When pectins and proteins meet: a new mechanism for assembling the plant wall

Plants have the extraordinary ability to use solar energy to convert atmospheric CO2 into sugars. These sugars constitute an almost inexhaustible source of energy and also serve as building blocks in the form of polysaccharides (long chains of sugars). With these bricks, the plants build a rampart around each cell: the wall. This wall serves both as a protective barrier and as support for a pressurized skeleton. The latter gives rigidity to the organs of the plant, like an inflatable mattress. Indeed, the wall is so robust that it can withstand considerable pressures inside the cell, reaching up to 10 times that of our atmosphere. Curiously, the presence of this wall does not prevent the enlargement of the cells when the plant grows.

A question then arises: how can this wall grow, without losing its integrity, which would cause the cell to explode? To understand this mechanism, scientists analyzed in detail the assembly process of this wall.

To do this, they studied the growth of the pollen tube of the thale cress, a model plant. Its cell wall includes two major components, including fibers and a matrix, mainly made up of pectins. Pectins are well known for their role as a gelling agent in the making of jams!

The IJPB "Primary Cell Wall" PAR team had already discovered that once deposited in the wall, pectins swell, following an enzymatic transformation giving them negative charges, and allow the expansion of the wall.

Here the same team in collaboration with researchers from the BIA Unit, the IINS, the Universities of Lausanne & Zurich in Switzerland and the University of Antwerp in Belgium, shows that these negatively charged pectins form a complex with positively charged peptides present on the surface of a wall protein. This interaction induces the condensation of the pectin, thus creating a cross-linked pattern which confers both strength and extensibility to the wall. The pollen tube wall of plants carrying mutations in peptides, or in their protein partner, does not show this cross-linked structure and disintegrates prematurely. The formation of the peptide-protein-pectin complex and the condensation of pectins have also been demonstrated in vitro. Legend: When bound to its LRX8 receptor (grey), the RALF4 peptide (pink) exposes an alkaline surface rich in lysine (K70, K87, K88) and arginine (R78, R89, R90), © Julia Santiago Cuellar, University of Lausanne

This study, which reveals the dual signaling and structural role of this peptide-protein-pectin complex to shape cell wall extension, is a source of inspiration for rationalizing new mechanisms of protein-polymer interactions. Beyond this, these results should facilitate the selection of new plant varieties adapted to climate change.

Finally, this work is a fine example of collaboration between the TRANSFORM and BAP INRAE divisions, which resulted in a first-rate scientific production.

Projects involved: 
> ANR « HOMEOWALL » coordinated by Herman Höfte PAR team leader
> ERC Starting grant « STORMTHEWALL » from Kalina Haas, PAR team 

Partners:
> The Plant Signaling Mechanisms Laboratory, Department of Plant Molecular Biology, University of Lausanne, 1015, Lausanne, Switzerland
> Université Paris-Saclay, INRAE, AgroParisTech, Institut Jean-Pierre Bourgin (IJPB),78000, Versailles, France
> Department of Plant Molecular Biology, University of Lausanne, 1015 Lausanne, Switzerland
> Electron Microscopy Facility, University of Lausanne, Lausanne, Switzerland
> UR Biopolymères, Interactions et Assemblages INRAE, UR1268 BIA, F-44300 Nantes, France
> Integrated Molecular Plant Physiology Research (IMPRES), Department of Biology, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
> Department of Plant and Microbial Biology & Zurich-Basel Plant Science Center, University of Zurich, 8008 Zurich, Switzerland
> IINS, CNRS UMR5297, University of Bordeaux, 33000 Bordeaux, France