excitotoxic degeneration in the retina.