Overexpression of SbMyb60 in sorghum bicolor impacts both primary and secondary metabolism

Authors: Scully, Erin; Gries, Tammy; Palmer, Nathan - Nate; Sarath, Gautam; Funnell-Harris, Deanna; BAIRD, LISA - University Of San Diego; TWIGG, PAUL - University Of Nebraska; SERAVALLI, JAVIER - University Of Nebraska; CLEMENTE, THOMAS - University Of Nebraska; Sattler, Scott

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2017
Publication Date: 9/25/2017
Publication URL: http://handle.nal.usda.gov/10113/5867544
Citation: Scully, E.D., Gries, T.L., Palmer, N.A., Sarath, G., Funnell-Harris, D.L., Baird, L., Twigg, P., Seravalli, J., Clemente, T.E., Sattler, S.E. 2017. Overexpression of SbMyb60 in sorghum bicolor impacts both primary and secondary metabolism. New Phytologist. 217(1):82-104. doi:10.1111/nph.14815.
DOI: https://doi.org/10.1111/nph.14815

Interpretive Summary: Sorghum is a climate resilient grass commonly grown in the United States as a forage crop and is currently being developed as a crop for biofuels production. Lignin is the major structural component of plant cell walls (biomass) and the abundance and composition of this compound determines how plant biomass can be used for production of biofuels and other natural products. Despite the importance of lignin for biomass quality, few factors have been identified in sorghum that can be used to manipulate the amount of lignin in plant cell walls for use in thermal biofuels production. Previously, the SbMyb60 gene was identified as an activator of lignin synthesis. Increasing the expression of this gene activated the biochemical pathway that synthesizes subunits of lignin, which ultimately led to higher lignin levels in sorghum biomass. In the current study, the effects of SbMyb60 on other biochemical pathways were examined in order to better understand its role in controlling lignin synthesis. The results of this study showed that SbMyb60 not only affects lignin synthesis, but that it also redirects carbon and nitrogen towards lignin production. SbMyb60 increases the synthesis of the amino acid phenylalanine and other cofactors required for lignin synthesis. Therefore, SbMyb60 affects biochemical pathways unrelated to lignin synthesis. This study increases our understanding of how plants direct resources to synthesize cell walls, which are necessary for the structure and strength of plants and influence their usability for biofuels. Our future goal is to use these results to improve biomass composition for forage and bioenergy uses in sorghum and other crops.

Technical Abstract: Few transcription factors have been identified in C4 grasses that either positively or negatively regulate monolignol biosynthesis. Previously, overexpression of SbMyb60 in sorghum (Sorghum bicolor (L.) Moench) was shown to induce monolignol synthesis, which led to elevated lignin deposition and altered cell wall composition. To determine how SbMyb60 overexpression impacts other metabolic pathways, RNA-Seq and metabolite profiling were performed on stalks and leaves. 35S::SbMyb60 was associated with transcriptional activation of genes involved in aromatic amino acid, SAM, and folate biosynthetic pathways. High coexpression values between SbMyb60 and genes assigned to these pathways indicates that SbMyb60 may directly induce their expression. In addition, 35S::SbMyb60 altered the expression of genes involved in N assimilation and C metabolism, which may redirect C and N towards monolignol synthesis. Genes linked to UDP-sugar biosynthesis and cellulose synthesis were also induced, which is consistent with the observed increased cellulose deposition in the internodes of 35S::SbMyb60 plants. However, SbMyb60 had low coexpression values with these genes and is not likely a direct regulator of cell wall polysaccharide synthesis. These findings indicate that SbMyb60 can activate pathways beyond monolignol biosynthesis including pathways that synthesize substrates and cofactors required for lignin synthesis.