Editors' ChoicePlant biology

Rapidly Reorienting Microtubules

Sci. Signal.  09 Dec 2014:
Vol. 7, Issue 355, pp. ec343
DOI: 10.1126/scisignal.aaa4351

Plant cells grow directionally through cell expansion by elongation, which requires a specific orientation of the microtubule cytoskeleton. Elongating and expanding cells have transverse microtubules. Reorientation of the microtubules longitudinally inhibits cell expansion. Auxin is a plant hormone that mediates various developmental and growth processes through the activation of two distinct pathways; one (TIR/AFB-AUX/IAA) triggers transcriptional responses and another (auxin-binding protein 1 [ABP1]) stimulates both transcriptional and nontranscriptional responses. Chen et al. found that in the roots or hypocotyls of Arabidopsis thaliana microtubules (tagged genetically with a fluorescent protein fused to a microtubule-associated protein or to tubulin, which forms the microtubules) were predominantly transverse, indicating cell elongation. Addition of natural or synthetic auxin stimulated the realignment of root or hypocotyl microtubules to a longitudinal orientation. Roots respond to gravity such that auxin concentrations are highest where cell expansion is inhibited at the lower side closest to the gravitational source. In roots that were reoriented by 90˚ relative to gravity both auxin distribution and the trajectory of microtubule elongation rapidly changed with auxin accumulating and microtubules growing longitudinally at the lower side within 10 minutes. Kinetic analysis of the microtubule trajectories revealed a very fast (within 30 seconds) response to auxin, consistent with a nontranscriptional response. Roots in plants in which the TIR/AFB-AUX/IAA pathway was genetically disrupted had normal microtubule orientation and responded to auxin, but required higher concentrations than wild-type plants. Roots in plants with genetic mutations in ABP1 or its downstream signaling components, the guanosine triphosphatase ROP6, RIC1, and the microtubule-severing protein katanin (KTN1), exhibited impaired auxin-induced microtubule reorientation. These data indicated that auxin inhibits cell expansion by rapidly stimulating longitudinal microtubule reorientation through the ABP1 pathway.

X. Chen, L. Grandont, H. Li, R. Hauschild, S. Paque, A. Abuzeineh, H. Rakusová, E. Benkova, C. Perrot-Rechenmann, J. Friml, Inhibition of cell expansion by rapid ABP1-mediated auxin effect on microtubules. Nature 516, 90–93 (2014). [PubMed]