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Romanesco broccoli grows in a geometric shape called a fractal. The central structure is a meristem that produces small primordial meristems that induce new meristems, which gives rise to the fractal shape. © INRA, LAUFS Patrick

Shaping up: how plants take form

The first manifestation of cell growth: cell wall expansion

Plant cell walls are characterised by their rigidity, which begs the question: how are they able to change shape as cells grow? Research has shed light on certain molecular mechanisms that are involved in this phenomenon. Herman Höfte offers some insight.

By Pascale Mollier, translated by Teri Jones-Villeneuve
Updated on 02/10/2016
Published on 12/15/2015

. © INRA

Why are you looking into the molecular aspects of cell wall growth?

Herman Höfte: Under certain environmental stresses – such as osmotic stress related to excess salts – the cell wall is the first to feel the effects. Our latest research has shown the crucial importance of a particular enzyme family that acts on pectin in the cell wall. These enzymes, called pectin methylesterases (PME) play a very early role in cell wall growth, increasing both elasticity and extensibility. They are also involved in the salinity stress response as they create a reserve of demethylated pectins that trap salts as well as heavy metals. We’re currently working on a project with the French National Research Agency (ANR) to study the relationships between the activity of these PMEs and abiotic stress tolerance. For instance, there are willow populations that are naturally capable of fixing heavy metals by activating PMEs. These enzymes also play a role in pest attacks. In this case, they are suppressed, which in turn leads to a hardening of the cell walls, preventing nematodes from entering or insect gall formation (e.g., phylloxerans). These examples show that very basic mechanisms can help us understand highly complex interactions between plants and the environment.

How do these enzymes work?

H. H.: PMEs (there are around sixty just in the model plant Arabidopsis!) are expressed as soon as the cell wall starts expanding – they are responsible for increasing their elasticity. These enzymes do this by breaking down methyl groups in an acidic compound in pectin, directly altering the pH. This results in changes in the cell wall’s elasticity via another well-known category of molecules, expansins, which were discovered in the early 2000s. Strangely enough, although expansins are not associated with enzyme activity, they are able to disrupt adhesion of hemicelluloses, which hold cellulose fibrils together and make the cell wall more rigid. We believe that the process occurs in this order: PME synthesis, influence on the pH, expansion activation, increased membrane elasticity. Auxin is one of the factors that triggers PME synthesis via its action on pH, because PME synthesis is reliant on the pH level

How is the direction of cell wall growth – and therefore that of the cell – determined?

H. H.: We know that the synthesis of cellulose microfibrils (each microfibril consists of 18 cellulose chains) happens on the outer side of the cell wall. The enzyme that synthesises cellulose – cellulose synthase – follows the microtubules, which are located along the inside of the cell wall. As the film shows (1), cellulose synthase moves along the microtubules by extruding the cellulose chains, like a spider extrudes its silk. This results in cellulose being deposited in the same direction as the microtubules, and we know that this directionality is a result of the mechanical forces the cells exert on one another, similar to a metal hoop around a barrel (2). From this hoop and based on the effect of the cell’s elongation, the cell wall’s polymers (pectin and cellulose) rearrange themselves and stretch perpendicular to the rigid hoop. We can conclude from this that the direction of cell wall growth depends on that of the microtubules. In reality, there is another phenomenon that occurs before all this, during which the cell wall loses its symmetry and acquires localised, or anisotropic, properties. In 2015, we were able to show that PMEs (yet again!) also play a role in this mechanism, even if we don’t quite know the specifics. The direction of cell wall growth precedes the reorientation of the microtubules, which simply reinforce it.

Where is research headed in this area?

H. H.: There are still many things we don’t understand in the overall process of cell wall expansion. We need to create a model that integrates the mechanical forces (that redirect the microtubules); auxin, which affects pH; and the control over cell wall extensibility, via its two main influences, PMEs and expansins. There are surely others that remain to be discovered. Finally, there are also mechanisms of depolymerisation/repolymerisation involved in redirecting the microtubules that are still not well understood.

(1) Film by Samantha Vernhettes

(2) See section 3

Scientific contact(s):

Associated Division(s):
Plant Biology and Breeding, Science for Food and Bioproduct Engineering
Associated Centre(s):


-  Peaucelle A. et al., 2015. The control of growth symmetry breaking in the Arabidopsis hypocotyl. Current Biology 25, 1746–1752.

-  Peaucelle A., Braybrook S. and Höfte H. 2012 Cell wall mechanics and growth control in plants: the role of pectins revisited. Frontiers in plant science 3-121, DOI: 10.3389/fpls.2012.00121

-  Peaucelle A. et al. 2011. Pectin-induced changes in cell wall mechanics underlie organ initiation in Arabidopsis. Current Biology 21, 1720–1726.

. © Creative Commons Wikipedia

Cell wall structure

Diagram of the pectocellulosic cell wall structure. The diagram shows the cellulose fibres, covered with pectins (in green) and hemicelluloses (in red).After de-methyl-esterification, the pectins, now negatively charged, can form networks linked by bridges with positively charged calcium ions. The structure is woven together by HRGP (hydroxyproline-rich glycoproteins, in black), which reduces extensibility.