OUP accepted manuscript

The production of Arabidopsis seed mucilage involves complex polysaccharide biosynthetic pathways and developmental processes in seed epidermal cells. Although the polysaccharide components of Arabidopsis seed mucilage have been clarified, their regulatory mechanism requires further investigation. Here, we show that the Class II KNOX gene family members KNAT3 and KNAT7 play an essential role in regulating mucilage production in the early developmental stages of Arabidopsis seeds. Mutations in both KNAT3 and KNAT7 resulted in defective seed mucilage production and columellae formation, whereas knat3 showed a normal phenotype compared with wildtype and the thickness of mucilage of knat7 was slightly disturbed. Rhamnogalacturonan I (RG-I) and its biosynthetic substrate GalA and Rha were reduced in both the adherent and soluble mucilage of knat3kant7. Comparative transcriptome analysis on the whole seeds suggested that polysaccharide, glucosinolate and anthocyanin biosynthetic pathways were specifically repressed in knat3knat7. Transient co-expression of KNAT3 and KNAT7 with promoter region of candidate genes in Arabidopsis protoplast revealed that both KNAT3 and KNAT7 act as positive regulators of the RG-I biosynthetic gene MUM4. Collectively, our results demonstrate that KNAT3 and KNAT7 are multifunctional transcription factors in cell wall development and at least partially redundantly modulate mucilage biosynthesis in Arabidopsis seed.