Eva E. Deinum
Department of Biomolecular Systems
FOM institute AMOLF
Amsterdam, The Netherlands
Presented in the Embryo Physics Course, October 26, 2011
Abstract
The shape and development of plant cells and their tissues is largely controlled by the cell wall: the structural and mechanical anisotropy of the wall feeds back on the shape of the cell and determines how it can expand. A key factor in establishing this anisotropy is the cortical microtubule array: it plays a major role in directing the motion of the cellulose synthase complexes, the enzyme machinery that deposits the cellulose microfibrils that form the structural support of the cell wall.
In elongating interphase cells, the cortical microtubules are organised in a highly aligned transverse array, which de facto determines the direction of cell expansion. Thus, understanding the principles behind microtubule organisation and orientation forms the key to understanding single cell shape. The cortical microtubules are immobilised by attachment to the inside of the cell membrane, but still display “movement” through their intrinsic (de)polymerization dynamics. Because this system is effectively 2D these dynamic microtubules can interact through collisions.
Recent experimental observations suggest that the observed self-organization may arise from these collisions, which can have three possible angle-dependent outcomes — cross-overs, induced catastrophes and zippering. Using mathematical models and computer simulations several groups have shown that these interactions are indeed sufficient.
These models are mostly based on isotropic nucleation of new microtubules. Experiments, however, show that most nucleations (in a steady state array) occur from existing microtubules, with a typical bias on the relative nucleation angles.
We have implemented this in our simulation model based on the known dynamics of individual microtubules and their collisional interactions. Combining simulations with theory derived concepts we study the impact of orientationally biased microtubule-bound nucleation. We find that it increases the speed and degree of alignment and greatly expands the parameter regime of spontaneous alignment. The natural positive feedback that occurs between alignment and nucleation (partially) congruent with this alignment turns out to be an important factor in explaining these results.
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