KNAT 2/6b , a class I KNOX gene, impedes xylem differentiation by regulating NAC domain transcription factors in poplar

Wood formation is the terminal differentiation of xylem mother cells derived from cambial initials, and negative regulators play important roles in xylem differentiation. The molecular mechanism of the negative regulator of xylem differentiation Pag KNAT 2/6b was investigated. Pag KNAT 2/6b is an ortholog of Arabidopsis KNAT 2 and KNAT 6 that is highly expressed in phloem and xylem. Compared to nontransgenic control plants, transgenic poplar plants overexpressing Pag KNAT 2/6b present with altered vascular patterns, characterized by decreased secondary xylem with thin cell walls containing less cellulose, xylose and lignin. RNA sequencing analyses revealed that differentially expressed genes are enriched in xylem differentiation and secondary wall synthesis functions. Expression of NAM / ATAF / CUC ( NAC ) domain genes including Pag SND 1‐A1 , Pag SND 1‐A2 , Pag SND 1‐B2 and Pag VND 6‐C1 is downregulated by Pag KNAT 2/6b , while Pag XND 1a is directly upregulated. Accordingly, the dominant repression form of Pag KNAT 2/6b leads to increased xylem width per stem diameter through downregulation of Pag XND 1a . Pag KNAT 2/6b can inhibit cell differentiation and secondary wall deposition during wood formation in poplar by modulating the expression of NAC domain transcription factors. Direct activation of Pag XND 1a by Pag KNAT 2/6b is a key node in the negative regulatory network of xylem differentiation by KNOX s.