A novel rice fragile culm 24 mutant encodes a UDP-glucose epimerase that affects cell wall property and photosynthesis

UDP-glucose epimerases (UGEs) are essential enzymes to catalyze UDP-glucose (UDP-Glc) conversion into UDP-galactose (UDP-Gal). Although UDP-Gal is examined as the substrate for the biosynthesis of carbohydrates, glycolipids and glycoproteins, much remains unknown about the biological function of UGEs in plants. In this study, we selected a novel rice fragile culm 24 (Osfc24) mutant and identified it as a nonsense mutation of the FC24/OsUGE2 gene. The Osfc24 mutant shows a typical brittleness phenotype with significantly altered cell wall composition and disrupted cellulose microfibril orientation. We also examined much less arabinogalactan protein (AGP) accumulation into cell walls, which may consequently affect plant growth, cell wall deposition and cellulose microfibril orientation in the Osfc24 mutant. Meanwhile, this mutant exhibits dwarfism and less-green-leaf phenotypes with remarkably decreased galactolipids and chlorophyll contents, resulting in defect of plant photosynthesis to some degree. Based on all findings obtained in this study, we propose a mechanism model highlighting how OsUGE2 participates in two distinct metabolism pathways to co-modulate cellulose biosynthesis, wall assembly by dynamically providing UDP-Gal and UDP-Glc substrates. Therefore, this study has provided insights into plant photosynthesis and cell wall biosynthesis for plant growth and biomass production in rice and beyond.