Reorientation of Cortical Microtubule Arrays in the Hypocotyl of Arabidopsis thaliana Is Induced by the Cell Growth Process and Independent of Auxin Signaling

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.

Combining tensile testing and microscopy to address a diverse range of questions

“…In addition, many of the proteins, protein polymers and signalling networks that control the mechanical properties of cells are sensitive to mechanical stresses that build up during development, for example, by altering their orientation to align with the stress direction (Hush et al ., ; Hejnowicz & Sievers, ; Hejnowicz et al ., ; Hamant et al ., ; Heisler et al ., ; Nakayama et al ., ; Jacques et al ., ; Sampathkumar et al ., ; Hervieux et al ., ; Louveaux et al ., ; Bringmann & Bergmann, ; Robinson & Kuhlemeier, ), thus feeding back on to the process of development itself. However, there is also experimental evidence and modelling predictions that many of the same proteins and molecules change their localisation in response to light or hormonal cues (de Reuille et al ., ; Jönsson et al ., ; Smith et al ., ; Abley et al ., ; Peaucelle et al ., ; Mansfield et al ., ; Adamowski et al ., ); how this integrates with mechanical responses is unclear. It is also not well understood why, in some cases, mechanical conflict is resolved by buckling whereas in other cases, they are resolved by mechanical feedback on the growth machinery.…”

A coupled mechano-biochemical framework for root meristem morphogenesis

“…Auxin distributes along the root through a tightly controlled mechanism and its disruption results in organ growth failure [16][17][18] . Auxin synthesis and homeostasis is thought to be the other major contributor to cell elongation 19 . The main source of auxin during globular root embryogenesis comes from the shoot, and tends to accumulate in vascular tissue, root tip and epidermis 20,21 .…”

“…• Regulator and polarizer diffuses along the cell membrane according to the following equations: (19) (20) D(POLmem) and D(REGmem) are the amount of regulator and polarizer diffused over the membrane, respectively; Dreg and Dpol are regulator and polarizer diffusion rate, respectively; POLmem and REGmem are the amount of regulator and polarizer on membrane section mem, respectively; The polarizer is displaced by the presence of regulator molecules towards the zone where the concentration of the regular is the lowest. To achieve this, we apply a simple stochastic algorithm: for a given membrane section mem, a fixed batch amount of polarizer is reserved for possible displacement; then one of the two adjacent membrane segments is selected randomly; if the selected segment contains less amount of regulator than the current segment, the batch of polarizer is moved to that segment, otherwise, nothing is performed.…”

“…Consequently, this auxin-mediated cell expansion requires the proper deposition of cytoskeleton elements such as cortical microtubules (CMTs). CMTs contribute to cell polarity maintenance by constraining the directional trafficking of membrane proteins, small signalling molecules and cell wall elements 5,6,18,19 , thereby integrating mechanical stresses into the organ morphogenesis 20,21 . Mechanisms of polar auxin transport and CMT deposition are therefore essential for the root development but the integration of these processes is poorly understood.…”

Complementary Superresolution Visualization of Composite Plant Microtubule Organization and Dynamics

“…The plant interphase cortical array is a widespread microtubule system lying at the close vicinity of the plasma membrane, and it is intimately associated with cell growth and differentiation (Elliott and Shaw, 2018b). It can promptly reorganize in response to physical (reviewed in Lindeboom et al, 2013;Nakamura, 2015;Hamant et al, 2019), or hormonal (Vineyard et al, 2013;Elliott and Shaw, 2018a;Adamowski et al, 2019;True and Shaw, 2020) signals, in order to redefine cell growth directionality by blueprinting the orientation of cellulose deposition in the overlying cell wall (Chen et al, 2016).…”