Research Article

A Mechanism for Reorientation of Cortical Microtubule Arrays Driven by Microtubule Severing

Science  06 Dec 2013:
Vol. 342, Issue 6163,
DOI: 10.1126/science.1245533

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Structured Abstract

Introduction

Organization of the cortical cytoskeleton guides the growth and morphogenesis of organisms, from bacteria to higher plants, that depend on cell walls. By positioning wall-building enzymes, the cytoskeleton acts as an interior scaffold to direct construction of the cell’s exterior. In plants, environmental and hormonal signals that modulate cell growth cause reorganization of cortical microtubule arrays. These arrays do not appear to be remodeled by moving individual microtubules, but rather by rules that govern how microtubules are assembled or disassembled. In this Research Article, we investigate the mechanism by which blue light, an important signal from the environment, causes a rapid 90° reorientation of cortical arrays in growing cells of the plant axis.

Embedded Image

Blue light perception stimulates generation of a cascade of newly oriented microtubules by katanin severing. A confocal microscopy time series of the cortical microtubule array (white, preexisting; blue, newly assembled) in an Arabidopsis epidermal cell is shown. Perception of blue light by phototropin receptors has stimulated severing at microtubule intersections. Growth of the new ends creates new and co-oriented microtubules. Together, the organization of the preexisting array and the statistical behavior of severing favor the growth of longitudinal microtubules, driving array reorientation.

Methods

We used spinning-disk confocal microscopy to image the reorganization of cortical microtubule arrays in real time and visualize functional proteins tagged with fluorescent proteins. We developed image-analysis methods to measure changes in array organization and behaviors of individual microtubules during array reorientation. To test hypotheses about signaling and reorganizational mechanisms, we analyzed mutants in light-perception pathways and in activity of the microtubule-severing protein katanin. Finally, we conducted photomorphogenesis assays in plant seedlings to place our findings in a physiological context.

Results

We discovered a mechanism, based on microtubule severing by the protein katanin, that reorients cortical microtubule arrays in response to perception of blue light. Specifically, we observed that katanin localized to microtubule crossovers, where it was required to preferentially catalyze the severing of the newer microtubule, an activity that was stimulated by the function of phototropin blue light receptors. New plus ends created by severing were stabilized and immediately grew at a high frequency, resulting in the effective creation of new microtubules. Most microtubules generated during reorientation were created by this mechanism, producing ~83% of new longitudinal microtubules. Cortical arrays failed to reorient in a mutant lacking the katanin protein. Microtubules produced by severing at crossovers can make new crossovers and, thus, opportunities for further rounds of severing and initiation, constituting a molecular amplifier that rapidly builds a new population of microtubules orthogonal to the initial array. Further experiments put this mechanism in a physiological context by revealing that katanin function is required for directional blue light to stimulate bending of the plant axis toward the light source.

Discussion

Cortical microtubule arrays in higher plants are being recognized as systems with self-organizing properties arising from rules governing the outcomes of microtubule interactions. In this Research Article, we present evidence that one outcome of microtubule interaction, katanin-mediated severing at crossover sites, is regulated by light perception and acts to reorient the array. Severing is thought to help build microtubule arrays in neurons and meiocytes, but it has been difficult to test this idea directly because of imaging limitations. With live imaging of plant cell cortical arrays, we have been able to investigate the cellular function of severing at the level of individual molecular events, revealing how generation of microtubules by severing is used to construct a new array.

Light Turns the Array

The organization of cortical microtubule arrays in higher plant cells is essential for organizing cell and tissue morphogenesis, but it is not clear how specific architectures are acquired and reconfigured in response to environmental cues. Lindeboom et al. (10.1126/science.1245533, published online 7 November; see the Perspective by Roll-Mecak) used live-cell imaging and genetic studies to show that the microtubule-severing protein, katanin, plays a crucial role in reorienting cortical arrays from transverse to longitudinal in Arabidopsis seedlings in response to blue light perception. Katanin localized to microtubule intersections where, stimulated by blue light receptors, it preferentially catalyzed the severing of the newer microtubule. The microtubule “plus” end created by severing were observed to grow preferentially, effectively building a new population of microtubules orthogonal to the initial array. The net effect of this process steers the growing seedling toward light.

Abstract

Environmental and hormonal signals cause reorganization of microtubule arrays in higher plants, but the mechanisms driving these transitions have remained elusive. The organization of these arrays is required to direct morphogenesis. We discovered that microtubule severing by the protein katanin plays a crucial and unexpected role in the reorientation of cortical arrays, as triggered by blue light. Imaging and genetic experiments revealed that phototropin photoreceptors stimulate katanin-mediated severing specifically at microtubule intersections, leading to the generation of new microtubules at these locations. We show how this activity serves as the basis for a mechanism that amplifies microtubules orthogonal to the initial array, thereby driving array reorientation. Our observations show how severing is used constructively to build a new microtubule array.

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