SECONDARY WALL ASSOCIATED MYB 1 is a positive regulator of secondary cell wall thickening in Brachypodium distachyon and is not found in the Brassicaceae

Grass biomass is comprised chiefly of secondary walls that surround fiber and xylem cells. A regulatory network of interacting transcription factors in part regulates cell wall thickening. We identified Brachypodium distachyon SECONDARY WALL ASSOCIATED MYB 1 ( SWAM 1 ) as a potential regulator of secondary cell wall biosynthesis based on gene expression, phylogeny, and transgenic plant phenotypes. SWAM 1 interacts with cellulose and lignin gene promoters with preferential binding to AC ‐rich sequence motifs commonly found in the promoters of cell wall‐related genes. SWAM 1 overexpression ( SWAM ‐ OE ) lines had greater above‐ground biomass with only a slight change in flowering time while SWAM 1 dominant repressor ( SWAM 1‐ DR ) plants were severely dwarfed with a striking reduction in lignin of sclerenchyma fibers and stem epidermal cell length. Cellulose, hemicellulose, and lignin genes were significantly down‐regulated in SWAM 1‐ DR plants and up‐regulated in SWAM 1‐ OE plants. There was no reduction in bioconversion yield in SWAM 1‐ OE lines; however, it was significantly increased for SWAM 1‐ DR samples. Phylogenetic and syntenic analyses strongly suggest that the SWAM 1 clade was present in the last common ancestor between eudicots and grasses, but is not in the Brassicaceae. Collectively, these data suggest that SWAM 1 is a transcriptional activator of secondary cell wall thickening and biomass accumulation in B. distachyon . Feed‐forward transcription regulatory loops activate the thickening of secondary cell walls in Arabidopsis thaliana . Here we report on the MYB transcription SWAM1, which is present in grasses and many eudicotyledons, but not the Brassicaceae. It directly activates grass cell wall biosynthesis and transgenic lines exhibit rather striking and agronomically important phenotypes.