Computational models of the role of pectins in plant cell wall structure

While it is established that pectins play a significant role in the structure of plant cell walls, little is known about the mechanisms pectins use to control cell architecture and growth. Recent experiments have shown that mutations in Rhamnose Biosynthesis 1 in A. thaliana give rise to a cellular phenotype characterized by a left-handed helical twist. These mutations suppress the synthesis of the pectic polysaccharide, rhamnogalacutonan I (RG-I). Mutations corresponding to other pectic polysaccharides fail to produce chiral phenotypes. It has also been widely observed that cellulose nanocrystals form left-handed chiral nematic phases. We hypothesize that the affinity of RG-I with cellulose microfibrils prevents the formation of a helical phase in wildtype cells. We have developed a multi-scale modeling approach to probe this system. Our all-atom models are able to recapitulate the backbone dihedral angles of cell wall polysaccharides and assess how efficiently pectins and cellulose intertwine and pack. To understand the formation of the left-handed helical twist of the mutants, we perform MD simulations of coarse-grained cellulose nanocrystals and explore the self-assembly of left-handed chiral nematic phases over a range of concentrations of pectic polysaccharides.