KNAT7 modulates xylan biosynthesis in Arabidopsis seed coat mucilage.

As a major hemicellulosic component of plant cell walls, xylan plays a determinative role in maintaining cell wall structure. However, the transcriptional regulation mechanisms of xylan biosynthesis remain largely unknown. The Arabidopsis seed mucilage represents an ideal system for studying polysaccharide biosynthesis and modifications of plant cell walls. Here, we identify KNOTTED ARABIDOPSIS THALIANA 7 (KNAT7) as a positive transcriptional regulator of xylan biosynthesis in seed mucilage. Xylan content was significantly reduced in the knat7-3 mucilage accompanied by significantly reduced expression of xylan biosynthesis-related genes IRREGULAR XYLEM 14 (IRX14) and MUCILAGE MODIFIED 5/MUCILAGE-RELATED 21 (MUM5/MUCI21). The electrophoretic mobility shift assay (EMSA), yeast one-hybrid (Y1H), and chromatin immunoprecipitation with quantitative PCR (ChIP-qPCR) assays verified the direct binding of KNAT7 to the KNOTTED1 (KN1) binding site [KBS,TGACAG(G/C)T] in promoters of IRX7, IRX14 and MUM5/MUCI21 in vitro, in vivo and in planta. Furthermore, KNAT7 directly activated the expression of IRX14 and MUM5/MUCI21 in transactivation assays in mesophyll protoplasts. Additionally, overexpression of IRX14 or MUM5/MUCI21 in knat7-3 partially rescued the defects in mucilage adherence. Collectively, our results revealed that KNAT7 positively regulates xylan biosynthesis in seed coat mucilage via directly activating the expression of IRX14 and MUM5/MUCI21.