Brittle culm 25, which encodes an UDP-xylose synthase, affects cell wall properties in rice

Because plant mechanical strength influences plant growth and development, the regulatory mechanisms underlying cell-wall synthesis deserve investigation. Rice mutants are useful for such research. We have identified a novel brittle culm 25 (bc25) mutant with reduced growth and partial sterility. BC25 encodes an UDP-glucuronic acid decarboxylase involved in cellulose synthesis and belongs to the UXS family. A single-nucleotide mutation in BC25 accounts for its altered cell morphology and cell-wall composition. Transmission electron microscopy analysis showed that the thickness of the secondary cell wall was reduced in bc25. Monosaccharide analysis revealed significant increases in content of rhamnose and arabinose but not of other monosaccharides, indicating that BC25 was involved in xylose synthesis with some level of functional redundancy. Enzymatic assays suggested that BC25 functions with high activity to interconvert UDP-glucuronic acid (UDP-GlcA) and UDP-xylose. GUS staining showed that BC25 was ubiquitously expressed with higher expression in culm, root and sheath, in agreement with that shown by quantitative real-time (qRT)-PCR. RNA-seq further suggested that BC25 is involved in sugar metabolism. We conclude that BC25 strongly influences rice cell wall formation.