Deciphering plant cell WALL remodeling using mechanical measurements and bioMIMEtic models – WALLMIME

The plant primary cell wall is a complex structure composed of polysaccharides and proteins. It plays a central role in the control of plant growth and development, therefore in the production of biomass. Pectins are major components of the primary cell wall, representing up to one third of the wall dry mass, and are widely used in the food industry, as gelling agents. Among pectins, homogalacturonan (HG) is a homopolymer of ?-1,4-linked-D-galacturonic acid units that can be methylesterified and acetylated. Over the recent years, HG-type pectins have been reported as major actors of the modulation of the mechanical properties of the cell wall. They can mediate changes in growth, through the action of remodeling enzymes that fine-tune the degree of polymerization (DP) - polygalacturonase (PG) or pectin/pectate lyases (PLLs) - or of methyesterification/acetylation (DM/DA) of the polymer - pectin methylesterase, PME or pectin acetylesterase, PAE. Up to now, the combination of developmental biology and biophysics has started to shed light on the cell wall mechanical effectors that mediate changes in cell shape. However, our understanding of the mode of action of pectin remodeling enzymes is impaired by gene redundancy (enzymes are all encoded by multigenic families of 12 to 69 members), as well as by the occurrence of compensation mechanisms among gene families and cell wall polymers. The objectives of the WALLMIME project will be thus to: i) Biochemically characterize a set of HG remodeling enzymes (PME, PAE, PG, PLL), ii) Comprehensively follow their effects upon application on both etiolated hypocotyls and biomimetic model of cell wall, iii) Build a model of the dynamics of HG-pectin remodeling. To tackle these objectives, the WALLMIME project will use two distinct models. On one hand, mimetic membranes/films combining cellulose microfibrils and pectins of various structures (DM, DA, DP), will be used as a simplified well-controlled system to determine the role of pH, ionic strength and calcium ions concentration, which are key effectors of apoplast homeostasis, on the dynamic of the pectic network. On the other hand, Arabidopsis dark-grown hypocotyl, a standard developmental model that is solely characterized by cell elongation, will be used to relate the modifications of the pectin network to phenotypical changes. The WALLMIME project will be organised in 4 scientific Work Packages (WP) and one management WP. The originality of the organization lies in the fact that the three firsts scientific WP (WP1: Consequences of HG-enzymes action on polymer structure and cell integrity, WP2: Consequences of HG-enzymes action on mechanical properties, and WP3: Consequences of HG-enzymes action on elongation/deformation) will be performed on the Arabidopsis hypocotyl and on mimetic cell wall in parallel. This will allow getting the best benefit from the highly complementary expertise of the three partners. WP4 (Integrated model of the dynamics of pectic network) will build a model of the dynamics of pectic network using the experimental data generated in WP1 to 3. Bringing experts from various fields (enzymology, biophysics, polysaccharide chemistry, glycomaterials), WALLMIME will drastically increase our knowledge on how plants can fine-tune their cell wall pectic matrix and how this can mediate changes in plant growth. This is of the utmost importance to understand how plants can respond to the environment and can produce biomass. The project will further challenge the current concepts of plant cell wall and will contribute to setting new standards for dynamic cell wall models. This is currently a highly competitive field for which the project will be an original contribution, as it combines the characterization of the effects of enzymes on plant material and biomimetic models. It will thus contribute to a better understanding of the enzymes’ mode of action on substrates, paving the way for novel use of plant enzymes in glycomaterials.