Abstract
Cortical microtubule arrays in elongating epidermal cells in both the root and stem of plants have the propensity of dynamic reorientations that are correlated with the activation or inhibition of growth. Factors regulating plant growth, among them the hormone auxin, have been recognized as regulators of microtubule array orientations. Some previous work in the field has aimed at elucidating the causal relationship between cell growth, the signaling of auxin or other growth-regulating factors, and microtubule array reorientations, with various conclusions. Here, we revisit this problem of causality with a comprehensive set of experiments in Arabidopsis thaliana, using the now available pharmacological and genetic tools. We use isolated, auxin-depleted hypocotyls, an experimental system allowing for full control of both growth and auxin signaling. We demonstrate that reorientation of microtubules is not directly triggered by an auxin signal during growth activation. Instead, reorientation is triggered by the activation of the growth process itself and is auxin-independent in its nature. We discuss these findings in the context of previous relevant work, including that on the mechanical regulation of microtubule array orientation.
Highlights
In interphase plant cells, microtubules are found just underneath the plasma membrane
We study the processes in elongating cells of the hypocotyl, during the promotion of growth by auxin which is associated with the reorientation of microtubules into transverse arrays
Under Hypothesis C (Figure 1C), only growth is directly activated by auxin signaling, while the reorientation of microtubules into transverse arrays happens as a downstream consequence of the anisotropic cell growth
Summary
Microtubules are found just underneath the plasma membrane. In elongating epidermal cells, such as those in roots, hypocotyls or coleoptiles, these cortical microtubules are aligned into ordered arrays that, especially under the outer plasma membrane, have the ability to conspicuously reorient in relation to the long cell axis. Put most growing cells typically exhibit microtubules transverse to the long cell axis, and non-growing cells have longitudinally oriented microtubules [1]. This simple correlation doesn’t encompass all observed cases. Dark grown hypocotyls exhibit only limited transverse orientation in the outer epidermal domain [5]. A single cell’s array doesn’t necessarily exhibit a single orientation, but it can be subdivided into domains with microtubules at distinct angles [2]
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