In vivo measurement of the Young's modulus of the cell wall of single root hairs

Abstract

Root hairs are cells from the root epidermis that grow as long tubular bulges perpendicular to the root. They can grow in a variety of mechanical or chemical environments. Their mechanical properties are mainly due to their stiff cell wall which also constitutes a physical barrier between the cell and its environment. Thus, it is essential to be able to quantify the cell wall mechanical properties and their adaptation to environmental cues. Here, we present a technique we developed to measure the Young's (elastic) modulus of the root hair cell wall. In essence, using custom-made glass microplates as cantilevers of calibrated stiffness, we are able to measure the force necessary to bend a single living root hair. From these experiments one can determine the stiffness and Young's modulus of the root hair cell wall.

Keywords: Bending; Cell wall; Mechanics; Root hair; Stiffness; Young’s modulus.

Fig. 1

(A) Schematic representation of the experiment. The microplate is placed vertically against the tip of the RH. Then, thanks to a piezo-actuator, the microplate base is moved by a distance D resulting in the bending of the root hair. (B) Schematic representation in the XY plane. Top, before the measurement. Bottom, the root hair is deflected by a length d. The microplate deflection is thus (D-d) and it exerts a force F = k (D-d) on the root hair, with k the stiffness of the microplate. (C) Sequence of images displaying deflection increments applied on a single RH. Between each image the base of the microplate is moved by 5 µm. (scale bar = 30 µm). (D) data corresponding to (C). The force exerted is expressed as a function of $\frac{3d}{L^3}$. The blue curve represents the linear fit of the data. Its slope y represents the factor EI. (E) Boxplot showing the distribution of the measured factor EI for n = 16 root hairs. (F) Schematic representation of a root hair viewed as a hollow cylinder of radius R and negligible width h. (H) Boxplot showing the distribution of the stiffness Eh (n = 16) (I) Boxplot showing the distribution of the cell wall Young Modulus E when h is taken as equal to 250 nm (n = 16). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

(A) Sequence of images displaying deflection increments applied on a single ERU RH. Between each image the base of the microplate is moved by 5 µm. (scale bar = 50 µm). (B) Boxplot showing the distribution of the cell wall Young Modulus E when h is taken as equal to 250 nm for wild-type RH and equal to 500 nm for ERU loss-of-function (n_WT = 16, n_ERU = 18, seedlings of about two weeks old). Data sets were compared using a Wilcoxon rank-sum test (**** p < 0.0001).