Unraveling the role of KNOTTED OF BRACHYPODIUM 7 in grass-specific cell wall development

The synthesis and deposition of cellulose, hemicellulose, and lignin in xylem and fiber cells is a key aspect of plant growth. KNOTTED-LIKE HOMEOBOX OF ARABIDOPSIS THALIANA 7 (KNAT7) is a transcriptional repressor of secondary cell wall thickening. A rabidopsis thaliana mutants have thicker interfascicular fiber walls and more lignin. Mutants of the closest rice ortholog show thicker interfascicular fiber walls, but without a reported change in lignin content. To further investigation the function of this genes in regulating secondary cell wall synthesis in grasses, mutant and overexpression lines of the Brachypodium distachyon KNAT7 ortholog, KNOTTED OF BRACHYPODIUM 7 (KNOB7) were generated. Stem interfascicular fiber wall thickness increased in the mutants and decreased in KNOB7 overexpressing plants. A similar trend was observed in the mutant related to the lignin composition, hydroxycinnamic acids and cell wall polysaccharide content. DNA affinity purification sequencing revealed 3,228 binding sites. Target genes were significantly enriched for phenylpropanoid pathway genes. The DNA target sequence of KNOB7 protein was a W-box like sequence, TTGAC. Our findings reveal that KNOB7 interacts with the W-box element to repress lignin biosynthesis and wall thickening.