Cellulose microfibril alignment recovers from DCB-induced disruption despite microtubule disorganization

The Arabidopsis sku6/spiral1 gene encodes a plus end-localized microtubule-interacting protein involved in directional cell expansion

The sku6-1 mutant of Arabidopsis thaliana exhibits altered patterns of root and organ growth. sku6 roots, etiolated hypocotyls, and leaf petioles exhibit right-handed axial twisting, and root growth on inclined agar media is strongly right skewed. The touch-dependent sku6 root skewing phenotype is suppressed by the antimicrotubule drugs propyzamide and oryzalin, and right skewing is exacerbated by cold treatment. Cloning revealed that sku6-1 is allelic to spiral1-1 (spr1-1). However, modifiers in the Columbia (Col) and Landsberg erecta (Ler) ecotype backgrounds mask noncomplementation in sku6-1 (Col)/spr1-1 (Ler) F1 plants. The SPR1 gene encodes a plant-specific 12-kD protein that is ubiquitously expressed and belongs to a six-member gene family in Arabidopsis. An SPR1:green fluorescent protein (GFP) fusion expressed in transgenic seedlings localized to microtubules within the cortical array, preprophase band, phragmoplast, and mitotic spindle. SPR1:GFP was concentrated at the growing ends of cortical microtubules and was dependent on polymer growth state; the microtubule-related fluorescence dissipated upon polymer shortening. The protein has a repeated motif at both ends, separated by a predicted rod-like domain, suggesting that it may act as an intermolecular linker. These observations suggest that SPR1 is involved in microtubule polymerization dynamics and/or guidance, which in turn influences touch-induced directional cell expansion and axial twisting.

Microtubule Defects and Cell Morphogenesis in the lefty1lefty2 Tubulin Mutant of Arabidopsis thaliana

lefty1 and lefty2 are semi-dominant left-handed helical growth mutants of Arabidopsis thaliana, which result from identical dominant-negative amino acid substitutions in alpha-tubulin 6 and alpha-tubulin 4, respectively. Here we characterized the expression patterns of the affected tubulin genes and the phenotypes of the lefty double mutant to address increasing effects of microtubule defects on cell morphogenesis. Both tubulin genes were expressed ubiquitously in examined tissue and cell types, and the alpha-tubulin 2/4/6 subfamily transcripts predominated over other alpha-tubulin transcripts in Arabidopsis seedlings. The lefty double mutant seedlings showed helical growth in hypocotyls and radial cell expansion in the root elongation zone where mutant cortical microtubule arrays were more fragmented and less well aligned than wild-type arrays. Branching of leaf trichomes was highly reduced. In adult mutant plants, anisotropic growth of anther filament cells was severely impaired. These results suggest that left-handed twisted elongation is an intermediate state that leads to full isotropic expansion as the cortical microtubules are increasingly destabilized.