Cell wall Microdomain molecular scaffolds control plant cell Wall dynamics – MicroWall

MicroWall aims (i) at establishing the proof of concept by characterizing a pioneer example of molecular scaffold (PMEI6/HG/PRX36) within a cell wall microdomain implicated in Arabidopsis seed development, for which we had preliminary evidence (WP1 and WP2), and (ii) to provide proofs that this example belong to a wider universal concept with other combinations of molecular interactions between members of the same multigenic families in Arabidopsis as well as in flax (WP3).

Granting of MicroWall in July 2018 enabled pre-founding of experiments required by reviewers leading to the publication of a first article in January 2019 (Francoz et al (2019) Dev Cell). This pioneer article set the basis of the concept of MicroWall and answered to part of WP1. We demonstrated through multidisciplinary approaches (bioinformatics, genetics, cell biology), the relationship between 2 proteinaceous actors (PMEI6 and PRX36) and a polysaccharidic actor (partially demethyesterified HG pattern): During Arabidopsis seed development, and within a cell wall microdomain located in mucilage secretory cells, PMEI6 controls the formation of a partially demethylesterified HG pattern that enables the anchoring of PRX36. This peroxidase can then act as a loosening enzyme specifically at this microdomain position. The consequence is a «pre-fragilization« of the surrounding of seeds during their development enabling to properly release a mucilage during mature seed imbibition, in turn favouring correct germination.

The project is currently focused on further demonstration of these molecular interactions using biochemical approaches and by screening candidate combinations of other members of the studied multigenic families.

1. Francoz E, Ranocha P, Le Ru A, Martinez Y, Fourquaux I, Jauneau A, Dunand C, Burlat V (2019) Pectin demethylesterification generates platforms that anchor peroxidases to remodel plant cell wall domains. Dev Cell. 48(2):261-276. doi: 10.1016/j.devcel.2018.11.016.
2. Viudes S, Burlat V, Dunand C (2020) Seed mucilage evolution: Diverse molecular mechanisms generate versatile ecological functions for particular environments. Plant Cell Environ. doi: 10.1111/pce.13827.
3. Jemmat AM, Ranocha P, Le Ru A, Neel M, Jauneau A, Raggi S, Ferrari S, Burlat V, Dunand C (2020) Coordination of five class III peroxidase-encoding genes for early germination events of Arabidopsis thaliana. Plant Sci. 298:110565. doi: 10.1016/j.plantsci.2020.110565.

The industrial use of plant cell walls (CWs) is impacted by their heterogeneity and dynamics. Indeed, CW composition (e.g. polysaccharides, proteins) displays multiscale spatiotemporal specificities (e.g. evolutionary, developmental, cellular, subcellular). In some instances, this heterogeneity positively impacts CW use (e.g. cotton or flax fibers have been selected for millennia for their mechanical properties, now understood as corresponding to particular cellulose-enriched CWs). In other cases, CW heterogeneity negatively impacts CW use (e.g. the pulp and paper or biofuel industries need abundant homogeneous and easy-to-process CW material). CW heterogeneity is still poorly understood at the subcellular scale. Indeed, CWs may be viewed as the assembly of multiple microdomains that are growingly described through various (immuno)labellings. However, the molecular interactions and functions of these microdomains remain obscure.
‘MicroWall’ will uncover scaffolds of molecular interactions within CW microdomains and will provide functional roles for these polarised CW molecular scaffolds. The molecular components of particular interest will be CW proteins encoded by multigenic families and various patterns of a highly variable CW polysaccharide. The 3 multigenic families particularly studied will be Class III peroxidases (PRXs) for their dual roles of CW loosening or CW stiffening, pectin methylesterases (PMEs) and pectin methylesterase inhibitors (PMEIs) that control the methylesterification degree of homogalacturonan (HG) pectin domains. The specific HG patterns constitute the highly variable CW polysaccharide hypothesized to enable positioning of specific PRXs to specific individual CW microdomains through specific molecular interactions. These specific PRX localisations will contribute to CW dynamics through either polarized CW loosening or stiffening at the position of the individual CW microdomains. MicroWall aims at (i) establishing the proof of concept through the extensive characterization of one particular CW microdomain molecular scaffold involved in Arabidopsis seed development and for which we have convincing preliminary data, and (ii) providing evidence that this example is part of a more universal concept. The first goal of this project (WP1 and 2) will be achieved through the extensive pluridisciplinary characterization of a partially methylesterified HG microdomain created, during Arabidopsis seed development in the outer CW of mucilage secretory cells, by a yet-to-be discovered PME that is regulated by PMEI6. In turn, this HG microdomain is expected to enable PRX36 specific anchoring during seed mucilage secretory cell development. This accurate PRX36 anchoring will sequentially allow (i) loosening this CW microdomain during seed development, (ii) proper rupture of the loosened polarised CW during mature seed imbibition and (iii) correct mucilage release and efficient germination. The second goal of this project will be achieved through the transposition of this proof of concept HG/PMEI6/PRX36 model to flax mucilage secretory cell development (WP1), and through the search of additional similar CW microdomains involving other HG methylesterification patterns and other PRXs, PMEs and PMEIs co-expressed during mucilage secretory cell development (WP3). Indeed, the rationale is that individual members of these multigenic families that are co-expressed in a single cell may have non redundant functions because of their accurate positioning in CW microdomains of this individual cell. Beyond, these examples dedicated to the understanding of CW dynamics during seed mucilage secretory cell development, and since the molecular actors of these scaffolds are universal along plant development and evolution, this fundamental knowledge proposing putative functions for hundreds of protein and polysaccharide CW components, will be crucial for future industrial use of plant CWs in various contexts.