MicroWall

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.