KNAT2/6b, a class I KNOX gene, impedes xylem differentiation by regulating NAC domain transcription factors in poplar

Abstract

Summary 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 PagKNAT2/6b was investigated. PagKNAT2/6b is an ortholog of Arabidopsis KNAT2 and KNAT6 that is highly expressed in phloem and xylem. Compared to nontransgenic control plants, transgenic poplar plants overexpressing PagKNAT2/6b 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 PagSND1‐A1, PagSND1‐A2, PagSND1‐B2 and PagVND6‐C1 is downregulated by PagKNAT2/6b, while PagXND1a is directly upregulated. Accordingly, the dominant repression form of PagKNAT2/6b leads to increased xylem width per stem diameter through downregulation of PagXND1a. PagKNAT2/6b 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 PagXND1a by PagKNAT2/6b is a key node in the negative regulatory network of xylem differentiation by KNOXs.

Contribution of phenylpropanoid metabolism to plant development and plant–environment interactions

“…MYB6 competes with MYB75 for the binding of KNAT7 and disrupts the interaction of MYB75 and KNAT7 to repress the lignin biosynthesis and promotes flavonoid biosynthesis (An et al, 2019c). In hybrid poplar ( Populus alba × glandulosa ), PagKNAT2 and PagKNAT6b repress the expression of NAC domain genes such as PagSND1‐A1 , PagSND1‐A2 , PagSND1‐B2 , and PagVND6‐C1 , and inhibit cell differentiation, lignin biosynthesis, and secondary wall deposition (Zhao et al, 2020). The NAC TFs are also under the feedback regulation control mediated by MYB proteins.…”

Overexpression of PtrMYB121 Positively Regulates the Formation of Secondary Cell Wall in Arabidopsis thaliana

“…The ARK1 gene from the ARBORKNOX family reduced the lignin content by downregulating lignin synthesis-related genes [ 59 ]. KNAT 2/6b, one member of the class I KNOX genes, inhibited the xylem differentiation by regulating NAC TF in poplar [ 60 ]. The overexpression of PdOLP1 , an osmotin-like protein gene in poplar, caused a reduction in the radial width and cell layer number in xylem and phloem zones, and downregulated expression of genes involved in lignin biosynthesis, and in the fibers and vessels of xylem cell walls [ 61 ].…”

Molecular Changes Concomitant with Vascular System Development in Mature Galls Induced by Root-Knot Nematodes in the Model Tree Host Populus tremula × P. alba

“…Heterodimers between BLH and KNAT are able to repress the expression of monolignol biosynthetic genes [54], while BLH6 is able to repress SCW formation [55]. Another TF from the KNOX family, KNAT2/6b, inhibits the differentiation of xylem cells through repression of several NAC and MYB TFs [56]. The development of the gall vascular system may therefore be associated with a repression of genes preventing the differentiation of these specialised cells, possibly guaranteeing sugar and water sources throughout the nematode life cycle.…”

“…The development of the gall vascular system may therefore be associated with a repression of genes preventing the differentiation of these specialised cells, possibly guaranteeing sugar and water sources throughout the nematode life cycle. Since these genes are, during normal development, co-expressed with the positive regulators of this biological process [50,55,56], we may hypothesise that their down-regulation in the galls is tuned by an exogenous factor, as for example a compound secreted by the nematode [57]. Finally, one can hypothesise that the down-regulation of these repressors leads to drastic xylem development to support the completion of the nematode life cycle.…”