Xylan Biosynthesis in Grasses: Uncovering Specific Protein-Protein Interactions (PPIs) between Rice Members of the GT43 and GT47 Families and their Implication in Plant Development

Characterization of xylan synthase complexes (XSCs) responsible for xylan biosynthesis is currently lacking. Unlike Arabidopsis, multiprotein xylan synthase complexes have been isolated from monocots which have a central core complex made-up of members of the GT43 and GT47 families. The fact that monocots have expanded members of these GT43 families opens the question of whether there is a functional diversification in xylan synthesis in grasses through the formation of various "GT43/GT47" complexes. In this dissertation, a combination of gene association network (GAN) and co-expression approaches in rice was used to identify clusters of four GT43 and four GT47 members that potentially interact to form at least three distinct “OsGT43/OsGT47” core complexes of different XSCs. Using bimolecular fluorescence complementation (BiFC), I demonstrated that these “OsGT43/OsGT47” core complexes are the result of specific protein-protein interactions (PPIs) between OsGT43s and OsGT47s within each complex. I also demonstrated that these core complexes assemble in the endoplasmic reticulum before export to the Golgi. To investigate the physiological function of these complexes, allelic knockout mutants in the four OsGT43s (OsGT43B, OsGT43F, OsGT43I, and OsGT43J) were generated using CRISPR/CAS 9 technology. While mutations in OsGT43F (closest homolog to Arabidopsis IRX9L) resulted in stunted plant growth and smaller panicles, mutations in OsGT43B and OsGT43J (closest homologs to Arabidopsis IRX9 and IRX14/IRX14L, respectively) had mild phenotypes. Surprisingly, only mutations in OsGT43B and OsGT43J resulted in an irregular xylem (IRX) phenotype observed in Arabidopsis. Thus, the stunted plant growth in rice mutants does not correlate with the IRX phenotype. My results indicate a functional differentiation between OsGT43s within the IRX9/IRX9L group, as the rice homolog to IRX9L seems to play a more important role in rice plant growth, while in Arabidopsis, IRX9L performs a minor function and can partially substitute for IRX9. These new insights advance our understanding of heteroxylan biosynthesis and regulation in grasses.