Plant cell wall expansion

© K.T. Haas. Fig. 1. The «Expanding Beam» model explains the shape of the cotyledon epidermal cells. A CryoSEM image of Arabidopsis cotyledon showing an undulating pavement cell. B (top) fluorescence image of pavement cell lobe, (bottom) 3D dSTORM pointillist picture of the same anticlinal wall segment in 3D view showing ; HG^HM-orange-violet colormap encoding Z position; and HG^LM-green nanofilaments detected with LM20 and 2F4 antibody respectively. C In silico implementation of the ‘expanding beam’ model using nonlinear FEM representing 60 growth iterations. After 25 iterations, simulation continuous as either WT (wild-type) tissue, or inducible overexpresser (oe) of PME5 or inhibitor of PME (PMEI3). PME5oe and PME3Ioe present impaired growth and lobe morphogenesis. D Schema explaining lobe formation: (1) HG^HM are inserted in the wall, (2) more pectin is demethylated on the convex side, leading to (3) lobed anticlinal wall.

© K.T. Haas. Fig. 2 A Putative fine structure of HG nanofilaments. (left) Hexagonal lattice of HG^HM polymers, (right) square lattice of HG^LM polymers. B A diagram explaining the current growth model as the remodelling of the xyloglucan ‘hot spots’ connecting cellulose microfibrills. (left) before, and (right) after the growth. C Expanding beam model; HG demethylation leads to radial swelling of HG nanofilaments and thus cell wall expansion.

Cell wall architecture and growth

The presence of a rigid pectocellulosic extracellular matrix (the cell wall) is one of the defining characteristics of the plant cell. Although its microscopic observation was a pivotal moment in cellular biology in the 17^th century, today still little is known about the structure and function of the cell wall. At the interface between cell and its environment, it fulfills a plethora of fundamental functions in plant cell physiology, signaling and most notably the growth process

Plant growth depends not only on the expansion of the cell, but the capacity of cell to modulate its immediate environment, which is the cell wall. Throughout development, extracellular fibers and polymers are placed strategically outside the cell to locally permit or restrict the expansion leading to morphogenesis. The primary cell wall (PCW) constitutes a rigid and resilient scaffold, whose primary role is to provide mechanical strength, and protection against external environment. It is an essential relay system, sensor, and transducer of the information occurring at the cell’s interface with its environment.

One can ask a simple, but nontrivial question: how does this rigid wall can grow at all? The answer to this question lies in a sophisticated architecture, which is modified by the cell itself during the growth. To understand growth, it is mandatory to understand the organization of the cell wall.

The expanding cell wall is composed of three groups of polysaccharides: cellulose, hemicellulose, and pectins, small number of proteins, water and phenolic compounds. Today cellulose is placed at the heart of plant growth mechanism and morphogenesis (anisotropic expansion). In the current view, the loosening of the cellulosic network by rearrangement of tethers (so called hot-spots) connecting cellulosic microfibrils is responsible for cell wall expansion. Anisotropic expansion is achieved by the local cell wall reinforcement through the guided deposition of the cellulose microfibrils transverse to the growth axis. In this canonical view, pectins are reduced to an amorphous matrix forming a ‘spacer’ that modulates cellulose fiber movements during turgor driven expansion. Over the last two decades however, a different picture gradually emerged that showed a critical role for pectins in cell wall expansion, beyond the simple cell wall padding. Nowadays the role for pectin in cell wall expansion is well accepted, but we only begin to understand the mechanism by which pectins contribute to growth. The major pectin polymers are the linear homogalacturonans (HGs). HGs are addressed to the cell wall in a high methylesterified form (HG^HM) Once incorporated in the cell wall, HG can be de-methylesterified by pectin methylesterase enzymes (PME) to form poly-anionic low-methylated HG (HG^LM). Over the last decade it was shown that this in muro pectin chemistry change plays a critical role in cell wall expansion and organogenesis. Such drastic importance for a simple chemical change of the polymer considered until then as merely a cell wall padding is complicated to understand in the current framework.

Methodological limitations

Revision of current models needs elaboration of new approaches.The polymeric structure of the cell wall is usually explored using quantitative biochemistry, a method that requires destruction of the native structure of the tissue. The cell wall architecture has been observed using electron microscopy (EM), X ray diffraction, and AFM. Any form of fibrous structure observed is usually assigned to as cellulose microfibrils by default and little is known about the organization of the matrix polysaccharides. Electron microscopy provides high resolution but without reliable information on the chemical composition, nor on the three-dimensional structure.

Expanding beam model of plant growth revolution or anomaly?

Using multicolor 3D nanoscopy on anticlinal walls of pavement cells, we were able to observe that HG do not form an amorphous matrix within the cell wall but instead axially aligned nanofilaments with a diameter of ~20-30 nm (Figure 1). This is consistent with X-ray diffraction data obtained in the early 1980’s, which showed that, in vitro, HG could form a fibrous structure composed of axially aligned polymers. Interestingly, they also showed that de-methylesterification induced a transition from a compact hexagonal lattice to a more expanded rectangular lattice (Figure 2). These observations led us to formulate a new “Expanding Beam” growth model in which the cell wall has an intrinsic expansion capacity driven by HG de-methylesterification. We confirmed this model by showing that HG de-methylesterification with exogenous PME and in the absence of turgor pressure led to tissue expansion by a factor of 1.4 (Figure 2A), in accordance with the predicted volume change due to lattice reorganization. Polymer volume phase transitions are well-studied processes in polymer physics, but were rarely explored in the context of plant growth ⁸. It is potentially a universal growth and morphogenesis mechanism of walled organisms, and perhaps of any extracellular matrix in general. It solely requires a polymer that can acquire a charge in a controlled fashion leading to conformational change. Examples other than HG include alginates of brown algae, heparin sulphate and chitin, a major component of the extracellular matrix of fungi and insects. This discovery opens up a range of possibilities to understand, predict and control plant growth. If proven to be ubiquitous, it would be a major shift in the paradigm of plant growth.

Our model is the first one to propose a molecular mechanism for the role of pectins in growth. So far their role was explained merely in terms of changes in the cell wall elasticity. What are the microscopic determinants of the cell wall that project to macroscopic phenomenological properties, such as elasticity, is not known.

The observation of complex HG quaternary structures may spark a drift from curently acepted paradigm turgor pressure-driven reorganization of cellulosic network. Wheter it is an anomaly or a revolution should be a prioritized question in plant cell biology.

To learn more:

• Pectin homogalacturonan nanofilament expansion drives morphogenesis in plant epidermal cells, Science 2020, DOI: 10.1126/science.aaz5103. K. T. Haas^#, R. Wightman, E. M. Meyerowitz & A. Peaucelle^#;