Never Let a Good Crisis Go to Waste: The Kinesin ARK1 Promotes Microtubule Catastrophe during Root Hair Development

Microtubules are rigid, hollow rods that help determine cell shape and play roles in a variety of dynamic processes, such as organelle transport and mitosis. These roles require microtubules to be highly dynamic. Microtubules are composed of heterodimeric tubulin subunits that assemble into a polar, cylindrical polymer with a dynamic plus end and a less dynamic minus end. Addition or loss of tubulin subunits at the ends of microtubules causes them to exist in persistent phases of growth or shrinkage, respectively, with abrupt transitions between these two states (first described in Mitchison and Kirschner, 1984). The transition from growth to shrinkage is often referred to as catastrophe, while the transition back to growth is known as rescue. In plants, microtubules play an important role in tip growth, an extreme form of polarized growth that results in cylindrical cells with rounded tips at which growth activity takes place. Such growth is typified by root hairs. Indeed, treatments that destabilize (oryzalin) or stabilize (taxol) microtubules cause crooked and branched root hairs to develop, demonstrating their importance in tip growth, although their exact function is currently unclear.

The dynamic process of microtubule catastrophe and rescue is orchestrated by microtubule-associated proteins, such as kinesins. These highly conserved motor proteins transport cargo along microtubules and modulate microtubule dynamics and organization. Of the 61 putative kinesins identified in Arabidopsis thaliana to date, only one, Kinesin-13A, has been shown to be a catastrophe factor, but it functions in the secondary cell wall pits of developing xylem cells and not during tip growth (Oda and Fukuda, 2013). Another kinesin, ARMADILLO-REPEAT KINESIN1 (ARK1), is thought to promote microtubule catastrophe in Arabidopsis root hairs, as ark1 mutants have wavy/branched root hairs with highly abundant endoplasmic microtubules. However, the role of ARK1 in modulating microtubule dynamics has not been confirmed.

In a pivotal study, Eng and Wasteneys (2014) used a variety of techniques to demonstrate that ARK1 plays a crucial role in modulating microtubule dynamics during root hair growth. First, using a green fluorescent protein (GFP)-labeled microtubule plus-end tracking protein, they determined that the ark1-1 mutant exhibits reduced microtubule catastrophe frequency and slow growth velocities, which are partially restored by low concentrations of oryzalin; treatments with this drug, but not taxol, partially rescued the root hair phenotype of the mutant (see figure). Then, the authors transformed the mutant with an ARK1-GFP gene construct, revealing that ARK1 predominantly localizes to the plus ends of growing microtubules. Finally, to further explore microtubule catastrophe in the ark1-1 mutant, the authors transiently transformed germinating ark1-1 seedlings harboring GFP fused to a microtubule marker with an ARK1-RFP (red fluorescent protein) construct driven by the UBIQUITIN10 promoter. This system made it possible to compare microtubule dynamics in cells overexpressing ARK1-RFP versus nontransformed (negative control) cells within the same cotyledon. Cells expressing ARK1-RFP had a significantly higher catastrophe frequency than the control. Moreover, while ARK1-RFP microtubules spent a greater amount of time in the shrinkage phase, the microtubule shrinkage velocity was not significantly altered in these cells, although the growth velocity was reduced.

The root hair phenotype of ark1-1 is partially rescued with oryzalin but not taxol. Bright-field light images of wild-type and ark1-1 root hairs treated with taxol or oryzalin. Bars = 50 μm. (Reprinted from Eng and Wasteneys [2014], Figure 2A.)

Thus, ARK1 accumulates at (and tracks along) growing microtubule plus ends and promotes microtubule catastrophe. Complementary biochemical studies could help determine whether ARK1 functions alone in such a process. Nonetheless, by promoting such catastrophe, ARK1 facilitates microtubule turnover, perhaps by maintaining the levels of free tubulin required to sustain rapid microtubule polymerization. Such polymerization would lead to proper unidirectional tip growth in root hairs, which is hardly a catastrophe.