Postdoctoral Researcher Tomonobu Nonoyama and Assistant Professor Satoru Tsugawa from Akita Prefectural University, along with Professor Minako Ueda from Tohoku University and their colleagues, have quantitatively elucidated the mechanism of microtubule band structure formation and its movement phenomenon during anisotropic growth of Arabidopsis zygotes through numerical simulations. They demonstrated that microtubule bands are formed with greater width than the directional cue regions that change microtubule orientation, and that there can be two different mechanisms when microtubules form bands. The numerical simulation results suggest the potential to control the orientation formation and movement of microfibers, with expected applications in the design of fiber-reinforced materials. The group's findings were published in Scientific Reports.
(A) In Arabidopsis, the zygote elongates unidirectionally and anisotropically to establish the plant's meristematic axis. It is thought that the zygote within the ovule undergoes anisotropic growth and unequal division along the meristematic axis, thereby determining the plant's meristematic axis.
(B) Microscopically captured images of the elongation of an Arabidopsis zygote. Over time, microtubules form a circular band structure and migrate towards the cell tip (white).
Provided by Akita Prefectural University
Plants possess intracellular fibrous structures called the cytoskeleton that maintain cell morphology and generate the physical forces necessary for movement both inside and outside the cell. Ueda's group at Tohoku University had discovered that cortical microtubules of plant zygotes align circumferentially to form band structures, but the detailed mechanism of this band formation was not well understood. Furthermore, while observations of zygotes over approximately 24 hours revealed that microtubule bands move toward the cell apex accompanying cell growth, how nanometer-scale microtubules undergo orientational ordering and alignment had long remained a mystery.
The research group aimed to elucidate the mechanism of cellular-scale band movement phenomena from molecular-scale behavior by combining imaging analysis of microtubules with agent-based models, a method that treats microtubules as agents.
First, based on fluorescence observations using two-photon excitation microscopy, they quantified the width and movement speed of microtubule bands within zygotes. As a result, it was revealed that microtubule bands are maintained near the cell apex and move toward the apical side at approximately the same speed as cell growth.
To explain this experimental data using molecular-level behavior, they constructed an agent-based model that considers the molecular biological behavior of cortical microtubules. In this method, experimental facts such as microtubule plus-end elongation at 0.08 micrometers per second, the phenomenon of microtubules bundling when they collide with each other, and the phenomenon of microtubule shortening under certain conditions are incorporated through 12 main parameters. Through these parameter settings, it becomes possible to analyze how microtubules behave at the cortex inside zygotes.
Using this model, the research group hypothesized that microtubule band formation requires a directional cue that changes the direction of microtubule elongation and investigated when and how microtubule bands form when the strength of the directional cue is changed in stages.
As a result, they obtained the non-trivial result that microtubule bands are formed in regions wider than the directional cue regions, regardless of the strength of the directional cue. They also found that depending on the strength of the directional cue, there are two different mechanisms: cases where microtubule bands align rapidly and cases where they align gradually. These results mean that the control of microtubule direction as input differs from the actual microtubule collective movement.
In addition to these differences between input and output regarding the width and formation of microtubule bands, interesting results were also obtained when moving the directional cue region toward the cell apex. When the speed of the directional cue region was set to an input speed close to experimental data, microtubule bands were formed, whereas at high input speeds outside the experimental data range, microtubule bands could not keep up with the directional cue region and were left behind. This result suggests a possible factor for why microtubule bands are not formed in root hairs and pollen tubes, which have faster cell growth rates than zygotes, similar apical growth cells.
While microscopic observations are effective for understanding physiological phenomena occurring within plant zygotes, there are limitations to the precise observation of nanoscale microtubules that are even smaller than the cellular scale. Agent-based models that incorporate molecular-level experimental data, as in this study, are effective for inferring the behavior of molecules that are difficult to observe and represent an important method for connecting cellular and molecular scales in a cross-cutting manner. Such collective movement simulations are expected to find applications not only in explaining molecular behavior in the developmental biology of zygotes and other systems, but also in fiber-reinforced materials and composite materials manufactured by controlling fine fibers.
Journal Information
Publication: Scientific Reports
Title: Agent-based simulation of cortical microtubule band movement in arabidopsis zygotes
DOI: 10.1038/s41598-025-11078-8
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.