Encounters between Dynamic Cortical Microtubules Promote Ordering of the Cortical Array through Angle-Dependent Modifications of Microtubule Behavior[W]

Abstract

Ordered cortical microtubule arrays are essential for normal plant morphogenesis, but how these arrays form is unclear. The dynamics of individual cortical microtubules are stochastic and cannot fully account for the observed order; however, using tobacco (Nicotiana tabacum) cells expressing either the MBD-DsRed (microtubule binding domain of the mammalian MAP4 fused to the Discosoma sp red fluorescent protein) or YFP-TUA6 (yellow fluorescent protein fused to the Arabidopsis a-tubulin 6 isoform) microtubule markers, we identified intermicrotubule interactions that modify their stochastic behaviors. The intermicrotubule interactions occur when the growing plus-ends of cortical microtubules encounter previously existing cortical microtubules. Importantly, the outcome of such encounters depends on the angle at which they occur: steepangle collisions are characterized by approximately sevenfold shorter microtubule contact times compared with shallowangle encounters, and steep-angle collisions are twice as likely to result in microtubule depolymerization. Hence, steep-angle collisions promote microtubule destabilization, whereas shallow-angle encounters promote both microtubule stabilization and coalignment. Monte Carlo modeling of the behavior of simulated microtubules, according to the observed behavior of transverse and longitudinally oriented cortical microtubules in cells, reveals that these simple rules for intermicrotubule interactions are necessary and sufficient to facilitate the self-organization of dynamic microtubules into a parallel configuration.

Computer simulation and mathematical models of the noncentrosomal plant cortical microtubule cytoskeleton

“…Under low CMT anchoring conditions, this model results in a limited collision-induced depolymerization probability, similar to the experimental observations in Arabidopsis petiole cells [Wightman and Turner, 2007]. The authors suggest that tighter CMT anchoring to the plasma membrane might explain the higher probability for collision-induced depolymerization observed in tobacco BY-2 cells [Dixit and Cyr, 2004b]. Based on these data, regulation of CMT anchoring to the plasma membrane is an important mechanism for controlling array organization.…”

“…They developed a Monte Carlo simulation with a limited number of CMTs in the system where the nucleation process is not considered. Their simulations show that simple rules for CMT interactions extracted from real-cell experiments can result in a parallel CMT arrangement from a randomly arranged population [Dixit and Cyr, 2004b]. They found that both bundling and collision-induced depolymerization are necessary and sufficient for CMT organization, although this conclusion might be related to the restricted size of the simulations, as stated by the authors.…”

“…Encounters between CMTs have been observed to lead to different outcomes depending on the encounter angle [Dixit and Cyr, 2004b]. In particular, if the encounter angle is shallow (<40 ), then the plus-end of the encountering CMT almost invariably reorients and grows along the impeding CMT, resulting in CMT bundling.…”

“…The necessity of CMT interactions for parallel array organization is commonly agreed upon among different modeling studies [Dixit and Cyr, 2004b;Baulin et al, 2007;Allard et al, 2010b;Eren et al, 2010;Hawkins et al, 2010;Tindemans et al, 2010]. However, among the studies that consider both bundling and collision-induced depolymerization interactions, there are controversial results regarding the relative contribution of these two mechanisms.…”

“…Based on these data, regulation of CMT anchoring to the plasma membrane is an important mechanism for controlling array organization. However, this model fails to explain the angle-dependence of collision-induced depolymerization observed experimentally in Dixit and Cyr [2004b]. The effect of the encounter angle is considered while modeling the bundling mechanism.…”