Overexpression of SbMyb60 in Sorghum Bicolor Alters Secondary Cell Wall Composition and Impacts Transcriptional Expression of Both Primary and Secondary Metabolic Pathways

C4 grasses are rapidly emerging as dedicated resources for the production of energy, biofuels and bioproducts, including Sorghum bicolor (sorghum), a climate resilient feedstock that can be grown on low quality or marginal lands with minimal agricultural inputs. The phenylpropanoid pathway that generates lignin subunits represents a significant target for altering the abundance and composition of lignin in sorghum and other C4 grasses. Previously, brown midrib (bmr) mutants have been used for lowering lignin to improve biomass for cellulosic ethanol and more recently, a myb transcription factor (SbMyb60) was identified as a positive regulator of monolignol biosynthesis in sorghum. Ectopic expression of SbMyb60 was associated with higher expression of genes involved in monolignol biosynthesis, and led to higher abundances of syringyl lignin, significant compositional changes to the lignin polymer and increased lignin concentration in biomass. Moreover, SbMyb60 plants also displayed ectopic lignification in leaf midribs and stems, differential distribution of cellulose in stems, and elevated concentrations of soluble and wall-bound phenolic compounds in biomass. Further, RNA-Seq profiling of SbMyb60 leaves and stalks revealed that several metabolic pathways responsible for synthesizing several cofactors required for monolignol biosynthesis were also stimulated (ie, SAM, folate, CoA, NADPH and NADH, and methionine). Metabolite profiling using LC/MS confirmed accumulation of several primary metabolites. This study elucidates the role of SbMyb60 in secondary wall biosynthesis, and its impacts on primary metabolism in sorghum. SbMyb60 represents a target for modification of plant cell wall composition, which has the potential to improve biomass for renewable uses.